Starter Thermal Elements
Overload relays protect motors, controllers and branch-circuit conductors against excessive heating due to prolonged motor overcurrents. Protection of the motor, and the other branch-circuit components, from higher currents is a function of the branch-circuit fuses, circuit breakers or motor short-circuit protectors.
Thermal overload relays are very effective in providing motor running overcurrent protection. They will produce a shorter trip time at a higher current, similar to the way a motor will reach its temperature limit in a shorter time at a higher current. In a high ambient temperature, a thermal overload rely will trip at a lower current, allowing the motor to be used to its maximum capacity in its particular ambient temperature. Once tripped, the thermal overload relay will not reset until it has cooled, allowing the motor to cool before it can be re-started.
Schneider Electric melting alloy thermal overload relays allow the motor current to pass through a small heater winding. Under overload conditions, the heat causes a solder to melt causing a ratchet shell to spin free, opening the control circuit contacts. Bimetallic thermal overload relays utilize a U-shape bimetal strip associated with a current carrying heater coil. When an overload occurs, the heat will cause the bi-metal to deflect and operate a control circuit contact. Bimetallic overload relays are used where the controller is remote or difficult to reach. Melting alloy and bimetallic overload relays are designed to approximate the heat actually generated in the motor. As the motor temperature increases, so does the temperature of the thermal unit.
Motor Control Basics
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.