kannansiyer
Member
- Location
- Bangalore,India
Need help/advice on using standard VFD on a low input DC voltage of 72V DC. I have a custom built 54V 3 phase induction motor!
Thanks in advance.
Thanks in advance.
vfd for low voltages
- Thread starter kannansiyer
- Start date
- Status
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Any VFD can be programmed to output any RMS voltage at or below it's input, in fact that's how they work. All you would need to do is determine the design voltage and frequency ratio of that motor, then find the parameters in the VFD to program it to do that. They are all capable of it, some are easier to do than others.
The biggest issue will be with the motor winding insulation. The VFD will take in AC and rectify it to DC, where the DC bus voltage will be roughly 150% of the AC RMS input voltage. So if you put in 208V, the lowest you will find here, the DC bus will be over 300V, which means that all of the PWM pulses going to your motor will therefore be over 300VDC. If your motor is custom wound but used standard 1200V magnet wire, no problem. But you are going to have to confirm that with whomever made that weird motor. If for instance they used magnet wire normally used on small 90V DC motors, the insulation may be only rated for 150-200V and the 300VDC pulses will damage it in short order.
The second half of this issue is that most VFDs will have a minimum input voltage that is 2/3 of the rated voltage class, maybe more. So if you buy a 240V class VFD, it will likely work down to 150VAC input, but below that it will trip on Low Voltage because the electronics are getting low voltage too. Don't be fooled by the VFDs that say they can accept 120VAC input. While true, that will not help your DC bus voltage problem because those drive use what is called a "Voltage Doubler" on the input, so even though the AC input is 120V, the DC bus will still be over 300V. That might be an good option however if you find out the motor insulation can take the 300V+ pulse level.
In theory, you COULD get a transformer and step the VFD output voltage down to a level your motor can handle. The problems there will be getting the right ratio, and finding a transformer that can deal with the PWM output and varying frequency coming from the VFD. If you have one made, it will likely cost you more than just trashing that weird motor and starting over.
This is unfortunately just the biggest issue, there are more. VFDs are hard on motors that were not designed for them. Custom wound motors like this one are typically not made to be run by a VFD, so the other issues, such as bearing EDM damage, reflected wave voltage spikes, low speed cooling loss, etc. etc. etc. are going to be problematic too.
So why is it important to use an odd duck like that?
PS:
Sorry, I missed that you want a 72VDC input and you are in India, so the comment about 208V is irrelevant. I didn't want to retype the whole thing though. I think you are out of luck finding a VFD that will run with that low of a DC input.
The biggest issue will be with the motor winding insulation. The VFD will take in AC and rectify it to DC, where the DC bus voltage will be roughly 150% of the AC RMS input voltage. So if you put in 208V, the lowest you will find here, the DC bus will be over 300V, which means that all of the PWM pulses going to your motor will therefore be over 300VDC. If your motor is custom wound but used standard 1200V magnet wire, no problem. But you are going to have to confirm that with whomever made that weird motor. If for instance they used magnet wire normally used on small 90V DC motors, the insulation may be only rated for 150-200V and the 300VDC pulses will damage it in short order.
The second half of this issue is that most VFDs will have a minimum input voltage that is 2/3 of the rated voltage class, maybe more. So if you buy a 240V class VFD, it will likely work down to 150VAC input, but below that it will trip on Low Voltage because the electronics are getting low voltage too. Don't be fooled by the VFDs that say they can accept 120VAC input. While true, that will not help your DC bus voltage problem because those drive use what is called a "Voltage Doubler" on the input, so even though the AC input is 120V, the DC bus will still be over 300V. That might be an good option however if you find out the motor insulation can take the 300V+ pulse level.
In theory, you COULD get a transformer and step the VFD output voltage down to a level your motor can handle. The problems there will be getting the right ratio, and finding a transformer that can deal with the PWM output and varying frequency coming from the VFD. If you have one made, it will likely cost you more than just trashing that weird motor and starting over.
This is unfortunately just the biggest issue, there are more. VFDs are hard on motors that were not designed for them. Custom wound motors like this one are typically not made to be run by a VFD, so the other issues, such as bearing EDM damage, reflected wave voltage spikes, low speed cooling loss, etc. etc. etc. are going to be problematic too.
So why is it important to use an odd duck like that?
PS:
Sorry, I missed that you want a 72VDC input and you are in India, so the comment about 208V is irrelevant. I didn't want to retype the whole thing though. I think you are out of luck finding a VFD that will run with that low of a DC input.
kannansiyer
Member
- Location
- Bangalore,India
Thanks Jraef.
I am compelled to use 72V DC as input as it is battery source.Any other methods of doing it?
I am compelled to use 72V DC as input as it is battery source.Any other methods of doing it?
Rampage_Rick
Senior Member
- Location
- British Columbia, Canada
I'd investigate the motor controllers used for small electric vehicles. They take a 72V DC input and generate a 3ph output (not sure of the output voltage though) The unit I'm familiar with is a Cushman Hauler Pro and the speed controller is made by Curtis.
*edit* The Curtis 1238-7501 does 72-96V @ 550A @ 0-300 Hz
*edit* The Curtis 1238-7501 does 72-96V @ 550A @ 0-300 Hz
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Excellent idea!
this
http://curtisinstruments.com/?fuseaction=cProducts.DownloadPDF&file=50209_1232E_RevD.pdf
used in many applications ( other mfgs are available )
rectification factor is 1.35 (3 sqrt2 / Pi) for 3 phase in bipolar out
eg 480 = 650 or +/- 325
the invertor is the inverse
so 72/1.35 ~ 54 AC
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