How do you calculate the RPM of a 3 phase VFD motor

Variable frequency drives (VFDs) and electric motors are strange companions: The VFD is a static device, delicate, intolerant of wide variations in environmental conditions; extremely adjustable and controllable by microprocessors; capable of being monitored and controlled from remote locations; and a product of modern electronic engineering and precision—the beauty.

Calculating RPM for a three phase VFD is relatively simple… 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 VFD 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 VFD can be calculated at any input frequency. So how fast will a four pole three phase Delta VFD run when operated at 45 Hz? The three phase VFD 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

When selecting a three phase motor, the number of poles is chosen to achieve the speed of rotation that you require. Here are two tables, one for a 50 Hz power supply and one for a 60 Hz power supply:

The formula is n = 60 x f /p where n = synchronous speed; f = supply frequency & p = pairs of poles per phase. The actual running speed is the synchronous speed minus the slip speed.

For a 50 Hz three phase supply:

2 poles or 1 pair of poles = 3,000 RPM (minus the slip speed = about 2,750 RPM or 6 -7% n)
4 poles or 2 pairs of poles = 1,500 RPM
6 poles or 3 pairs of poles = 1,000 RPM
8 poles or 4 pairs of poles = 750 RPM
10 poles or 5 pairs of poles = 600 RPM
12 poles or 6 pairs of poles = 500 RPM
16 poles or 8 pairs of poles = 375 RPM

For a 60 Hz three phase supply:

2 poles or 1 pair of poles = 3,600 RPM (minus the slip speed = about 2,750 RPM or 6 -7% n)
4 poles or 2 pairs of poles = 1,800 RPM
6 poles or 3 pairs of poles = 1,200 RPM
8 poles or 4 pairs of poles = 900 RPM
10 poles or 5 pairs of poles = 720 RPM
12 poles or 6 pairs of poles = 600 RPM
16 poles or 8 pairs of poles = 450 RPM

To figure out the amount of poles, you are able to study the information plate straight or calculate it in the RPM stated around the information plate or you are able to count the coils and divide by three (poles per phase) or by six (pairs of poles per phase). Exactly where the energy from the induction motor is continuous, the torque increases in the price that the speed decreases.

Using the advent of variable frequency drive (VFD), you are able to have any frequency / rated volts you want. I frequently see name plates with issues like 575VAC, 42.five Hz and so on. When these "specials" are produced I generally see six pole machines - but that might be just a manufacturer's preference.