Electrical and Energy

In a Synchronous AC Motor, the rotating magnetic field of the stator imposes a torque on the magnetic field of the rotor, causing it to rotate steadily. It is called synchronous because at steady state, the speed of the rotor matches the speed of the rotating magnetic field in the stator. By contrast, an induction motor has a current induced in the rotor; to do this, stator windings are arranged so that when energised with a polyphase supply they create a rotating magnetic field that induces current in the rotor conductors. These currents interact with the rotating magnetic field, causing rotational motion of the rotor.

For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (ns), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. If this happens while the motor is operating, the rotor slightly slows down, and consequently a current is induced again. 

The ratio between the speed of the magnetic field as seen by the rotor (slip speed) and the speed of the stator's rotating field is unitless and it is called slip. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as induction generator, or it can be unrolled to form the linear induction motor which can directly generate linear motion.

To understand the behaviour of induction motors, it is helpful to understand their distinction from a synchronous motor. A synchronous motor always runs at a shaft rotation frequency that is an integer fraction of the supply frequency; the synchronous speed of an induction motor is the same. It can be shown that ns in rpm is determined by

where f is the frequency of the AC supply in Hz and p is the number of magnetic poles per phase. Some texts refer to the number of pole pairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of pole pairs, takes the place of p in the equation.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. 

Many single-phase motors having two windings and a capacitor can be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-phase supply and feeds it to a separate motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field. There are three types of rotor: squirrel cage rotors made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ring rotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel.

Typical torque curves for different line frequencies. By varying the line frequency with an inverter, induction motors can be kept on the stable part of the torque curve above the peak over a wide range of rotation speeds.

The theoretical unloaded speed (with slip approaching zero) of the induction motor is controlled by the number of pole pairs and the frequency of the supply voltage.

When driven from a fixed line frequency, loading the motor reduces the rotation speed. When used in this way, induction motors are usually run so that in operation the shaft rotation speed is kept above the peak torque point; then the motor will tend to run at reasonably constant speed. Below this point, the speed tends to be unstable and the motor may stall or run at reduced shaft speed, depending on the nature of the mechanical load.

Before the development of semiconductor power electronics, it was difficult to vary the frequency, and induction motors were mainly used in fixed speed applications. However, many older DC motors have now been replaced with induction motors and accompanying inverters in industrial applications.

The equivalent circuit of an induction motor has the equivalent resistance of the stator on the left, consisting of the copper and core resistance in series, as Rs. During operation, the stator induces reactance, represented by the inductor Xs. Xr represents the effect of the rotor passing through the stator's magnetic field. The effective resistance of the rotor, Rr, is composed of the equivalent value of the machine's power and the ohmic resistance of the stator windings and squirrel cage.

The induction motor equivalent circuit when idle is approximately Rs + Xs, which is mostly reactive. Induction motors generally have a poor power factor (pf), which can be improved by a compensation network.

The idle current draw is often near the rated current, due to the copper and core losses existing without load. In these conditions, this is usually more than half the power loss at the rated load. If the torque against the motor spindle is increased, the active current in the rotor increases by Rr. Due to the construction of the induction motor, the two resistances induce magnetic flux, in contrast to synchronous machines where it is induced only by the reactive current in the stator windings.

The current produces a voltage drop in the cage factor of Rr and a slightly higher one in the stator windings. Hence, the losses increase faster in the rotor than in the stator. Rs and the copper factor of Rr both cause I2R losses, meaning the efficiency improves with increasing load and reduces with temperature.

increases engine power losses. In continuous operation, this is an approximation because a nominal torque generated by the cooling of the rotor and stator is not included in the calculation. Above the rated speed or frequency, induction motors are more effective at higher voltages. 

Today, Rs and Rr are measured automatically and thus can be used on a motor to automatically configure itself and thus protect it from overload. Holding torques and speeds close to zero can be achieved with vector controls. There can be problems with cooling here, since the fan is usually mounted on the rotor.