Exhaust fan motor with auto xfrmr starter sagging 2

[govolsdb]

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?

 

[jbartos]

Suggestion: It may need troubleshooting for any irregular connections.

 

[ScottyUK]

I wonder if a bad connection is causing your starter to change taps? I am surprised you can't hear this if it is occurring, but you need to be there to listen which can be awkward if it is intermittent.

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.

 

[tmahan]

I think a decrease in current typically indicates a reduced load (or an increase in voltage if power output remained constant)

Sounds to me like the fan is doing less work.....what is the pressure in the area being exhausted doing??

 

[Sanditech]

The Transformer will be out of circuit on the running drive as it is only switched in during the starting period. So it should be running on a Main Contactor only.
I would check load variations fist then the Motor Power Circuit connections.

 

[rhatcher]

I think that tmahan is on the right track suggesting load variation. How 'airtight' is the area that the fan is serving? I have some experience on shipboard applications where duct fans are used to supply or exhaust compartments that are essentially watertight. In either case, the fan load is at a minimum when the compartment is sealed because there is no flow. When a hatch is opened, the load (and flow) momentarily surges then steadies out at a higher level. Is it possible that this is the case for you? If you feel significant flow into the area when at doorways or windows, it may be that when these are closed the load on the exhaust fan is reduced due to reduced flow.

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.

 

[jbartos]

Suggestion: Some analogy might be made with a vacuum cleaner. If the cleaner is lifted from the carpet, the motor RPM are increasing and motor current is decreasing; if the cleaner is heavily pressed on the carpet, the motor RPM are down and the motor current is increased. Any poor connection, e.g. very long and small AWG "thin" cord is reducing the motor RPM and current in the cord.

 

[rhatcher]

jbartos, your vacuum cleaner analogy is somewhat different than govolsdb's situation. The reason your vacuum cleaner motor rpm decreases and current increases when it is pressed into the carpet is because you are increasing the load on the brushing mechanism due to friction with the carpet. That is why you don't see the same effect on wood or tile floors. govolsdb is describing a case of reduced load on a fan due to reduced flow.

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.

 

[jbartos]

Comment on the previous posting:
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.