How does an electric motor starter protect against motor overload?
1 Answer
An electric motor starter is designed to protect the motor from various electrical faults, including motor overload. The primary mechanism used to protect against motor overload is called an overload relay. The overload relay is a crucial component in motor starters, and it works based on the principle of thermal protection.
Here's how an electric motor starter protects against motor overload:
Overload Relay: The overload relay is an electro-mechanical device that measures the current flowing through the motor's windings. It is typically connected in series with the motor circuit. When the motor starts, the current passes through the overload relay.
Heating Element: Inside the overload relay, there is a heating element that is designed to heat up when current flows through it. The heating element is typically made of a bi-metallic strip or a heater coil.
Trip Time: The overload relay is set to a specific trip current level, which is based on the motor's rated current and its thermal characteristics. If the current drawn by the motor exceeds the trip current level over a certain period, the heating element in the overload relay heats up.
Bi-Metallic Strip: In the case of a bi-metallic strip, the heating causes the strip to bend due to differential expansion of the metals. This bending action triggers the relay to trip, opening the motor circuit and disconnecting it from the power supply.
Heater Coil: In the case of a heater coil, the coil heats up and causes the relay to trip, similar to the bi-metallic strip mechanism.
Tripping Mechanism: Once the overload relay trips, it usually has a latch or lockout mechanism that keeps it in the tripped position until manually reset. This ensures that the motor cannot be re-energized until the cause of the overload is addressed.
Reset: After addressing the cause of the overload (e.g., removing the excessive load, fixing a mechanical issue), the operator or an automated system must manually reset the overload relay before the motor can be restarted. Some modern motor starters may have automatic reset features as well.
By using an overload relay in the motor starter, the motor is protected against excessive current, which could otherwise lead to overheating and potentially damage the motor or its windings. This protection mechanism is crucial in preventing motor failure and ensuring the safe and reliable operation of electric motors in various applications.
Here's how an electric motor starter protects against motor overload:
Overload Relay: The overload relay is an electro-mechanical device that measures the current flowing through the motor's windings. It is typically connected in series with the motor circuit. When the motor starts, the current passes through the overload relay.
Heating Element: Inside the overload relay, there is a heating element that is designed to heat up when current flows through it. The heating element is typically made of a bi-metallic strip or a heater coil.
Trip Time: The overload relay is set to a specific trip current level, which is based on the motor's rated current and its thermal characteristics. If the current drawn by the motor exceeds the trip current level over a certain period, the heating element in the overload relay heats up.
Bi-Metallic Strip: In the case of a bi-metallic strip, the heating causes the strip to bend due to differential expansion of the metals. This bending action triggers the relay to trip, opening the motor circuit and disconnecting it from the power supply.
Heater Coil: In the case of a heater coil, the coil heats up and causes the relay to trip, similar to the bi-metallic strip mechanism.
Tripping Mechanism: Once the overload relay trips, it usually has a latch or lockout mechanism that keeps it in the tripped position until manually reset. This ensures that the motor cannot be re-energized until the cause of the overload is addressed.
Reset: After addressing the cause of the overload (e.g., removing the excessive load, fixing a mechanical issue), the operator or an automated system must manually reset the overload relay before the motor can be restarted. Some modern motor starters may have automatic reset features as well.
By using an overload relay in the motor starter, the motor is protected against excessive current, which could otherwise lead to overheating and potentially damage the motor or its windings. This protection mechanism is crucial in preventing motor failure and ensuring the safe and reliable operation of electric motors in various applications.