We are in the process of choo

[HamidEle]

We are in the process of choosing the control power supply for motors. Our client is a Eroupen company which dictates the applications. They object use of control transformer and want to use ac or dc ups for motor control power. They want to make sure the control power is always available to enable the motors reaccelrating if the power outages revovers within 0.5s. My concerns are, if there are so many motors recelerating simutaneously, large voltage dips may occur which may bring the system voltage further down and shutdown the whole plant. I don't have any operation experience with this applications. I appreciate it for any inputs.

 

[GTstartup]

Makes no sense to me, how can you restart the motor if the power is lost even if you have UPS control power? When the power is restored then the contol power is availble from the control transformer.

 

[waross]

On large motors, a restart within 0.5 seconds is a good way to twist shafts and create severe current surges. In 1/2 second, any motor/load combinations with very much inertia will still be turning at a reasonable speed and the back EMF will be high. An out of sync closer will cause a current transient and a companion torque transient that may be in excess of normal starting torque and may do serious mechanical damage. Also, if the motor is ahead of sync, the torque may be negative and gear train backlash may cause damage.
Conventional wisdom is to either use manual starting after an outage, or if starting is automatic, a time delay may be programmed so that all motors have enough time to slow down and the back EMF may decay to a safe level.
The next step may well be to get your legal advisers to prepare ironclad disclaimers to go with this one.
I seem to remember this being discussed here at other times so you may want to spend a few minutes searching this site.
PS. Where are you located these days?

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

 

[HamidEle]

Good points, Waross. That was our bigest concern. I may ask pur operations to write the disclaimer that can move this away.

 

[HamidEle]

To make it clear to everybody here, the typical practice here in North America is to feed the motor control power from the feeder contactor circuit. In the event of power outage, the contactor will drop out and the control power will be lost as well. If the power is recovered, the operator will have to manually start the motors. If the control power if from UPS, the motor will be reaccerated automatically .

 

[GTstartup]

Why would the opertor have to manually start the motors if the control power is not from a UPS? The control power might be lost in an outage but the control system is still operational. Once the main power is restored so is the control power and if the control system is still calling for the motor to run, it will. Or if it is just a pulse to start it can reissue the pulse.

I think this comes from the European concept of a "reclosing device" which dependng on the length of the power outage will issue a command to the control system and the control system will restart selected motors that are essential to the safe operation of the equipment. Not all motors have to be restarted.

 

[ScottyUK]

A sag or momentary interruption can cause all the motors on the bus to slow down as the slip increases. If the voltage then recovers the motors will try to simultaneously re-accelerate, drawing a very large current in the process. Many supply systems are not robust enough to allow recovery if all motors accelerate together, so the restart of some drives is intentionally delayed slightly.

 

[waross]

Another issue with leaving the motors connected to the line is that they will tend to stay close to the same speed. If you have some large motors with high inertia loads they may tend to hold their speed, Back EMF will be shared with all motors and will drive those motors albeit at a decreasing speed. An out of sync reclosure may well effect almost all motors in the plant and may result in a serious current surge on the grid. Much worse than a single motor subject to a fast reclosing.


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