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Reversing Three Phase Induction Motors

Three phase induction Motor rotation can be reversed by changing any two of the three lines feeding power to the motor. It is common standard practice to switch line 1 and line 3. When the motor needs to be run in a clockwise and a counterclockwise rotation, a reverse starter is needed.

Reverse Starter

Reverse starters are two-three pole contactors in which one of the contactors contains one set of overloads.    

Both contactors must contain a set of normally closed auxiliary contacts along with the normally open seal-in contacts. The normally closed contacts will be used to provide a means of interlocking. The auxiliary interlocking is wired in series with the opposite coil. This method of wiring the coils through the opposite auxiliary normally closed contacts prevents the coils of the starters from being energized at the same time, which could be very dangerous even if it is wired incorrectly.   

Most reversing starters also contain a mechanical interlocking device that also serves as a means to lock out the coils from being pressed in at the same time. Often technicians will use screwdrivers to manually engage the coils of the starter. This method of troubleshooting is prohibited shortcutting and very dangerous.

Pushbutton Interlocking

Another method of interlocking is the use of pushbutton interlocking. Pushbutton or button interlocking is a method of control wired done by the control technician. The station consists of three pushbuttons one pushbutton is a normally closed stop pushbutton and the other two are make and break pushbuttons containing both normally open and normally closed contacts.

The pushbuttons are wired in such a way that allows the motor to be started in either “forward or reversing” rotation. If the load is required to spin in the opposite direction while one of the contactors is energized, pressing the opposite rotation pushbutton will cause the energized coil to lose power to its seal-in contact. At this moment both contactors are now de-energized allowing the pushbutton to engage the non-running coil.  

The built-in interruption of the control circuit allows time for both coils to be de-energized before the opposite coil can be started. To ensure the utmost safety for personnel and equipment, all three methods of interlocking should be implemented.

Reversing Starter Operation

Reversing Three Phase Induction Motors

Configuration 1: Reversing Three Phase Induction Motor

The Reversing Starter in Configuration 1 operates as follows:

  • The contactors controlling the forward or the reverse rotation of the motor can be started by pressing the forward pushbutton or the reverse pushbutton.
  • If the forward pushbutton is pressed power will be sent to the coil through the reverse auxiliary contact.
  • The coil will be energized through the forward normally open seal-in contact.
  • At this time the reverse pushbutton has been isolated out of the circuit because the forward auxiliary contact is open so the reverse contactor cannot be energized simultaneously or at the same time the forward contactor is running.
  • In order for the reverse pushbutton to be pressed and turn the motor in the reverse direction, the stop pushbutton must be pressed releasing the power from the normally open seal-in contact of the forward motor starter.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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