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VFD driven motor on Generator power mismatch

12/04/2021 11:16 PM

We have a 500 HP motor driven pump. Using a VFD powered from a 300Kw generator.

The pump is operating at the lower end of the operating curve at 38Hz. The drive is showing 150 A at 38HZ. (119Kw)

The generator has a Deep Sea engine controller which shows a load of 42 Kw and 15% load, (57A).

Why does the generator show such a difference in load from the VFD. If we calculated it by 3X (57A) we get closer (141Kw) but why doesn’t the generator controller show a load closer to the VFD value? The only other loads from the motor is a controller at about 30 watt.

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#1

Re: VFD driven motor on Generator power mismatch

12/05/2021 1:02 AM

..."In an inductive circuit, when frequency increases, the circuit current decreases and vice versa"...

..." in case of AC circuits:

I = V / Z

Where “total resistance of AC circuits = Impedance = Z = √ (R2 + (XL – XC2)”

In case of Inductive circuit:

  • Z = √ (R2 + XL2)
  • I = V / XL or I = V / Z

It shows that in inductive circuit, Current is inversely proportional to the inductance “L” as well as inductive reactance “XL” as inductance and inductive reactance are directly proportional to each others."...

https://www.electricaltechnology.org/2019/09/inductive-circuit-current-increases-frequency-decreases.html

http://www.davidsonsalesshop.com/catalog/files/Products/Deep%20Sea/DS%20Download%20Instructions.pdf

https://oaktrust.library.tamu.edu/handle/1969.1/192571?show=full

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#2
In reply to #1

Re: VFD driven motor on Generator power mismatch

12/05/2021 1:25 AM
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#3

Re: VFD driven motor on Generator power mismatch

12/05/2021 9:29 AM

There are two factors that may not be taken into account in your calculations.

Firstly, you can't multiply voltage by current to get power in an AC circuit with an inductive load. The current lags the voltage by some angle θ, and power equals voltage x current x cos(θ). The factor "cos(θ)" is the power factor.

Secondly, because the impedance of an inductive load decreases with decreasing frequency, a VFD reduces the output voltage at lower than rated frequency, as shown in the plot below, to prevent the load from drawing excessive current.

So in calculating the power output, the reduced voltage and power factor need to be taken into account.

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#4

Re: VFD driven motor on Generator power mismatch

12/05/2021 2:19 PM

They can theoretically both be correct under certain load conditions that from rough calculations I would say motor would be considerably overloaded (about max current al rouglly 2/3 full power rated RPM plus considerably worse air cooling) But in this specific case I would just trust the drive readings being correct (considering actual power factor on your working conditions) because between gen and drive chopping cirquitry, you have rectification bridge that makes the VFD input current a full-of-peaks mess of a waveshape that not all instruments can read correctly. If you are puzzled by the VFD current being higher than input, also understand that VFD also outputs lower voltage for lower RPM, not only lower frequency. Actualy programming VFD for pump applications, V/RPM out ratio is more or less linear. You can use an AC RMS clamp meter on VFD input to verify what I said. S.M.

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#5
In reply to #4

Re: VFD driven motor on Generator power mismatch

12/05/2021 3:23 PM

The VFD output voltage is lower- 460.

I know with all the harmonics generated some measurements are not accurate.

But the VFD rectifier should be the only load the generator sees so is that an inductive load? How does the generator see an inductive, ie. motor load?

I was thinking the work done by the drive output I*E*1.73/1000 would be the KW output (roughly) and the generator input would have to be similar (plus conversion losses)

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#7
In reply to #5

Re: VFD driven motor on Generator power mismatch

12/06/2021 3:47 PM

Well, the current the generator sees drawn is a bit more complicated than that. It is not exactly phase leading or lagging. (Still does not have a good power factor). On each phase and for simplicity reasons showing the positive part only, the gen sees something like the pic below. The current is the green wave, not a sine, not half a sine, nothing of the sort. It's width and peak value depends on gen output impedance, VFD input and in-between wiring impedance, and of course the VFD loading, i.e. how much the bulk capacitors (their voltage is the red line here) have discharged since the last cycle. You can only measure that current correctly with an RMS meter.

Now advanced vector control VFDs sense the motor current and load and for total power saving reasons at lower than expected load can lower their output voltage even lower than the initial V/F scheme and increase the output frequency a bit to compensate for the increased slip. This lowers the total consumed power somewhat, but unfortunatly this can increase motor copper losses. This looks like an oxymoron but it really isn't. The output current just increases more than the voltage lowers. Tricky eh? And all this behavior is on most times programmable. If it were that simple everybody would design VFD's, no? (LOL) Have fun. S.M.

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#6

Re: VFD driven motor on Generator power mismatch

12/05/2021 8:00 PM

The generator seems undersized for a 500hp motor....or the motor is substantially oversized.....is the pump operating properly? What is the gpm? What is the generator voltage?

https://www.eng-tips.com/viewthread.cfm?qid=448686

https://www.generatorsource.com/Power_Calculator.aspx

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#8
In reply to #6

Re: VFD driven motor on Generator power mismatch

12/07/2021 7:21 AM

Increasing the speed of the original poster's pump may well result in the generator tripping on overload, as the power drawn by a centrifugal pump varies by something like the cube of the tip speed of the impeller.

Just a thought.

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#9

Re: VFD driven motor on Generator power mismatch

12/07/2021 10:19 PM

The 119 KW is based on 460 volts to the motor 460*150*√3/1000=119 KW. At 38 Hz the voltage is about 460/60*38 = 291 V; 291*150*√3/1000 = 76 KW. The ratio of power in verses power out is about 55%. The power supplied by the generator is lower, in part, because the generator AC is converted to DC in the VFD and there are reactors between the VFD and generator to take into account. As the load gets closer to the full power the discrepancy will become less and less until the efficiency reaches the maximum.

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#10

Re: VFD driven motor on Generator power mismatch

12/08/2021 1:43 AM

VFDs have internal losses. When VFD is operated at a low load, the losses tend to be higher than those at rated VFD capacity in % terms. Generator has to cater to those VFD losses as well (apart from the motor power).

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#11

Re: VFD driven motor on Generator power mismatch

12/08/2021 2:59 PM

"The drive is showing 150 A at 38HZ. (119Kw)"

No, you are calculating that wrong. If the drive is at 38Hz, and assuming it is a centrifugal pump and the VFD is programmed for Variable Torque loads, the V/Hz ratio of the VFD output will be based on the SQUARE of the speed change. 38Hz out of 60Hz is 63.33% speed, so the voltage at the motor terminals is 40% (63.3 squared). Then assuming a motor nameplate of 460V, 40% makes it 184.5V at 38Hz. So 185V and 150A, assuming a .8PF = 38.45kW of shaft power. But from the generator side, it is going to have to add in some losses and at 38kW of load out of 373kW, the load is only 10% of the motor rating and the VFD throughput efficiency will be around 90%, so there you are at 42kW on the generator.

Totally normal.

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#12
In reply to #11

Re: VFD driven motor on Generator power mismatch

12/09/2021 10:52 PM

Thanks, that makes much more sense. This is a stacked horizontal centrifugal pump. First one I've messed with, but the vendor came out and verified we were running on the pump curve.

So is that rule of the VFD output being the square of the speed change apply to Variable Torque applications or just a function of the V/Hz ratio?

We have noticed that we have to keep the speed below 40 Hz to avoid high vibration and we load the generator to 130 Kw (Drive showing 39 Hz at 209 A)

Thanks everyone, this was a great learning experience. Even the generator folks couldn't explain this, though they did admit it was something going on inside the drive.

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#13
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Re: VFD driven motor on Generator power mismatch

12/28/2021 6:19 PM

So is that rule of the VFD output being the square of the speed change apply to Variable Torque applications or just a function of the V/Hz ratio?

It is based on the LOAD being Variable Torque. You tell the VFD it is a VT load, so the VFD uses a V/Hz2 curve in order to conserve energy.

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