What are the differences between the two types of thermal overload relays?

Motor Starters and Contactors

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An has two basic parts:

  1. A heater element that is connected in with the power line to the motor. All current drawn by the motor must pass through the heater element.
  2. A set of that are connected in series with either the lines feeding the motor (manual starters) or the coil of the magnetic contactor (magnetic starters). The types of relays most commonly seen are the bimetallic strip and the melting-solder pot assembly.

Bimetallic Strip

The bimetallic strip consists of two dissimilar metals with different heating coefficients. As they heat, they expand at different rates, which causes them to bend or deform at a preset temperature. This bending action can open or close a set of .

When used in an overload device, the bimetallic strip is mechanically linked to a set of normally closed electrical contacts. When an overload occurs, the bending action opens the set of normally closed contacts, interrupting the current to the circuit.

What are the differences between the two types of thermal overload relays?
Bimetallic contact in a normally closed position

Here the normally closed contacts are allowing current to pass through them, while the heat source is beginning to deform the metal.

What are the differences between the two types of thermal overload relays?
Bimetallic strip in open position

The heat source has caused the grey-shaded metal (the piece on the bottom) to expand faster than the blue-shaded metal (the piece on the top) and so has opened the set of normally closed contacts, thus interrupting the current flow to the motor.

Melting Solder Pot

What are the differences between the two types of thermal overload relays?
Solder pot closed contacts

The melting solder pot consists of a heater element, a solder-pot assembly, ratchet wheel, and a set of normally closed contacts.

A spring is held under tension by the ratchet wheel. If the wheel is allowed to spin, then the spring will push upwards and open the set of normally closed contacts. The wheel is held in place by the solder inside the solder-pot assembly. Different levels of tin and zinc in the solder change the melting temperature, allowing for use at many different current ratings and ambient temperature settings.

If an overload current is sensed by the heater elements for too long a time, then the alloy becomes a liquid, allowing the spring to push open the normally closed contacts. This causes the line contacts to trip open and interrupt current flow to the motor.

What are the differences between the two types of thermal overload relays?
Solder pot open contacts

Both the bimetallic strip and melting solder-pot rely on thermal energy to trip their elements. As such, a cool-down period is required before the contacts can be reset. Once the relay cools, the bimetallic strip will return to its normal position, or the melted solder will solidify and the ratchet wheel can be reset to close the line contacts again.

Other Types of Overload Relay

Some modern motor control systems incorporate real-time current transformer monitoring applications that use integrated computer to protect against motor overloads. These systems can be interconnected with networked PLC’s and other safety equipment.

A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.

In electrical terms, refers to a connection where current has only one path to flow.

Loads connected in series will have the the same value of current flowing through them, and share the total voltage between them. Switches and overcurrent equipment is connected in series with equipment to control and protect it.

A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.

The conducting part of a switch that makes or breaks a circuit.

In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.

Heat is a major factor in the performance and life of a motor, and one of the primary sources of motor heating is current running through the motor windings. Since heating is an unavoidable condition of motor operation, it’s important to protect the motor from overheating, or thermal overload.

In a previous post, we described several types of sensors that can measure the temperature of motor windings directly. But in some cases — particularly for AC induction motors — motor heating can be measured indirectly by thermal overload relays, which determine motor temperature by monitoring the amount of current being delivered to the motor.

Thermal overload relays are wired in series with the motor, so the current flowing to the motor also flows through the overload relay. When the current reaches or exceeds a predetermined limit for a certain amount of time, the relay activates a mechanism that opens one or more contacts to interrupt current flow to the motor. Thermal overload relays are rated by their trip class, which defines the amount of time for which the overload can occur before the relay responds, or trips. Common trip classes are 5, 10, 20, and 30 seconds.

Taking time, as well as current, into account is important for AC induction motors because they draw significantly more than their full rated current (often 600 percent or more) during startup. So if the relay tripped immediately when the overload current was exceeded, the motor would have difficulty starting.

There are three types of thermal overload relays — bimetallic, eutectic, and electronic.

Bimetallic thermal overload relays (sometimes referred to as heater elements) are made of two metals, with different coefficients of thermal expansion, that are fastened or bonded together. A winding, wrapped around or placed near the bimetallic strip, carries current.

What are the differences between the two types of thermal overload relays?
In a bimetallic thermal overload relay, heating due to current flow causes the bimetallic strip to bend to one side, activating a trip mechanism.
Image credit: Siemens

As the current running through the relay (and, therefore, through the motor) heats the bimetallic strip, the two metals expand at different rates, causing the strip to bend toward the side with the lower coefficient of thermal expansion. When the strip bends, it actuates a normally closed (NC) contactor, causing it to open and stopping the current flow to the motor. Once the bimetallic relay has cooled and the metal strips have reverted to their normal state, the circuit is automatically reset and the motor can be restarted.

Eutectic thermal overload relays use a eutectic alloy (a combination of metals that melts and solidifies at a defined temperature), housed in a tube and connected to a heater winding. The supply current to the motor flows through the heater winding and heats the alloy. When the alloy reaches a sufficient temperature, it transforms rapidly to a liquid.

What are the differences between the two types of thermal overload relays?
In a eutectic thermal overload relay, heating due to current flow causes a eutectic alloy to rapidly liquify, activating a mechanical device that trips the relay.
Image credit: Rockwell Automation

As a solid, the alloy holds a mechanical device, such as a spring or ratchet, in place. But when the alloy melts, the mechanical device releases, opening the overload contacts. Like the bimetallic design, a eutectic thermal overload relay cannot be reset until the alloy has sufficiently cooled and returned to its original, solid state.

What are the differences between the two types of thermal overload relays?
Electronic thermal overload relays are more accurate and reliable than heater designs, and they can provide data for diagnostics and preventive maintenance.
Image credit: ABB

Electronic thermal overload relays measure current electronically, rather than relying on a heater mechanism, and so are insensitive to changes in ambient temperature. They’re also less prone to “nuisance,” or false, tripping. Electronic overload relays can provide data such as the percentage of thermal capacity utilization (%TCU), percentage of full-load amps (%FLA), time-to trip, current RMS, and ground fault current — information that can help operators conduct diagnostics and predict when the relay is at risk of tripping.

Electronic designs can also protect motors against phase loss (also referred to as phase failure), which occurs when one phase of current equals zero amps, often due to a short circuit or blown fuse. This causes the motor to draw excessive current on the remaining two phases and leads to significant motor heating.

Thermal overload relays are typically part of the motor starter, which includes the overload relay plus contacts. It’s important to note that thermal overload relays are only designed to protect the motor from overheating, and won’t trip if there’s a short circuit, so additional fuses or circuit breakers are necessary to protect the circuit.