3 phase motor speed control??

Thread Starter


Joined Mar 1, 2008
how would i go about controling the speed on a 3 phase motor?


Joined Feb 29, 2008
Hendo, there are a couple of types of 3 phase motors, syncronous and induction. With syncronus the rotor keeps up with the speed of the rotataging magnetic field. With induction the rotor (squirel cage) has some slip. For example a typical 4 pole 3 phase induction motor at 60 Hz will rotage at 1750 rpm. The 60 Hz produces a magnetic field that rotates 1800 rpm, but because of the slip the rotor is turning slightly slower. With a syncronous motor perminant magnets are used in the rotor and it turns at the same speed as the field. In the case of the induction motor, the frequency controls the speed. In the case of a syncronous motor the position of the rotor is used to control when the poles in the field reverse and so the rotor and field stay in lock step. In syncronous an increase in power to the motor, voltage and current, enable the field to pull the rotor to the next position faster so the overall power in the field is what controls the speed of the motor. Of course as the rotor speeds up the frequency goes up as well. With induction you still need more power so the rotor doesn't slip to a stall, but you must increase the frequency also or the speed will not change.

In either case you need a variable frequency drive also called an inverter, or a, variable speed drive, or an AC drive.

By the way Brushless DC permenant magnet motors are syncronous motors. Another kind of brushless is a slip ring motor, these are similar but use an electro magent for the rotor magnet which is powered by contacts on slip rings. The slip rings take the place of brushes. And since the commutation occurs by way of alternating current the armiture is "always" active with no intruputions that you would have with brushes.

The first step is to define what motor type you are using.
Next you need to know the HP and RPM.
You also need to know the class of wire, and the voltage rating.
You can get all of this from the nameplate.

You also need to know what the operating conditions are for your motor, indoor, outdoor. And the type of load you plan to apply, constant torque or variable torgue.

Once you answer these questions, there are about 50 vendors that make VFD's. I hope this helps.