Voltage Monitoring Relay(VMR) Connection Diagram and Wiring

Hi, in this article, we are going to see the Voltage Monitoring Relay(VMR) Connection Diagram and Wiring Procedure. A Voltage Monitoring Relay (VMR) is an electrical or electronic device used to monitor the voltage levels within an electrical system. It ensures that the voltage stays within predefined limits to protect electrical equipment from damage due to overvoltage, under voltage, or phase loss. According to the voltage level in the electrical system and the predefined voltage limits it actuates its relay contacts to generate the control output signal.

When an abnormal voltage condition is detected, the relay output can be used to trigger other devices or systems, such as circuit breakers or alarms, to take protective actions. VMRs are used in various applications, including industrial automation, power distribution, HVAC systems, and any environment where voltage stability is critical for the safe operation of equipment. In this article, we are going to see the Connection of a Voltage Monitoring Relay or VMR with a Motor.

Voltage Monitoring Relay(VMR) Connection Diagram

Here, you can see the connection diagram of a Voltage Monitoring Relay(VMR) with a Motor Control circuit so it protects the motor from abnormal voltage-related problems. 

Here, the VMR used is an advanced device where we can set the Under voltage and Overvoltage range and even we can set the delay for ON. This VMR also shows so many indications such as whether there was an Under voltage Fault or Overvoltage Fault, or Phase Loss.

When there is no fault or the system is healthy the Green Color ON indication lamp will glow. When there is an Under voltage fault, the red color UV indication lamp will glow. When there is an Overvoltage fault, the red color OV indication lamp will glow. When the phase is reversed the bottom side red color indication lamp will continuously glow. When there is an unbalanced voltage or phase failure the bottom side red color indication lamp will blink.

Connection Description

• The three-phase power supply that is powered by the motor is also connected to the VMR(L1, L2, L3, and N terminals).
• The output NC contacts of the VMR are connected to the control circuit of the contactor.

In the above voltage monitoring relay (VMR), we have OV% knob. This is over voltage percentage setting, which is the limit above the normal voltage at which the relay will trip, for example if your normal voltage is 230V and you set OV% to 10%, then the relay will trip if voltage goes above about 253V.

Here also we have UV% Knob which means under voltage percentage setting. It is the limit below the normal voltage at which the relay will trip, for example if UV% is set to 10% then the relay will trip if voltage drops below about 207V.

The VMR also have On delay setting knob which means the time delay before the relay turns ON (or allows output) after the voltage becomes normal again. This is used to avoid frequent ON/OFF due to small fluctuations, so the relay will wait for the set time (like 5 or 10 seconds) before energizing its output once voltage is within the safe OV and UV limits.

Voltage Monitoring Relay(VMR) Wiring

Here, you can see the wiring of a simple Voltage Monitoring Relay. It doesn't give any function to set the predetermined Undervoltage and overvoltage levels manually.

Here, the three-phase power input to the VMR is taken from the contactor or Overload Relay output where the motor is connected. This is because if there is any fault in the contactor or overload relay then also it can protect.

Here, the output NO contacts is used for controlling. The output NO contact is connected to the contactor holding circuit. So, when the user presses the start push button the motor will start running and the VMR also gets the power supply. If there are no abnormal conditions in the power supply the VMR will make its NO contact NC and the contactor will hold. But if there are any abnormal conditions in the VMR, it will keep its NO contact NO so the contactor will not hold.

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