Under Voltage Trip Relay
Under Voltage Trip Relay
(OP)
Currently working on the design of a power system for a small cement plant, the main incoming breakers are going to use under voltage trip relays with a 0.5 to 3 second time delay.
The reason for the time delay is the maintenance guys do not want to reset all the breakers every time there is a lightning strike that causes a very short power outage. The utility already told me that they try once to automatically reclose at 2 seconds.
My question comes with up with what happens when the voltage does not go to 0 and is really just a sag. Will the motors get damaged if the time delay is set to 3 seconds and the relay is set to trip at a 20% drop in voltage. Motors could continue to run at 70%, 60% or possibly a lower voltage for 3 seconds before the UVT trips, will damage occur? Plant will have some across the line high torque loads along with some large VFDs.
Thanks
The reason for the time delay is the maintenance guys do not want to reset all the breakers every time there is a lightning strike that causes a very short power outage. The utility already told me that they try once to automatically reclose at 2 seconds.
My question comes with up with what happens when the voltage does not go to 0 and is really just a sag. Will the motors get damaged if the time delay is set to 3 seconds and the relay is set to trip at a 20% drop in voltage. Motors could continue to run at 70%, 60% or possibly a lower voltage for 3 seconds before the UVT trips, will damage occur? Plant will have some across the line high torque loads along with some large VFDs.
Thanks





RE: Under Voltage Trip Relay
I always ask in this case: For what you need UV function on the main breaker. Are you have some back-up generator?
You need protect your motors, every motor or one UV relay per bus, as you want.
In additional, MCC contactors will opened by UV in about 0.7Un.
Best Regards.
Slava
RE: Under Voltage Trip Relay
The motors will generally not be damaged by short term voltage sags. Damage can occur if maintained ON/OFF controls are used and the motors try to start back up immediately after a short power blip. The motors should stay off until things settle down and power is restored and stable.
The VFDs will likely shut themselves down before the UV device trips the main breakers.
RE: Under Voltage Trip Relay
Bill
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"Why not the best?"
Jimmy Carter
RE: Under Voltage Trip Relay
The PLC will have to be programed to keep loads de-energized until the power system is stable after a bump and start loads on a prioritized basis.
Thanks