Is FLC for motor the same with or without VFD?

[skbutler]

I'm a boiler guy and not really up on VFDs.
I have a 3 HP standard induction motor driving a combustion air fan powered by a VFD. (This is all new equipment.) The VFD is powered by 208 Vac 1ph and the motor is 208 Vac 3ph. The name plate FLC of the motor is 8.6 amps. The VFD is programmed to ramp up to speed in about 30 seconds. Putting my Fluke on the motor leads shows the motor drawing around 8.5 amp when the frequency reaches 52 Hz and 14-a5 amps when the frequency reaches 60 Hz. I've been afraid to run the motor longer than about 45 seconds at full speed for fear of damaging the motor but after 45 seconds at 60 Hz I don't see the current going down. is this normal?

Thanks
Steve

 

[itsmoked]

It is your meter lying to you. The waveform out of a VFD is very complex and your Fluke is misinterpreting it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[CJCPE]

If the motor is sized to reach full load at 60 Hz, it should draw about 6-1/2 to 7+ amps at 52 Hz and 8.6 amps at 60 Hz. In addition to a problem with the ammeter, you might consider:

1. If the VFD is not designed for single phase input, the DC bus capacitance might not be adequate to maintain proper voltage at full load.

2. The motor might be under sized.

3. The belt ratio between the motor and fan might be wrong.

4. The motor might not be connected for the right voltage.

5. The line voltage might be extremely low.

 

[skbutler]

Wow, thanks for the quick response. Let me add the following.

This is a new boiler from a Canadian company and I have not worked on their equip,ment before but they have a good reputation. The VFD is properly sized and the supply voltage is spot on the spec. The single phase voltage to the VFD does not drop while the motor goes to full speed and neither does the three phase voltage from the VFD to the motor.

The motor is rated at 3450 rpm and the fan impeller is mounted directly to the motor shaft. The motor is properly sized.

Steve

 

[DickDV]

Leave your tools in your toolbox and let the drive tell you what's going on out there on the motor leads.

You will be able to see output amps, output frequency, and output voltage on the keypad. These numbers are trustworthy, the test equipment data is usually not.

 

[waross]

If you have air control dampers on the fan, they may be opened too wide. This is in addition to the other good suggestions.
respectfully.

 

[machmech]

Hello

Here is a brief explanation of drives and how they operate.

http://en.wikipedia.org/wiki/Variable_Frequency_Drive#See_also

Chuck

 

[ScottyUK]

steve,

The comment about DC voltage ripple may still be valid event though your 208V is correct. In many drives the capacitors on the DC link are sized to reduce the ripple from the output of a three phase rectifier. The ripple from a single phase rectfier, as you have, is significantly higher. Is this drive specifcally designed for single phase supply, or is it a three phase input type being supplied by a single phase supply?


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[jraef]

At 3HP the chances are that the VFD IS capable of 1 phase input. Probably 90% of VFDs in that size and voltage range sold in the US are.

As itsmoked said at the outset, don't trust your Fluke on the output current of a VFD, it is almost impossible to measure correctly that way. VFDs (typically) use Hall Effect transducers internally along with a complex algorithm to determine the output current and heating effect that changes with frequency. Because the VFDs must now also contain motor thermal overload protection, this measurement is highly tested and verified by UL before the VFD can be listed. I would trust it better than any external device. Just make sure that your motor nameplate information is correctly entered into the VFD programming, then forget the external verification.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

I am not so sure that the Fluke misinterprets the motor current. John Fluke Mfg Co has been very careful and made some real good instruments that tell the truth, even when measuring the output from a frequency inverter. But, not all Flukes are created equal, and some are notorious liars.

The fact that you measure correct current at 52 Hz and a lot more at 60 Hz implies, at least to me, that there may be a problem with the base speed setting. If it has been set to 50 Hz (instead of 60 Hz) you will have constant voltage above 50 Hz. I.e. a very low V/Hz ratio at 60 Hz. That increases slip enormously and your motor will get hot very quickly.

But that wouldn't explain the high current drawn. Anyhow, I would check that setting. Export is export and the manufacturer (FW?) may have thought it was for Europe. ;-)

Gunnar Englund

http://www.gke.org

 

[jraef]

OK skbutler, I'll concede to Gunnar that Fluke makes meters that ARE capable of reading correctly, but you must know when purchasing it that this is what you want.

First off it must be a "True RMS" reading meter, AND the True RMS must apply to current, not just voltage as some are. For instance, if you buy a DMM that says TRMS, but is requires a clamp-on probe for current measurement, that current input will not be TRMS unless it specifically says so. A classic example of that is the ubiquitous Fluke 87 (now the 787) which says True RMS all over the box and packaging. That is TRMS for voltage only. The other most common Fluke meter is the 73/77 series, and neither of those are TRMS in any way. Here is a paper from Fluke on the difference of TRMS or Average reading meters. Especially helpful is the chart at the end that gives you an idea of how the readings may differ (Note: the automatic link code won't work on this URL because of character conflicts so you will have to copy and paste the entire line, then click on "Why True RMS").

http://us.fluke.com/usen/support/ap...ukeProducts)&parent=APP_NOTES(FlukeProducts)#

Second, the meter must specifically say is is rated for use on PWM output circuits, because the extreme noise can cause interference on the measurement. There is another Fluke paper on that subject at the above site called "Electrical Measurements on Adjustable Speed Drives". It might be noted that only their most expensive meters are discussed here.


Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[bigbillnky]

Hello skbutler

My suggestion would be to check shaft speed at 60hz(a high quality stroboscope is the best, most reliable means of checking speed that I have found). Compare this reading to motor nameplate speed. It is also best to measure the voltage, current, and frequency readings at the same time(it isn't real easy to do simultaneously). Slower than nameplate shaft speed= higher than nameplate current draw at rated voltage and frequency. Nameplate speed= nameplate current at rated voltage and frequency.

You could also try to bypass the drive and connect across the line using a standard starter to see what effect it has on the motor current.

You may simply have an overloaded motor. Blowers loads vary with shaft speed. Higher speed= higher load current.



Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[itsmoked]

skbutler; What does the VFD display for current at 60Hz?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[aolalde]

Hi skbutler.

Check the line voltage feeding the motor. At 52 HZ it should be 180 volts and at 60 HZ 208 Volts. If the voltage is larger the motor magnetic circuit saturates increasing the line current for a given load.

 

[sck25jr]

i think i have seen this before with 2x normal current, with ABB drives the default motor rpm parameter is 1800rpm. you should check to see if it has been correctly entered at 3450. also ABB derates their drives by 50% when using single phase input.