Melting Alloy Overload Relay

The melting alloy overload relay consists of a heater coil, a eutectic alloy, and a mechanical mechanism to activate a tripping device when an overload occurs. Usually, a eutectic alloy tube is used in combination with ratchet wheel to activate a tripping device when an overload occurs. A eutectic alloy is a metal that has fixed temperature at which it changes directly from a solid to a liquid. A heater coil converts excess current into heat which is used to determine whether the motor is in danger.


In melting alloy overload relay, the motor current pass through a small heater winding. Under overload conditions, 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. The magnitude of the current and the length of time it is present determine the amount of heat registered in the heater coil.


Melting alloy overload relays must be reset by deliberated hand operation after they trip. A reset button is usually mounted on the cover of overload relay.

The heating units of melting alloy overload relay are interchangeable. The heater units are rated in amperes and are selected on the basis of motor full load current, not horsepower.

One of the fundamental application differences between the bimetallic and melting alloy overload relay is that bimetallic overload relay is available in ambient compensated forms. Only 5-10% of all installations require ambient compensation. Ambient compensated overload relays are designed for applications where the motor is in a constant ambient and the controller is in a varying ambient. Applications where the controller and motor are in the same varying ambient do not require compensation. Also, applications where the controller is at a constant temperature and motor is in a varying ambient do not require compensation. In all cases, the melting alloy or bimetal units should be sized according to the motor full load ampere (FLA).

The fundamental difference between bimetallic and melting alloy overload relay is the design and operation of the trip mechanism. Because 90-95% of all applications do not require ambient compensation, the choice of bimetallic or melting alloy overload relay is usually a matter of user preference, and not an absolute application requirement. In these cases, bimetallic or melting alloy overload relay works equally well.

The bimetallic Overload Relay

Overload protection of the relay is accomplished with the use of a bimetal strip. The bimetallic overload relay consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip level. A bimetal strip is made up of two dissimilar metals permanently joined together. The two metals have different thermal expansion characteristics, so the bimetal bends at a given rate when heated.

Under normal operating conditions the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay.

Under overload conditions a current rises and heat also rises. The hotter the bimetal becomes, the more it bends. In an overload condition the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor.


Bimetallic overload relays are available in manual reset and automatically reset.

Once the tripping action has taken place, the bimetal strip cools and reshapes itself, automatically resetting the circuit. If the cause of the overload still exists, the motor restarts, and the motor will trip and reset again. This cycle will repeat and eventually the motor will burn out due to the accumulated heat from the repeated inrush and overload current. Care must be exercised in the selection of this type of overload relay. More important is the possibility of danger to personnel. The unexpected restarting of a machine may find the operator or maintenance man in a hazardous situation as he attempts to find out why machine has stopped.

Different heaters give different trip points. In addition, most bimetallic overload relays are adjustable over a range of 85% to 115% of the nominal heater rating.

Ambient Compensated Overload Relay

Ambient compensated overload relays are designed for the situation when motor is at a constant ambient temperature and controller is located separately in a varying ambient temperature. In certain applications, such a submersible pump, the motor may be installed in a location having a constant ambient temperature. The motor control, along with the overload relay, may be installed in a location with varying ambient temperature. The trip point of the overload relay will vary with the temperature of the surrounding air as well as current flowing through the motor. This can lend to premature and nuisance tripping. To compensate for temperature variations an ambient compensated overload relay is used.

Ambient compensated overload relays are designed to overcome this problem. A compensated bimetal strip is used along with a primary bimetal strips. As the ambient temperature changes, both bimetal strips will bend equally and the overload relay will not trip the motor. However, current flow through the motor and heater element will affect the primary bimetal strip. In event of an overload condition the primary bimetal strip will engage the trip unit.

Overload Relay

Overload relay is a device that prevents an electric motor from drawing too much current, overheating, and literally burning out. Overload relay is necessary to prevent burn out and to ensure maximum operating life.

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 long enough period of this, the motor can melt its insulation and burn out. This condition is called overload.

In fact, the motor could stop turning under a large enough load (called a lock rotor). 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.

Overload may be caused by low line voltage, excessive voltage unbalance, or by an open line in a polyphase system, which results in single-phase operation. Under any condition of overload, motor draws excessive current that causes overheating. Since motor winding insulation deteriorates when subjected to overheating, there are established limits on motor operating temperature. To protect a motor from overheating, overload relays are employed in a motor control to limit the amount of current drawn.

Although the overload current is not enough to blow the fuses or trip circuit breakers, it can produce sufficient heat to burn up the motor. The operation of overload relay should be such that the motor is allowed to carry harmless overloads but is quickly removed from the line when an overload has persisted too long.

What is difference between overload protection and overcurrent protection?

The overload relay dose not provide short circuit protection. This is the function of overcurrent protective equipment like fuses and circuit breakers. An overload protection device is required that does not open the circuit while motor is starting, but opens the circuit if the motor gets overloaded and the fuses do not blow.

Fuses are not designed to provide overload protection. Their basic function is to protect against short circuits (overcurrent). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and damaging overload. Such fuses, chosen on the basic of motor full-load current, would blow every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.

The idea overload protection for motor is an element with current –sensing properties very similar to the heating curve of the motor (in figure) which would open the line to motor when full load current is exceeded.
An overload relay consists of:
- A current sensing unit (heater).
- A mechanism to breaker the circuit, either directly or indirectly.

Overload relay has a time delay to allow temporary overload without breaking circuit. It has a trip capability to open the control circuit if overload current continue over period of time. Overload relay have some means of resetting the circuit once the overload is removed.

Type of overload relay

1. Bimetallic Overload Relay
2. Melting Alloy Overload Relay
3. Solid State Overload Relay

Motor Starters

Whenever motors are used, they must be controlled. The most basic type of motor control involves turning a motor on and off. This is often accomplished by using motor starters.

Motors starters are a device that is used to start a motor from stop. Motor starters consist of the switching means necessary to start and stop the motor and overload protection device.

Motor starters let you turn an electric motor on and off, while providing overload protection. Now, we have a power control device that offers more than just a manual on/off control, such as a knife blade switch. The manual motor starters also provide a means to protect the motor from overload conditions.

There are two main types of motor starters:

1. Manual Motor Starters
A starter which the force for closing the main contacts is provided exclusively by manual energy. Operating manual motor starters is fairly simple and straightforward. You press a button or toggle (mounted directly on the starters) to start or stop the motor. Mechanical linkages from the buttons or toggle force the contacts to open and close, starting and stopping the motor.

2. Magnetic Motor Starters
A starter which the force for closing the main contacts is provided by an electromagnetic energy.Magnetic motor starters consist of magnetic contactor and overload relay. Magnetic motor starters offer some additional capabilities not available in manual motor starters, most importantly, remote and automatic operation.

Types of magnetic motor starters

Motor starters are classified as either full voltage or reduce voltage starting.

1. Full Voltage Starting Motor Starters
This starter connects the incoming power directly to the motor. We also call across the line motor starters or direct on line motor starters.

1.1 Full Voltage Non-Reversing Motor Starters (FVNR)
It can be used in any application where the motor runs in only one direction, at only one speed, and starting the motor directly across the line. This type of motor starters is the most commonly used general purpose starters.

1.2 Full Voltage Reversing Motor Starters (FVR)
A starter intended to cause the motor to reverse the direction of rotation by reversing the motor primary connections while it is running. The motor starters reverse motor by reversing any two lead to the motor. This accomplished with two contactors and one overload relay. One contactor is for the forward direction and other is for reverse. It has both mechanically and electrically interlocked sets of contactors to prevent line shorts and energizing both contactors simultaneously.

1.3 Full Voltage Two-Speed Motor Starters
Motor windings in two-speed motor require special starter which can be connected to the motor to obtain different motor speeds.

Two-speed motor starters are designed to control re-connectable squirrel case induction motor for operation at two different constant speeds depending on the construction of the motor. The speed of an induction motor is a function of the supply frequency and the number of poles of the motor windings. To obtain different speeds with fixed supply frequency, the number of magnetic poles of the motor must be changed.

1.3.1 Two Speed One Winding Motor Starters (2S1W)
A single winding two-speed motor carries current in various configurations to provide either constant torque, variable torque, or constant horsepower at each speed. This type of starter is designed for motor which has a single winding for two speeds. Extra winding taps are brought out for reconnection for different number of stator poles. We also call this type of motor as consequent pole motor.

1.3.2 Two Speed Two Winding Motor Starters (2S2W)
A two-winding two speed motor carries current in only one of its windings at each speed.This type of starter is designed for motor which has separate winding for each speed. This motor construction is relatively simple. Separate winding motor with delta connected motor winding require one corner to be opened on each un-used winding.

2. Reduced Voltage Motor Starters
This starter intended to start and accelerate a motor to normal speed by connecting the line voltage across the motor terminals in more than one step or by gradually increasing the voltage applied to the terminals.Two main reasons to use reduced voltage motor starters are reducing the inrush current and limiting the torque output and mechanical stress on the load.

2.1 Reduced Voltage Auto-Transformer Motor Starters (RVAT)
Reduced voltage auto transformer motor starters use an auto transformer to reduce the voltage applied to a motor during start. Reduced voltage auto-transformer motor starters should be used with hard to start loads such as reciprocating compressors, grinding mills, and pumps.

2.2 Part Winding Motor Starters (PW)
Part winding motor starters first connect a part of the motor winding to the supply lines as the first starting step and then connect the remaining portion of the winding to the supply lines as the second step. Part winding motor starters are suited to low starting torques loads such as fans, blowers.

2.3 Wye Delta Motor Starters (YD)
Wye delta motor starters connect the phase windings first in wye relationship for the effect of reduced voltage starting and, subsequently, reconnect these phase winding in delta relationship for running. With wye delta starting, the starting current on the wye connection is equal to 0.33 times the locked rotor current on the delta connection. Wye delta motor starters are applicable to high inertia loads with long acceleration times such as centrifugal compressors.