A three-phase reversing motor starter is an electrical device used to control the direction of rotation of a three-phase electric motor. It enables the motor to run in both forward and reverse directions. This type of starter is commonly employed in applications where the motor needs to perform tasks in two opposite directions, such as conveyor systems, winches, cranes, and more.
The operation of a three-phase reversing motor starter typically involves the use of contactors, overload relays, and control circuits. Here's a step-by-step breakdown of how it works:
Main Power Supply: The starter is connected to a three-phase AC power supply. This supply typically consists of three lines - L1, L2, and L3 - which provide the necessary voltage and current to the motor.
Contactor Coils: The motor starter contains two main contactors - one for forward rotation and one for reverse rotation. Each contactor has a coil that is energized to close the contactor's main power contacts.
Control Circuit: The control circuit consists of switches, pushbuttons, and relays. These components are used to control the direction of the motor. There are typically two control pushbuttons for "Start" and "Stop," one for each direction of rotation.
Forward Operation:
To start the motor in the forward direction, the operator presses the "Forward Start" pushbutton.
This action energizes the coil of the forward contactor. The contactor closes its main power contacts, connecting the motor to the three-phase power supply.
Additionally, an overload relay is connected in series with the motor to protect it from excessive current. If the current drawn by the motor exceeds a set threshold, the overload relay trips and disconnects the power supply to the motor.
Reverse Operation:
To start the motor in the reverse direction, the operator presses the "Reverse Start" pushbutton.
This action energizes the coil of the reverse contactor. The contactor closes its main power contacts, connecting the motor to the three-phase power supply in reverse phase sequence. This reversal of phase sequence causes the motor to rotate in the opposite direction.
Stopping the Motor:
To stop the motor, the operator can press the "Stop" pushbutton. This de-energizes both the forward and reverse contactor coils, opening the main power contacts and disconnecting the motor from the power supply.
Additionally, the overload relay provides an extra layer of protection. If the motor draws excessive current due to a fault or overload condition, the overload relay trips and stops the motor.
Overall, a three-phase reversing motor starter allows for convenient and controlled operation of a motor in both forward and reverse directions. The combination of contactors, control circuits, and overload protection ensures safe and reliable motor operation while also offering the flexibility to switch between directions as needed.