There are four operating modes for the induction motor – high slip motoring, low slip motoring, low slip generating and high slip generating.

When the motor is driving the load, it is "motoring" and when the load is driving the motor, it is generating. When the motor is operating close to synchronous speed, it is operating at low slip, and when the motor is running at a speed that is not near synchronous speed, it is operating at a high slip. Typically, high slip operation is where the difference in rotational speed and the driving speed is greater than ten percent of the rated speed of the motor.

Motors that are controlled by a variable speed controller should only ever operate in low slip motoring and low slip generator modes. Motors not controlled by a variable speed controller always operate in high slip motoring mode during start.

High slip operation is used to accelerate the motor and drive load up to full speed and occurs every time a motor is "started".

Under high slip operation, the induction motor draws a very high current, typically six to eight times the rated current of the motor when full voltage is applied. The efficiency of motors operating under high slip conditions is very low.

High slip characteristics are indicated by the Locked Rotor Current (LRC or LRI) and the Locked Rotor Torque (LRT).

The Locked Rotor conditions are the current drawn and torque developed by the motor when full voltage is applied and the rotor is held stationary. The LRC of the induction motor is typically in the range of 550 to 900 percent of the rated current of the motor, but extreme motors will fall between 450 and 1200 percent of the rated motor current.

LRTs are typically between 120 and 240 percent of the rated torque of the motor with extreme motors within the range of 60 to
450 percent.

During start, the motor draws a high overload current until the motor reaches about 80 percent full speed when the current begins to drop with speed. The start current is dependent on the motor design, motor speed and terminal voltage. The start current is independent of the shaft load.

The induction motor comprises two major components, the stator and the rotor.

The stator is made up of a stack of punched laminations with a set of coils arranged to make up a number of magnetic poles. The arrangement of the coils determines the number of poles and therefore the speed of the motor. The flux in the iron influences the torque, power factor and efficiency of the motor.
The rotor is made up of a stack of laminations pressed onto the shaft of the motor. Embedded in the rotor are a series of bars around the circumference of the rotor running parallel to the shaft and shorted at each end by a shorting ring. The rotor is effectively a short circuited secondary winding. The shape and position of the rotor bars determine the starting characteristics of the motor.
Electrically, the motor behaves like a short circuited transformer with a variable frequency voltage applied to the secondary. When the rotor is locked, the current is limited by the resistance of the rotor, the reactance of the rotor and the leakage reactance of the stator rotor coupling. The primary limiting components are the rotor resistance and the reactance.

The torque is developed by the interaction of the stator and rotor fields. If these fields are in phase, then the torque is maximum. If these fields are at 90 degrees, there is no torque.

For maximum torque under high slip conditions it is important that the current has a high resistive component to ensure an in phase component. If the current is limited by reactance only, there could be a very high start current with no torque.

The starting characteristics of the motor are determined by the rotor design. The primary influence being the shape and position of the rotor bars. Deep rotor bars exhibit a high reactance whereas shallow bars exhibit a low reactance. The cross sectional area and the material of the bar determines its resistance.

For optimum running efficiency, a low resistance bar is required (preferably zero resistance). This is the opposite of the starting requirements. By using a thin bar that penetrates radially into the rotor, it is possible to get the best of both worlds. The inner sections of the bar have a high reactance, so during start all the current is concentrated on the outer edge of the bar. This increases the effective resistance of the bar during start yet allows for a low resistance during run.

High efficiency motors can exhibit poor starting characteristics, but there are some that offer high efficiency at start and at run.

There are very large variations between motors in regard to their starting efficiency. If you are working with difficult starting conditions (such as high torque, high inertia, and weak supply) then it is important that the motor selection is optimized for the application – both start time and run time.

A good measure of comparison between motors is to divide the LRT (in percentage) by the LRC (in percentage) – the bigger the number, the better the result.

Typical figures for LRT are 100 to 270 percent with values as low as 60 percent and as high as 450 percent. The LRC is typically in the range of 550 to 900 percent but can be less than 350 percent and greater than 1200 percent. The trend is for high LRCs to be associated with low LRTs and lower LRCs with higher LRTs.

If reduced voltage or reduced current starting is applied to the motor, the start torque reduces with the square of the voltage/current reduction. This exaggerates the difference between different rotor designs and makes the motor selection more important.

Another factor in rotor design is the thermal capacity of the rotor. During operation, there is power that is dissipated in the rotor. This power is the slip loss and is a product of the shaft torque and the slip. The slip is the difference in speed between the rotor speed and the synchronous speed of the motor. At start up, the slip is 100 percent. As the motor accelerates the slip reduces down to the running slip which may be only a few percent.

During start, the slip losses are very high and this causes a temperature rise in the rotor bars. If the thermal capacity of the rotor bars is very low, the temperature will rise very quickly and the motor will only withstand a short starting time. If the load has a high inertia, it is very important that the rotor can withstand the starting time.

The thermal capacity of the rotor is usually expressed as either the "maximum locked rotor time" or the "maximum load inertia". Both ratings refer to the ability of the motor to start a high inertia load.

If the motor is not able to withstand the start, there will be rotor damage causing a premature rotor failure. The power dissipated in the rotor during start, is equal to the full speed kinetic energy of the load.

If a motor is unsuitable for the load, either because it has insufficient start torque, or it has insufficient rotor thermal capacity, the characteristics can often be changed by changing the rotor only. Some manufacturers have different rotors for a particular stator. This may be a cheaper option that a complete motor replacement.

Remember, for the best starting results, you must have the
best motor.