Help with Motor data to program a VFD

skyfox

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Nov 2006
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Trying to get a piece of equipment put back in to service. OLD VFD from this system has been scavenged so trying to add a new one to get it going again. Looking at what limited operator instructions that are available, it had two speed set points of 3,000 and 6,000 RPM's. (I am guessing 50hz and ~100Hz). Unit is only cycled for short periods of time. Short as in less than 5 to 10 minutes each hour. I need help in figuring which set of parameters to enter in to the VFD. Do I go with the 110Hz, 6600 RPM and 13KW values and expect the FLA to be around 27A?



Also, am I correct in my assumption that this is a 2 Pole motor based on 2,920 RPM value at 400VAC 50Hz?


Thanks.

NAME PLATE.jpg
 
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Not 100% sure but i read that plate as:

400v 50Hz 13.5A(at 400v) 2920Rpm 7.5kw.

If i was programming a VFD to run it I would be sizing it at the 13kw 27A end of the scale but setting the motor nameplate data to match the above. But allowing the drives max frequency to be 100Hz expecting that the current would be Circa 27A. Depending on if you need the faster speed with it only being a rcirc pump?

But then the 87Hz number rang a bell, a quick goole found me this.

https://inverterdrive.com/HowTo/Increase-Motor-Power-Output-and-Speed-by-73-delta/

If any of that is of any help i don't know, And i'm sure there will be someone who knows a lot better than me.
 
When running above base frequency at the maximum available voltage, the flux strength is weakening, so the current will actually go down as you lose torque. The motor is running at a constant power, not constant torque any more.

You always program the drive for the motor nameplate data, don’t play with that because that is what sets up the protection of the motor.
 
Thank you smiley125 and jaref,


Followed your advice and went ahead and used the name plate data for 400/50Hz operation. All's well during testing carried out over last 4 days and customer is happy with the end results while not exceeding the 50Hz mark. However, I am still curious about this picture that was in the old operator instructions manual, so I took a scanned picture of it myself. This is where I get confused with this V/Hz thing. Looking at the picture below (6,000 RPM at 99.9Hz), it is clear we no longer have the V/Hz ratio we programmed the VFD for. Also note, the current draw is 11.95A at 400VAC at 99.9 Hz and at 6,000 RPM. About 1.5 AMPS below that of name plate FLA for 400Vac 50Hz at ~3,000 RPM. Still trying to clear the cob webs in my head on this last part. Any thoughts on how they were able to achieve this without generating smoke?

Thanks again to both of you for your help.

Cheers.

99.9 Hz 6,000 RPM.PNG
 
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Torque required is determined by load, torque produced is going to be synonymous with current up to and including the output power (HP or kW) of the machine. So once you get to the maximum V/Hz ratio and have reached the maximum torque capability but continue to increase the frequency, your power remains flat from that point on and your current will as well; it's just that the torque PRODUCED by that current will start to drop.


When I first answered I was not able to see that nameplate because the image was being blocked by my firewall for some reason. Now that I can, I see another aspect of it. The line below the Voltage and Current data, where is says "400V D 87Hz" is indicative of how they may have been using it.


A "trick of the trade" in VFD implementation that can get you around the limits of the V/Hz ratio issue is to use a dual voltage motor, strap it for the LOWER voltage, and connect it to the HIGHER voltage. So in this case the motor connections are Delta (D) for 230V, Star (Wye) for 400V normal use. But if you connect it in Delta and they SUPPLY it with 400V, then program the VFD to reach 50Hz at 230V, you can continue to increase the Hz and Volts together, up to 87Hz (400/230 = 1.73, x 50Hz = 87Hz). This is what the next line below that is indicating; that if you connect it in Delta, the speed range is 10-87Hz with full torque capability (they are indicating that you should not operate that motor below 10Hz). So then taking it from 87 to 100Hz would mean you would NOT lose as much torque when over speeding it.
 
^ thanks for the explanation, jraef. I've heard of this trick before, but wouldn't it require that the load be extremely low in general, or you would risk overloading the motor? Because you're essentially doubling to voltage across the windings once the motor is up to the rated frequency, right?

This trick seems like a good card to have up your sleeve in a pinch, but I'm curious what all precautions one needs to take to implement it safely. Would the standard VFD overload settings protect the motor, I'm assuming?
 
You aren't really over-voltaging the motor in this scheme. Since you hook it up for low voltage, you program the v/f ratio for 2x the voltage and 2x the frequency.

At base frequency, it's putting out base voltage. At 2x, the voltage and frequency are 2x as well, so the magnetics are fat & happy (technical term) :)

Been doing machine tool spindles this way since the 70's...
 
Thanks jraef & Gene Bond,


That was an excellent bit of information I was not able to find on-line to date. That is until your explanation. Really appreciate the help.


Cheers!!!
 

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