# Frequency Vs Max Load current (Motor)

## Main Question or Discussion Point

Have a 15 HP motor whose full load current (@ 50 Hz - India norms) is calculated at around 21 Amps. Have added a VFD (Variable Frequency drive) and would like to always run the machine @ 90% of full load current for maximum efficiency.

Can anyone assist in helping me calculating full load current as frequency is changed? What would be the full load current @ 40 Hz, 35 Hz and 45 Hz?

Thanking everyone in advance. Appreciate the help, time and effort.

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NascentOxygen
Staff Emeritus
This is a 3ɸ induction motor, is it? I looked over a few articles, and it seems it's not particularly straight-forward. For example, are you intending that the motor start on a reduced frequency under full load?

Generally, where variable frequency control is used to slow a motor, it is also necessary to reduce the voltage proportionately to avoid magnetic saturation, increased losses, and aggravating high pitched noise from the motor. See http://www.most.gov.mm/techuni/media/EP05028_6.pdf [Broken]

My reading of this is that as you lower the frequency, you must simultaneously reduce the voltage so as to maintain the motor's current constant at its rated value (or, in your case, at 90% of that value).

(Should you intend increasing the frequency to speed the motor up, this is going to be a different kettle of fish.)

I'll monitor this thread to see whether someone with first-hand experience can help.

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russ_watters
Mentor
Full load current is full load current. It is a constant, based on the physical properties of the motor.

all i want to ensure is i do not lose my blast wheel efficiency which is being run by the motor. All experts recommend running the wheel @90% to get max utilization.

Blasting is controlled by amperes - with 50 HZ i can get 19-20 Amps easy (90% of 21 A) but when i bring the freq. down using VFD to 35 Hz it is hard to get my amps back to 19-20 Amps.....not much of a electrical guy but was thinking there should be some relation.

Am afraid to push the amps back to 19 Amps @ 35 Hz might overload the motor.....

Just trying to explain my reasoning...then there are the experts who know much better...

NascentOxygen
Staff Emeritus
Is this a 3ɸ induction motor?

russ_watters
Mentor
Yes, reducing the frequency will reduce the amperage because you're reducing the mechanical power output.

ty russ......i believe the same too....how would we calculate what reduction is achieved though?

If your load torque is constant, then reduction in speed (w) results in reduction in output power (P = T*W), almost linearly with speed. So, If you did keep the Voltage constant, then current would reduce linearly as well. (Not quite linearly, but its cos(phi) component should reduce linearly with speed).
However, like in most VFD, if voltage is decreased with frequency (speed), then it appears that the current shouldn't decrease but remain constant.
abhipatel said:
Am afraid to push the amps back to 19 Amps @ 35 Hz might overload the motor.....
Do you have a separate control of frequency and Voltage or Are they linked?
How exactly do you plan to 'push the amps back @ 35Hz' ?

russ_watters
Mentor
ty russ......i believe the same too....how would we calculate what reduction is achieved though?
It depends entirely on the nature of the load. What is the load?

NascentOxygen
Staff Emeritus
Generally, where variable frequency control is used to slow a motor, it is also necessary to reduce the voltage proportionately to avoid magnetic saturation, increased losses, and aggravating high pitched noise from the motor. See http://www.most.gov.mm/techuni/media/EP05028_6.pdf [Broken]

My reading of this is that as you lower the frequency, you must simultaneously reduce the voltage so as to maintain the motor's current constant at its rated value (or, in your case, at 90% of that value).
Only just noticed the reference I included above is nothing like I intended, seems I must have clicked on the wrong link. Here is a better reference, the torque graphs showing voltage being reduced proportionately as frequency is reduced, in order to maintain flux at design value.

http://itee.uq.edu.au/~mmme2104/Lecture%206a%20-%20VVVF%20IM%20Speed%20Control.pdf [Broken]

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jim hardy
Gold Member
2019 Award
Dearly Missed
I think that Nascent and Russ have nailed it. That little tutorial on VFD was great, by the way......

If i understand what is a "blast wheel", it is akin to a centrifugal pump but it's slinging a granulated abrasive instead of a fluid like water or air.

Do a search on "Fan Laws" and you'll find that the power to drive any pump varies with the square or cube of the speed depending whether mass flow rate changes.....

so - at reduced speed, your V/F control keeps the motor's magnetizing current nearly constant
but the load current due to motor's mechanical load depends on the mechanical work being drawn by the blast wheel.
You may not be able to force full load current into that motor if it is lightly loaded because of the reduced speed.
Can I assume the abrasive's mass flow rate drops off at lower speed? Certainly its exit velocity would, hence power drawn from motor is also less .

Try controlling current by adjusting the abrasive feed rate(which controls mechanical work being done) instead of adjusting voltage( which just controls flux inside the motor).

all - is my thinking straight here? did i address right question?

anyhow if you force extra current into a lightly loaded motor by raising voltage, you make extra magnetic flux which overheats the iron, and in large machines can melt the corners off your stator laminations. Try a search on "fringing flux".
Forcing current up to 21 amps (rms not average) shouldn't hurt the copper provided the cooling fan is sized for lower speed operation, but you may well hurt the iron.
So Recall Thoreau's "Different Drummer". Let the motor draw just whatever current it needs to provide the required mechanical work.

old jim

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jim hardy