Overload Relays: Types & Tripping & What is Overload Protection?

Introduction to Motors

Electric motors are an integral component of industrial equipment, toys, vehicles, and electronic devices. They are designed to convert electrical energy into mechanical energy. These devices may be powered by AC or DC sources. Blowers, fans, compressors, cranes, extruders, and crushers are a few important devices equipped with electric motors.

What is an Induction Motor?

An induction motor, also referred to as a synchronous motor, is one of the main types of AC electric motors used in commercial and industrial environments. These motors feature armortisseur windings, and work on the principle of electromagnetic induction. The electro-magnetic field in the rotor is produced by the rotating field of the stator. In short, the power is transferred to the rotor winding by stator through induction. There are two main types of induction motors — single-phase induction motors and three-phase induction motors.

Introduction to Three-Phase Induction Motors

It is one of the most widely used types of electrical motors; and, is an integral part of almost 80% of the industrial applications. Its popularity is due to the rugged construction, excellent operating characteristics, speed regulation, and absence of commutator. Like any regular induction motor, this motor also comprises a stator and rotor.

• Stator: This is the stationary element of the induction motor. The stator is a small cylindrical frame which carries the cylindrical core of the rotor. It features different slotted stampings to carry three-phase windings. The windings of the stator have 120 degrees separation.
• Rotor: This is the rotating part of the motor. The rotor features laminated cylindrical slots with copper or aluminum conductors that have joined ends. It is the shaft of the motor.

The rotor of the three-phase induction motor is classified as phase wound rotor or slip ring rotor and squirrel cage rotor. Among the two, the squirrel cage rotor is one of the most common ones.

Squirrel Cage Induction Motors

Induction motors equipped with a squirrel cage rotor are known as squirrel cage induction motors. They get their name because the rotor resembles the rotating cylindrical “cage” that you might find in a pet squirrel or hamster cage. These motors are available in sizes ranging from fractional horsepower (HP) less than one kilowatt to 10,000’s HP (tens of megawatts). Factors such as simplicity, rugged construction, and constant speed in different load sizes have contributed to their popularity. Like other induction motors, the squirrel cage motor consists of:

• Rotor: It is a cylindrical-shaped component mounted on a shaft. It contains longitudinally organized conductive bars. The bars are made of copper or aluminum, and are set into grooves, which are connected at ends to form a cage-like structure. The rotor has a laminated core, which helps avoid power loss due to hysteresis and Eddy currents. Conductors of the rotor are skewed, which helps prevent cogging during the start of the equipment. Also, this skewing assures improved transformation ratio between the rotor and stator.
• Stator: It consists of a three-phase winding along the core. The stator is placed in a metal housing. The windings in the stator are organized such that they are 120-degree apart in space, and mounted on a laminated iron core. This iron core provides reluctance path for flux generated by AC currents.

What is Overload Protection?

When the motor draws excess current, it is referred to as an overload. This may cause overheating of the motor and damage the windings of the motor. Because of this, it is important to protect the motor, motor branch circuit, and motor branch circuit components from overload conditions. Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat from the overload condition. Overload relays are part of the motor starter (assembly of contactor plus overload relay). They protect the motor by monitoring the current flowing in the circuit. If the current rises above a certain limit over a certain period of time, then the overload relay will trip, operating an auxiliary contact which interrupts the motor control circuit, de-energizing the contactor. This leads to the removal of the power to the motor. Without power, the motor and motor circuit components do not overheat and become damaged. The overload relay can be reset manually, and some overload relays will reset automatically after a certain period of time. After which, the motor can be restarted.

How an Overload Relay Works

The overload relay is wired in series with the motor, so the current that flows to the motor when the motor is operating also flows through the overload relay. It will trip at a certain level when there is excess current flowing through it. This causes the circuit between the motor and the power source to open. The overload relay can be manually or automatically reset after a predetermined time duration. The motor can be restarted after the cause of the overload has been identified and rectified.

Types of Overload Relays

Bimetallic Overload Relay

Many overload relays include bimetallic elements or bimetallic strips, also referred to as heater elements. The bi-metallic strips are made of two types of metals – one with a low coefficient of expansion, and another with a high coefficient of expansion.These bimetallic strips are heated by a winding around the bimetal strip, which carries the current. Both of the metal strips will expand due to the heat. However, the metal with a high coefficient of expansion will expand more in comparison to the metal with a low coefficient of expansion. This dissimilar expansion of the bimetallic strips causes the bimetal to bend towards the metal with a low coefficient of expansion.As the strip bends, it actuates an auxiliary contact mechanism and causes the overload relay normally closed contact to open. As a result, the contactor coil circuit is interrupted.The amount of heat generated can be calculated by the Joule’s Law of Heating. It is expressed as H ∝ I2Rt.

• I is the overcurrent flowing through the winding around the bimetal strip of the overload relay.
• R is the electrical resistance of the winding around the bimetal strip.
• t is the time period for which the current I flows through the winding around the bimetal strip.

The above equation defines that heat produced by the winding will be directly proportional to the time period of the flow of overcurrent through the winding. In other words, the lower the current, the longer it will take the overload relay to trip and the higher the current, the faster the overload relay will trip, in fact it will trip much faster because the operation of the relay is a function of the current squared.

Bimetallic overload relays are often specified when automatic reset of the circuit is required, and occurs because the bimetal has cooled and returned to its original state (form). Once this happens the motor can be restarted. If the cause of overload is not rectified, the relay will trip again, and reset at predetermined intervals. It is important to be careful during the selection of an overload relay, because repeated tripping and reset can reduce the mechanical life of the relay and may cause damage to the motor.

In many applications, the motor is installed at a location with a constant ambient temperature, and the overload relay and motor starter may be installed in a different location, which experiences different ambient temperatures. In such applications, the trip point of the overload relay can vary depending on multiple factors. The current flow through the motor and the temperature of the surrounding air are two factors, which may cause premature tripping. In such cases, ambient compensated bimetallic overload relays are used. The relays of this type feature two types of bi-metal strips – a compensated bi-metal strip and a primary non-compensated bi-metal strip. At ambient temperatures, both these strips will bend equally, thereby preventing the overload relay from nuisance tripping. However, the primary bi-metal strip is the only strip that gets affected by the current flow through the heater element and the motor. In the condition of an overload, the trip unit will be engaged by the primary bi-metal strip.

Eutectic Overload Relay

This type of overload relay is comprised of a heater winding, a mechanical mechanism for activation of a tripping mechanism, and a eutectic alloy. A eutectic alloy is a combination of two or more materials, which solidifies or melts at a specific known temperature.

In the the overload relay, the eutectic alloy is contained in a tube, which is often used along with a spring loaded ratchet wheel to activate the tripping mechanism during the overload operations. The motor current passes through the small heater winding. During the overload, the eutectic alloy tube is heated by the heater winding. The alloy melts due to the heat, thereby releasing the ratchet wheel, and allowing it to turn. This action initiates the opening of the closed auxiliary contacts in the overload relay.

Eutectic overload relays can only be manually reset after tripping. This reset is usually done through a reset button, which is positioned on the cover of the relay. The heater unit installed on the relay is chosen on the basis of the full load current of the motor.

Solid State Overload Relay

These relays are commonly referred to as electronic overload relays. Unlike the bimetallic and eutectic overload relays, these electronic overload relays measure current electronically. Although available in various designs, they share common features and benefits. The heaterless design is one of the main advantages of these relays. This design helps reduce the costs and efforts of installation. In addition, the heaterless design is insensitive to the change in ambient temperatures, which helps minimize nuisance tripping. These relays also provide protection from phase loss – more effectively than bimetallic or eutectic alloy overload relays. These relays can easily detect a loss of phase, and operate an auxiliary contact to open the motor control circuit. Solid state overload relays enable easy adjustment of trip times and set points.

Overload Relay Tripping

The tripping time of an overload relay will decrease when the current increases. This function is plotted on the inverse time curve below, and is termed as the trip class. The trip class also indicates the time taken by the relay to open in an overload condition.

Trip Classes 5, 10, 20, and 30 are common. These classes suggest that the overload relay will trip in 5, 10, 20, and 30 seconds. This tripping usually occurs when the motor is running 720% of its full load. Trip Class 5 is suited for motors that demand fast tripping, whereas Class 10 is usually preferred for motors of low thermal capacity like submersible pumps. Class 10 and 20 are employed for general purpose applications, whereas Class 30 is employed for loads with high inertia. Class 30 relays help avoid nuisance tripping.

We hope that this short paper has given you a good, basic understanding of overload relays. Look for other informative papers from c3controls at c3controls.com/blog.

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