Machine Design

# AC Motors

A synchronous and synchronous motors are the two main categories of ac motors. The induction 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 motor.

Induction motors: Induction 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 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.

Rotors typically 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 "squirrel cage" rotates 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: ns = 120f/p, where ns = synchronous speed,    f = frequency, and     p = the number of poles.

Synchronous speed is the absolute upper limit of motor speed. If the 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 running, the rotor always rotates 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. The speed difference between the 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 motors: Polyphase squirrel-cage motors are basically constant-speed machines, but some degree of flexibility in operating characteristics results from modifying the rotor slot design. These variations produce changes in torque, current, and full-load speed. Evolution and standardization have resulted in four fundamental types of motors.

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

Design C: High starting torque with normal starting current and low slip. This design is 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.

Design D: High slip, very high starting torque, low starting current, and low full-load speed. Because of the 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.

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

Wound-rotor motors: Squirrel-cage motors are relatively inflexible with regard to speed and torque characteristics, but 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. Performance characteristics are obtained by inserting different values of resistance in the rotor circuit.

Wound-rotor motors are generally started with secondary resistance in the rotor circuit. The 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. This 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.

Multispeed motors: Consequent-pole motors are designed for one speed. By physically reconnecting the leads, a 2:1 speed ratio can be obtained. Typical synchronous speeds for a 60-Hz motor 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 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 motor size and weight. Single-phase multispeed motors are usually variable-torque design, but constant-torque and constant-horsepower motors are available.

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

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

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

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

Single-phase motors: Single-phase induction 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 motors are split-phase, capacitor-start, permanent split-capacitor, and shaded pole.

The 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 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 motors may be operated in either direction, but the standard rotation is counterclockwise when facing the end opposite the drive shaft. Motors can be reconnected to reverse the direction of rotation.

Universal motors: The universal motor operates with nearly equivalent performance on direct current or alternating current up to 60 Hz. It differs from a dc series motor because of winding ratios and thinner iron laminations. A dc series motor runs on ac, but with poor efficiency. A universal motor can operate on dc with essentially equivalent ac performance, but with poorer commutation and brush life than for an equivalent dc series motor.

An important characteristic of a universal motor 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 motor.

When operated without load, a universal motor tends to run away, speed being limited only by windage, friction, and commutation. Therefore, large universal 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 motor, speed control is simple, since motor speed is sensitive to both voltage and flux changes. With a rheostat or adjustable autotransformer, motor speed can be readily varied from top speed to zero.

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

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

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

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

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

Dc-excited 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 motor shaft.

Single-phase or polyphase synchronous motors can't start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the field is rotating at synchronous speed, the 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 designs, fhp sizes use starting methods common to induction motors (split-phase, capacitor-start, repulsion-start, and shaded-pole). The electrical characteristics of these motors cause them to automatically switch to synchronous operation.

Although the dc-excited 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 motor protection while starting, applies dc field excitation at the proper time, removes field excitation at rotor pull out (maximum torque), and protects the squirrel-cage winding against thermal damage under out-of-step conditions.

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.

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 motor pull-in and pull-out torques because the unexcited salient-pole rotor tends to align itself with the stator magnetic field to maintain minimum magnetic reluctance. This reluctance torque may be sufficient to pull into synchronism a lightly loaded, low-inertia system and to develop approximately a 30% pull-out torque.

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 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 motors, and 175 to 200% for 0.8-leading-power-factor motors.

Pull-in torque of a synchronous 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 the motor changes from induction to synchronous operation. It is usually the most critical period in starting a synchronous 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 motors: Timing motors are rated under 1/10 hp and are used as prime movers for timing devices. Since the motor is being used as a timer, it must run at constant speed.

Ac and dc motors can be used as timing motors. Dc timing motors are used for portable applications, or where high acceleration and low speed variations are required. Advantages 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 servomotors: Ac servomotors are used in ac servomechanisms and computers which require rapid and accurate response characteristics. To obtain these characteristics, servomotors 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.

Servomotors are wound with two phases physically at right angles or in space quadrature. 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 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 servomotor, torque at any speed is directly proportional to control-winding voltage. In practice, however, this relationship exists only at zero speed because of the inherent inability of an induction motor to respond to voltage input changes under conditions of light load.

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