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Motor Control FAQs:

What is contactor control voltage? Where do I find it?
The main switching contacts of a contactor can be any voltage up to its maximum rating. This is typically 600V for US Breaker contactors. The control coil is an electromagnet that pulls in the main contacts when energized. The coils voltage connections are typically marked A1 and A2, and need to be fed by the voltage the coil is designed for. US Breaker contactors pick up between 85 and 110% of their coil voltage. As an example, a contactor with a 120V control coil will pick up when a voltage between 102 and 132V is applied to the coil. When replacing a contactor, the control voltage is usually marked near the A1-A2 terminals, not main ratings label. Examples may be found at the attached link:
Is it OK to oversize a contactor and overload relay? Where do I find it?
A contactor is simply opening and closing a circuit so it can be rated for a higher amperage and horsepower than the application. A higher amperage contactor can extend its life as long as there is sufficient control power to pull in the larger coil. An overload relay is a protective device and should be sized so it can be set at the full load amps of your motor. The NEC does allow for higher settings of up to 15 or 20% of your motors full load amps in certain conditions. The ideal situation is when your overload relay range covers both your full load amps and has a 20% upside in the setting range.
What causes contactors to chatter?
Low voltage to the contactor coil (A1 and A2 connection) will allow the contactor to chatter. One item to check is the VA output and size of the transformer feeding it if there is one. The other would be a voltage sag due to simultaneous startup of another piece of equipment. Contactor chatter can also occur due to worn switches, pushbuttons or loose connections in the control circuit. A low voltage situation can also occur due an unbalanced loading of the motor. Some causes of an unbalance load might be a single phasing condition or higher resistance in one of the phases caused by inadequate wire sizing or poor/loose connections. The higher resistance will cause a voltage lag which in turn could lead to contactor chattering. Tech Tip: Try disconnecting the motor from the load side at first startup. If there is no chatter, try again with the motor load connected. This will narrow the issue between the control circuit and feed to the motor load.
How to select a contactor and overload by motor Hp?
Contactor and overload relay selection by motor Hp may be found at the following link:
Where to find wiring diagrams for a contactor and overload?
Common wiring may be found at the attached link:
How do I wire a 3 phase starter for single phase application?
Many motor overloads include protection for when three phase motors single phase or become unbalanced. When 3 phase contactors and overloads are used on a single phase motor, looping one of the phases back up and through the unused pole will balance the load among all three phases. If control power is being tapped off of the incoming power, it should be connected to the original incoming feed terminals. An example may be found at the following link:
How to test an overload relay?
Below is that summary of how the overloads aux contacts will operate with different operations.

Start by removing it from power and put the reset in manual mode (twist the blue button out). Hit the stop button. Next, hit the yellow test button behind the window until the trip flag appears. After you get the trip flag you will manually reset the device with the blue button. You can then check the following to be sure things are operating properly.

Test (Fully depress test button until the yellow trip flag appears on the right side of the dial. Push slightly past the initial resistance point.)
- NO aux has continuity, NC does not

Hit Reset
- NO aux loses continuity, NC aux has continuity

Stop Button
- NO aux has no reaction to the stop button
- NC loses continuity while the stop button is depressed and goes back when it is released.
How to test a contactor coil?
Below is a summary of how to test a contactor coil.

To begin testing first identify the L and T sides of the contactor. You will be testing the resistance through each of the legs of the contactor (L1-T1, L2-T2, etc.) Once these have been identified you can provide power to the contactor.

Once power has been supplied, there should be a click. If not shut it down because there is an insufficient voltage supply. If it does click, use a volt ohmmeter to test the resistance in each of these legs. Each of them should read 0 ohms and direct short. If not the contactor is bad and needs to be replaced.

Once that has been checked switch the volt ohmmeter to check for current in each of the legs. Put each lead into the volt connector of the meter and see if the voltage is close to or at the appropriate voltage.
What's the difference between a normally open contact and a normally closed contact?
A Normally Open (NO) contact is off and separates the circuit until control power is applied to the coil, causing the contacts to engage. A normally closed (NC) contact is engaged until power is applied to the coil, causing it to separate and cut the flow of power through it.
What's the difference between a mechanical and electrical interlock for reversing application?
A mechanical interlock is a mechanism that mounts between two contactors and physically prevents both from being pulled in at the same time. An electrical interlock is created by wiring the control power for each contactor through a Normally Closed contact of the opposing contactor. This electrically cuts off control power to the opposing contactor when one is on.
How do you change a contactor coil??
Our contactor coils are changed by removing two screws from the front and swapping out the coil that is sandwiched between the front and back of the contactor. The external coil tag identifying the voltage also changes. Visit our YouTube channel for a video demostration at:
What causes a contactor to weld closed or fails prematurely?
There can be many reasons and we will review a few common ones. The most common is insufficient voltage to the control coil, causing arcing between the contacts if they are not pulled in tightly. This can also happen if a control power transformer feeding the control circuit is undersized or failing. Other reasons include an undersized contactor, an unprotected overcurrent condition, loose connections, bad switches in the controlling circuit, or debris built up between the contacts.

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