Momentary Loss of MCC Control Power

[jliu1]

Our foreign client has a question on momentary loss of MCC control power.

They want to know the US practice of countermeasure against instantaneous power loss for MCC.

MCC uses step-down power from main power for control power. Contactor will open in instantaneous power failure condition on MCC. ON demand signal needs to be kept in order to operate motor load after recovery of power. They usually use retention of demand signal within a few second timer to automatically restart the motor if the power is recovered within the time delay. If the power is recovered after the time delay, the motors will be started in sequenced order.

Is the countermeasure against instantaneous power loss used (required) for
all non reversible starter circuits in US?

Or, is this used for limited circuits(loads)?
If so, what kind of circuits should have such countermeasure?

Any comments are appreciated.

 

[waross]

It depends on the load. Safety codes prohibit some types of equipment from automatically restarting after a power failure. Table saws are a good example.
Some loads restart automatically even on simple control schemes. A sump pump controlled by a float switch is a good example.
Some large motors are prohibited from immediately restarting after a short power interruption.
A large motor (over about 40 HP, but possibly smaller motors) with power factor correction capacitors connected to the motor terminals is a good example. Such motors may be damaged if they are reconnected to the grid before the back EMF decays. Such installations may force a delay of several seconds before the motor is restarted to allow time for the back EMF to decay to a safe level.
This may be summed up in two words. "It depends"

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Shell Standards stipulate that all the vital and essential drives shall have auto restart facility so as to ensure safety and production requirements in any project.

 

[juleselec]

Like waross said, it depends. Auto-restart is something that depends on the plant and the loads. I once spent 6 months retro-fitting auto-restart facilities on selected motor starters for an oil refinery.

The loads were selected by the process / operations engineers to maintain production after short supply interruptions or brownouts (which were costing the plant a fair bit of money). These were obviously process engineering decisions and they were careful to choose only the drives that could be restarted safely.

There are a couple of ways you can implement this:

1) As you mentioned, you could program it into the DCS start signal, but this would only work if the drive has a DCS start signal in the first place.

2) You can hard-wire an auto-restart module (e.g. Holec ERM restart module) into the control circuits, which can hold in the contactor during a short outage or voltage dip.

3) Quite a few integrated motor protection relays (e.g. GE MM2 relay) have in-built auto-restart facilities that will ride through short power supply interruptions.

4) If you have a separate control supply (e.g. UPS), then you can hard-wire an auto-restart with basic timer relays in the motor start circuit.

 

[dpc]

This has to be determined on a case by case basis, depending on the nature of the load. Some motors have maintained RUN commands, while other have three-wire START/STOP.

We have had clients run critical motor starters from UPS for control power to avoid contactor drop-out on voltage dips and momentary outages. This too must carefully evaluated to make sure it is not creating more problems than it is trying to solve.



David Castor

http://www.cvoes.com