The Official Variable Frequency Drive (VFD) Distributing Blog

If you do not yet know how a Variable Frequency Drive (VFD) works, it is recommended you visit our post entitled: How Does A Variable Frequency Drive (VFD) Work? prior to reading this post.

AC Three Phase Induction Motor RPM is determined by the formula:

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

Since the number of poles of a three phase induction motor is established when it is manufactured, the only way to change the speed of the motor is to change the Frequency.

For Example:

A four pole three phase induction motor when operated at 60 Hz will be very close to 1800 RPM (synchronous speed). The rated full load speed will be less than synchronous speed by the value of “Slip”. A four pole three phase induction motor with a rated full load speed of 1750 has a slip rating of 2.7%. By formula:

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

When using this information and the above formulas the running speed of an AC three phase induction motor can be calculated at any input frequency.

So how fast will a four pole three phase induction motor run when operated at 45 Hz? The three phase induction motor has a Full Load RPM rating on the nameplate of 1760 RPM.

• 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
• ((1800 RPM - 1760 RPM) / (1800 RPM)) * 100% = (40 RPM / 1800 RPM) * 100%
• (40 RPM / 1800 RPM) * 100% = .022 * 100%
• Slip Rating = 2.2%

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

• RPM Slip = RPM * Slip Rating
• RPM Slip = (1350 * .022) = 27.7 RPM
• RPM Slip = 27.7 RPM

So full load RPM of this motor at 45 Hz will be calculated as such:

• Full Load RPM = RPM - RPM Slip
• Full Load RPM = 1350 RPM - 27.7 RPM
• Full Load RPM = 1322.3 RPM

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

Joe Chamberlin
http://www.vfddistributing.com

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