Motors pulling different amps on same load?

[Gatorcason]

I have an issue I want to know if anyone else has dealt with. I have a large exhaust fan that is driven by (2) 150 hp, 1750 rpm motors. The exhaust fan has a common pulley that is belt driven equally by each motor. The motors are both controlled via the same soft starter and each protected by their own overload. The issue is one motor pulls about 30% higher amperage. Tried a new motor and it too pulls more load. Any ideas?

 

[ptonsparky]

I have an issue I want to know if anyone else has dealt with. I have a large exhaust fan that is driven by (2) 150 hp, 1750 rpm motors. The exhaust fan has a common pulley that is belt driven equally by each motor. The motors are both controlled via the same soft starter and each protected by their own overload. The issue is one motor pulls about 30% higher amperage. Tried a new motor and it too pulls more load. Any ideas?

Your motors and or belt system are not exactly equal.

My WAG would also be that a VFD would have been a better choice.

 

[Ingenieur]

Did you tach each motor?
one is making 1 pu torque
the other 0.7 pu
the first >40% more! (0.3/0.7)
if offset one is making 0.3 the other 0.0, it's being pulled

could be belt slip/tension
differences in motors, mostly speed
sheaves could be slightly off

 

[JFletcher]

What are the line to line voltages and amperages at the motors with both of them running?

Motor #1:

V_AB =
V_BC =
V_AC =
I_A =
I_B =
I_C =

Motor #2:

V_AB =
V_BC =
V_AC =
I_A =
I_B =
I_C =

Are both motors fed by one feeder? Are both motors wired the same?

Mechanically, one motor's belts could be tighter than the other's. Misalignment or bearing issues could also be responsible. As ingenieur mentioned, are both motors rated for the same speed? Measuring the pulley size wouldnt be a bad idea either.

 

[Jraef]

Your two motors do not have the same slip characteristics. No two motors will be exactly the same, but two motors made by the same mfr at the same factory will be as close as you can get. When two motors run at different speeds, one will drag the other as a load until the speeds match, so that one will pull more current. The higher the load the less difference there will be.

 

[ptonsparky]

Is the one motor overloaded? Pulling above FLA?

 

[Gatorcason]

Your motors and or belt system are not exactly equal.

My WAG would also be that a VFD would have been a better choice.

Why would a VFD be better for this operation? We either want the fan on or off, we never have a need to vary the speed. One soft start drives both motors that are turning the same pulley. In other locations I normally notice the amperage is only slightly different, maybe 2-3%. Just odd that a new motor has the same issue as the old one we replaced.

 

[K8MHZ]

I have an issue I want to know if anyone else has dealt with. I have a large exhaust fan that is driven by (2) 150 hp, 1750 rpm motors. The exhaust fan has a common pulley that is belt driven equally by each motor. The motors are both controlled via the same soft starter and each protected by their own overload. The issue is one motor pulls about 30% higher amperage. Tried a new motor and it too pulls more load. Any ideas?

Does each motor have its own belt? That would be the only way to drive the fan 'equally'. If there is only one belt, the configuration of the belt will most likely change how the load is distributed by the two motors. Also, the belt will have a tight side and a loose side. The motor pulling on the fan pulley will have more dynamic belt tension than the side pulling on the motor and 'pushing' into the fan pulley.

 

[Fulthrotl]

I have an issue I want to know if anyone else has dealt with. I have a large exhaust fan that is driven by (2) 150 hp, 1750 rpm motors. The exhaust fan has a common pulley that is belt driven equally by each motor. The motors are both controlled via the same soft starter and each protected by their own overload. The issue is one motor pulls about 30% higher amperage. Tried a new motor and it too pulls more load. Any ideas?

i'd call it different belt tensions.
one motor is pulling both it's part of the shared load,
and dragging the other motor along with it, as it's trying
to turn the shared load a bit faster than the other motor.

assuming synchronous motors, this is reducing the slip
the slacking motor needs to develop to do the work.

it wouldn't take very much of a difference in tension
to make a fair bit of difference in running loads.

i'd get two new belts, from someone like gates, not
cheap import stuff, and tension them the same.

you'll need a decent tension gauge... here is one:

http://www.gates.com/products/autom...fessional-tools/krikit-v-belt-tensioner-gauge

i'm not sure if that one will handle the belt size you have.
there are others available, but that one is reasonable.... about $30

with two separate v belts driving a common shaft, it's going to be hard to have them both
perfectly sharing the load. as the belts wear, things will change. gilmer belts would solve
the problem, but cost a lot more. i'm assuming the equipment doesn't have spring loaded
belt tensioners. if it has tensioners, i'd set the belts up without tensioners, with a gauge,
then drop the tensioners into place, run the belts a half hour, stop and measure the tension
of both belts at that point.

 

[ptonsparky]

Were the motors matching to begin with?

Maybe you replaced the wrong motor.

 

[Besoeker]

I have an issue I want to know if anyone else has dealt with. I have a large exhaust fan that is driven by (2) 150 hp, 1750 rpm motors. The exhaust fan has a common pulley that is belt driven equally by each motor. The motors are both controlled via the same soft starter and each protected by their own overload. The issue is one motor pulls about 30% higher amperage. Tried a new motor and it too pulls more load. Any ideas?

Does this happen during steady speed operation i.e. after the soft starts are out of circuit (bypassed) or only during the acceleration phase?

 

[Jraef]

It could be belt tension for sure, but 30% is fairly extreme if that's the case. It would likely have to be a case where one is too loose AND the other is too tight. A lot of mechanics /riggers don't pay enough attention to belt tension and when there is only one motor, they never really notice the effects directly. In a situation like this though, it's something that shows up immediately. But it's not the mechanics that see it, it's the electricians, so it's an "electrical issue". But before telling the mechanics to fix it, make sure it's not an issue with slip difference. Look at the motor nameplate speeds. If it's a 4 pole motor and one says 1750 RPM, but the other says 1725 RPM, the slower motor will contribute no torque until after the faster one is loaded down to the point of spinning 1725 RPM, so it is ALREADY pulling more current by the time the other motor even begins to contribute.

So one way you could help tell is if the motor pulling more current is the one that lists a faster speed. It still COULD be just the belt tension, but if the higher current is on the slower motor, then it (almost) HAS TO be the belt tension.