Overload protection of motors


Before you knowing about overload protection check out the post about contactors link given below

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Overload protection prevents an electric motor fromdrawing too much current, overheating, and literally burning out

How Motors Work?

A motor goes through three stages during normal operation: resting, starting, and operating under load.

A motor at rest does not require current because the circuit is open. But once the circuit is closed, the motor starts generating a tremendous input current; up to 6-8 times its operating current.
Here’s the problem: this large input current can cause the immediate trip of the circuit breaker. A fuse or circuit breaker sized to handle the normal operating load of the motor will open the circuit during start-up.
You might think that sizing the fuse or circuit breaker for the peak in current consumption would solve the problem. But if you did this, once the engine was running, only the most extreme overload would open the circuit. Minor overloads would not trip the switches, and the motor would shut down


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.

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.

Overload relay

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.

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

Overload relay types:

  • Eutectic (melting alloy)
  • Bimetallic
  • Solid State

1.The Eutectic Overload Relay

The fusion alloy (or eutectic) overload relay consists of a heating coil, a eutectic alloy, and a mechanical mechanism to activate a triggering device when an overload it happens The relay measures the motor temperature by controlling the amount of current that is drawn. This is done indirectly through a heater coil. There are many different types of heater coils available, but the operating principle is the same: a heater coil converts the excess current into heat that is used to
determine if the engine is in danger. The magnitude of the current and the the time that is present determines 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.

2.The Bimetallic Overload Relay

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
Once the firing action has taken place, the bimetal strip cools and reshapes in itself, automatically restarting the circuit. The motor restarts even when the overload it has not been erased, and it will fire and restart again and again. (This assumes an automatic restart This type of relay can also be equipped with a manual reset).

The Solid State Overload Relay

Unlike the other two types of relays, the solid state overload relay does not generate heat to facilitate a trip. Instead, measure the current or a change in resistance. The advantage of this method is that the overload relay does not waste energy that generates heat and does not increase the cooling requirements of the panel.

The current can be measured through current transformers, then it becomes a voltage which is stored in the memory inside the overload relay. If the relay notices that the the current is higher than it should be for a too long period of time, it shoots up. Another type of solid state overload relay uses sensors to detect the heat generated in the engine When the sensor also detects heat above the preset value for a long period of time, it shoots up.

The solid state overload relay also provides some advanced functions.

  1. It is possible to provide proactive functionality and improved protection against special conditions. For example, when the conditions of high ambient temperature exist, devices that use sensors can detect the effect of room temperature taking in the engine.

  2. Some solid state overload relays offer programmable trip time. This could be Useful when a load takes longer to accelerate than traditional overload relays will allow, or when you want a travel time between traditional travel classes.

  3. Some overload relays have a built-in emergency override to allow the motor to start even when you could damage the engine to do it. This could be useful in a situation where the process is more important than saving the engine.

  4. Some solid state overload relays can detect the change in current when an engine suddenly it is downloaded. In such situation, the relay will trigger to notify the user that there is an application problem. Normally, this indicates a system problem instead of a motor problem.

Also read
Motor control circuits - Types
Different types of Motor Enclosure