AC Motor Control and Protection

Motor Control and Protection

 How to Control an AC Motor 

Back in the days, after the invention of motors by Nikola Tesla, people needed a way to turn on and off an AC motor, so they created a knife switch to help control the motor. When an operator grabs the handle, he would move if from one side to another, connecting it to power. Although, this means of motor control wasn’t safe for the operator, because as the switch was about to close, it would cause some arcs around the edge, which would injure the operator. 

So they put the switch inside an enclosure to protect the operator. They put a handle on the outside and now they can turn on and off the motor and the arcs were contained inside the box. Meanwhile, this means of motor control wasn’t automatic. However, the problem is that someone has to stand there and physically operate the switch, If you have 40 operator motors spread across the plant, it became impossible for an operator to run up and down, operating the motors, clearly, this would not work for a lot of machine application.

Motor Control Using Contactors

So they embedded this thing called a CONTACTOR. Inside the contactor, there is a coil of wire wrapped around the metal. Above that piece of metal, there is another piece of metal attached to the electrical bridge that connects contacts from one side to the other side.

This top portion (the metal attached to an electrical bridge is called an ARMATURE) When you apply an electric current through the coil of wire, you end up turning the bottom piece of metal into a magnet (Electromagnet) 

AC Motor Control and Protection

Meanwhile, when you turn on the electromagnet, it attracts the top piece of metal, which is part of the armature, and pulls it down. Which in turn closes the electrical contacts on both sides. When the contact close, it allows current to pass through the contactor down to the motor. 

But how do we turn off the motor? First, We kill power to the coil, which will turn off the electromagnet, but that’s not enough to separate the electrical contacts, there is a spring inside the contactor. 


When the electromagnet is turned on, the top piece of metal overpowers and compresses the spring, but when the electromagnet is turned off, the spring is able to push apart the electrical Contactors. So, apparently, the motor can then be controlled without the need of an operator coming towards the motor in other to control it.

So what is the point of all this? It allows for remote operation of the contactor, which in turn allows remote control of the motor. 

If we put a push button on the other side of the circuit that is connected to the contactor’s coil. Then you can turn on the coil from wherever you want. So, we can have a control station with push buttons installed on it. Meanwhile, the operator can stand there and control al the motors around the facility without moving. 

Motor Control: Auxiliary Contacts

The design is made in a way that, from the top-down, we get 3-phase power coming in the toOP, three contacts on the inside, one for each phase, and three-phase power going out the bottom. However, there is a common accessory that gets added to the.contactor called the auxiliary contact, often, it is mounted to the side of the contactors. 

Inside the auxiliary contact module, we have an electrical contact that can be normally open (NO) or normally closed (NC) 

If you grab a normally open auxiliary off the shelf, it would not pass electricity through it, except an outside force changes it. When it gets attached to the side of a contactor, there is a mechanical connection between the auxiliary contact and the moving parts inside the contactor. 

So when the contactor pulls down, it causes the auxiliary contact to also pulls down. Which closes the contacts in the auxiliary. So what is nifty about this is that you can connect a circuit to the auxiliary contact, and it would open and close the circuit whenever the contactor turns on and off the motor. For example, we can connect a red light indicator to indicate that the motor is running. 

Basically, the auxiliary contact gives you a status of whether the contactors are open or close. 

What Protects a Motor? 

What can result to a short circuit in a motor?

For induction motors, one of the ways we can see a short circuit is a phase to phase short. So let’s say a fork lifter operator makes the wires accidentally go into a motor, it does not cut the wire in half but is enough to cause two of the phase to touch each other. Hence, the wires have unrestricted paths for current to flow and hence we get a very large amount of current very quickly along the wire.

Well, what’s wound around wires? INSULATION!!!

Insulators do not like heat. If you put a large amount of current along the wire, you build up heat very quickly, thus burning up the insulation and it can cause a fire.

Another way we get a short circuit is we can get a phase to ground short. Let’s say that wire ends up touching the rail of a garage door and now we have a path to the ground! Same problem. 

One of the ways we can protect against a short circuit is to use a molded case circuit breaker. Inside the circuit breaker, there are two forms of circuit protection. 

Types of Motor Protection 

  1. Magnetic Circuit Protection
  2. Thermal Protection

Magnetic Circuit Protection: 

A coil of wire wrapped around a metal core, next to the wire, we have a strip bar. When the current passes through the circuit breaker and through this coil of wire, it starts generating magnetism on the metal. When we get a large amount of current passing, it creates a magnet so strong that it pulls the strip bar towards it. Then causes the circuit breaker to trip. This is called Magnetic Circuit Protection. 

Motor Protection: Motor Overload

Motor Overload: When too much current is being drawn from the motor. This can be caused when the motor is trying to move a very heavy object, causing it to need torque greater than its breakdown torque. The motor will still try to move the load but it would pull more and more current as it tries to turn. Eventually, the motor would pull so much current, that it would cause too much heat inside and smoke the motor. 

Thermal Protection:

Circuit breakers have technology inside that protects against over-current. Moreover, the technology is based around two thin strips of metals bonded together. This is called a bimetallic strip. Next to the bimetallic strip, there is a trip bar. When the metal gets warm it expands. 

If we have two different types of metal bonded together, it is likely that one side will expand faster than the other, which will cause the strip to bend. If the current becomes too great, it bends towards the bar until it finally trips the circuit breaker. This is called Thermal Protection. 

Let’s plot a graph of current against time. When you start an electric motor from a dead stop, you get a big rush of current at the beginning, to get the motor up and running. After the big rush leaves out and runs at about 100% speed. At full speed, it will pull a fairly constant amount of current-the FULL-LOAD AMPS.


AC Motor Control and Protection

The big spike of current at the start of a motor is called INRUSH. INRUSH is usually about 6-8 times the full load amps of the motor. Going back to the circuit breaker, we have both magnetic and thermal protection built-in. Magnetic protection reacts to a very high level of current very quickly, so it’s suited to protect against short circuits. 

Thermal protection is slower reacting protection and correlates with the amperage you see in front of the circuit breaker. This is to protect against overcurrent. (overload) 

However, inrush current causes a large amount of current than usual, this looks a lot like over-current. 

Hence in some circuit breakers, they remove the thermal protection and leave the magnetic protection for short circuits. This kind of circuit breaker is called MOTOR CIRCUIT PROTECTOR (MCP) or HMCP (HIGH INTERRUPTING MCP). So if we get rid of the thermal protection, we still need to find a way to protect the motor from an overload condition. 


We can add a new component to the contactor called an Overload Relay. The Overload Relay have three bimetallic strips inside that are designed to heat up at the same rate as the windings of the motor. This is different from the bimetals inside the breaker because those bimetals did not know how to ignore the inrush when you start the motor. When you pair an Overload Relay with a contractor, we call this a STARTER.

Conclusion on Motor Control and Protection

In conclusion, we have seen that the manual control of an AC motor is very difficult in an application where more than one motor is present. So, it’s best that motors are automatically controlled. Meanwhile, there are two ways to achieve motor protection. There we have magnetic circuit protection and thermal protection. However, the magnetic circuit protection helps protect the motor against a short circuit. While Thermal protection protects the motor against overload. But there are cases where the thermal protection of a motor is removed, leaving only the magnetic protection. 

Well, that’s all we got on motor control and protection. We hope that you found this article helpful. Again, you can click here to check out our electrical related articles.

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