Intermittent Motor Overload on Start

[Timpi Electric Solutions]

Voltage unbalance on the service while the dust collector is not running is 1.02%. Technically, it exceeds NEMA standards, but only by 0.02%.
Voltage unbalance with the dust collector IS running is 0.95%.



Using the my motor's FLA of 222A, unbalance is only 4.58% while running.

While starting, motor current unbalance is only 0.49%.

 

[Timpi Electric Solutions]

I'm at a loss as to where to proceed next. The customer had a new and unused 400A circuit breaker in a new service that we installed a few years ago, and that did not resolve the starting overload. I've been attempting to hire a local company with motor circuit analysis equipment, but their communication has been basically nonexistent.

 

[Timpi Electric Solutions]

Here's another good info source:

Unbalanced Current

The current of a three motor line lead is not balanced.

www.ecmweb.com

Use the following procedures to assess and correct current unbalance. Current unbalance can be due to the motor or the supply line. To determine the source, arbitrarily label the supply lines A, B, and C — and the motor leads 1, 2, and 3. Then connect A to 1, B to 2, and C to 3, operate the motor, and measure the current in each line. Next, de-energize the motor and connect A to 3, B to 1, and C to 2, operate the motor, and again measure the current in each line.

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If the high current and low current readings follow the same line leads, the supply is the cause of the problem. If the high and low readings follow the motor leads, the motor is the source of the problem. This is illustrated in the Table.

I had thought about doing this but decided against it because the motor load should decrease, since the fan would be turning in reverse. The current should decrease, but should remain relatively proportionate in its unbalance, though. If I have time, I'll test this tomorrow.

 

[synchro]

The motor will not turn in reverse with that change in connections. The sequence of the phases is the same, it"s just that it was advanced by120 degrees. The rotation would be reversed if only two wires were exchanged

 

[TwoBlocked]

A few more things come to mind. What settings are on the 400A breaker? How are the bearings on the motor and on the fan? Is the fan belt driven? How do the belts and sheaves look? Can you disconnect the fan and get a current draw to compare? When disconnected, does the motor and does the fan spin easily?

A little voice in the back of my head is whispering: "It ain't electrical."

 

[Fred B]

A few more things come to mind. What settings are on the 400A breaker? How are the bearings on the motor and on the fan? Is the fan belt driven? How do the belts and sheaves look? Can you disconnect the fan and get a current draw to compare? When disconnected, does the motor and does the fan spin easily?

A little voice in the back of my head is whispering: "It ain't electrical."

Thinking the same. Mechanical issues will cause a significant rise in starting draw, Bad bearing or even debris getting into the vaccum from missing or mis-aligned filter will create a large spike in startup current to exceed the trip curve of the breaker. Manually turn the motor should give an idea if this is the problem.

 

[MTW]

You say there is no dampers installed on the main trunk, that's a normal setup, just blast gate valves at the drop locations. Does the discharge get duct-ed back to the building or does it discharge to the atmosphere outdoors? If the discharge is not returned back to the building, the building will be under extreme negative pressure, noted at the man doors while the system is running. Normally there should be a manual damper on the discharge side of the collector to balance the system flow to prevent pressure differentials from developing within the. building. Balancing the system is normally accomplished by restricting the output damper setting and locking it in place. If this setting has become loose or manually readjusted the current draw and airflow load may have been increased causing excessive loading on the fan both starting and running. Adjusting this damper is the manual way to control the load on the fan. If you have two of the same collectors, one fine and one not, compare the damper settings and the running load amps between the two. Just pointing out that the problem could be a mechanical one, affecting the blower drive. Sheave ratios would be another item to compare, are any of the sheaves adjustable and may have become changed?

 

[ActionDave]

A little voice in the back of my head is whispering: "It ain't electrical."

More than half the time that little voice is right

 

[ptonsparky]

I had never seen the FLA used in imbalance calculations before. Clicking No to the motor application would give identical results to my home grown spreadsheet.
I could decrease the percentage by rotating the leads often enough. It worked particularly well on Open Deltas.

 

[TwoBlocked]

OK, there is some current imbalance. The recommendation is to derate the motor to avoid it getting too hot. Looks like it's running at less than 60% FLA. I don't see the imbalance being a problem.

 

[Ziyad Alahrbi]

Hi,
I suggest to share the starter catalogue with checking the following:
What type of trip happened. Is it main breaker trip, or contactor trip or starter trip.
Is there any process related signal connected like ESD?
Check contactor latching circuit. The auxiliary relay might malfunctioning.
You might need to use thermal imaging all circuit terminals.

Voltage supply quality is very important also. Need to check for it.