The Basics of AC Motors

From tiny hysteresis motors that drive electric clocks to huge synchronous motors that develop more than 50,000 hp, the alternating current (AC) motor has become an indispensable part of everyday life. These omnipresent devices come in many varieties, but generally fall into two categories: synchronous and induction machines. The term “machine” emphasizes the fact that these devices can operate in two different modes. Electrical power causes the device to produce torque, making it a motor. But when the same machine receives input rotational energy, electricity is produced and it becomes a generator.


All rotating machines have two main sets of windings. One set of windings is mounted on a freely rotating shaft, or rotor. The other set, called the stator, is attached to the motor frame and remains stationary. In the case of a motor, the stator windings are energized with single-phase or polyphase voltage. Single-phase voltage is adequate for small machines up to about 5 hp, but polyphase voltages provide superior torque characteristics and simplify the starting process. Larger machines commonly use 3-phase voltage.

The current flowing through the stator windings produces a magnetic field, the polarity of which rotates about the center axis of the stator. When the rotor windings are magnetized, pairs of magnetic poles form and the rotor spins to keep the rotor poles aligned with the stator field. The number of poles determines the speed of the rotor, according to the equation

n = (120 x f)/p

where (n) is the synchronous speed in revolutions per minute, (f) is the electrical frequency in hertz, and (p) is the number of poles. When the machine is used as a generator, the “motoring” process basically is reversed.

Synchronous machines
Many generators and very large motors are categorized as synchronous machines, in which the rotor turns at the same speed as the stator field. This is desirable when the rotational speed or generated frequency must be precisely controlled. The synchronous design also lends itself to large devices. The largest synchronous generators are rated in excess of 2 million hp. But a means of applying voltage to the rotor windings is required, usually using carbon brushes and slip rings.

The slip ring is a conductive surface on the rotor shaft that is electrically connected to the rotor windings. The brushes are stationary and held snugly against the slip rings by spring pressure. These components complicate the motor construction, add cost to the motor design, and require considerable maintenance. As a result, a majority of the AC machines in service, particularly small- to medium-sized motors, belong to the other general category — the induction machine.

Induction machines
The induction machine uses electromagnetic induction to energize the rotor circuit. To further economize the design, most induction motors use cast rotor bars instead of a wound rotor. Because of their unique appearance, motors that use these rotor bars are called “squirrel cages” (Photo above).

The induction machine is essentially a transformer with the secondary winding free to rotate. The speed of rotation, however, isn't the same as that of the rotating stator field. The difference is called the slip, and determines the torque produced. A speed-torque curve illustrates the relationship between rotor speed and torque, and is essential when specifying a motor to drive a particular load.

Countless AC motor designs have come and gone over the years, but the principles of operation are common to all.

Induction motors

These motors are probably the simplest and most rugged of all electric motors. They consist of two basic electrical assemblies: the wound stator and the rotor assembly.

The rotor consists of laminated, cylindrical iron cores with slots for receiving the conductors. On early motors, the conductors were copper bars with ends welded to copper rings known as end rings. Viewed from the end, the rotor assembly resembles a squirrel cage, hence the name squirrel- cage motor is used to refer to induction motors. In modern induction motors, the most common type of rotor has cast-aluminum conductors and short-circuiting end rings. The rotor turns when the moving magnetic field induces a current in the shorted conductors. The speed at which the magnetic field rotates is the synchronous speed of the motor and is determined by the number of poles in the stator and the frequency of the power supply.



Where:
Ns = synchronous speed
f = frequency
P = number of poles

Synchronous speed is the absolute upper limit of motor speed. At synchronous speed, there is no difference between rotor speed and rotating field speed, so no voltage is induced in the rotor bars, hence no torque is developed. Therefore, when running, the rotor must rotate slower than the magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. This speed difference between the rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed:



Where:
s = slip
Ns = synchronous speed
Na = actual speed




Figure 4 - Typical speed-torque characteristics for Design A, B, C, and D motors.



Figure 5 - Rings in shaded-pole motor distort alternating field sufficiently to cause rotation.


Figure 6 - Split-phase windings in a twopole motor. Starting winding and running winding are 90 ° apart.


Figure 7 - Split-phase start induction motor.

Polyphase motors — NEMA classifies polyphase induction motors according to locked rotor torque and current, breakdown torque, pull up torque, and percent slip.

Locked rotor torque is the minimum torque that the motor develops at rest for all angular positions of the rotor at rated voltage and frequency.

Locked rotor current is the steady state current from the line at rated voltage and frequency with the rotor locked.

Breakdown torque is the maximum torque that the motor develops at rated voltage and frequency, without an abrupt drop in speed.

Pull up torque is the minimum torque developed during the period of acceleration from rest to the speed that breakdown torque occurs. Figure 4 illustrates typical speedtorque curves for NEMA Design A, B, C, and D motors.

Design A motors have a higher breakdown torque than Design B motors and are usually designed for a specific use. Slip is 5%, or less.

Design B motors account for most of the induction motors sold. Often referred to as general purpose motors, slip is 5% or less.

Design C motors have high starting torque with normal starting current and low slip. This design is normally used where breakaway loads are high at starting, but normally run at rated full load, and are not subject to high overload demands after running speed has been reached. Slip is 5% or less.

Design D motors exhibit high slip (5 to 13%), very high starting torque, low starting current, and low full load speed. Because of high slip, speed can drop when fluctuating loads are encountered. This design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve. These motors are usually available only on a special order basis.

Wound-rotor motors — Although the squirrel-cage induction motor is relatively inflexible with regard to speed and torque characteristics, a special wound-rotor version has controllable speed and torque. Application of wound-rotor motors is markedly different from squirrel-cage motors because of the accessibility of the rotor circuit. Various performance characteristics can be obtained by inserting different values of resistance in the rotor circuit.

Wound rotor motors are generally started with secondary resistance in the rotor circuit. This resistance is sequentially reduced to permit the motor to come up to speed. Thus the motor can develop substantial torque while limiting locked rotor current. The secondary resistance can be designed for continuous service to dissipate heat produced by continuous operation at reduced speed, frequent acceleration, or acceleration with a large inertia load. External resistance gives the motor a characteristic that results in a large drop in rpm for a fairly small change in load. Reduced speed is provided down to about 50%, rated speed, but efficiency is low.

Single-phase motors — These motors are commonly fractional-horsepower types, though integral sizes are generally available to 10 hp. The most common single phase motor types are shaded pole, split phase, capacitorstart, and permanent split capacitor.

Shaded pole motors have a continuous copper loop wound around a small portion of each pole, Figure 5. The loop causes the magnetic field through the ringed portion to lag behind the field in the unringed portion. This produces a slightly rotating field in each pole face sufficient to turn the rotor. As the rotor accelerates, its torque increases and rated speed is reached. Shaded pole motors have low starting torque and are available only in fractional and subfractional horsepower sizes. Slip is about 10%, or more at rated load.

Split phase motors, Figure 6, use both a starting and running winding. The starting winding is displaced 90 electrical degrees from the running winding. The running winding has many turns of large diameter wire wound in the bottom of the stator slots to get high reactance. Therefore, the current in the starting winding leads the current in the running winding, causing a rotating field. During startup, both windings are connected to the line, Figure 7. As the motor comes up to speed (at about 25% of full-load speed), a centrifugal switch actuated by the rotor, or an electronic switch, disconnects the starting winding. Split phase motors are considered low or moderate starting torque motors and are limited to about 1/3 hp.

Capacitor-start motors are similar to split phase motors. The main difference is that a capacitor is placed in series with the auxiliary winding, Figure 8. This type of motor produces greater locked rotor and accelerating torque per ampere than does the split phase motor. Sizes range from fractional to 10 hp at 900 to 3600 rpm.

Split-capacitor motors also have an auxiliary winding with a capacitor, but they remain continuously energized and aid in producing a higher power factor than other capacitor designs. This makes them well suited to variable speed applications.

Stepper Motor

Stepper motors offer many advantages. Although feedback is not usually required, stepper motors are compatible with feedback signals, either analog or digital. Error is noncumulative as long as pulse-to-step integrity is maintained by the stepper motor. A stream of pulses can be counted into stepper motors, and the stepper motor's final position will be known within a small percentage of one step.

Since maximum dynamic torque occurs at low pulse rates, stepping motors can easily accelerate a load. When the desired position is reached and command pulses cease, the stepper motor shaft stops and there is no need for clutches or brakes. The stepper motor is generally left energized at a stop position. Once stopped, the stepper motor resists dynamic movement up to the value of the holding torque. An additional feature of the PM stepper motor is that when all power is removed, it is magnetically detented in the last position. A wide range of step angles are available -- 1.8 to 80°, for example -- without logic manipulation. Stepper motors have inherent low velocity without gear reduction. A typical stepper motor driven at 500 pps turns at 150 rpm. The stepper motor's rotor inertia is usually low. Multiple stepper motors driven from the same source maintain perfect synchronization.

But the stepper motor's efficiency is low; much of the input energy must be dissipated as heat. Load must be analyzed carefully for optimum stepper motor performance. And inputs must be matched to the stepper motor and load. Damping may be required when load inertia is exceptionally high to prevent oscillation.

Stepper Motor - Excitation modes: Stepper motors can be excited in different modes, depending on stator winding and desired performance.

Stepper Motor - Two phase: One entire phase (stator winding) of the stepper motor, end-tap to end-tap is energized at a given moment in time. Input current and wattage are halved (compared to four-phase excitation), and heat dissipation is decreased. Output can be improved by as much as 10%. In the stepper motor's two-phase modified mode, both windings (end-tap to end-tap) are energized simultaneously. Energy input in this mode is the same as four phase, but output performance is increased by about 40%. The stepper motor control is complex and costly for this mode.

Stepper Motor - Three phase: Many variable-reluctance stepper motors use three-phase windings. In modified mode, two adjoining phases are excited simultaneously and the rotor indexes to a minimum reluctance position corresponding to the resultant of the two magnetic fields. Since two stepper motor windings are excited, twice as much power is required as the standard mode (one phase at a time). The stepper motor's output is not increased, but damping is improved.

Stepper Motor - Four phase: Each of the stepper motor's half winding is regarded as a separate phase, and phases are energized two at a time. Although this mode isn't very efficient, the controller is simple. Compared to single-phase excitation, twice the input energy is required. Torque output is increased by about 40%, and maximum response rate is increased.

Stepper Motor - Five phase: Five-phase stepper motors have 10 poles rather than the 8 poles typically used in other stepper motors. Rotor-to-stator offset becomes one-fourth to one-tenth the rotor tooth pitch. A 50-tooth rotor provides a full-step of 0.72°, and a 100-tooth version produces a 0.36° full-step (0.18° half-step). The stepper motors run at 500, 1,000, or 2,000 steps/rev with improved loaded-position accuracy and stiffer response. In addition to higher resolution, five-phase stepper motors produce less vibration than two to four-phase stepper motors with virtually no resonance effects.

Stepper Motor - Variable reluctance: These stepper motors have soft iron multipole rotors and a wound stator. The number of teeth on the rotor and stator, as well as the number of winding phases, determines the step angle. Variable-reluctance stepper motors are generally medium step-angle devices (5 to 15°) which operate at high step speeds. The stepper motor's torque is generally low. Rotor inertia and, thus, inertial load capacity are extremely low. Motors of this type operate at maximum pulse rates from 300 to 1,000 steps/sec and have a maximum load inertia capacity of about two-thirds of rotor inertia. When excited in an overlap mode, these stepper motors can move at half step angles and double pulse rates. These stepper motors produce a net output velocity, which remains the same.



Stepper Motor - Permanent magnet: PM stepper motors generally are thought of as low-torque, large step-angle devices. Torque developed by the stepper motors is far below that for equivalent-size hybrid stepper motors, and step angle generally is 90 or 45° . Position resolution, moreover, is on the order of +10% of step angle, a value that generally relegates the stepper motors to unsophisticated motion-control applications. Maximum pulse rates are for 100 steps/sec for large units to 350 steps/sec for small units. Stepper motors offer a rotor inertia, which is moderate between 5 and 75 gm-cm2.

Rare-earth magnets make possible PM stepper motors having a large number of poles. With a suitable number of poles, PM stepper motors develop more torque than either hybrid stepper motors or dc servomotors. Speed range for the stepper motors is less than that for dc types but much higher than that for hybrids.

Position resolution of the PM stepper motors is less than that for hybrids. But unlike hybrids, some PM stepper motors perform well in closed-loop systems.

Both cemf and iron losses are proportional to the number of poles in the stepper motor. Thus, available torque from a PM stepper motor falls off more slowly with speed than in hybrids and more rapidly than in dc motors. The result is that PM stepper motors operate effectively at higher speeds -- up to about 3,500 rpm -- than hybrids but not as high as dc types. The speed range for PM stepper motors, however, suits a wide range of servo applications.

Stepper Motor - Hybrid: Hybrid stepper motors are frequently chosen for a wide variety of motion-control systems because they are easy to use. Stepper motors can maintain accuracy and reliability in open-loop mode, requiring less complex drive electronics than closed-loop servocontrollers. And absolute positioning accuracy for stepper motors is comparable to closed-loop servocontrollers for many applications.
Conventional hybrid stepper motors are rarely used in closed-loop systems because torque falls rapidly as current increases above the peak torque point -- putting them outside typical control limits. The stepper motor's torque also decreases as speed rises. If driven too fast, hybrids lose position accuracy by skipping steps.

A stepper motor's peak torque is limited by the flux level that saturates the rotor and stator teeth. But an enhanced stepper motor is now available that reduces saturation effects and produces 50 to 100% more torque than conventional stepper motors for the same input power.

Both conventional and enhanced stepper motors develop maximum torque when the rotor teeth are offset by one-quarter tooth pitch from opposing poles in the energized phase. The stepper motor pole pairs develop appreciable torque even at zero current. Torque increases as current approaches the rated value.

At or near rated current in conventional stepper motors, a larger part of the air-gap flux traverses the gap from stator slot to rotor slot rather than from tooth to tooth, thus producing less torque.

The enhanced stepper motor uses a relatively new stator design to get around this problem. Here, samarium-cobalt or neodymium-iron-boron magnets are embedded in slots between the teeth. More concentrated flux lines result between the stepper motor's rotor and stator teeth with fewer flux lines lost to the slotted air gap. These new slot magnets focus the air-gap flux, reduce leakage, and allow the stepper motor to produce more torque.

Torque is also produced by a second pair of poles of the same phase placed 180° away from each other, and 90° away from the first pair. The second pole pair of the conventional stepper motor produces a torque that opposes the positive-acting pair. This negative torque is large at low currents but diminishes near rated current.

Enhanced stepper motors also have large negative torques at low current. But positive-acting flux from the permanent magnets in the stator overcomes the small negative torque generated at rated current. The resulting torque then aids the pole pair producing the primary positive torque.

The slot magnets in enhanced stepper motors provide peak torques reaching twice that of conventional stepper motors. Moreover, these stepper motors can handle three times rated current compared to only two times for conventional stepper motors. Depending upon the inertial load, these new stepper motors reach speeds of 5,000 to 10,000 steps/sec. Corresponding torques are 200 oz-in. to 3,100 oz-in. in 2 to 4-in.-diameter packages. Hybrid stepper motors also generally have high inertia (30 to 40,000 gm-cm2), small step angles (0.5 to 15°) and high accuracy (± 3%).

SERVO MOTORS

Servo motors are used in closed loop control systems in which work is the control variable, Figure 9. The digital servo motor controller directs operation of the servo motor by sending velocity command signals to the amplifier, which drives the servo motor.

Figure 9 - Typical dc servo motor system with either encoder or resolver feedback. Some older servo motor systems use a tachometer and encoder for feedback.


An integral feedback device (resolver) or devices (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal. Servo motors feature a motion profile, which is a set of instructions programmed into the controller that defines the servo motor operation in terms of time, position, and velocity. The ability of the servo motor to adjust to differences between the motion profile and feedback signals depends greatly upon the type of controls and servo motors used. See the servo motors Control and Sensors Product section.

Three basic types of servo motors are used in modern servosystems: ac servo motors, based on induction motor designs; dc servo motors, based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs.

DC Motors basics

Industrial applications use dc motors because the speed-torque relationship can be varied to almost any useful form -- for both dc motor and regeneration applications in either direction of rotation. Continuous operation of dc motors is commonly available over a speed range of 8:1. Infinite range (smooth control down to zero speed) for short durations or reduced load is also common.

Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting).

Dynamic braking (dc motor-generated energy is fed to a resistor grid) or regenerative braking (dc motor-generated energy is fed back into the dc motor supply) can be obtained with dc motors on applications requiring quick stops, thus eliminating the need for, or reducing the size of, a mechanical brake.

Dc motors feature a speed, which can be controlled smoothly down to zero, immediately followed by acceleration in the opposite direction -- without power circuit switching. And dc motors respond quickly to changes in control signals due to the dc motor's high ratio of torque to inertia.

DC Motor types: Wound-field dc motors are usually classified by shunt-wound, series-wound, and compound-wound. In addition to these, permanent-magnet and brushless dc motors are also available, normally as fractional-horsepower dc motors. Dc motors may be further classified for intermittent or continuous duty. Continuous-duty dc motors can run without an off period.

DC Motors - Speed control: There are two ways to adjust the speed of a wound-field dc motor. Combinations of the two are sometimes used to adjust the speed of a dc motor.

DC Motor - Shunt-field control: Reel drives require this kind of control. The dc motor's material is wound on a reel at constant linear speed and constant strip tension, regardless of diameter.

Control is obtained by weakening the shunt-field current of the dc motor to increase speed and to reduce output torque for a given armature current. Since the rating of a dc motor is determined by heating, the maximum permissible armature current is approximately constant over the speed range. This means that at rated current, the dc motor's output torque varies inversely with speed, and the dc motor has constant-horsepower capability over its speed range.

Dc motors offer a solution, which is good for only obtaining speeds greater than the base speed. A momentary speed reduction below the dc motor's base speed can be obtained by overexciting the field, but prolonged overexcitation overheats the dc motor. Also, magnetic saturation in the dc motor permits only a small reduction in speed for a substantial increase in field voltage.

Dc motors have a maximum standard speed range by field control is 3:1, and this occurs only at low base speeds. Special dc motors have greater speed ranges, but if the dc motor's speed range is much greater than 3:1, some other control method is used for at least part of the range.

Armature-voltage DC Motor Control: In this method, shunt-field current is maintained constant from a separate source while the voltage applied to the armature is varied. Dc motors feature a speed, which is proportional to the counter emf. This is equal to the applied voltage minus the armature circuit IR drop. At rated current, the torque remains constant regardless of the dc motor speed (since the magnetic flux is constant) and, therefore, the dc motor has constant torque capability over its speed range.


Armature-voltage DC Motor Control (cont.): Horsepower varies directly with speed. Actually, as the speed of a self-ventilated motor is lowered, it loses ventilation and cannot be loaded with quite as much armature current without exceeding the rated temperature rise.

DC Motors - Selection: Choosing a dc motor and associated equipment for a given application requires consideration of several factors.

DC Motors - Speed range: If field control is to be used, and a large speed range is required, the base speed must be proportionately lower and the motor size must be larger. If speed range is much over 3:1, armature voltage control should be considered for at least part of the range. Very wide dynamic speed range can be obtained with armature voltage control. However, below about 60% of base speed, the motor should be derated or used for only short periods.

DC Motors - Speed variation with torque: Applications requiring constant speed at all torque demands should use a shunt-wound dc motor. If speed change with load must be minimized, a dc motor regulator, such as one employing feedback from a tachometer, must be used.

When the dc motor speed must decrease as the load increases, compound or series-wound dc motors may be used. Or, a dc motor power supply with a drooping volt-ampere curve could be used with a shunt-wound dc motor.

DC Motors - Reversing: This operation affects power supply and control, and may affect the dc motor's brush adjustment, if the dc motor cannot be stopped for switching before reverse operation. In this case, compound and stabilizing dc motor windings should not be used, and a suitable armature-voltage control system should supply power to the dc motor.

DC Motors - Duty cycle: Direct current motors are seldom used on drives that run continuously at one speed and load. Motor size needed may be determined by either the peak torque requirement or heating.

DC Motors - Peak torque: The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Dc motor brush and commutator damage depends on sparking severity and duration. Therefore, the dc motor's peak torque depends on the duration and frequency of occurrence of the overload. Dc motor peak torque is often limited by the maximum current that the power supply can deliver.

Dc motors can commutate greater loads at low speed without damage. NEMA standards specify that machines powered by dc motors must deliver at least 150% rated current for 1 min at any speed within rated range, but most dc motors do much better.

DC Motors - Heating: Dc motor temperature is a function of ventilation and electrical/mechanical losses in the machine. Some dc motors feature losses, such as core, shunt-field, and brush-friction losses, which are independent of load, but vary with speed and excitation.

The best method to predict a given dc motor's operating temperature is to use thermal capability curves available from the dc motor manufacturer. If curves are not available, dc motor temperature can be estimated by the power-loss method. This method requires a total losses versus load curve or an efficiency curve.

For each portion of the duty cycle, power loss is obtained and multiplied by the duration of that portion of the cycle. The summation of these products divided by the total cycle time gives the dc motor's average power loss. The ratio of this value to the power loss at the motor rating is multiplied by the dc motor's rated temperature rise to give the approximate temperature rise of the dc motor when operated on that duty cycle.

AC Motor - Basics of AC Motor Design Engineering

A synchronous and synchronous electric motors are the two main categories of ac motors. The induction ac motor is a common form of asynchronous motor and is basically an ac transformer with a rotating secondary. The primary winding (stator) is connected to the power source and the shorted secondary (rotor) carries the induced secondary current. Torque is produced by the action of the rotor (secondary) currents on the air-gap flux. The synchronous motor differs greatly in design and operational characteristics, and is considered a separate class of ac motor.

Induction AC Motors: Induction ac motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction ac motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.

AC motors typically feature rotors, which consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This ac motor "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. The speed at which the ac motor magnetic field rotates is the synchronous speed of the ac motor and is determined by the number of poles in the stator and the frequency of the power supply: ns = 120f/p, where ns = synchronous speed, f = frequency, and p = the number of poles.

Synchronous speed is the absolute upper limit of ac motor speed. If the ac motor's rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. When ac motors are running, the rotor always rotates slower than the magnetic field. The ac motor's rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the ac motor's rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed, and na = actual speed.

Polyphase AC Motors: Polyphase squirrel-cage ac motors are basically constant-speed machines, but some degree of flexibility in operating characteristics results from modifying the rotor slot design. These variations in ac motors produce changes in torque, current, and full-load speed. Evolution and standardization have resulted in four fundamental types of ac motors.

AC Motors - Designs A and B: General-purpose ac motors with normal starting torques and currents, and low slip. Fractional-horsepower polyphase ac motors are generally design B. Because of the drooping characteristics of design B, a polyphase ac motor that produces the same breakdown (maximum) torque as a single-phase ac motor cannot attain the same speed-torque point for full-load speed as single-phase ac motors. Therefore, breakdown torque must be higher (a minimum of 140% of the breakdown torque of single-phase, general-purpose ac motors) so that full-load speeds are comparable.

AC Motors - Design C: High starting torque with normal starting current and low slip. AC motors are normally used where breakaway loads are high at starting, but which normally run at rated full load and are not subject to high overload demands after running speed has been reached.

AC Motors - Design D: High slip, ac motor starting torque, low starting current, and low full-load speed. Because of the high slip, speed can drop when fluctuating loads are encountered. This ac motor design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve.

AC Motors - Design F: Low starting torque, low starting current, and low slip. These AC motors are built to obtain low locked-rotor current. Both locked-rotor and breakdown torque are low. Normally these ac motors are used where starting torque is low and where high overloads are not imposed after running speed is reached.

Wound-rotor AC Motors: Squirrel-cage ac motors are relatively inflexible with regard to speed and torque characteristics, but a special wound-rotor ac motor has controllable speed and torque. Application of wound-rotor ac motors is markedly different from squirrel-cage ac motors because of the accessibility of the rotor circuit. AC motor performance characteristics are obtained by inserting different values of resistance in the rotor circuit.

Wound-rotor ac motors are generally started with secondary resistance in the rotor circuit. The ac motor resistance is sequentially reduced to permit the motor to come up to speed. Thus, ac motors can develop substantial torque while limiting locked-rotor current. This secondary ac motor resistance can be designed for continuous service to dissipate heat produced by continuous operation at reduced speed, frequent acceleration, or acceleration with a large inertia load. External resistance gives ac motors a characteristic that results in a large drop in rpm for a fairly small change in load. Reduced ac motor speed is provided down to about 50% rated speed, but efficiency is low.

Multispeed AC Motors: Consequent-pole ac motors are designed for one speed. By physically reconnecting the leads, a 2:1 speed ratio can be obtained. Typical synchronous speeds for 60-Hz ac motors are: 3,600/1,800 rpm (2/4 pole), 1,800/900 rpm (4/8 pole), and 1,200/600 rpm (6/12 pole).

Two-winding ac motors have two separate windings that can be wound for any number of poles so that other speed ratios can be obtained. However, ratios greater than 4:1 are impractical because of ac motor size and weight. Single-phase multispeed ac motors are usually variable-torque design, but constant-torque and constant-horsepower ac motors are available.

Power output of multispeed ac motors can be proportioned to each different speed. These ac motors are designed with output horsepower capacity in accordance with one of the following load characteristics.

AC Motors - Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.

AC Motors - Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.

AC Motors - Constant horsepower: These ac motors develop the same horsepower at each speed and the torque is inversely proportional to the speed. Typical applications for ac motors include machine tools such as drills, lathes, and milling machines.

AC Motors - Single-phase AC Motors: Single-phase induction ac electric motors are commonly fractional-horsepower types, although single-phase integral-horsepower are available in the lower horsepower range. The most common fractional-horsepower single-phase ac motors are split-phase, capacitor-start, permanent split-capacitor, and shaded pole.

The ac motors come in multispeed types, but there is a practical limit to the number of speeds obtained. Two, three, and four-speed motors are available, and speed selection may be accomplished by consequent-pole or two-winding methods.

Single-phase ac electric motors run in the direction in which they are started; and they are started in a predetermined direction according to the electrical connections or mechanical setting of the starting means. General-purpose ac motors may be operated in either direction, but the standard ac motor rotation is counterclockwise when facing the end opposite the drive shaft. AC motors can be reconnected to reverse the direction of rotation.

Universal AC Motors: Universal ac motors operate with nearly equivalent performance on direct current or alternating current up to 60 Hz. AC motors differ from a dc motors due to the winding ratios and thinner iron laminations. DC motors runs on ac, but with poor efficiency. Universal ac motors can operate on dc with essentially equivalent ac motor performance, but with poorer commutation and brush life than for an equivalent dc motor.

An important characteristic of universal ac motors is that it has the highest horsepower-per-pound ratio of any ac motor because it can operate at speeds many times higher than that of any other 60-Hz electric motor.

When operated without load, universal ac motors tend to run away, speed being limited only by windage, friction, and commutation. Therefore, large universal ac motors are nearly always connected directly to a load to limit speed. On portable tools such as electric saws, the load imposed by the gears, bearings, and cooling fan is sufficient to hold the no-load speed down to a safe value.

With a universal ac motor, speed control is simple, since electric motor speed is sensitive to both voltage and flux changes. With a rheostat or adjustable autotransformer, ac motor speed can be readily varied from top speed to zero.

Synchronous AC Motors: Synchronous ac motors are inherently constant-speed electric motors and they operate in absolute synchronism with line frequency. As with squirrel-cage induction ac motors, speed is determined by the number of pairs of poles and is always a ratio of the line frequency.

Synchronous ac motors are made in sizes ranging from subfractional self-excited units to large-horsepower, direct-current-excited ac motors for industrial drives. In the fractional-horsepower range, synchronous ac motors are used primarily where precise constant speed is required.

In large horsepower sizes applied to industrial loads, synchronous ac motors serve two important functions. First, ac motors provide highly efficient means of converting ac energy to mechanical power. Second, ac motors can operate at leading or unity power factor, thereby providing power-factor correction.

There are two major types of synchronous ac motors: nonexcited and direct-current excited electric motors.

Nonexcited Electric Motors are made in reluctance and hysteresis designs. These electric motors employ a self-starting circuit and require no external excitation supply.

Dc-excited Electric Motors come in sizes larger than 1 hp, and require direct current supplied through slip rings for excitation. Direct current may be supplied from a separate source or from a dc generator directly connected to the ac motor shaft.

Single-phase or polyphase synchronous electric motors can't start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the electric motor field is rotating at a synchronous speed, the electric motor must be accelerated before it can pull into synchronism. Accelerating from zero speed requires slip until synchronism is reached. Therefore, separate starting means must be employed.

In self-starting electric motor designs, fhp sizes use starting methods common to induction electric motors (split-phase, capacitor-start, repulsion-start, and shaded-pole). The electrical characteristics of these electric motors cause them to automatically switch to synchronous operation.

Although the dc-excited electric motor has a squirrel cage for starting, called an amortisseur or damper winding, the inherent low starting torque and the need for a dc power source requires a starting system that provides full electric motor protection while starting, applies dc field excitation at the proper time, removes field excitation at rotor pull out (maximum torque), and protects the electric motor's squirrel-cage winding against thermal damage under out-of-step conditions.

The electric motor's pull-up torque is the minimum torque developed from standstill to the pull-in point. This torque must exceed load torque by a sufficient margin so that a satisfactory rate of acceleration is maintained under normal voltage conditions.

Dc-excited AC Electric Motors (cont.) The electric motor's reluctance torque results from the saliency (preferred direction of magnetization) of the rotor pole pieces and pulsates at speeds below synchronous. It also has an influence on electric motor pull-in and pull-out torques because the unexcited salient-pole rotor tends to align itself with the stator electric motor magnetic field to maintain minimum magnetic reluctance. The electric motor's reluctance torque may be sufficient to pull into synchronism a lightly loaded, low-inertia system and to develop approximately a 30% pull-out torque.

The electric motor's synchronous torque is torque developed after excitation is applied, and represents the total steady-state torque available to drive the load. It reaches maximum at approximately 70° lag of the rotor behind the rotating stator magnetic field. This maximum value is actually the pull-out torque.

Pull-out torque is the maximum sustained torque the electric motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor electric motors, and 175 to 200% for 0.8-leading-power-factor electric motors.

Pull-in torque of a synchronous electric motor is the torque that it develops when pulling its connected inertia load into synchronism upon application of excitation. Pull-in torque is developed during transition from slip speed to synchronous speed, as electric motors change from induction to synchronous operation. It is usually the most critical period in starting a synchronous electric motor. Torques developed by the amortisseur and field windings become zero at synchronous speed. At the pull-in point, therefore, only the reluctance torque and the synchronizing torque provided by exciting the field windings are effective.

Timing Electric Motors: Timing electric motors are rated under 1/10 hp and are used as prime movers for timing devices. Since the electric motor is being used as a timer, it must run at a constant speed.

Timing Electric Motors (cont.) Ac and dc electric motors can be used as timing motors. Dc electric timing motors are used for portable applications, or where high acceleration and low speed variations are required. These electric motors offer advantages, which include starting torque as high as ten times running torque, efficiency from 50 to 70%, and relatively easy speed control. But some form of speed governor, either mechanical or electronic, is required.

Ac motors use readily available power, are lower in cost, have improved life, and do not generate RFI. However, ac motors cannot be readily adapted to portable applications, have relatively low starting torques, and are much less efficient than dc motors.

AC Servo Motors: Ac servo motors are used in ac servomechanisms and computers which require rapid and accurate response characteristics. To obtain these characteristics, servo motors have small-diameter high-resistance rotors. The small diameter provides low inertia for fast starts, stops, and reversals, while the high resistance provides a nearly linear speed-torque relationship for accurate control.

Servo motors are wound with two phases physically at right angles or in space quadrature. Servo motors feature a fixed or reference winding is excited from a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servoamplifier. The servo motor windings are usually designed with the same voltage-turns ratio, so that power inputs at maximum fixed-phase excitation and at maximum control-phase signal are in balance.

In an ideal servo motor, torque at any speed is directly proportional to the servo motor's control-winding voltage. In practice, however, this relationship exists only at zero speed because of the inherent inability of an induction servo motor to respond to voltage input changes under conditions of light load.

The inherent damping of servo motors decreases as ratings increase, and the servo motors have a reasonable efficiency at the sacrifice of speed-torque linearity. Most larger servo motors have integral auxiliary blowers to maintain temperatures within safe operating ranges. Servo motors are available in power ratings from less than 1 to 750 W, in sizes ranging from 0.5 to 7-in. OD. Most servo motors are available with modular or built-in gearheads.

ELECTRIC MOTORS

Electric motors, both ac motors and dc motors, come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. Thus, the first and most important step in electric motor selection is determining load characteristics -- torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost.


Starting and running torque are the first parameters to consider when sizing electric motors. Starting torque requirements for electric motors can vary from a small percentage of full load to a value several times full-load torque. Starting torque varies because of a change in load conditions or the mechanical nature of the machine, which the electric motor is installed in. The latter could be caused by the lubricant, wear of moving parts, or other reasons.

Electric motors feature torque supplied to the driven machine, which must be more than that required from start to full speed. The greater the electric motor's reserve torque, the more rapid the acceleration.

Electric motor drive systems that use gear reducers have parts that rotate at different speeds. To calculate acceleration torque required for these electric motors, rotating components must be reduced to a common base. The part inertias are usually converted to their equivalent value at the drive shaft. Equivalent inertia W2K22 of the load only is found from:



W2K22 =(W1K12)(N1/N2)2

where W1K21 = load inertia in lb-ft2, N1 = load speed in rpm, and N2 = electric motor speed in rpm.

Electric motors have bodies, which have a straight-line motion are often connected to rotating driving units by rack-and-pinion, cable, or cam mechanisms. For these electric motor parts, the equivalent WK2 is found from:



WK2 = W(S/2ΠN)2



where W = load weight, S = translation speed in fpm, Π is pi, and N = rotational speed in rpm.

Electric Motors - Acceleration time:
Acceleration time for electric motors is directly proportional to total inertia and inversely proportional to the electric motor torque. For electric motors with constant acceleration torque, acceleration time is:




where WK2 = rotational inertia in lb-ft2, (N2 - N1) = the speed difference, and Tx = acceleration torque in lb-ft. For translating bodies, acceleration time is:




where W = weight of the load in lb, (S2 - S1) = the translation speed difference, and Fx = translation force in lb.

An approximation method is necessary to find the electric motor's acceleration time if acceleration torque is not linear during speed increase. The quickest method is to break up the speed versus torque curves of the electric motor and the driven machine into segments and calculate acceleration time for each segment. Accurate electric motor acceleration times usually result.


Electric Motors - Power rating:
Electric motors offer the horsepower required to drive a machine, which is typically referred to as electric motor load. The most common equation for power based electric motors on torque and rotational speed is: hp = (torque X rpm)/5,250.

If the electric motor's load is not constant and follows a definite cycle, a horsepower versus time curve for the driven machine is helpful. From this curve both peak and rms the electric motor's horsepower can be determined. Rms load horsepower indicates the necessary continuous electric motor rating. Peak load horsepower is not necessarily an indication of the required electric motor rating. However, when a peak load is maintained for a period of time, electric motors feature a rating, which usually should not be less than peak load horsepower.

Duty cycle - Electric Motors:
Continuous steady-running loads over long periods are demonstrated by fans and blowers. On the other hand, electric motors installed in machines with flywheels may have wide variations in running loads. Often, electric motors use flywheels to supply the energy to do the work, and the electric motor does nothing but restore lost energy to the flywheel. Therefore, choosing the proper electric motor also depends on whether the load is steady, varies, follows a repetitive cycle of variation, or has pulsating torque or shocks.
For example, electric motors that run continuously in fans and blowers for hours or days may be selected on the basis of continuous load. But electric motors located in devices like automatically controlled compressors and pumps start a number of times per hour. And electric motors in some machine tools start and stop many times per minute.

Duty cycle is a fixed repetitive load pattern over a given period of time which is expressed as the ratio of on-time to cycle period. When operating cycle is such that electric motors operate at idle or a reduced load for more than 25% of the time, duty cycle becomes a factor in sizing electric motors. Also, energy required to start electric motors (that is, accelerating the inertia of the electric motor as well as the driven load) is much higher than for steady-state operation, so frequent starting could overheat the electric motor.

For most electric motors (except squirrel-cage electric motors during acceleration and plugging) current is almost directly proportional to developed torque. At constant speed, torque is proportional to horsepower. For accelerating loads and overloads on electric motors that have considerable droop, equivalent horsepower is used as the load factor. The next step in sizing the electric motor is to examine the electric motor's performance curves to see if the electric motor has enough starting torque to overcome machine static friction, to accelerate the load to full running speed, and to handle maximum overload.


Electric Motors - Service factors:
A change in NEMA standards for electric motor service factors and temperature rise has been brought about because of better insulation used on electric motors. For instance, a 1.15 service factor -- once standard for all open electric motors -- is no longer standard for electric motors above 200 hp.
Increases in electric motor temperature are measured by the resistance method in the temperature rise table. Electric motors feature a nameplate temperature rise, which is always expressed for the maximum allowable load. That is, if the electric motor has a service factor greater than unity, the nameplate temperature rise is expressed for the overload. Two Class-B insulated electric motors having 1.15 and 1.25 service factors will, therefore, each be rated for a 90°C rise. But the second electric motor will have to be larger than the first in order to dissipate the additional heat it generates at 125% load.

Electric motors feature a service factor, which indicates how much over the nameplate rating any given electric motor can be driven without overheating. NEMA Standard MGI-143 defines service factor of an ac motor as "...a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor..." In other words, multiplying the electric motor's nameplate horsepower by the service factor tells how much electric motors can be overloaded without overheating. Generally, electric motor service factors:

Handle a known overload, which is occasional.
Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors.
Obtain cooler-than-normal electric motor operation at rated load, thus lengthening insulation life.


Electric Motors - Efficiency:
Small universal electric motors have an efficiency of about 30%, while 95% efficiencies are common for three-phase machines. In less-efficient electric motors, the amount of power wasted can be reduced by more careful application and improved electric motor design.
Electric motor's feature an efficiency level, which also depends on actual electric motor load versus rated load, being greatest near rated load and falling off rapidly for under and overload conditions.