Undervoltage protection

[reactive]

I am trying to find 'typical' undervoltage settings for a SCIM. I have very few details on the machines (they are pretty old) apart from 4MW, 6,6kV and application ward leonard mine winder.

I don't want to pre-empt but would imagine an UV level of 80%Un would be acceptable (yes/no? lower?) but have no clue as to whether a time delay to trip would be applicable/acceptable.

 

[waross]

The first issue with ondervoltage on a motor is over current.
For the great majority of applications the overcurrent protection is all that is needed.
Another issue is reduced torque. You may want to monitor voltage and inhibit starting when the line voltage has fallen to the point that starting times will be excessive.
If the machines have been running for years with no undervoltage problems why add undervoltage protection now? It seems that the overload relays have been providing adequate protection.
If you are haveing issues with undervoltage, you are in the best position to determine the optimum setting depending on field conditions.
Older motors were typically designed to operate on 15% undervoltage. At less than 100% loading you could accept more undervoltage.
A time delay would be acceptable for overheating issues.
respectfully

 

[powerjunx]

reactive, i agree with waross. Older motor allows 15% UV, the fact i've seen old BBC (brown boveri corp) motor data having 15%UV. But since your motor is "pretty old", a derating factor will not tolerate that 15%UV, let say 10% is tolerable.

 

[oldfieldguy]

I may be missing something here, but if your motor is being fed from a mechanically latched source such as a power circuit breaker, you need undervoltage relaying to insure that the circuit breaker opens on loss of source. This makes sure that on power loss and subsequent restoration, the motor will not restart by itself.

If the motor is fed from a magnetically held contactor powered off a line-fed control power transformer, then this will typically insure that loss of source will opent the contactor and thereby prevent restarting on reapplication of source power.

And yes, the setpoint should consider the voltage levels needed for a safe start under your specific operating conditions. On strong sources, I see typical UV values of 90% of nominal to disable start and 75% of nominal to open the circuit breaker. Weak sources would require more study.

old field guy

 

[reactive]

Thanks guys.

There are undervoltage relays currently installed on the winders. Problem is that they are tripping spuriously due to dips. Relays are set to 80%Un instantaneous and I was wondering if it would be good practice to recommend a time delay to offer better dip ride through. Question is what is the max. time that could be recommended without knowing anything about the motor thermal curves.

 

[waross]

Good points oilfieldguy;
As to time delay settings, it depends.
What is the loading and duty cycle on the machine?
How firm is your supply?
The bottom line is how hot does the motor run? The cooler it runs and the more varied the duty cycle (allowing cooldown time) the more undervoltage time may be allowed. You may consider the time delay as a percentage of the start time.
10% to 50% of starting time will probably be safe. Possibly more if needed.
You may want to compensate for the delay and add some extra protection by setting the thermal trips a little lower. The main issue of undervoltage will be heating.
Also, the lower the voltage the shorter will be a safe time delay.
If the machine runs at close to 100% load for any length of time use caution.
If the machine never reaches 100% loading you may withstand more under voltage for longer periods of time.
You may also consider a circuit that will bias the Ward Leonard drive and reduce the speed during times of undervoltage so as to reduce heating. If the other issues are the worst case: 100% loading and fairly long periods of undervoltage, automatic load shedding may be the solution.
respectfully

 

[LionelHutz]

You need to consider the control side of things as well. What is the drop-out voltage of the motor contactor, as well as the drop-out voltage of all the other controls? What is the start/stop method? Where does the control power come from? All these, and others I'm sure, can play into the equation as well. Sure, you can set a 80% for 5 second undervoltage trip but if the controls will immediately drop the motor contactor with a 80% undervoltage then the relay setting means nothing.

I don't see you allowing a brownout event to persist for such a long period of time that the motor is damaged due to current overload. Besides, the overload protection should protect the motor from this happening. I would be more concerned about avoiding the torque transient that occurs on short power loss events than the thermal overload capability of the motor to handle the higher current for short periods.