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HIGH-PERFORMANCE MOTOR SYSTEM FOR WASHING MACHINES

3 SLIP CONTROL OF 3-PHASE INDUCTION MOTORS

3.1 PRINCIPLE OF INDUCTION MOTOR SLIP CONTROL

The motor efficiency curve versus torque shows a maximum value for a given voltage (see

Figure 1.). The most efficient and optimum operating point is located somewhere at the upper

right part of the curve that sets an optimum slip. The motor can be operated at its maximum

performance level by selecting the optimum slip value and deducting the stator frequency.

This is calculated as follows:

Stator Frequency = Rotor Frequency + Optimum Slip

All that remains to be done is to adapt the motor flux to the load line variations, which is done

by regulating the motor voltage to keep the slip constantly at its optimum value. When the load

tends to move the slip away from this value, the inverter immediately adjusts the motor voltage

so that the slip remains at its set point.

In case of excessive load variations, the instantaneous slip is limited and the stator frequency

is decreased to prevent loss of control. This avoids generating torque peaks that are source of

disturbing acoustic noise.

3.2 RESULTS

Figure 2. shows the maximum slip variation at its set-point value in an application for a hori-

zontal-axis washing machine. Using the most critical operating conditions, the washing ma-

chine was loaded with 2.5 kg of terry-cloth towels and 17 liters of water. The slip control gain

was set so as not to react abruptly to the load torque variation, in order to reduce the acoustic

noise of the motor / pulley / drum assembly as much as possible. The set-point value is 3Hz in

Figure 2..

Figure 3. shows the same curve but the set point is now adjusted from 3Hz at low speed to

6Hz at medium speed and 8Hz at maximum speed.