Exhaust fan motor with auto xfrmr starter sagging
Exhaust fan motor with auto xfrmr starter sagging
(OP)
There is an exhaust fan motor at one of my facilities which has a "sagging" problem. The motor will be running fine at about 50 amps and then, for no reason, current will drop to about 38-40 amps and hang out there for a minute or two. Flow also reduces during this time period. After a minute or two the current returns to normal. There is a small spike upon return to normal like you would expect from motor starting. The motor has an auto-transformer starter, could a malfunction of the starter be causing this? The control circuit has probably never been PM'd (I am new to this facility), and I have recommended PMing the MCC as a starting point. There doesn't appear to be any system load transients that could be causing this phenomenon. Any ideas?





RE: Exhaust fan motor with auto xfrmr starter sagging
RE: Exhaust fan motor with auto xfrmr starter sagging
A nice simple idea one of our technicians used to catch an intermittent contactor problem was to use a simple latching relay circuit driven by a spare auxiliary contact on the suspect contactor. It took about 15 minutes to make it up out of junk lying around the workshop. The activation of the latch proved that the contactor was definitely dropping out.
RE: Exhaust fan motor with auto xfrmr starter sagging
Sounds to me like the fan is doing less work.....what is the pressure in the area being exhausted doing??
RE: Exhaust fan motor with auto xfrmr starter sagging
I would check load variations fist then the Motor Power Circuit connections.
RE: Exhaust fan motor with auto xfrmr starter sagging
Also, Sanditech is right in saying that once the motor is up to speed, the autotransformer is out of the circuit and the motor is running off of a single across-the-line contactor.
RE: Exhaust fan motor with auto xfrmr starter sagging
RE: Exhaust fan motor with auto xfrmr starter sagging
However, I like the idea of using a vacuum cleaner to describe his situation since everyone has one and can try it for themselves. So...Disconnect the hose from the brushing mechanism. Turn on the vacuum and find where the exhaust is discharged. Note the motor rpm and the exhaust flow. Now, slowly cover the end of the hose. As you cover the hose, the motor rpm will increase and the exhaust flow will decrease. If you were measuring current during this experiment, it would decrease.
So, again, I suggest to govolsdb that it is possible that the situation he describes is caused by something affecting the fan flow. Either the duct is sometimes partially blocked or something is affecting the flow into the room that the fan is exhausting.
RE: Exhaust fan motor with auto xfrmr starter sagging
1. I agree with the second paragraph regarding the flow blocking, RPM increase, and current decrease.
Now, comment on a potential electrical cause of the temporary smaller current:
The small cord AWG gage, e.g. #18AWG and high voltage drop can cause the smaller current to flow and ultimately, the motor could stop running due to very high voltage drop. If the motor receives zero voltage, it will not turn, which is obvious. This might be an alternate cause of temporary low current, as posted in the original posting.