What Determines the Rotating Speed of a Motor?

Topics: Electrical Engineering, Motor Selection, Electric Motor, VFD

Jahnavi Sajip
Author : Jahnavi Sajip on June 19, 2020

Electric motors are characterized by their variety, ranging from fractional horsepower units for small appliances, to motors with thousands of hp for heavy industrial use. Other specifications found in motor nameplates include the input voltage, rated current, efficiency and speed in RPM.

The speed of an electric motor is determined by its physical construction and the frequency of the voltage supply. Electrical engineers select motor speed based on the needs of each application, similar to how horsepower is specified based on the mechanical load.

Make sure your building has the right motor for each application.


How the Power Supply Frequency Relates with Motor Speed

Depending on the country, the power supply can have a frequency of either 60 Hz or 50 Hz. Although a three-phase motor will rotate with both power inputs, there will be performance issues if a motor is specified for one frequency and used with the other.


Since a 60Hz power supply switches polarity 20% faster than a 50Hz supply, a motor rated for 50Hz will spin at 20% higher rpm if connected to 60Hz. Motor torque stays relatively constant, and for this reason a higher speed results in a higher shaft power. Although the motor releases more heat, this is compensated because the cooling fan also accelerates. However, the motor tends to draw more reactive current, reducing its power factor.

On the other hand, connecting a 60Hz motor to a 50Hz power supply is a more delicate matter. Reducing speed while keeping the same voltage may saturate the magnetic core of the motor, increasing current and overheating the unit. In this case, the simplest way to prevent saturation is by lowering the input voltage. Ideally, the V/Hz ratio should stay constant:

  • A 60Hz motor operating at 50Hz is at 83.3% of its rated frequency.
  • To keep the V/Hz ratio constant, the input voltage should also be reduced to 83.3%.
  • If the electric motor normally operates at 240V and 60Hz, the input voltage at 50Hz should be 200V to keep a ratio of 4 V/Hz.

Motor Wiring and Number of Poles

A permanent magnet has two poles, but motors can be wired so that their magnetic field has a higher number of poles. A two-pole motor completes a full revolution with one polarity change, while a four-pole motor only rotates 180° with one polarity switch. For these reason, more poles lead to a reduced motor speed: assuming all other factors are equal, a 4-pole motor will rotate at half the speed of a 2-pole motor.

  • A 60 Hz power supply changes polarity 60 times per second, and a two-pole motor will spin at 3,600 rpm when connected to this source. However, a four-pole motor will only rotate at 1,800 rpm.
  • For 50 Hz motors, the corresponding speeds are 3,000 rpm at 2 poles, and 1,500 rpm at 4 poles.

The concept can be summarized with the following equation:


Applying the equation above, a 4-pole motor at 60Hz has a speed of 1,800 rpm, while a 6-pole motor at 50Hz has a speed of 1,000 rpm. However, this is actually the speed of the magnetic field, called the synchronous speed, which does not always correspond with the shaft speed.

  • If the motor is synchronous, the rotor uses a permanent magnet or electromagnet to rotate at the calculated speed.
  • On the other hand, an induction motor will operate slightly below the calculated rpm. This is a natural consequence of the electromagnetic induction phenomenon, and should not be viewed as a malfunction.

If an electric motor has a nameplate speed of 1,800 rpm, one can conclude that the unit is a 4-pole synchronous motor rated for 60 Hz. On the other hand, if the nameplate speed is a lower value such as 1,760 rpm, the unit is an induction motor.

Variable frequency drives control motor speed by adjusting the input frequency, hence their name. They also modulate voltage to keep the V/Hz ratio below the point where the magnetic core is saturated. For this reason, a VFD does not damage a motor even when the speed is reduced below the nameplate value. The main drawback of VFDs is causing harmonic distortion, since they are nonlinear loads, but this can be compensated with harmonic filters.


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