Motor Speed Calculator
What is Motor Synchronous Speed?
The synchronous speed of an AC induction motor is the speed of the stator's magnetic field rotation. This theoretical speed is determined by the supply frequency and the number of magnetic poles in the motor's design. Unlike the actual rotor speed (which is slightly slower due to "slip"), the synchronous speed is a fixed mathematical constant for any given motor configuration.
How to Calculate Motor Speed
To calculate the synchronous speed of an electric motor, you need to know two primary variables: the electrical frequency (measured in Hertz) and the quantity of magnetic poles. The formula is as follows:
RPM = (120 × Frequency) / Number of Poles
In this formula, 120 is a constant derived from 60 seconds per minute multiplied by 2 (since poles always come in pairs). For example, a 4-pole motor operating on a 60Hz power grid will have a synchronous speed of 1,800 RPM. On a 50Hz grid, that same motor would run at 1,500 RPM.
Understanding the Impact of Poles and Frequency
The relationship between speed and the number of poles is inversely proportional. This means that as you add more poles to the motor design, the output speed decreases. Conversely, increasing the supply frequency (often achieved via a Variable Frequency Drive or VFD) will increase the motor's speed.
Frequently Asked Questions
What is the difference between synchronous speed and rated speed?
Synchronous speed is the speed of the magnetic field. Rated speed (or full-load speed) is the actual speed the shaft turns when the motor is under load, which is typically 2% to 5% slower than synchronous speed.
Why is my motor speed lower than the calculation?
This is known as "slip." Induction motors require slip to create torque. If the rotor turned at the exact same speed as the magnetic field, no current would be induced in the rotor bars, and no torque would be produced.
Does this calculator work for DC motors?
No, this formula is specifically for AC induction and synchronous motors. DC motor speed is controlled by voltage and field strength rather than frequency and pole count.