View Full Version : reverse rotation of an A/C motor

05-30-2006, 08:21 AM
I have an A/C motor which i intend to use to power a jointer. The motor turns clockwise and the jointer blade needs to turn counter-clockwise. I understand it is possible to reverse the polarity of the field coils and thereby reverse the direction of rotation of the motor. I can see a circuit board and some point to point wiring inside one end plate. The p to p wires all have slide on connectors so changing them around would not be a problem. Can anyone give me a brief outline of how this is done or direct me to a website which explains the procedure.

05-30-2006, 10:01 AM
Those straps could also be to change from 110 to 220. There should be a data plate on the motor with the strapping info. It may be inside the cover.

05-30-2006, 01:21 PM
While most induction motors will run equally well in either direction, some will not, depending upon the type of motor it is and it's intended application. The key is what direction the rotor starts to turn. This is determined by the polarity of the start windings relative to the run windings. In most, but not all motors the start winding is switched out of the circuit by a centrifugal switch or a current or potential relay once the rotor gets up to 70 - 80% of full speed.

As an economy measure, some motors intended for specific applications (so-called "definite-purpose" motors) do not bring all winding leads out to the terminal plate on the motor. In those cases, the motor cannot be reversed without major internal surgery to reconnect internal wiring. It would, however, be quite unusual for yours to be of this type.

If this is a salvaged motor, check to be sure it is not an "air-over" motor. Such motors are intended for fan applications in which the motor is cooled by the air moved by the fan. They are not suited to machinery applications without provision for external cooling. If the motor came off a machine tool, it would be ok. If it came off a fan, you may be in trouble. Look at the nameplate.

What you have to do is identify the various leads coming out from the windings to the terminal plate. It can get fairly complex if you have a multi-voltage, reversible motor, but is quite feasible. First and foremost, be sure that each lead has an identifying label or put your own labels on the leads and draw a diagram BEFORE disconnecting anything. That way, when you get confused with a handful of wires, you can at least restore normal operation.

The motor probably has a centrifugal switch. That centrifugal switch is in series with the start winding and, if present, a start capacitor under a pressed steel cap on the outside surface of the motor. However, not all motors have a start capacitor. Identify the wires at the terminal plate that connect to the series loop of start winding, centrifugal switch, and (probably, but not always) a start capacitor and reverse only those wires at the terminal plate. That should reverse the direction of rotation of the motor.

A good precaution is to measure the current drawn by the motor without load in its original configuration using a "tong tool" or clamp-on current meter around only one power input wire. Then, after your changeover, measure it again. It should be the same as before. If it isn't, more analysis is in order. Clamp-on ammeters are getting fairly inexpensive and are good to have on hand if you are messing with motors and power wiring. In the absence of an ammeter, listen carefully for any change in the sound of the motor after reversing that would indicate excessive current draw.

If this procedure is not successful, post pictures of the motor nameplate and terminal board.

Good luck.