Chapter 15: Reversing motors

Name and describe 4 basic methods of motor control wiring?

1) Point-to point - circuit components directly connected to one another as seen on a wiring diagram. 2) Hardwiring using terminal strips - wires to each circuit component are numbered and connected to a terminal strip instead of directly to components. Components are connected to each other via terminal strip. 3) PLC - control circuit wired to a PLC 4) Electric motor drive - control circuit wired to an electric motor drive.

How our 3 phase motors reversed? What is the industry standard way for doing this?

3-phase motors can be reversed by interchanging and two leads (among L1, L2, L3). The industry standard is to interchange L1 and L3.

What is a drum switch and how is it used to reverse a motor?

A drum switch is a manual switch made up of moving contacts mounted on an insulated rotating shaft. A drum switch can be used with a non-reversing starter in line with a drum switch to reverse the rotating of a motor.

How are DC motors reversed (all types: series, shunt, compound, permanent magnet)? What is the industry standard for doing this?

All DC motors can be reversed by either reversing the leads to the field windings or the armature. The industry standard is to reverse the leads to the armature.

Why can't a drum switch be used by itself to reverse a motor?

It does not contain overload protection like a motor starter.

What is a mechanical interlock? How is it represented in a wiring diagram?

It is an arrangement of contacts in such a way that both sets of contacts cannot be closed at the same time. Is used to ensure that forward and reverse operations cannot occur at the same time. Represented in a wiring diagram as a dashed line between the forward and reverse coils. See md2.27.

If a diagram is not available, how can you tell the starting winding from the running winding?

Measure resistance of the two windings. The running winding has a heavier gauge wire than the starting winding; therefore, it will have lower resistance.

What are three ways to prevent a motor from being activated in the forward and reverse direction at the same time?

Mechanical interlock, pushbutton interlock, auxiliary contacts (electrical interlock).

Which method of interlocking does not prevent a short (both forward and reverse coils from being activated) when there is a stuck contactor? Why?

Pushbutton interlocking. See md2.23. Assuming in this diagram that it does not have the auxiliary contact electrical interlocks, if the forward memory contact is stuck, when the reverse button is pressed the forward control circuit will be initially broke. However, when the reverse button is released the forward control circuit will re-energize causing a short.

What is an auxiliary contact electrical interlock?

See md2.23. It is an auxiliary contact that prevents the activation of the motor in both forward and reverse modes at the same time. An NC contact placed in front of the coil.

What is a pushbutton interlock? How is it represented in a wiring diagram?

See md2.23. The forward button is linked to a NC contact on the reverse control circuit. Pressing the forward opens the NC contact on the reverse control circuit preventing the motor from being reversed while the motor is running forward. Simultaneously the reverse button is linked to a NC contact on the forward control circuit. When it is pressed it opens the NC contact on the forward control circuit preventing the motor from running forward while the motor is reversed. It is represented by a dashed line between the button and NC contact in a wiring diagram.

In a reversing motor circuit there are two parts: the power circuit and the control circuit. Describe how to troubleshoot the power circuit.

See md2.25. 1) Test the voltage between each pair of power lines (L1-L2, L1-L3, L2-L3). Voltage must be within 10% of rated voltage of the motor. 2) Measure voltage out of each fuse or circuit breaker. Replace any blown fuse and reset tripped circuit breaker. 3) Using caution because manually operating starter contacts may start or stop a load without warning, test the motor starter by manually engaging the forward and reverse power contacts and measuring voltage (T1-T2, T1-T3, T2-T3). If voltage is not correct disconnect power and check motor starter contacts for burning or wear. 4) Check voltage at motor terminals. Voltage must be within 10% of rated voltage for the motor. If voltage is correct and motor does not run, there is a problem with the motor or mechanical connection.

How do you troubleshoot the control circuit in a reversing motor circuit?

See md2.26 1) Measure incoming supply voltage to the circuit at L1, L2. Voltage must be within 10% of control voltage rating. Test power circuit if voltage is not correct. 2) Measure voltage out of the overload contacts to ensure that they are closed. If no voltage present contacts are faulty or tripped. 3) Measure voltage into an out of the control switch or contacts (Start and stop buttons, memory contacts, ect). Should have voltage when closed, and no voltage when open.

How are single-phase motors reversed? What is the industry standard for reversing a single-phase motor?

Single-phase motors are reversed by reversing the leads on the starting winding or running winding. The industry standard is to switch the red and black wires of the starting winding. Example: Forward motor action: L1 to running winding and black lead of starting winding, L2 to running winding and red lead of starting winding. For reverse action: L1 to running winding and red lead of starting winding, L2 to running winding and black lead of starting winding. See md2.24 for a diagram showing the reversal of the starting winding.

How are 3-phase, single-phase, and DC motors reversed using magnetic motor starters?

They are wired the same as when using a manual motor starter. Like manual starters, magnetic starter also have a mechanical interlock to prevent simultaneously activating both reverse and forward operation at the same time. Magnetic starters also have a set of auxiliary contacts that can be used for the same purpose.

What are two advantages of wiring using a terminal strip vs. point-to-point wiring?

Troubleshooting and circuit modification are easier.