Motor Starter Dropout

[mls1]

We have an industrial facility that is losing critical motor loads on outside line faults. It appears to be a classic case of voltage sag that is dropping bus voltage to 50-70% for as much as 20 cycles. We are working on relay improvements to provide faster clearing but that won't change the level of voltage drop. For that there is not much that can be done. I'm exploring options to provide some level of ride through and in particular a time delay relay in parallel with the start button that will hold the start circuit long enough that the motor will pick back up. This has me concerned a little about the transient torque that can result. I'd appreciate other thoughts on the best way to approach this. An example load that I'm talking about is a boiler feed pump that once it trips causes the boiler to drop out and a serious loss of process.

 

[itsmoked]

OK, so you are not talking about damaged motors but just motors that have tripped off due to their start logic.

Your low voltage cases seem to be very specific. This may mean you can complain to the poco and they might have a chance of fixing the issue. It may be malfunctioning equipment like a slow voltage regulator transition or a nearby user that is abusing the local grid. e.g. have the power company fix their problem.

Otherwise, 20 cycle type dropouts are trivially carried thru by VFDs. You could use a VFD as a short term UPS for your boiler. Same with various other critical motors. You could use local (per motor) UPSes for other critical motors that are single phase.

In some cases you can use a small UPS to just power the control circuitry to keep it from dropping out. You do have to study the safety issues in this scheme. In most cases there will probably be no issues. Others may need THIS MACHINE MAY START AUTOMATICALLY placards, in other cases you will want to make sure indicator lights still show "running" while the control circuitry is asking for it.

You can also UPS the control circuits with various timers that will shut down after a couple of seconds if a power outage is actually long-term.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

DC control is another option, indeed you may even have this if your MCC is a relatively new one. Once you're using DC then battery support or a wide-input-range power supply become credible options.

 

[waross]

First, are the motors dropping out due to low control voltage or due to unbalanced currents or phase loss?
If this is an issue with just the low control power voltage dropping out the contactors, then a UPS on the control supply to critical motor starters will probably work well.
A UPS should feed all the critical load components, not just the motor starter. You don't want to provide protection for the feed water motor starter only to find the next point of failure in the control logic.
A time delay across the start contacts may be a bad idea and probably won't work.
At 50% voltage there will be enough torque so that rather than dropping out of sync, the slip frequency will increase. When the voltage recovers torque transients may not be greater than breakdown torque.

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

 

[Parchie]

Suggest you invest on a small steam turbine-driven boiler feed pumps and migrate your controls to DC.

 

[krisys]

Normally for the boilers, there will be two (2) or three(3) feed water pumps. One pump will be Steam Turbine (ST) driven and the other will be Electric Motor (EM) driven in the case of two pumps.
2ST + 1EM in the case of three pumps.
Where there are three pumps, the EM driven pump will be considered as standby. This works more economically, as the steam will be generated by the same boiler, which feeds the ST.

If OP's problem is only with the Boiler Feed Water (BFW) Pump this would be O.K. But it looks the entire facility is getting effected due to the voltage dip. This kind of voltage sag is also called as brown out. Here in this case, the voltage is not going to zero or even near zero. If that is the case, then you have to study the extent of voltage dip. Then have an objective study to evaluate which are the motors which can sustain the 20 cycle low voltage without entering the locked rotor region. These are typically, partially loaded motors, high inertia drives etc. These motors can be safely operated with the delayed trip for low voltages. Also called voltage dip ride thru circuit.
For other motors, if they are really critical, you may consider using some sort of active power factor capacitors. Which would provide instant voltage support for some extent. This is the solution which Schneider provides.

In my view providing VFDs or UPS would be quite expensive. Also some sort of power supply distribution optimization, for operation; such as improvement in the operating power factor, intake transformer tap position etc.

Additionally, you may improve the voltage profile, by building a good rapport with the neighboring consumers, by understanding their system etc.

 

[jraef]

Often if an AC motor is not on a VFD or something else that can sustained a 20ms loss, there will be a short cycle protection scheme to prenvent a motor from dropping out and having the starter re-close too quickly, risking a torque transient that can damage the equipment.

At risk of conflict (because this company is now owned by my employer), this product is specifically designed to address that issue.

http://www.softswitch.com/sst/dyscsolutions.do;jsessionid=E1824F1274DDB511B10D6FE1A107D1FA.B

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington