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VFD dc bus overvoltage
Power generation equipment control. topic
Posted by Dave on 26 September, 2013 - 1:25 pm
I have been reading up on DC bus overvoltage VFD faults. My VFD shuts down while running. From what I have read, this commonly happens when a VFD is decelerating, however we are running the VFD in local and selecting a Frequency to run at. There is no modulation to the frequency, no 4-20ma input to change the frequency while we are running. The over voltage trip point should be 810v, but when I do a fault trace, the VFD can show me what what happening at the time of the fault. My DC bus voltages (on three different fault occurrences) were 715, 745 and 785, which are all below the trip point but still higher that normal (typically around 640). Also I have two VFDs being fed by the same power source. The other unit has ran over 800 hours without a single fault. The nuisance VFD may run for several hours to a day without faulting but then may fault several times in a short time frame, always the same fault, dc bus over voltage.


Posted by Steve Myres on 26 September, 2013 - 8:27 pm
Saying that this condition exist during deceleration is often true, but a little misleading. Power is fed back from the load to the bus any time the motor is regenerating, meaning that it is producing braking torque or torque in the opposite direction of current rotation. It's common for this to happen while decelerating a massive load, but can occur at constant speed, or even while accelerating if the load conditions are that way, for example imagine a axis lowering a load vertically and partially or wholly without a counterbalance. The load will try to run away, and the drive is trying to maintain the commanded velocity. This is called an "overhauling load"

Conversely, on a horizontal axis with significant friction, you may be motoring (normal operation, torque in same direction as motion, opposite of regenerating) even while decelerating and not have bus voltage issues.

So, is this motor and the one that never faults driving exactly the same type of load? Or is this one more likely to encounter an overhauling load and thus be regenerating? Is it set for a more rapid decel ramp than the other drive?

Now I don't know why your drive is faulting on a lower than expected bus voltage, but perhaps there's some problem in the sensing circuit, or maybe the fault trip is faster than the number that's being recorded.


Posted by Dave on 27 September, 2013 - 9:18 am
Through a commonly fed control panel, each VFD's can provide power to its respective blower motor. These motors run a positive displacement blower which provide air to a sludge holding tank to aerate sludge at a waste water treatment plant. Only one blower at a time will run. The tank sludge level provides the load on the blowers and the plant will typically raise the tank level by adding sludge to the tank or lowering the tank level when the send sludge to their centrifuges to be processed. Typically the blower load will change when sludge is added and removed from the tank by only a couple of feet of head pressure and, the rise and fall of the sludge level is slow.
Long story short, both VFD's are experiencing the same marginal increases and decreases in load that occur through a typical day.
I am going to try to adjust the deceleration time, which is currently set at 10 sec, to 30 sec and see if that helps.


Posted by Curt Wuollet on 26 September, 2013 - 10:10 pm
Look for any reason the load would over run the VFD. Something is providing the energy that is charging the bus, find it and you've found your problem.

Regards
cww


Posted by Boy Coulomb on 27 September, 2013 - 2:47 am
What is the application of the motor being driven? Are there any breaking resistor installed? If so, does the brake chopper work properly engaging the breaking resistor during deceleration?


Posted by Dave on 27 September, 2013 - 9:23 am
The loads provide by the sludge process are marginal through a typical day. The electrician who wired the system is scheduled to come in first of the week to check his electrical connections. I don't if a loose electrical connection might be capable of causing fluctuations in loading.


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Posted by Karl Harshbarger on 28 September, 2013 - 12:50 pm
I've never seen bus regeneration faults on a VFD driving a fan that ran at a constant speed. It may be that the 3 phase feed to the control panel is fed from a Delta transformer and there is a ground fault on the motor connected to the trouble VFD. Some Delta transformers are ungrounded for fault tolerance. A single grounded phase won't trip a breaker and bring a whole system down. Ask the electrician to take voltage readings on the load side of the disconnect in the cabinet when the trouble VFD is running, also take another voltage reading with the good VFD running. Although some Delta transformers are ungrounded voltage testers will pick up stray capacitances to ground. If one of the phases goes high when the trouble VFD is running then you have a Delta transformer with bad wiring to the motor. I've seen grounded legs with a reading as high as 580V on a 480V 3 phase system. High input voltage will also cause a VFD to trip on DC over voltage.


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Posted by Phil Corso on 28 September, 2013 - 8:35 pm
Dave... you provided DC-Link voltages. Can you provide additional detail:

1) Input supply ph-ph voltages?

2) Are the trip events occuring at about the same time in a shift?

3) Are the two drives identical with regard to physical parameters:

a) connected to the same supply bus?

b) connected to the same utility xfmr?

c) same feeder circuit lengths?

d) Etc?

Regards, Phil Corso


Posted by Dave on 1 October, 2013 - 6:35 am
Thanks Phil,

My electrician is going to be onsite Wed. morning to 'Megger" the feeds and motors. I will pass this onto him to get additional info.

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