Transformer rating for VFDs
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  1. #1
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    Default Transformer rating for VFDs

    I have quite a few surplus VFDs rated for 480V.
    These models will take single phase and put out 3 phase.
    These VFDs allow one to set the output voltage back to 240.
    I'm thinking I can buy some 240 to 480 transformers to power the VFDs.
    This would give me fairly cheap 3 phase converters.
    So....as I look for some transformers to couple to the VFDs,
    my question is...at which voltage do I compute the power at?

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    O am not sure I am crazy-nuts about the plan, it seems a bit of a "long way around the barn", and has some disadvantages. However, that is not what you asked.

    VFDs, and other "inverters" and "switchmode power supplies", actually act like "transformers for power". The power in is essentially the same as the power out, less perhaps 5% for internal losses (many have more like 3% losses).

    What I would suggest, is to take the motor HP, look in the NEC or NEMA charts to find the input nominal rated current for a SINGLE PHASE motor of the same HP at 240V Divide that current by 0.6 (for the assumed power factor) to get a VFD input current.

    Multiply that current by your line voltage to find the VA (volt-amperes). Select a transformer which is at or above that VA rating. Above is better.

    This is not an absolute, but has enough fudge factors that it should work. It corresponds to my experience with motors of up to maybe 5 HP.

    I don;t care for the idea because you need a VFD of considerably higher power rating than the motor. That is because you need twice the current at 240 as at 480.

    Another thing I do not like is that if any of the motors are not 240/480 types, the voltage spike output of the VFD will correspond to 480V, and may be too much for a 240V motor.

    The last thing I do not like, is that the chance of bearing damage from the VFD pulses is minimal at 240V, but becomes quite significant at 480V. If you do not HAVE TO use 480V, I'd suggest that you NOT use it, 240V VFDs are fairly cheap. The cost of transformers may easily make up the difference vs using a plain 240V VFD.

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    Quote Originally Posted by JST View Post
    The last thing I do not like, is that the chance of bearing damage from the VFD pulses is minimal at 240V, but becomes quite significant at 480V.
    That's not totally true. Some years ago, I was doing ground brush effectiveness testing and I catastrophically fluted the bearings in a 1 hp 230 volt motor in 70 hours. Granted I set it up to push maximum available stray currents through the bearings.

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    240v to 480v transformer to supply single phase to a 480v vfd, 480v to 240v 3p transformer after the vfd.

    Ground the output after the transformer however you want, and there wont be any common mode voltage.

    You may find the transformer has enough leakage inductance to add capacitors on the output and you will get a clean sinewave.

    The vfd can be far larger than both or either transformer as long as they dont overheat long term, could still buy a 30hp vfd, but only 10kw transformers and dol start a 5hp motor as often as you want.

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    A 480V VFD set to 240V does not output 240V. It outputs 480/0.707= 679V chopped up to average 240V. I would be surprised to see anything rated over 600V on a 240V motor system.

    Second, you have to derate the VFD by 50% minimum.

    Are you positive the VFDs cannot be made to run on 240V?

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    Thanks for the replies.

    Derating the VFDs will put the kibosh on this since they are small to begin with (.75kw/1hp).

    I have very little money in them and was looking for a way to utilize them with no particular purpose in mind.

    These are Allen Bradley 160-BA03NSF1P1 VFDs
    The following snippet is from the manual, so I assume they will error out at 240v. (I'll still wire it up to verify)

    "For controllers rated at input voltage 380 – 460V AC, undervoltage trip occurs at 390V DC bus
    voltage (equivalent to 275V AC incoming line voltage)."

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    Quote Originally Posted by Strostkovy View Post
    .....

    Are you positive the VFDs cannot be made to run on 240V?
    Most 480V will work fine on 380V, bt will refuse to operate somewhere significantly above 240VAC input. Being a doubler type (presumably), that may be different.


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