Motor Starter Basics

Motor Starter/Control Basics

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What is a Motor Starter?

The primary purpose of a motor starter is to start and stop a motor which it is connected to safely. It is essentially a switching device.

Allows for remote operation of the motor.

Starters are made from two building blocks, contactors and overload protection: (however are commonly used with some form a circuit protection as well)

Contactors control the electric current to the motor. Their function is to repeatedly establish and interrupt an electrical power circuit.

Overload Protection protects motors from drawing too much current and overheating, from literally “burning out.”

Contactor

Functions much like a relay, in that when control voltage is applied to a coil (or electromagnet) the contacts are pulled closed allowing current to pass through the contacts to the motor. (for more information on relay operations, please click on the link in the description below.)

A magnetic contactor is operated electromechanically without manual intervention. This means that the contactor can be operated remotely, without the need for putting a person in potentially dangerous location. Magnetic contactors use a small control current to open and close the circuit.

Auxiliary contact

This can be normally or normally closed contact.

Allows us to control another circuit, which we can use to monitor the state of the contactor, and whether it is open or closed. This can also be referred to as “Run Status”

How do we protect the motor?

Starter are commonly used in conjunction with a motor circuit protector (MCP)

This is a type of circuit breaker specifically designed for motor starters, that does not utilize thermal protection.

It operates to prevent large spikes of current that could be caused by a short circuit.

It uses Magnetic protection

For more info on circuit protection, please refer to our other video.

We could use a fused disconnect switch in place of this device.

Overload Protection

What is an overload

Overloads are designed to protect against prolonged overcurrent of a motor which could cause the motor to overheat and burn up.

An overload relay consists of:

A current sensing unit (connected in the line to the motor).

A mechanism to break the circuit, either directly or indirectly.

To meet motor protection needs, overload relays have a time delay to allow harmless temporary overloads without breaking the circuit. They also have a trip capability to open the control circuit if mildly dangerous currents (that could result in motor damage) continue over a period of time. All overload relays also have some means of resetting the circuit once the overload is removed.

Review

We have a Motor Circuit Protector (this is a type of breaker that is designed for motor control)

It allows to protect the motor against short circuits, while also giving us the ability to disconnect power to the circuit if needed.

Contactor

Which gives us the ability to apply power to the motor when needed, and monitor the state of the motor (Run Status) using Aux. contacts.

Overload

Which provides the motor form sustained over current that can cause overheating.

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