Interface for Induction Motor

Question

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)?

Answer 1

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.

Answer 2

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).