Single Phase Transformer to de-rated 3 Phase VFD

Hello All, I am a woodworker and new to the site, but this seems to be the place to go when you have transformer and phase converter questions! I have a new tablesaw that I am trying to get running which has a 5hp 3 phase direct drive motor that draws 6.4 FLA and would like to run it on single phase 220v.

I've read ToolNut's suggestion to run 440 3 phase motors on single phase through a transformer/vfd and would like to go this route utilizing a general purpose Hevi-Duti 10kva single phase 440/220 step down transformer that I would like to back feed, the 440 of which will go to a 10 HP 440v GE AF-300P11 VFD.

My questions are, when wiring a single phase transformer in reverse, how many amps will the transformer be pulling? Also, do the grounding issues from back feeding transformers only apply to three phase delta-wye units or all units including single phase? How do I ground this properly?

I understand the calculation for the "new" primary in my case to be: 10kva/220v = 45.45A*125% overcurrent protection = 56.81A, but have also read that you should increase the overcurrent protection by 250% to compensate for the higher inrush! Any help would be greatly appreciated, thanks!

I use that system on my RPC, that of stepping up the 240 to 480 before feeding the idler motor.

In theory, yes, you are correct, there is a difference between the design primary and secondary in the that design primary must carry the magnetizing current in addition to the load current. The primary is usually the innermost set of windings which increases the coupling and hence improves regulation.

Now, all that aside, there is no practical reason that you cannot reverse the transformer. I seriously doubt you will ever come close to loading the transformer to its max continuously. As far as currents are concerned, the transformer is 10kva, the current at 240 is 41.7 amps and at 480, 20.8 amps.

As far as the grounding is concerned, the transformer completely isolates the load from the distribution system. To keep noise and capacitively coupled voltages out the load, you should ground the secondary of the transformer. Follow all standard safety grounding instructions, such as grounding the equipment frame and such.

As far as overcurrent protection, follow your local codes and/or the National Electric Code. Also use any information the drive manufacturer has recommended as high speed fuses may be recommended.

Tom

Here is how you can estimate your current.

You have now, a 5HP 3 phase 460V motor, 6.4 FLA.

Convert 3 phase current to single phase current, multiply by the sq. rt. of 3, so 6.4 x 1.732 = 11.08A at 460V single phase.

Now if you REALLY have "220V", then take the ratio of 220 to 460V (460/220 = 2.09:1) and multiply that to your 460V current. so 11.08 x 2.09 = 23.16A is the theoretical motor FLC. The VFD is about 97% efficient, the transformer is about the same, so assuming 6% total losses, that puts your current at 24.15A at 220V single phase.

Your transformer is bigger than it needed to be, you could have gotten away with a 7.5kVA, because the VFD means no inrush current from the motor. But if you already have the 10kVA xfmr, you have to protect it as a 10kVA. So the FLC on the low voltage side will be rated as 41.6A, which means a 50A circuit breaker or 50A time delay fuses on the low voltage side.

On the other (480V) side, you are required to size the conductors feeding the VFD for 125% of the VFD maximum input amps, which you can get from the VFD. You can then size the high voltage protection devices to protect those cables, as long as you don't go over the maximum sizes listed for that circuit in the NEC. So for example, assuming you are using a 10HP rated VFD, the maximum input amps will likely be around 14A. So you will need to use #12 wire to feed it, and #12 can be protected by a 20A CB or fuse. The NEC would allow a larger OCPD on the transformer circuit, but don't do it because the VFD will likely be listed with a smaller size.

Thanks for the great information, I'll get together with my electrician and get it wired up! Thanks again!

Although some would wrinkle nose at using a VFD to power a table saw motor, I'm certain you're really, really, really gonna like it. You'll not only be able to adjust your motor speed for best blade and material performance, you'll have a nice soft start ramping (no appreciable lights blinking when you flip the switch), and you'll have dynamic braking to slow that blade down in a big hurry when you shut it off. Please do make sure there's a lock-out/tag-out or disconnect plug that will assure safety when needing to change the blade, etc...