Different Horsepower, Same Load

[emoltzan]

Scenario: We have 2 motors, different horsepower, driving a vacuum pump that is loaded the same.
-Motor #1 is a 3phase, 460V, 60hp motor
-Motor #2 is a 3phase, 460V, 75hp motor

Motor #1 runs at about 66 amps and then climbs to 80 amps and eventually to 100 amps then the overloads trip out, as expected.
Motor #2 runs at about 66 amps and then climbs to 80-85 amps and does not trip out, as expected.

My main question is; with 2 different sized hp motors on equal loads, should we expect to see similar or different amp draw and why?

-Thanks

 

[waross]

The first order of business is to determine the reason for the gradual increase in current or load. It could be from either direction: load increasing or grid voltage dropping.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Late to the thread . . .

What Bill just said.

The way I would have posed the question is to go back to first principles:

Motor #1 runs at about 66 amps and then climbs to 80 amps and eventually to 100 amps then the overloads trip out, as expected.

Why, oh why, does this happen? Has there been any attempt to throttle the pump discharge so as to drill down to whether there is [a] a pump problem or a motor problem, or [c] an operational problem where something the system was not designed to do is being attempted.

Is this a new problem?

Has anything been modified recently?

Has any maintenance/cleaning been improperly / incorrectly performed?

Are these liquid ring pumps? Losing water will increase the displacement and increase the motor load. Are you adding enough makeup water?
It may be a pump/operation issue rather than a motor issue.

The OP has never answered this question . . .




CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[emoltzan]

My main question is; with 2 different sized hp motors on equal loads, should we expect to see similar or different amp draw and why?

I'm not asking whether it's a pump or motor issue.

 

[crshears]

Fair enough.

That being the case:

My main question is; with 2 different sized hp motors on equal loads, should we expect to see similar or different amp draw and why?

In reading through this thread, I believe your main question has been satisfactorily answered a few different ways.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

@OP

Your main question has been answered here by many by questioning your premise of equal loads.

Muthu

http://www.edison.co.in

 

[petronila]

Hi emoltzan,

"My main question is; with 2 different sized hp motors on equal loads, should we expect to see similar or different amp draw and why?"

The answer is on the pump efficiency. For pumps, the hp depends on the pump efficiency, GPM (or PSI), ft(head), and Specific gravity. If both pumps are pumping the same fluid and nearly GPM (equal loads), and the efficiency of the pumps are pretty close then will need the same hp and the motors will be pulling nearly amps. For your example or case, assuming 2 pole motors, rated amps will be around 72 A (60 hp) and 90 A (75 hp) which explains why the motors are performing well without peak loads, and drawing 66 Amps. For the 75 hp motor, the 90 rated Amps will allow going up to 90 Amps so won´t trip. However, if you are experiencing peak loads in the 60 hp motor I believe the actual motor is rated with some Services Factor (1.1 or 1.15) that will help to handle temporary overloading but this motor would not last as expected because the insulation life will be degraded by the overheating due to overloading thus this motor is prone to fail quickly. If so, my recommendation is to upgrade that motor to 75 hp-SF 1.15.

Best Regards,

Petronila

 

[eeprom]

This is normal.

If you look at the current/speed curve of an induction motor, the current goes up quickly as the speed slows (slip increases) down due to load. It is a very sharp increase towards the breakdown torque. If you overload the motor with more load than it can handle, the slip increases and the current spikes. That's what you are seeing on the smaller motor.