1/2 HP Motor Overload Tripping

[Little Bill]

Had a fellow electrician call me about a problem with a restaurant exhaust fan motor. From what he said the original motor quit. It was a 1/2 HP variable speed motor without cap. start. He (owner) replaced the motor with one he bought at a farm store (Name Brand). It was a single speed 1/2 HP cap. start.
Also, the motor/fan are belt driven.

My friend said it would run for about 15 minuites then the thermal overload would trip. The owner gets another motor and it does the same. Also he said they bypassed the variable speed switch.

I told him the motor was not the right one for the fan and probably not a continious run motor. I think the motor was pulling too many amps from the load and over heating. I told him he would probably have to order the same motor as the original.

Does this sound right with the advise given considering the circumstances? Or is there a way to get the replacement motors to work for this?

 

[kwired]

Had a fellow electrician call me about a problem with a restaurant exhaust fan motor. From what he said the original motor quit. It was a 1/2 HP variable speed motor without cap. start. He (owner) replaced the motor with one he bought at a farm store (Name Brand). It was a single speed 1/2 HP cap. start.
Also, the motor/fan are belt driven.

My friend said it would run for about 15 minuites then the thermal overload would trip. The owner gets another motor and it does the same. Also he said they bypassed the variable speed switch.

I told him the motor was not the right one for the fan and probably not a continious run motor. I think the motor was pulling too many amps from the load and over heating. I told him he would probably have to order the same motor as the original.

Does this sound right with the advise given considering the circumstances? Or is there a way to get the replacement motors to work for this?

My first guess is the speed is higher than the original.

Many fan motors for this kind of application are often 1200 RPM and the common motors found at a place like you mention are 1800 RPM. The original was also probably a PSC motor which works with speed control but a capacitor start motor will not. But it is more unusual to see a PSC for a belt driven load. Anyway my point is check the speed, HP demand increases inversely with increase in speed for most blowers.

 

[Jraef]

My first guess is the speed is higher than the original.

Many fan motors for this kind of application are often 1200 RPM and the common motors found at a place like you mention are 1800 RPM. The original was also probably a PSC motor which works with speed control but a capacitor start motor will not. But it is more unusual to see a PSC for a belt driven load. Anyway my point is check the speed, HP demand increases inversely with increase in speed for most blowers.

I think maybe you meant exponentially. HP increases at the cube if the speed increase. But the reasoning is good and the first thing I thought of too.

Vent hoods often use 1200RPM motors so they don't make as much wind noise. If he replaced it with an off the shelf 1800RPM motor, that's 50% faster, so the motor load would increase by 1.5³ to 337.5% load, the motor would indeed trip off line, or start a fire if it didn't.

It could also be that the speed controller was set up to never allow it to run at full speed because they knew of the potential OL. When he bypassed it, the same issue happens.

It could also be that the motor base is slightly different and the center is off now, so the belt is too tight. Or similarly, he had that issue but it was too loose so he changed the sheave to make up for it, but that then changed the fan speed and increased the load.

 

[kwired]

I think maybe you meant exponentially. HP increases at the cube if the speed increase. But the reasoning is good and the first thing I thought of too.

Thanks that is what I intended.

 

[Besoeker]

I think maybe you meant exponentially. HP increases at the cube if the speed increase. .

Cube, yes. But not exponential.

 

[steve66]

Cube, yes. But not exponential.

A cube is an exponential. :? At least over here it is.

Edit: Or at least that's how I've always used the term. A number raised to a power is an exponential, and the power is the exponent.


Are you thining he meant the natural exponent?

 

[Besoeker]

A cube is an exponential. :? At least over here it is.

Edit: Or at least that's how I've always used the term. A number raised to a power is an exponential, and the power is the exponent.


Are you thining he meant the natural exponent?

Cube law is y=X³
Exponential is y=e^x

 

[Jraef]

Cube law is y=X³
Exponential is y=e^x

hmy:


Technically, you are correct. I have been caught as an unwitting victim of the MBS, the Mathematics Bureau of Semantics (or the Math BS for short).

But in my defense your honor, I was simply searching for a term similar to "inversely" that was closer to what I had assumed (correctly as it turned out) that he had intended to say. Please do not revoke my priveledges, I promise to be good.
:angel:

 

[Little Bill]

Thanks everyone that replied!:thumbsup:
I didn't get any further involved yesterday so I didn't know what they did to resolve the issue until today. I think he said the original motor was 1700 RPM. You guys were probably correct in that the speed control probably kept the fan from turning full RPMs.
My friend said they went and got a 1-1/2 HP motor and it didn't trip, but this is/was just a temp. replacement until he can either get another motor or have the old one repaired.

He did say the curtains were almost pulled off the wall from the faster turning motor/fan.:lol:

 

[Besoeker]

Technically, you are correct.

Actually, just correct..............
Slightly more seriously and a little off topic (be kind mods)...
I think it's an important distinction.
Exponential functions are important in electrical engineering as I'm sure you know.
As are polynomials.
But not the same.

I have been caught as an unwitting victim of the MBS, the Mathematics Bureau of Semantics (or the Math BS for short).

Nah - just the PROFs - pedantic recalcitrant old Fhaaarts.............

But in my defense your honor, I was simply searching for a term similar to "inversely" that was closer to what I had assumed (correctly as it turned out) that he had intended to say. Please do not revoke my priveledges, I promise to be good.
:angel:

I'm sure your postings will continue to be beyond reproach.