First off, if you need a variable frequency drive for your AC motor, make sure you check out our web store. Calculating RPM for a three phase induction motor is relatively simple… ACThree Phase Induction MotorRPMis determined by the formula:

- RPM = (120 * Frequency) / # of poles in the motor

Since the number of poles of athree phaseinduction motoris established when it is manufactured, the only way to change the speed of the motor is to change theFrequency.

**For Example:** A four polethree phase induction motorwhen operated at 60 Hz will be very close to 1800RPM(synchronous speed).The rated full load speed will be less than synchronous speed by the value of Slip.A four polethree phase induction motorwith a ratedfull loadspeed of 1750 has a slip rating of 2.7%.By formula:

- ((Synchronous Speed – Rated Full Load Speed) / (Synchronous Speed)) * 100% = Slip Rating
- ((1800RPM-1750RPM) / 1800RPM) * 100%= (50RPM/ 1800RPM) * 100%
- (50RPM/ 1800RPM) * 100%= .027 * 100%
- .027 * 100% = 2.7%
- Slip Rating = 2.7%

When using this information and the above formulas the running speed of an ACthree phaseinduction motorcan be calculated at any input frequency. So how fast will a four polethree phase inductionmotorrun when operated at 45 Hz?Thethree phase induction motorhas a Full LoadRPMrating on the nameplate of 1760RPM.

- RPM= (Frequency * 120) / # of poles in the motor
- RPM= (45Hz * 120) / 4
- RPM= 1350

Next, we calculate the Slip Rating:

- ((Synchronous Speed – Rated Full Load Speed) / (Synchronous Speed)) * 100% = Slip Rating
- ((1800RPM– 1760RPM) / (1800RPM)) * 100% = (40RPM/ 1800RPM) * 100%
- (40RPM/ 1800RPM) * 100% = .022 * 100%
- Slip Rating = 2.2%

Instead of using a percentage, we will convert the Slip Rating into how manyRPMs actually slip using the following formula:

- RPMSlip =RPM* Slip Rating
- RPMSlip = (1350 * .022) = 27.7RPM
- RPMSlip = 27.7RPM

So full loadRPMof this motor at 45 Hz will be calculated as such:

- Full LoadRPM=RPM–RPMSlip
- Full LoadRPM= 1350RPM– 27.7RPM
- Full LoadRPM= 1322.3RPM

Note:If the load is less than the available torque output from the motor theRPMwill be higher thanfull loadRPMbut will never be at synchronous speed for any given frequency.All of this information pertains to Squirrel Cage type ACthree phase induction motors.