Now that you have a good understanding of what a contactor is and how it works, let’s move on to a discussion of overload protection. As we mentioned at the beginning of this module, overload protection prevents an electric motor from drawing too much current, overheating, and literally “burning out.”

Now that you have a good understanding of what a contactor is and how it works, let’s move on to a discussion of overload protection. As we mentioned at the beginning of this module, overload protection prevents an electric motor from drawing too much current, overheating, and literally “burning out.”

Like a contactor, overload protection is a building block of starters. Remember the starter equation:

Figure 8. A Starter is Made Up of a Controller (Most Often a Contactor) and Overload Protection

Let’s begin this section by learning how a motor works, and why overload protection is needed. Then we will move on to the different types of overload protection.

Part of understanding overload protection is understanding how motors work. A motor goes through three stages during normal operation: resting, starting, and operating under load.

But once the circuit is closed , the motor starts drawing a tremendous Inrush current; as much as 6-8 times its running current.

Here is the problem: this large inrush current can cause immediate tripping of the circuit breaker . A fuse or circuit breaker sized to handle the normal running load of the motor will open the circuit during startup.

You might think that sizing the fuse or circuit breaker for the spike in current draw would solve the problem. But if you did this, once the motor was running, only the most extreme Overload would open the circuit. Smaller overloads would not Trip (Tripping) the breakers, and the motor would burn out.

So, what is an overload? The term literally means that too much load has been placed on the motor. A motor is designed to run at a certain speed, called its synchronous speed. If the load on the motor increases, the motor draws more current to continue running at its Synchronous Speed.

It is quite possible to put so much load on a motor that it will draw more and more current without being able to reach synchronous speed. If this happens for a long enough period of time, the motor can melt its insulation and burn out. This condition is called an overload.

In fact, the motor could stop turning altogether (called a Locked Rotor) under a large enough load. This is another example of an overload condition. Even though the motor shaft is unable to turn, the motor continues to draw current, attempting to reach its synchronous speed.

Although the running motor may not draw enough current to blow the fuses or trip circuit breakers, it can produce sufficient heat to burn up the motor. This heat, generated by excessive current in the windings, causes the insulation to fail and the motor to burn out. We use the term Locked Rotor Amps to describe when the motor is in this state and is drawing the maximum amount of current.

So, because of the way a motor works, an overload protection device is required that does not open the circuit while the motor is starting, but opens the circuit if the motor gets overloaded and the fuses do not blow.

The Overload Relay is the device used in starters for motor overload protection. It limits the amount of current drawn to protect the motor from overheating.

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.

The blower motor on this furnace uses an overload relay to protect the motor when the blower turns on and current inrush begins.

The inrush continues until the blower fan reaches full speed, or, more technically, the motor’s synchronous speed.

Let’s take a look inside a few overload relays to see how they work. We’ll review the following overload relay types:

Eutectic (melting alloy)

Bimetallic

Solid State

The melting alloy (or eutectic) overload relay consists of a Heater Coil, a Eutectic Alloy, and a mechanical mechanism to activate a tripping device when an overload occurs. The relay measures the temperature of the motor by monitoring the amount of current being drawn. This is done indirectly through a heater coil.

Many different types of heater coils are available, but the operating principle is the same: A heater coil converts excess current into heat which is used to determine whether the motor is in danger. The magnitude of the current and the length of time it is present determine the amount of heat registered in the heater coil.

Usually, a eutectic alloy tube is used in combination with a ratchet wheel to activate a tripping device when an overload occurs. A eutectic alloy is a metal that has a fixed temperature at which it changes directly from a solid to a liquid. When an overload occurs, the heater coil heats the eutectic alloy tube. The heat melts the alloy, freeing the ratchet wheel and allowing it to turn. This action opens the normally closed contacts in the overload relay.

Figure 11. Eutectic Overload Relay: Ratchet Wheel and Eutectic Alloy Combination

A bimetallic device is made up of two strips of different metals. The dissimilar metals are permanently joined. Heating the Bimetallic Strip causes it to bend because the dissimilar metals expand and contract at different rates.

The bimetallic strip applies tension to a spring on a contact. If heat begins to rise, the strip bends, and the spring pulls the contacts apart, breaking the circuit, as shown in Figure 12.

Figure 12. Bimetallic Overload Relay: The Warping Effect of the Bimetallic Strip

Once the tripping action has taken place, the bimetallic strip cools and reshapes itself, automatically resetting the circuit. The motor restarts even when the overload has not been cleared, and will trip and reset itself again and again. (This assumes an automatic reset. This type of relay can also be equipped with a manual reset.)

As we mentioned, an overload relay is designed to prevent the motor from overheating. The heat comes from two sources: heat generated within the motor, and heat present in the area where the motor operates (Ambient heat). Although ambient heat contributes a relatively small portion of the total heat, it has a significant effect on the operation of the overload relay bimetals. A properly designed ambient-compensating element reduces the effects of ambient temperature change on the overload relay.

This type of overload relay is commonly found in applications such as walk-in meat coolers, remote pumping stations, and some chemical process equipment, where the unit is operated in environments with varying ambient temperatures.

Unlike the other two relay types, the Solid State overload relay does not actually generate heat to facilitate a trip. Instead, it measures current or a change in resistance . The advantage of this method is that the overload relay doesn't waste energy generating heat, and doesn't add to the cooling requirements of the panel.

Current can be measured via current transformers, then converted into a voltage which is referenced by the overload relay. If the relay notices that the current is higher than it should be for too long a period of time, it trips.

Another type of solid state overload relay uses sensors to detect heat generated in the motor. Heat in excess of the preset value for too long a period of time trips the motor offline.

It is possible to provide proactive functionality and improved protection against special conditions. For example, when high ambient temperature conditions exist, devices that use sensors can sense the effect the ambient temperature is having on the motor.

Some solid state overload relays offer programmable trip time. This can be useful when a load takes longer to accelerate than traditional overload relays will allow, or when a trip time in between traditional Trip Classes is desired.

Some overload relays have a built in emergency override, to allow the motor to be started even when it could be damaging to the motor to do so. This can be useful in a situation where the process is more important than saving the motor.

Some solid-state overload relays can detect the change in current when a motor suddenly becomes unloaded. In such a situation, the relay will trip to notify the user that there is an application problem. Normally, this indicates a system problem rather than a motor problem.

Many overload protection devices have a trip indicator built into the unit to indicate to the operator that an overload has taken place.

Overload relays can have either a manual or an automatic reset. A manual reset requires operator intervention, such as pressing a button, to restart the motor. An automatic reset allows the motor to restart automatically, usually after a “cooling off” period, as in the case of the bimetallic strip.

Overload relays also have an assigned trip class. The trip class is the maximum time in seconds at which the overload relay will trip when the carrying current is at 600% of its current rating. Bimetallic overload relays can be rated as Class 10 , meaning that they can be counted on to break the circuit no more than 10 seconds after a locked rotor condition begins. Melting alloy overload relays are generally Class 20 .

You will get motor protection with either a manual or a magnetic starter. However, the actual mechanics of the overload protection work differently for each type of starter.

When a manual starter experiences an overload, an overload trips a mechanical latch, causing the contacts to open and disconnect the motor from the electrical line.

In a magnetic motor starter (which we will discuss in the next section), an overload results in the opening of a set of contacts within the overload relay itself. This set of contacts is wired in series with the starter coil in the control circuit of the magnetic motor starter. Breaking the coil circuit causes the starter contacts to open, disconnecting the motor from the line. The motor is stopped and saved from burning out.