Automation Notes: On/Off Controls for AC Motors

Most control systems require objects to move, and this usually involves motors. Lifting, pumping, robots, conveyor belts, fans-almost everything uses some kind of motor. Universal one or three-phase AC motors are great for simple on/off systems; inverter motors are specifically designed for operation with variable frequency drives or VFDs.

General motors are usually connected to the main power circuit through a main circuit breaker or fuse, and use a contactor to enable and disable the power to the motor; overload protects your equipment from accidental overcurrent/overheating, which may be caused by blockage or Caused by malfunction.

1. What is an engine starter?

A motor starter is a combination of equipment that starts, runs, and stops induction motors according to the commands of the operator or controller. A motor starter must have a contactor and an overload relay to operate properly: a contactor for opening and closing the flow of energy to the motor, and an overload relay for providing an overload protection.

2. What is a contactor?

The contactor is a three-pole electromechanical switch whose contact point is closed by applying a voltage to the coil. The coil will remain energized until the contact is de-energized, so that the contacts are closed as well. Since the motor has inductance, it is more difficult to break the current, so the contactor has horsepower and rated current, which needs to be adhered to.

An overload relay is a device that has three current sensing elements to protect the motor from overcurrent. Each phase from the contactor to the motor must pass an overload relay current sensing element. If the overload current exceeds the set time of the relay long enough, a set of contacts will open to protect the motor from damage.

The AC motor starting circuit can be controlled from a simple button or remote signal, such as PLC.

To learn more, check out the chapter on AC motor on/off control in our automated video cooking book. Understand the types of AC motors and basic control schemes.

The “Technical Tips” video starts with an overview of AC motors and how to choose the right size according to system requirements. Other topics include:

1)How to specify the contactor

2)Choose overload protection

3)Select the circuit breaker

4)How and why to use surge protection

5)Setting up and using latching contactors

6)How to use auxiliary contacts

7)Use contactor to commutate the motor

The “How to” video shows step by step how to wire and use programming logic and various types of motor control circuits to connect to CLICK, Do-more or Productivity series plc.


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