What is Motor and Phase Rotation Indicator?
Motor and Phase Rotation Indicator is an instrument capable of indicating the correct rotation of motor corresponding to the marked phase connection on motor terminal box. It can also indicates the correct phase sequence of three phase system.
Motor terminal box have terminals marked as R, Y and B for external connection. If the external power supply is connected to these terminals in correct sequence then motor should rotate in the correct direction. Correct direction of rotation (DOR) is the direction of rotation of the coupled equipment like pump.
Identification of correct DOR must be performed during the pre-commissioning of motor. This means that, a motor should be confirmed for its correct DOR before extending power supply to it. This necessitates the requirement of Motor and Phase Sequence Indicator.
Motor and Phase Rotation Indicator can truly identify the DOR of three phase as well as single phase unpowered motor.
Why Motor and Phase Rotation Indicator Required?
The direction of rotation of a motor can be checked and identified visually by operating it. So why do we need an instrument like Motor and Phase Rotation Indicator to ascertain it? Why to invest money to purchase this instrument? These are the questions which need to be answered to understand the need and importance of the instrument. Let us address these questions.
First question tells about visual identification of DOR by running the motor. This means the cable connection at Switchgear end as well as motor terminal box is complete. Now, if it happens that motor rotates in opposite direction, we need to reverse the connection either at motor terminal box or at Switchgear end. Since the cable connection at both the ends were done at applicable torque, we again need to open the connection at any one end and ensure that reconnection is done at the applicable torque. This is quite time consuming and have the potential for mistake.
If we carry out identification of DOR as a pre-commissioning activity and prior to cable connection, such mistake can be avoided and a lot of time will be saved. Thus it is not wastage of money to purchase the indicator.
How to Identify the DOR by Motor and Phase Rotation Indicator?
The method of identification of DOR of unpowered motor is using the instrument is shown below.
R, Y and B terminals at motor and Indicator end are shown by L1, L2 and L3 respectively. We connect the L1, L2 and L3 leads of indicator to the corresponding terminals of the motor terminal box. Now just rotate the shaft of rotor in the correct direction by hand. If Green LEDs glow on the indicator, this means that motor DOR will be correct if R, Y and B phase are connected at the marked terminals in motor terminal box.
If Red LEDs glow on indicator, this means DOR is not correct and the connection to the terminals should be changed. So where to change the connection i.e. at motor terminal box or at Switchgear end?
The best way is to change the connection at motor terminal box but not that of power cable. But the windings leads coming to the terminal box should be changed. This is beneficial as the R, Y and B marking will remain the same and we do not need to change it. In case we change the power cable connection, the R, Y and B marking at Motor Terminal Box will be changed and hence we need to change this marking to avoid any mistake during the maintenance in future.