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jacojan
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Join Date: Feb 2008
Posts: 36

VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 2:15 AM

Hello !

My previous workplace we had kilns driven by 22kW, 1450rpm 3ph induction motor with eddy current coupler to reduce/control speed. But this drive system has used for charcoal activation where charcoal density is lower than some other materials. So it has monitored that even at kiln's full load condition, 22kW motor has only consumed 48 % of its full capacity. Therefore using an oversized motor could lead to unnecessary losses and power factor could drop and could also lead rise reactive power. Therefore it has recommended using of 11 kW motor in replacement of 22kW motor, after carry out trial with 11 kW & 7.5 kW motor. It has monitored 11kW motor operated at 60% of it's full capacity.

Further more, for the same application, instead of using eddy current coupler for speed controlling/ I did trial with a VFD. So the motor speed reduced via VFD to get the required rpm of the kiln. That mean VFD Frequency set to 15 Hz. Then the power consumption became 2.7kW @ 15Hz. And earlier it consumed 5.6kW with motor running at full speed (50Hz) and speed controlled through eddy current coupler.

Please comment with your expertise engineering knowledge about my trial.

Can I take 15 Hz = 435 rpm as 50 Hz = 1450 rpm ? Or how should I calculate ?

Is Affinity law P1/P2= (n1/n2)3 valid for above trial ? Already try seems not matching with trial result.

Highly appreciate your inputs.

Thanks in advance.
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Yahlasit
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Location: Cd. Juarez, Chihuahua, Mexico.
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#1

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 8:20 AM

Can I take 15 Hz = 435 rpm as 50 Hz = 1450 rpm ? Or how should I calculate ?

RPM= Drive frequency by 120 over number of motor poles:

RPM= (drive freq. * 120)/motor poles

Example 1 RPM= (15 * 120)/4 = 450

Example 2 RPM= (50 * 120)/4 = 1500

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jacojan
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Join Date: Feb 2008
Posts: 36
#4
In reply to #1

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 10:54 PM

Thank you for the reply. Even voltage and frequency both varying by VFD (v/f) therefore can I apply the above formula. ?
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Tornado
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Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

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#2

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 2:31 PM

The given affinity law applies to fans and centrifugal pumps (variable torque), but your numbers sound more like a conveyor or drum (constant torque).

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jacojan
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Join Date: Feb 2008
Posts: 36
#5
In reply to #2

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 11:07 PM

Thank you Tornado. Yes you are correct it's like drum. This is a rotary kiln used in charcoal activation process. I am writing a technical report, so how can I technically explain my trial. I have to explain how speed reduction of the motor reduced energy consumption.

Thank you.
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Wal
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Join Date: Jun 2011
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#6
In reply to #5

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 11:56 PM

The audience of the report are most likely been counters and don't care why it saves energy.

Collect and tabulate data.

Energy consumption before V energy consumption after.

That they will understand. They don't care why.

If they ask why it wasn't done more efficiently in the first instance tell them it is now 2015 and way back whenever it was done the way it was done was because that's how it was done back then.

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RAMConsult
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#3

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/28/2015 3:18 PM

The thing that you have ignored is the application of the affinity laws to the internal cooling of the motor. The flow rate is reduced by 70% and the pressure to push the air through the motor is reduced by 91%.

Since you have determined that the power consumption was reduced by 52%, but your fan is providing only 30% of its previous air flow, how are you going to ensure that you can remove all the excess heat from the motor? Proper heat removal at reduced speeds is one of those things that allows a motor to be listed for "inverter duty" on the nameplate.

Get out your infrared thermometer and start looking for hotspots around the motor housing, and let us know what you find.

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jacojan
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Join Date: Feb 2008
Posts: 36
#7
In reply to #3

Re: VFD Control Motor Energy Consumption vs Motor Speed

05/30/2015 12:51 PM

Thanks for reply. Additional cooling fan has attached to the motor for cooling. And this is not a fan application. The motor has used to drive rotary kiln (a drum) that is why I need to know, what formula I must use for calculations, since it is not a fan or pump I cannot apply Affinity Law.
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