# How to calculate RPM using frequency

I am working with VFD (CT-2002ES-A75). It shows output frequancy on it's seven segment display. However It has a parameter(Cd46 on page 27 in the manual) for display the current RPM of the motor.

For display the current RPM need to enter some factor as parameter input. How to calculate that scaling factor for my motor?

This is the nameplate of the motor that controlled by the VFD. Motor is connected as star system. Working on 230VAC supply. • Haven't you forgotten to tell us which way the motor is wired? Delta or star ('Y' or wye)? Once you figure that out you read the current from the rating plate. The number before the comma is the delta-mode current. The number after the comma is the Y-mode current. – Transistor Dec 26 '17 at 11:48
• @Transistor It's star. RPM = (120 * Frequency) / # of poles in the motor how can I get # of poles in the motor? – user_fs10 Dec 26 '17 at 11:54

• If a motor has two poles it will rotate once per mains cycle. At 50 Hz this will be 50 x 60 = 3000 RPM.
• If a motor has four poles it will rotate half a revolution per mains cycle. At 50 Hz this will be 50 x 60 / 2 = 1500 RPM.
• If a motor has six poles it will rotate one third of a revolution per mains cycle. At 50 Hz this will be 50 x 60 / 3 = 1000 RPM.

This can be generalised as RPM = 120 x f / poles.

Therefore # of poles = 120 x f / RPM.

Since induction motors slip the full-load RPM will be somewhat less than the calculated speed. From the rating plate we can see that the min -1 (revolutions per minute) is 930 to 950 at 50 Hz. This is just below 1,000 RPM so it must be a six-pole motor.

• Did you calculate like this? # of poles = 120 x f / RPM=120*50/930=6.45 – user_fs10 Dec 26 '17 at 13:35
• Kind-of. Induction motor slip is typically a few percent. Pick the nearest standard RPM, 1000, 1500 or 3000, above the rating plate RPM and calculate the number of poles from that. – Transistor Dec 26 '17 at 13:58
• Got It..Thank you – user_fs10 Dec 26 '17 at 14:04

Inside of the VFD, you can change the display to read in actual RPMs instead of Hz. look at Cd46 Speed multiplier (Settable range 0.01~500). Most VFDs can do this.

• What is the speed muliplier that I want to enter to cd46? How to calculate it for my specific motor? – user_fs10 Dec 28 '17 at 13:03