Load Side Monitoring:
Broken contact springs, worn and pitted contacts,
or loose and corroded wire connections
are common causes of unbalanced voltages and currents. These lead to overheating and eventual
equipment failure. Voltage monitor #1 in the diagram cannot
protect the motor from these if they occur at or after the contractor. For complete protection, a
voltage monitor should be connected to the motor leads (Voltage Monitor #2).
Voltage monitor #1 protects the motor control panel from faults associated with main power
supply. It detects phase reversal, phase loss, undervoltage voltage, and unbalanced voltage faults
before motor startups. Without voltage monitor #1 in the system, dangerous reverse rotation
or locked rotor starts could occur. Voltage Monitor #2 provides protection against loose wiring, worn contacts, and
long undersized cables.
Voltage monitor #2’s contacts open each time the motor contactor opens, a bypass
timer is required to restart the motor. It should be connected as close to the motor loads as possible. The bypass time delay should be kept as short as
possible (1-2 seconds maximum). A short bypass delay allows restarting but returns control
to voltage monitor #2 as rapidly as possible. Voltage Monitor #2 protects against phase loss
and reversal, unbalance, undervoltage, and short cycling.
Some electricians install voltage monitor #1 in the main motor supply panel. This approach
ensures prestart protection for all motor control panels in the building. Then voltage monitors and bypass
timers are installed in each motor control panel and connected like voltage monitor #2 in the
diagram.
The RLM, TVM and TVW series are good choices for the load side monitor because of their
superior protection from the high voltages caused when the contactor disconnects the rotating motor. |
