Question

What is the RPM of a 3 phase motor?

Answer 1

The RPM (Revolutions Per Minute) of a 3-phase motor depends on two key factors:

1. The number of poles in the motor: Motors come with different numbers of poles, such as 2, 4, 6, etc. The number of poles affects the motor's speed.
   

1. The supply frequency: In most countries, the supply frequency is either 50 Hz (Hertz) or 60 Hz. This is the frequency of the alternating current (AC) that powers the motor.
   

The formula to calculate the synchronous speed (RPM) of a 3-phase motor is:

\[
\text{Synchronous Speed} = \frac{120 \times \text{Frequency (Hz)}}{\text{Number of Poles}}
\]

For example:

1. For a 4-pole motor on a 50 Hz supply:
   

\[
\text{Synchronous Speed} = \frac{120 \times 50}{4} = 1500 \, \text{RPM}
\]

1. For a 2-pole motor on a 50 Hz supply:
   

\[
\text{Synchronous Speed} = \frac{120 \times 50}{2} = 3000 \, \text{RPM}
\]

However, the actual RPM of the motor is slightly less than the synchronous speed due to the slip (which is a result of the load on the motor). The difference between the synchronous speed and the actual speed is known as the slip speed.

1. For example, a 4-pole motor running at 50 Hz may run at around 1450 RPM instead of 1500 RPM, due to slip.
   

So, to summarize:

1. The RPM of a 3-phase motor depends on the supply frequency and the number of poles.
   

1. The synchronous speed is calculated using the formula above, and the actual RPM will be a bit less due to slip.