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Electric motors: Connection between slip and speed

Did you know that the motor speed tells you almost everything about the rotor losses of an electric motor? Slip is responsible for this. It directly reflects the proportion of losses in the air gap power, i.e. the effective input power of the stator minus the stator copper and iron losses.

Bargteheide , 2021-08-10

The slip is easily calculated: Slip = field rotation speed minus nominal speed divided by field rotation speed.

Example: At a field rotation speed of 1,500 rpm and a nominal speed of 1,460 rpm, the slip S is 0.02666. That is 2.67% slip.

If the air gap power Pδ is already known from the stator losses and the input power, the remaining losses can be deduced: In the rotor iron these are low due to the minimal frequency and in the bearings they are usually only a few watts. The main proportion is due to ohmic losses of the rotor cage. If the air gap power Pδ – for example 3,140 W – is multiplied with calculated slip S, the rotor cage losses result in 83.73 W.

A good identifying feature of low loss motors is therefore low slip, because this means that the speed of the motor is very close to the field rotation speed. Thus, the 4-pole 3 kW motor (50 Hz) from NORD DRIVESYSTEMS in its IE1 version, upon which the example is based, with for example a speed of 1,415 rpm can easily increase to a speed of 1,460 rpm as an IE3 version and the slip reduces from 5.67% to only 2.67%.

However, in ventilation and pumping applications the driven machine must be modified for this! Otherwise, the saving of losses by the energy saving motor results in higher energy consumption due to higher speed of the fan characteristic curve because the air flow power increases.

In case of any queries regarding the correct selection and dimensioning of drives, your personal contact partners at NORD DRIVESYSTEMS will be glad to answer your questions.