Thermal overload relay 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 relay 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 relay 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 relay are conversely designed to measure a motor's heating profile; therefore, overload must occur for an extended period before the circuit is interrupted.
Overload Relay Type
- Bimetallic Thermal Relay
As their name implies, bimetallic thermal relay 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. - Solid State Thermal Relay
Solid state relay are electronic devices that have no moving or mechanical parts. Instead, the relay calculates thermal overload relaythe average motor temperature by monitoring its starting and running currents. Solid state relay 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 Relay
Temperature control relay 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 Relay
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.
Motor Failure and Protection
Motor failure may be the result of electrical or mechanical factors. A study commissioned by the Electrical Research Associates (ERA) of the United Kingdom in 1986 indicated the most common causes of motor failure are:
- Overcurrent » 30%
- Contamination » 18%
- Single Phasing » 15%
- Bearing Failure » 12%
- Aging (natural wear) » 10%
- Rotor Fault » 5%
- Miscellaneous » 7%
Failure modes 1, 3 and 7 are attributable to electrical issues. Modes 2, 4, 5 and 6 are the result of mechanical (and some manufacturing) issues.
Historically, motor protection provided with the controller was only able to address the electrical causes of motor failure. These electrical issues account for at least 45% of the most common causes of motor failure. Motor branch circuits are protected against short circuits (instantaneous overload currents) and steady state or low level, sustained overload relays. In the U.S., this protection is provided by the short circuit protective device (SCPD) and the motor overload relay, when they are applied according to the National Electrical Code (NEC).
Trip Class Designation
Regardless of the product style (NEMA or IEC), overload relays respond to overload relay conditions according to trip curves. These trip curves are defined by the class of protection required.
| Class Designation | Tripping Time |
|---|---|
| Class 10 | 10 Seconds or less |
| Class 20 | 20 Seconds or less |
| Class 30 | 30 Seconds or less |
IEC components are typically application rated. This means the controller is sized very close to its operational limit for a given application. IEC motors are also generally more application rated. For these reasons, Class 10 trip is most common on IEC applications. Because NEMA products are applied with more built-in excess capacity, the Class 20 trip is most common.
Typical trip curves
Trip Current Rating
Trip current rating is a nominal value which approximates the minimum current to trip an overload relay in an ambient temperature, outside of the enclosure, of 40°C (104°F).
Protection Level
Protection Level is the relationship between trip current rating and full load current. Protection level, in percent, is the trip current rating divided by the motor full load current times 100. National Electrical Code, Section 430-32, allows a maximum protection level of 125% for the motor.
Minimum Trip Current
Also called ultimate current may vary from the trip current rating value, since ratings are established under standardized test conditions. Factors which influence variations include: the number of thermal units installed, enclosure size, proximity to heat producing devices, size of conductors installed, ambient (room) temperature, and others.
Except for ambient temperature-compensated overload relays, an ambient temperature higher than 40°C would lower the trip current, and a lower temperature would increase it. This variation is not a factor in selecting thermal units for the average application, since most motor ratings are based on an ambient temperature of 40°C, and motor capacity varies with temperature in about the same proportion as the change in trip current. Temperature-compensated relays maintain a nearly constant trip current over a wide range of ambient temperature, and are intended for use where the relay, because of its location, cannot sense changes in the motor ambient temperature.
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