# Interface for Induction Motor

I'm trying to make a board for controlling a reversible single phase induction motor. The board simply needs to be able to turn on/off the motor and control the direction.

My understanding is that for single phase motors some kind of capacitor is used to shift the phase of the AC input. So by switching which "line" the capacitor is connected to, direction control is established.

Going from a black box view of the motor would look something like this:

simulate this circuit – Schematic created using CircuitLab

To this:

simulate this circuit

Does that seem correct?

In reality it seems that 1 phase motors come with more complicated configurations. Some have 7 wires and support different voltages.

How should a control board interface (connections) look like if it is intended to support a variety of motors?

Maybe something based on this:

simulate this circuit

Would the above be considered a general interface to such motors (allowing the user to select his own)?

• I believe that typically, reversible single-phase induction motors have two caps, but the caps are for starting only. A centrifugal switch disconnects them from the circuit after the motor gets up to run speed. Not sure whether you need or want to show the centrifugal switch. Anyway, you should be aware that the direction control only works during start up. Once the motor is spinning in one direction, it will not start up in the opposite direction unless it is first stopped altogether. If you want instant reversing, you need three phase. Oct 29, 2015 at 6:58
• @mkeith no instant reverse needed in this case. Its pick a direction and go! Oct 29, 2015 at 7:01

For a single phase motor to be reversible, it generally must have two windings and you must have access to connections that will allow you to reverse the connection of one winding with respect to the other winding. Switching the capacitor connection has no effect. A two pole double throw switch or relay is required. If the capacitor is switched out of the circuit when the motor is at full speed, the motor must be allowed to coast to a stop or at least to a low enough speed for the switch to reconnect the capacitor before reversing. The diagram below shows the basic connections. If there is more than one capacitor, more than one voltage connection or more than one speed, reversing is more complicated.

Re last diagram in the question:

This reversing scheme will work if the auxiliary winding is identical to the main winding. The switch will put the main winding in series with the capacitor and connect the auxiliary winding directly across the power source. Some motors may be specifically designed for this reversing scheme, but it is not the typical design for this type of motor. See diagram below.

• I am trying to understand the simple case without speed and where direction change would only happen when stopped. So does this mean the circuit in my last diagram would not work? Oct 31, 2015 at 0:08

There are two general classes of single-phase induction motor: 1) Capacitor Start and 2) Capacitor Run.

The motor you have shown in your drawing is a Capacitor Run type motor. As you suggest, to reverse the direction, you simply move the AC Line connection from one winding / capacitor connection to the other.

The Capacitor Start motor is significantly different. These motors usually have two completely independent windings: the Run winding and the Start winding. The Start winding usually has some means of disconnecting the winding when the motor reaches near operating speed and often has a series capacitor to increase the phase shift.

To reverse the direction of a Capacitor Start type motor, you have to physically swap the ends of the start winding with respect to the run winding. This requires a DPDT switching arrangement. This can be a physical switch or a relay or even just swapping the wires on the motor terminal board (inside the motor junction box).

• I don't believe you can simply move the AC line connection from one end of the capacitor to the other. That will work if the auxiliary winding and the main winding are the same, but I don't believe that is usually the case.
– user80875
Oct 30, 2015 at 0:53