
Overload Relays

NEMA overload relays are devices used to protect motors from overload conditions. They are typically thermal overload relays, which detect excessive motor current and respond by interrupting the power supply. These relays are often panel-mounted, allowing for easy installation and maintenance. They may include auxiliary contacts for additional control and signaling purposes.
NEMA overload relays come in different types, including thermal overload relays and combination starters. They provide essential protection for motors, ensuring their safe operation and preventing damage caused by excessive current or power surges. By interrupting the circuit, they safeguard the motor from overheating, which can lead to reduced performance, shortened motor life, or even catastrophic failure.
When selecting a NEMA overload relay, factors such as the motor's power, size, and type need to be considered. The relay must be sized appropriately to match the motor's requirements and provide reliable protection. It is important to ensure that the relay is designed for the intended motor application and that it meets the necessary certifications and conformity standards.
Products
More Information about NEMA Overload Relays
NEMA overload relays are compatible with contactors, which are responsible for controlling the power supply to the motor. The overload relay is connected in series with the motor and the contactor, ensuring that it can detect and respond to any overload condition. By monitoring the motor current, the relay can trip and disconnect the power if it exceeds the predetermined threshold, effectively protecting the motor from harm.
NEMA overload relays are designed to meet specific product specifications and standards set by NEMA. They may have different trip classes, which determine the response time of the relay to an overload condition. Trip classes allow for customization based on the motor's characteristics and the desired level of protection.
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.