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Energy Efficiency Calculations

03/15/2017 7:21 AM

Dear sir,

In our cement plant,We are having a RABH fan for which we connected a 730KW motor.The details of the motor are Make:Crompton Greaves Limited. KW:730. Volts:690V.Rotor Amps:0.Rotor Voltage:0. RPM:990. Amps:730A Efficiency:95.8%.Frame:BTPC500F .Year of Mfg.03/2009.

This motor is connected to a ABB VFD.ACS800-04-1060-7. 730KW.

The power is fed to VFD through a transformer.The details of the transformer are Make:PETE Transformers.KVA:1000.HV voltage:6600.LV1 voltage:690V.LV2 Voltage:690V.HV Amps:87.5A.LV1 Amps:418.4 Amps.LV2 Amps:418.4A, 3Phase,50Hz,Connection Symbol:DYN11d).Impedance:4.48We have measured KW at different points.KW at HT braker of Transformer:726.2 KW.KW at the input to VFD:695KW.KW at the output of the VFD:647.65KW.Motor RPM at that time=925.8RPM.We want to increase motor speed further ,at least to 960-970 RPM.But,we are not sure whether motor withstands or not.The power meter at the transformer incoming is showing 726.2 KW.Please help me as my production can be improved if we can run the motor at higher speeds. Thanks,NVRSrinivas

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#1

Re: Energy Efficiency Calculations

03/15/2017 9:18 AM

Generally, the power consumed by any air-mover is proportional to about the cube of the fan blade tip speed. So, (970/926)3 = 115% or so, multiplied by 726kW gives 834kW at the transformer and multiplied by 648kW gives 745kW at the motor. Therefore at first glance, the motor would be overloaded at the highest speed:

  • Provided the overload protective device(s) is/are set correctly, the motor would be protected against overload. However, it may be inconvenient for the motor to have tripped inadvertently while in production. Raising the overload trip setting to obviate it is not recommended at it may present hazards to the equipment, the operation and the people associated with it, with fire and catastrophic self-disassembly of the equipment being among the possibilities.
  • Running at these powers might present overload to the motor circuit cabling, the detail of which has been withheld from the forum and needs to be checked. Fire risk is the obvious candidate.
  • Running at these powers might present overload to the transformer, the detail of which has been withheld from the forum and needs to be checked. Fire risk is the obvious candidate.

<...production can be improved if...motor at higher speeds...> There must be other ways as well, and a brainstorming session might flush out these other opportunities, some of which might obviate any need to change this particular part of the equipment.

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#7
In reply to #1

Re: Energy Efficiency Calculations

03/16/2017 4:18 PM

But 647.85kW is the power input to the motor. The shaft power output is 647.65 x motor efficiency 95.8% = 620.4kW.

620.4*115% = 714kW estimated fan input power at 970rpm. The motor is rated 730kW shaft power at 990rpm, and at 970rpm it will do 730*970/990 = 715kW. So it looks just about doable, but I’d keep a close eye on it.

The fan supplier’s data for kW at various speeds would be of interest, to compare with the measured figures.

Also worth checking that the motor is designed for use with a VFD.

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#8
In reply to #7

Re: Energy Efficiency Calculations

03/16/2017 4:48 PM

Didn't this guy have another thread right after that clearly stated they burned up the motor?

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#9
In reply to #8

Re: Energy Efficiency Calculations

03/16/2017 4:57 PM

That was a different motor. I couldn't be bothered to read the full story. It doesn't affect anything I said in #7.

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#12
In reply to #7

Re: Energy Efficiency Calculations

03/22/2017 6:33 AM

Dear sir,Thanks for your reply.I like to add some more information and request your advise.We have increased the motor speed to 945RPM and we have taken different readings.The details are as follows.Power measurement at transformer primary is 759 KW.Power measured at the input of the VFD is 730 KW.Power measured at VFD output is 672 KW.Considering the efficiency of the motor as 95.8,as given on the motor name plate,the power delivered by the motor is 640 KW.I have noted the following parameters from the VFD.

Speed of the motor:945 RPM

Frequency:47.58 Hz

Current:750 A

Power:672 KW

Torque:95.41%

Since the motor delivered power output is only 640 KW against the name plate of 730 KW,I am of the opinion that motor can be loaded further.
kindly give your opinion.

The next point is motor current is shown on VFD as 750Amps,but motor name plate amps is only 730 Amps.I am of the opinion,since it is driven by VFD at reduced frequency,the current measurement of 750 Amps is not correct.Generally the current measurement is calculated for 50 Hz frequency. Kindly give your advise.

Thanks and Regards,

NVRSrinivas

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#13
In reply to #12

Re: Energy Efficiency Calculations

03/22/2017 11:59 AM

Apart from the 750 amps, it looks OK for further increase. As my #7, 970 rpm could be near the limit. But you can’t compare the 640 kW directly with the name plate 730 kW. Available motor output power (and voltage) vary directly with speed (torque and current constant) up to 50Hz, so at 945rpm power = 730*945/990 = 697kW (slip doesn’t vary much with speed, so cancels). Still OK, but by a smaller margin. Above 50Hz, power, amps and volts are constant, torque decreases.

Don’t confuse available motor output power varying directly with speed, with the fan shaft power varying as speed3.

But the 750 amps at 645rpm does look high. The motor nameplate data gives calculated PF 730kW/(730amps*0.69kV*95.8%eff'y*√3) = 0.87. New measurements show PF ~ 0.79.

Calculated fan power at 645rpm = 620.4*(645/625.8)3 = 660kW in reasonable agreement with your 672kW.

Presumably there were no signs of distress at 645rpm? If you want to try higher speeds, I would do it in easy stages and monitor things carefully. And have a chat with the motor and VFD suppliers, to get their opinion.

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#2

Re: Energy Efficiency Calculations

03/15/2017 10:35 AM

Has this motor ever been used without VFD? How was the starter configuration?

This is a substantial motor, 'twould be a shame to let the smoke issue forth from it.

Is it really the motor, or the VFD that is limited out at the present time?

Is there no opportunity to install a speed up gear on the output shaft?

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#3
In reply to #2

Re: Energy Efficiency Calculations

03/15/2017 11:42 AM

Ahem - <...a speed up gear on the output shaft...> would present the motor with a similar overload condition than were the inverter run at the highest speed, similar to #1⇑, as it is the fan tip speed and its rough cube that determines the motor power. A <...speed up gear...> is therefore not the solution to the original poster's quest.

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#4
In reply to #3

Re: Energy Efficiency Calculations

03/15/2017 12:18 PM

OK, OK, air ball on Mr. Stewart.

It seems in his next thread, they already burned up the motor, or VFD, or both.

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#5

Re: Energy Efficiency Calculations

03/15/2017 1:49 PM

The general rule I have always used on electric motor responding is that as long as the new settings is not running it beyond the motors rated amps, service factor and thermal limits life is good.

Until you either electrically blow through the motor windings insulation due to over voltage or the thing flies apart from over speed.

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#6
In reply to #5

Re: Energy Efficiency Calculations

03/15/2017 8:18 PM

Just a side note to that post; motor mfrs will tell you that if using a VFD, motors that HAD a Service Factor must be used at a SF of 1.0. The added heat from the PWM output essentially "consumes" the SF, (if there was one).

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#10

Re: Energy Efficiency Calculations

03/17/2017 2:46 AM

Best way is to take VFD at Local mode and start motor at set point greater than 925 rpm ( keeping air loop same i-e position of dampers). May be first time at 935 rpm and if power is in limits, increase it to 945 rpm and so on.

All related calculation were already shared

http://cr4.globalspec.com/thread/108421#comment1163500

By the way it is Kiln ID fan motor, Cement mill fan motor etc ?

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#11
In reply to #10

Re: Energy Efficiency Calculations

03/17/2017 8:04 AM

I hadn't noticed those calculations from last August, but they incorporate the same mistake as #1 - they relate to motor electrical input power, not output (shaft) power.

I don't know why you bring torque into it explicitly. For a squared-torque drive eg a fan as in this case, absorbed power varies as speed3. Gives the answer directly.

And you say When Speed increased from 926 RPM to 970 RPM, Torque also increases. No! Ignoring very low Hz conditions, up to 50Hz torque is constant, power proportional to speed. Above 50Hz, power is constant, torque decreases. (can't see where he gives supply frequency, but it's almost certainly a 6-pole 50Hz motor for 990rpm rated speed).

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Aghvel Niazi (1); Codemaster (4); James Stewart (3); JRaef (1); nvrsrinivas (1); PWSlack (2); tcmtech (1)

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