# Undersized motor?



## John Valdes (May 17, 2007)

How is the motor mechanically attached to belt? Gear reducer? Pulleys? Chains? Magnetic starter? VFD? What do you have?
If this motor is running, it is highly unlikely you have issues that a megger will find. It is a load issue it seems. That is where you start. Put a current clamp on it and take readings at different loads. This is what you present to the powers that be. The current readings. 
The right gearing can make any motor that is to small just right.
So you are going to check? CURRENT. Right!


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## MDShunk (Jan 7, 2007)

Different belting material types can make all the difference in the world if you are simply overloading the motor and don't want to upgrade the motor/gearbox/chains/sprockets and whatever all else.


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## Cram Renraff (Feb 10, 2012)

I've already checked the amperage. Interestingly, of the five 20 hp motors we have running conveyors, with no freight, this one pulls the lowest amount of current. None of the others ever have standing freight on them though.


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## micromind (Aug 11, 2007)

Two more tests;

1) Measure voltage under load. At the load side of the starter is fine. If you get anywhere from 420 to 500, you're good.

2) Measure current, all 3 legs. Load side of the starter is fine here too. If the current is balanced within about 10% or so, and the highest reading is over the SFA (service Factor Amps) listing on the nameplate, then the motor is being asked to produce more HP than it's designed to. 

If there's no SFA, you can use about 10% over the full load amps.


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## mrmike (Dec 10, 2010)

Cram Renraff said:


> I've already checked the amperage. Interestingly, of the five 20 hp motors we have running conveyors, with no freight, this one pulls the lowest amount of current. None of the others ever have standing freight on them though.


 


I think you have answered your own Assumption that the motor is undersized with this reply. And this is the only one with freight on it answers the overload. The motor probably is over the limit of the starting torque it needs to move that freight, when the belt is loaded over halfway............. 

I think this is entirely a mechanical overload.........


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## JRaef (Mar 23, 2009)

mrmike said:


> I think you have answered your own Assumption that the motor is undersized with this reply. And this is the only one with freight on it answers the overload. The motor probably is over the limit of the starting torque it needs to move that freight, when the belt is loaded over halfway.............
> 
> I think this is entirely a mechanical overload.........


I agree. There are simplistic calculators to determine the HP requirements of conveyors if you know the weight, slope, speed and type of conveyor, I suggest trying to find one of them on-line if you know some useable values. If not, chalk it up to experience and bump the motor size. 
Example (click to link) 

But before you do that, what MDShunk said is also true, you may have a different belt material being used from what the original design was based on, or someone may have over tightened it, or a bearing may be going bad etc. etc. Eliminate all the easy things first.


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## Cram Renraff (Feb 10, 2012)

Well thanks everybody for your advice. The belting, I'm 99% certain, is the same as what was designed and might even be the original belt. The conveyor has a gravity counterweight so overtightening of the belt wouldn't be an issue but I'll check the bearings out. It's low speed, chain driven off of a gear reducer. I'll have to check the voltage under load as well. I did not consider that as a possible issue. We do have a sister facility a couple of miles down the road and they have a belt which operates under the same conditions as ours but theirs has a 25 hp motor.


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## micromind (Aug 11, 2007)

Another possibility here is to slow the belt down a bit. 

If the rest of the system is OK with it and it's easy to do, slowing it down will increase the motors effective torque delivered to the belt. 

A lot of conveyers are Vbelt driven from the motor to the gearbox. Simply change the motor sheave to a smaller one or the gearbox sheave to a larger one.


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## wdestar (Jul 19, 2008)

Horsepower = torque x speed

Assuming the speed that you're currently running the conveyor at is the minimum acceptable speed, you'll need more horsepower. Which translates to not only a bigger motor but also a bigger drive train and more money.


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## Cram Renraff (Feb 10, 2012)

Our motors are connected to the gearbox with a woods coupling and insert. Since the parent company has their own engineering department, the most I can really do is make them aware of what is going on and give a recommendation.


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## micromind (Aug 11, 2007)

One thing working in your favor here is that a standard NEMA frame 20 and 25HP motor share almost all the same dimensions. 

The shaft is the same diameter, length and height, and the mounting holes are the same width, but the back 25HP holes are further from the shaft end.

I can't remember though, when (or if) I've seen the larger base not drilled for both mounting dimensions. 

If you replace a standard NEMA frame 20HP motor with a 25, it'll fit.


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## John Valdes (May 17, 2007)

micromind said:


> One thing working in your favor here is that a standard NEMA frame 20 and 25HP motor share almost all the same dimensions.
> 
> The shaft is the same diameter, length and height, and the mounting holes are the same width, but the back 25HP holes are further from the shaft end.
> 
> ...


In most cases. Not every case. You are correct regarding those particular dimensions. But the "C" dimension and "O" dimension I believe are not NEMA requirements since Epact went into effect.

I have been bit one time on these two dimensions. Even motors on the same frame do not need to meet a standard for "C" or "O" dimension.
It can vary from manufacturer to manufacturer. Or even within the framework of one manufacturer.
This is direct result of Epact and the governments insistence that motors meet certain efficiency requirements.

In this case I would consult the manufacturers frame listing to see if indeed it will fit. 

Looking at Baldors 501 catalog (motor bible) I see the 20 Hp on a 256T frame, at 1800. And then I see the 25 Hp on a 284T frame at 1800 also.
But the "C" dimension (over all length) on the 20hp is 23.16" and on the 25hp it is 27.76.
As you can see the 25 hp will only work if he has 27.76" of clearance. If he does not have 27.76", this motor may not fit into the space.

The mounting holes and most everything else does align. But make sure all the dimensions are correct before you order anything.


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## Cram Renraff (Feb 10, 2012)

Turns out, I did convince the powers that be. They actually engineered the conveyor to be right on the edge for some reason but have given in and sent us a new motor and coupler. Thanks for the advice. Now I have another issue. We have a photo control on two signs and a 150 watt HPS fixture. All three loads work fine but the photo control keeps burning up and shorting the contacts out. In less than a day of the lights being on. The circuit is 277 and for about 10 feet, they ran another 277 line (for the office lighting) in the same conduit before they hit a box and split. That circuit is inducing about 12 volts into my circuit. The loads together are only pulling 1.9 amps. The photo control is rated at 15 amps tungsten and 8 amps ballast. I have never heard either of those terms before (in 29 years as an electrical mechanic). What do they mean in real terms? I also read, with the load side pulled off of the breaker, 0 ohms to ground on the load wire. That would ordinarily mean a short to ground but it also would ordinarily mean a tripped breaker, right? What am I missing?


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## piperunner (Aug 22, 2009)

Cram Renraff said:


> Turns out, I did convince the powers that be. They actually engineered the conveyor to be right on the edge for some reason but have given in and sent us a new motor and coupler. Thanks for the advice. Now I have another issue. We have a photo control on two signs and a 150 watt HPS fixture. All three loads work fine but the photo control keeps burning up and shorting the contacts out. In less than a day of the lights being on. The circuit is 277 and for about 10 feet, they ran another 277 line (for the office lighting) in the same conduit before they hit a box and split. That circuit is inducing about 12 volts into my circuit. The loads together are only pulling 1.9 amps. The photo control is rated at 15 amps tungsten and 8 amps ballast. I have never heard either of those terms before (in 29 years as an electrical mechanic). What do they mean in real terms? I also read, with the load side pulled off of the breaker, 0 ohms to ground on the load wire. That would ordinarily mean a short to ground but it also would ordinarily mean a tripped breaker, right? What am I missing?


Wow 15 amps tungsten and 8 amps ballast ive heard the terms but i think 
you can start by explaining how you came up with 12 v induced into that circuit first. You say 29 years never heard these before well ive never heard these used in this manner before . Explain in more detail ?


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## Cram Renraff (Feb 10, 2012)

I assume the induced voltage is because they are different phases. I looked up the tungsten and ballast references and the person said the tungsten was for tungsten lamps. No explanation for ballast. That is what the product literature give for a load rating though. I will add that the original control I took out showed no damage but the one I just took out looked like there had been an short across the sensor elements. They were were all black and sooty, showing the signs of an arc. I haven't looked at the last one yet. Kind of tired of climbing the ladder to the roof without an answer.


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## Cram Renraff (Feb 10, 2012)

what I mean about the 12 volts is with this circuit breaker off, I still read 12 volts. When I turn the other breaker off, it disappears. I know that I am not going to get an accurate resistance with voltage on the line and because I work nights and the other circuit is the lights for the area, I can't really turn that breaker off.


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## Blue (Nov 28, 2010)

Cram Renraff said:


> I assume the induced voltage is because they are different phases. I looked up the tungsten and ballast references and the person said the tungsten was for tungsten lamps. No explanation for ballast. That is what the product literature give for a load rating though. I will add that the original control I took out showed no damage but the one I just took out looked like there had been an short across the sensor elements. They were were all black and sooty, showing the signs of an arc. I haven't looked at the last one yet. Kind of tired of climbing the ladder to the roof without an answer.


Does the photocell have load on it ? Or are you using it to close a lighting contactor?


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## Cram Renraff (Feb 10, 2012)

it directly feeds the load, two fluorescent lit signs and a 150 watt HPS fixture.


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## mbednarik (Oct 10, 2011)

I am assuming you have a 277 volt photo eye, not just a 120. It stands for 15 amp of resistive incandescent load or 8 amps of ballasted load. Every photoeye has these ratings. What is the load of the circuit when you bypass the photoeye? Some of those fluorescent signs draw more than you think, your photoeye is limited to 8 amps (ballast).


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## Cram Renraff (Feb 10, 2012)

I haven't read it without the photoeye yet. That'll have to wait until Monday along with a another resistance reading with the other circuit off.


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## Blue (Nov 28, 2010)

mbednarik said:


> I am assuming you have a 277 volt photo eye, not just a 120. It stands for 15 amp of resistive incandescent load or 8 amps of ballasted load. Every photoeye has these ratings. What is the load of the circuit when you bypass the photoeye? Some of those fluorescent signs draw more than you think, your photoeye is limited to 8 amps (ballast).


Yes. I would take an amp draw and proceed from there. Also, I have gotten bad photo cells from the supply house. This is also a possibility.


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## Cram Renraff (Feb 10, 2012)

yeah, I've gotten bad ones too and I don't remember the second one actually testing ok but the third definately tested ok but didn't shut the lights off the next day.


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