
Starter Overload Relays

Overload relays protect the motor and its components from an overload situation, which can result in excessive heat and damage. The overload relay is wired in series with the motor, so that the electrical current that flows to the motor will flow through the overload relay as well. When the electrical current flowing in the monitored circuit exceeds a certain preset limit over a certain period of time, the overload relay will trip, causing an auxiliary contact to interrupt the motor control circuit. This de-energizes the contactor, removing power to the motor, avoiding overheating and damage. The overload relay will reset automatically after a certain period of time or can be reset manually after the cause of the overload has been rectified.
When selecting an overload relay for electric motors, it is important to take into consideration time, as well as current. During start up, a motor will draw significantly more current than it is rated for, in a short period of time. Therefore, a relay would trip immediately when the overload current occurred, making starting the motor difficult. Thermal overload relays are rated by their trip class (5, 10, 20, and 30 seconds), which defines the amount of time an overload can occur before the relay trips.
The three types of thermal overload relays are: bimetallic, eutectic and electronic. Bimetal overload relays contain a bimetal strip that when overheated, bends to close and trip the circuit. Eutectic overload relays contain a eutectic alloy that when overheated, melts, releasing a mechanical device that opens the overload contacts, tripping the circuit. Electronic overload relays do not rely on a heater mechanism to detect an overload, but rather use temperature sensors or current transformers to sense the amount of current flowing to the motor. Temperature is sensed using positive temperature coefficient (PTC) thermistors and used to trip the circuit in overload situations.
Products
Motor control allows operational control of electrical motors in various environments. Motor control circuits provide a safe way to operate electrical motors. Back when motor control circuits were in their infancy, it would have been common to see a simple disconnect switch that would be used to turn on and off a motor. Depending on the size of the motor and how much voltage was required, operating this disconnect would have been dangerous, with a very high possibility of arcing or electrocution. Not to mention that when the disconnect was actuated, the large amount of inrush current would have damaged the motor over time.
In today's motor control circuits, there are a few common pieces of hardware. A circuit breaker is used to protect the motor and any hardware downstream. A contactor and an overload relay are connected together and function in tandem to allow for remote and safe operation of the motor. The contactor functions much like a relay, allowing for a smaller electrical circuit to remotely close the motor contacts, starting the motor. The overload relay is designed to protect the motor in the case of a prolonged overcurrent event. These two devices are wired in series, so that if the overload relay detects an overcurrent event, the contactor will open the motor contacts, shutting off power to the motor.
The other two most common types of motor control hardware are a soft starter and a Variable Frequency Drive (VFD). Both of these devices function in a similar way to the motor circuit with some added functions. The soft starter is designed to reduce large inrush current to the motor upon startup. This “soft starting” of the motor will prolong its life and allow for safer operation. The VFD performs the soft start functions, but also allows for speed control of the motor. This speed control is critical in many different environments and has made VFDs one of the most common and safe to use motor control circuits today.