How to Select Thermal Overload Relays

Image credit:  Eaton Corporation | Benshaw, Inc. | ABB Low Voltage Products

Thermal overload relays prevent an electric motor from drawing too much current and overheating. 

Thermal Overload Relays - How They Work

Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed (NC) relay. When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay temperature, or sensed overload current, depending on the relay type. 

Thermal overload relays are similar to circuit breakers in construction and use, but most circuit breakers differ in that they interrupt the circuit if overload occurs even for an instant.  Thermal overload relays are conversely designed to measure a motor's heating profile; therefore, overload must occur for an extended period before the circuit is interrupted.

Specifications

The GlobalSpec SpecSearch database contains information about various thermal overload relay specifications, including type, electrical rating, switch details, and features.

Type

Buyers may choose between several different kinds of relays, including bimetallic thermal, solid state, or temperature control types.

Bimetallic Thermal Relays

As their name implies, bimetallic thermal relays use a bimetallic strip to mechanically open the contacts.  Bimetallic strips consist of two conjoined pieces of metal which expand at different rates when exposed to heat.  This difference forces the strip to bend when heated.  In a thermal relay, the strip is attached by a spring to a contact.  When excess heat from overcurrent causes the strip to bend and pull the spring, the contacts are pulled apart and the circuit is broken.  When the strip is cooled it then returns to its original shape.

This video illustrates the use of bimetallic switch, with the bimetallic strip highlighted in the middle of the video.  When the flame is applied to the switch, the strip bends and the switch opens.  Note that when the strip cools, the strip returns to its original position and the switch closes. 

Video credit:  tonsilol

Solid State Thermal Relays

Solid state relays are electronic devices that have no moving or mechanical parts.  Instead, the relay calculates the average motor temperature by monitoring its starting and running currents.  Solid state relays tend to be faster than electromechanical ones, and also feature adjustable set points and trip times.  Because they are incapable of generating a spark, they can be used in explosive environments. 

Temperature Control Thermal Relays

Temperature control relays directly sense a motor's temperature using a thermistor or resistance thermal device (RTD) probe which is embedded in the motor winding.  When the nominal temperature of the probe is reached, its resistance increases rapidly.  This increase is then detected by a threshold circuit, which opens the relay contacts.   

Melting Alloy Relays

A melting alloy (or eutectic) overload relay consists of a heater coil, a eutectic alloy, and a mechanical mechanism for breaking the circuit. Using the heater coil, the relay measures the motor's temperature by monitoring the amount of current drawn.

Electrical Rating

Relay electrical specs include current range, trip information, phase, and control voltage.

Trip Specifications

Tripping is used to describe the circuit interrupting action of overload relays and circuit breakers.  Thermal overload relays may include several specifications about this action.

Full load current range refers to a range of current values for a relay to be set to.  A motor's rating plate will include a full load current rating for that particular motor.  In order for the thermal overload relay to trip, the relay's full load current point must be set to match the value on the rating plate.

Temperature trip range applies to relays which are designed to measure temperature instead of current, such as solid state or temperature control relays. 

Trip class refers to the maximum time, in seconds, that a relay can withstand 6 times its current rating before tripping.  For example, a Class 10 relay can carry 600% of its rated current for 10 seconds until it trips.  Trip class is an important specification because a motor's starting circuit spikes the current draw for short periods of time each time a motor is started.  An overload relay must be able to handle these high starting currents without tripping.  Trip class timing might be said to allow a relay to "distinguish" between normally high starting currents and abnormally high overload currents. 

Poles

The term "pole" describes the number of separate circuits controlled by a switch.  The number of circuits determines the number of switch contacts, which in turn determines the poles needed to make or break the contacts.  Switches typically have between one and four poles.

The image series below illustrates, from left to right, a single pole (SP), double pole (DP), and triple pole switch (3P).  Note that, in the last image, the switch is connected to three separate circuits and has three contacts.

 Image credit:  Enasco| Skycraft Surplus | Frank Alapini

Control Voltage

Control voltage is an important specification because the control circuit voltage is often different from a motor's specified voltage.  This is known as "separate control."  The control voltage is usually less than the motor voltage, and an overload relay should be selected according to this specification. 

Features

Buyers may choose a relay featuring a number of special attributes.

• A relay with automatic reset will return to its original "closed" position after a specified period of time.  If the motor is still overloaded after the reset, the relay will trip again.
• Relays with ambient temperature compensation operate efficiently over a wide range of ambient temperatures. 
• Some relays feature various degrees of phase monitoring.  These products may check for phase loss, reversal, or imbalance.  When any phase problems are detected, the relay would trip and cut power to the motor.  Phase imbalance in particular can cause dangerous fluctuations in a motor's voltage or current flow and lead to motor damage.

• Underload detection refers to a relay's ability to detect a drop in current as a result of unloading.  This may occur if, for example, a pump begins to run dry.  These relays are designed to detect these differences and trip as they would if detecting overload.

• Relays with visual indicators are products that feature light-emitting diodes (LEDs) or other status indicators.

References

 Power Quality and Drives - Overload Relay Class Time Delay

• A crystal ball for motor controls
  such as jam, stall, ground fault, and phase imbalance. More recently, overload relays have been given an ability to communicate. This lets users monitor current, see how much thermal capacity is utilized, and identify the reason for a trip. These overload relays may even include built-in input points...
• Trends in circuit protection
  as power-control relays. Programmability has also come to circuit breakers. Smart protectors sport programmable trip values and overload time delays. More-sophisticated protectors measure and report voltage and current values, alert control. Trends in circuit protection | Machine Design. Skip to Content. Home...
• Self-Protecting Starters
  protects against both thermal and magnetic overload. The self-protected starter can be used in single or multiple installations and satisfy Article 430 of the National Electric Code (NEC) which addresses the safe installation of motors, circuits, and controllers. Power-control equipment comprises motor...