# How to increase the speed of a specific AC motor

Here are details of this project: The motor being used is a 2 phase Slo-Syn model SS400 (1/4 HP). The motor has two 120V windings that are physically 90 degrees to one another. It is operated by applying 120V signals 90 degrees out of phase. Winding current is 1 amp and motor speed is 72 RPM @ 60 hz. It is a synchronous motor with permanent magnet rotor.

The two 120 voltage sources come from 208 volt 3 phase power which comes into the building. A 208V to 120V transformer is connected line to line that gives one output. The second output which is 90 degrees out of phase is connected line to neutral. Line to neutral voltage is 120. The 3 phase is Y connected configuration.

This motor is part of the positioning mechanism for a 5.5M parabolic dish antenna to be used for an EME radio setup. We need the motor to run 2-3 times faster than it currently does so the applied frequency needs to be 120-180 Hz. This will be required for the antenna to 'track' the moon. The problem with raising the frequency is that the reactance of the motor windings increase as the frequency is increased. The result is lower winding current and thus lower torque.

An experiment using a variable frequency control (VFC) device was done. These are often used for running 3 phase motors from a single phase source. The output of these devices is delta connected which means there is no neutral and everything must be connected line to line. We needed the neutral connection to get the 90 degree phase shift so we tried using three 208V to 120V transformers. The 208V sides were connected line to line to the VFC output and the 120V secondaries were connected in the Y configuration. The VFC did not like this setup and would trip on overload and not operate the motor.

So the question is, how can we get 2-3 times more speed from this motor without losing too much torque.

Using a variable frequency quasi sine wave source is being considered. Also possibly using high power class D audio amplifiers and a step up transformer.

Understanding that the torque will indeed be a factor, we would still like to attempt to speed the motor up. Changing to another motor and/or gear box would be difficult due to the mechanics of the dish assembly. Ideas to electrically increase the frequency/motor would be helpful and worth a try.

• Power is torque * speed. If you increase the speed by 3x and keep the same torque, you will be delivering 3x the power. Generally, this is not possible. My suggestion is to replace the motor with a servo-motor. – mkeith Nov 6 '18 at 3:34
• Get a 3x bigger motor and reduce the transmission ratio of the gearbox by the same factor (you can add an additional gearbox). Watch out that the antenna and/or its construction does not break under the tripled dynamic forces. – joe electro Nov 6 '18 at 5:04
• You simply cannot increase the speed and maintain the torque. Even if this were a three phase motor, operating on a VFD, as you pass base speed, the motor goes from a constant torque operation mode to a constant power operation mode; ie. Torque drops off linearly as speed increases. – R Drast Nov 6 '18 at 10:12