Need Help! Voltage Drop RPC and Transformer

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Thread: Voltage Drop RPC and Transformer

  1. #1
    Member MyToyz's Avatar
    Join Date
    Sep 2020

    Angry Voltage Drop RPC and Transformer

    Need some input - I have a Haas VF2, brand new, it has the internal 480V high voltage step down transformer. I was told that i could feed it between 240V and 480V 3ph with no problem. Well, not the case, I am having issues with voltage drop and Haas tech is telling me it is my RPC, but when i spoke with my RPC mfg they tell me the voltage drops do not make sense and that it has to be the machine. Hoping someone here has seen this and can help. Below is my equipment and problems i am having.

    Electrical Company - 50KVA transformer on pole with 400amp 240V 1PH service
    Machine - Haas VF2 - 10kRPM - 30HP (has internal high voltage step down transformer)
    RPC - 30HP rated RPC - (two 30 HP 3PH motors) providing 240V 3PH
    Transformer - 45KVA 240V-480V step up transformer.

    Now, because the machine will not take 240V due to the internal transformer - i have to step up the 240V to 480V with an external 45KVA transformer. I have a 600V 40A fused switched between the external transformer and machine.

    When i start the spindle at 4000RPM it is fine, it is when i go to 4500RPM or higher that the machine shuts down. Now, i can run it and starting at 4000RPM and step it up to 10000RPM stepping 10% each time. Machine also runs, with load, up to 10kRPM with no problem or alarms

    Voltage readings at startup, above 4000RPM, are as follows: at machine - drops from 482V to 433V. at RPC single phase side 241V to 236V. The amps on the machine side at start up jump from 35 to over 200 but does not blow the 40amp fuses on my disconnect

    Can anyone here help me - i need someone neutral that can give me suggestions as to what the problem could be. Haas is saying that it is the RPC causing the drop between the transformer and machine. My RPC company is saying the drop does not make sense because a 50V drop at the machine and 5V drop at the RPC does not compute.

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  2. #2
    Community Moderator Jim Dawson's Avatar
    Join Date
    Dec 2013

    Default Re: Voltage Drop RPC and Transformer

    My best guess based on what you posted is that the power factor is so skewed that there just isn't enough power left to run the machine. It's possible that the voltage and current are so out of phase that the effective available power is very low.

    Also check the leg to leg voltage, as well as the current on each leg on both sides of the 45KVA transformer when running. Best guess is that they are way out of balance. 5% or so imbalance is normally acceptable.

    So I think both the RPC manufacturer and Haas are both right and wrong.

    It would be interesting to put a scope on the legs and look at the current and voltage waveforms.

    Additional capacitors or inductors might be needed to correct this.

    Jim Dawson
    Sandy, Oregon, USA

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