AC MOTORS

Most of the world's motor business is addressed by AC motors. AC motors are relatively constant speed devices. The speed of an AC motor is determined by the frequency of the voltage applied (and the number of magnetic poles). There are basically two types of AC motors: induction and synchronous.

INDUCTION MOTOR - If the induction motor is viewed as a type of transformer, it becomes easy to understand. By applying a voltage onto the primary of the transformer winding, a current flow results and induces current in the secondary winding. The primary is the stator assembly and the secondary is the rotor assembly. One magnetic field is set up in the stator and a second magnetic field is induced in the rotor. The interaction of these two magnetic fields results in motion. The speed of the magnetic field going around the stator will determine the speed of the rotor.

The rotor will try to follow the stator's magnetic field, but will "slip" when a load is attached. Therefore induction motors always rotate slower than the stator's rotating field. Typical construction of an induction motor consists of 1) a stator with laminations and turns of copper wire and 2) a rotor, constructed of steel laminations with large slots on the periphery, stacked together to form a "squirrel cage" rotor. Rotor slots are filled with conductive material (copper or aluminum) and are short-circuited upon themselves by the conductive end pieces. This "one" piece casting usually includes integral fan blades to circulate air for cooling purposes. The standard induction motor is operated at a "constant" speed from standard line frequencies. Recently, with the increasing demand for adjustable speed products, controls have been developed which adjust operating speed of induction motors. 

Microprocessor drive technology using methods such as vector or phase angle control (i.e. variable voltage, variable frequency) manipulates the magnitude of the magnetic flux of the fields and thus controls motor speed. By the addition of an appropriate feedback sensor, this becomes a viable consideration for some positioning applications. Controlling the induction motor's speed/torque becomes complex since motor torque is no longer a simple function of motor current. Motor torque affects the slip frequency, and speed is a function of both stator field frequency and slip frequency.

Induction motor advantages include: Low initial cost due to simplicity in motor design and construction; availability of many standard sizes; reliability; and quiet, vibration free operation. For very rapid start-stop positioning applications, a larger motor would be used to keep temperatures within design limits. A low torque to inertia ratio limits this motor type to less demanding incrementing (start-stop) applications.

SYNCHRONOUS MOTOR - The synchronous motor is basically the same as the induction motor but with slightly different rotor construction. The rotor construction enables this type of motor to rotate at the same speed (in synchronization) as the stator field. There are basically two types of synchronous motors: self excited ( as the induction motor) and directly excited (as with permanent magnets).

The self excited motor (may be called reluctance synchronous) includes a rotor with notches, or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator. Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the rotating field.

A directly excited motor (may be called hysteresis synchronous, or AC permanent magnet synchronous) includes a rotor with a cylinder of a permanent magnet alloy. The permanent magnet north and south poles, in effect, are the salient teeth of this design, and therefore prevent slip. In both the self excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a small distance behind the stator field. This angle will increase with load, and if the load is increased beyond the motor's capability, the rotor will pull out of synchronize.

The synchronous motor is generally operated in an "open loop" configuration and within the limitations of the coupling angle (or "pull-out" torque) it will provide absolute constant speed for a given load. Also, note that this category of motor is not self starting and employs start winding's (split-phase, capacitor start), or controls which slowly ramp up frequency/voltage in order to start rotation. A synchronous motor can be used in a speed control system even though a feedback device must be added. Vector control approaches will work quite adequately with this motor design.

However, in general, the rotor is larger than that of an equivalent servomotor and, therefore, may not provide adequate response for increment applications. Other disadvantages are: While the synchronous motor may start a high inertial load, it may not be able to accelerate the load enough to pull it into synchronize. If this occurs, the synchronous motor operates at low frequency and at very irregular speeds, resulting in audible noise. Also for a given horsepower, synchronous motors are larger and more expensive than non-synchronous motors.