Max HP for a VFD fed by Single Phase

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mhulbert

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I've been looking at specs for a few different VFD's (Variable Frequency Drive), and many of them will run up to a 3HP (3 phase) motor with a 240V 1 phase input. Does anybody know of any makes/models that go higher than this 3HP limitation? It seems like these would be a nice alternative to an Add-a-phase, and you get variable speed, soft start, OL's, etc out of the deal.

I'm not familiar with the innards of these things, is there a reason they all stop at 3 HP?
 
We installed 65oo horsepower VFD's (@66oovac)this summer for the Trans Alaskan Pipeline this summer, is that big enough for you? Caterpillar I believe makes a 5000 HP unit too.


Editted to reflect -
Single phase.... oops, never mind.
 
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You can use single phase input up to over 100 hp if you do your homework and calulations first.
The VFD rectifies the input voltage to a DC bus bar. Across the DC bus are high power transistor called IGBT's. It is these that modulate the waveform to turn the motor. If you apply single phase power, only half of the input diodes are available to energize the DC bus, so you need to upsize the VFD. Usually double the ampacity rating. Then you can get the 3-phase output for you motor load. Always use amps to calculate which VFD you select to install. HP ratings can cause a lot of problems if you use them.

There are several other threads covering this topic also.
 
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The real limiting factor is the input rectifiers. They are generally rated for what the current is on 3 phase. On single phase it takes more current to get the same HP out of the inverter section.

There are some manufacturers who would be perfectly happy to make you a custom input rectifier section so you could do single phase input as high as you wanted. Probably not a standard product.

It is also possible to stack drives by tying the DC buses together. You could take two drives, tie the DC buses together, and use just one of them to run a motor.
 
Here is the math behind VFD derating for 1 phase input.

The power pulled from the 1 phase source will be greater than the 3 phase motor power by the square root of 3, so to make sure the input rectifier can handle the additional current you would take the 3 phase motor FLA x 1.732. However in addition to the power rating issue, a VFD must also deal with ripple in the DC bus caused by rectification of the AC line. The output transistors can be damaged by having too much ripple to contend with, so the VFD includes a bank of capacitors to smooth it out. When rectifying 1 phase power, each cycle has a period of time with no power on it, so the ripple is much higher and you need additional capacitance to smooth it . The 1.732 rule may not provide enough, depending on the VFD internal design criteria, so a better rule-of-thumb is to double (2x) the size of the VFD relative to the size of the motor.

Any VFD is theoretically capable of being used this way, but there are a few issues. Some manufacturers build in Phase Loss protection because they have sized their components so marginally (for an expected 3 phase application) that they must protect them from even the chance of an input phase loss. If you are purposely oversizing that VFD to use 1 phase input, you must be able to disable that phase loss protection. There are a few manufacturers out there who do not allow the phase loss protection to be disabled, therefore you cannot use their drives for this purpose. Another relatively minor issue is that the VFD is providing the motor overload protection, and when using the drive on a motor that is 1/2 the rating of the drive, the factory default settings for overload protection are too high. A very few manufacturers for some reason do not allow their motor current settings to go low enough to do this.

The reason why you don't need to do this on some brands of 230V drives 3HP and under is that the cost of components is so low for the manufacturers that they just build in that extra capacity anyway. The only manufacturer that I know of who goes higher with that concept is AC Tech, who has a 5HP drive that can accept 1 phase input without derating. By the way, if you want to do this on a 480V drive, you ALWAYS must double the size.
 
Thank you for the replies! Very informative.

It sounds like I will have to talk to the manufacturers engineers before commiting to one unit.
 
I don't have a problem with the low end drives.
The one we bought for shop practice does way more than what I see us using it for. Another brand for a couple hundred dollars more has an internal PLC function. A drive we installed for a submersible well had over 600 parameters that could be set by the user. It would not only display the normal Amps, Hz, Freq but also PSI, GPM and/or whatever text message you wanted. The low end drive as it turns out could have done the same job on the wells, minus the display of PSI. Some include input and output reactors.

We are new at this type of control. I know we have only scratched the surface.

Thanks for the replies, they have helped.
 
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If you don't mind, what brand of dirve had a PLC built in? I could see that coming in handy for some simple applications, like a pressure booster pump.

I too am new to the VFD drive world, I'm looking to laern a little about what's out there, etc

Thanks,
Mike
 
The new Teco MA7200 drives have a built-in PLC function as well. Actually a lot of VFDs have these capabilities built in now, just some don't bother to call them "PLC". It just means that you can control the I/O completely independantly, i.e. without being associted with the motor.
 
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