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.