Phase converter currents out of balance
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  1. #1
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    Default Phase converter currents out of balance

    Hello all. I have a WNY phase converter model RP10. This is a 10HP rotary single to 3 phase converter. I am powering a 1/4" shear and the unloaded current measurements are not balanced. When the shear is activated with foot pedal the current shoots up too high and causes breaker to trip. My question is, are the unloaded current readings supposed to be off or should they be balanced or equal. Your help is appreciated, Thanks.

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    Which leg(s) are high? Assuming all there is is the RPC and the motor, the motor is a more or less constant impedance. If the voltages are balanced, the currents will be reasonable balanced. For those of the EE persuasion, I know, things are not that simple. Typically, the unloaded RPC generated leg will be higher than the others, but will sag due to the impedance of the RPC motor. When I set my systems up I try for about %5 unbalance or better.

    Tom

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    Currents cannot be balanced unloaded because there is no load on the generated leg to draw current...... other than balance capacitors.

    Voltages can only be balanced by using the "balance" (actually power factor correcting) capacitors on the generated leg. The balance will then change with load.

    The idler inherently CANNOT produce a balanced output by itself, because the back EMF which is used as the generated leg MUST be less than the input voltage, it's part of how motors work. So, aside from added impedance, the "best" way to balance is either by adding turns to the winding providing the generated leg, OR using a boost transformer to increase the output voltage. The transformer could be either on the input to the motor, or on the henerated leg output.

    One could also balance the output by providing series impedance to the direct legs that equals the motor impedance. That would provide a lower, but pretty well balanced output. I do not claim it is the best approach.... but it would be well balanced.

    Another way is to bag the balance capacitors, and just use a very oversized idler. The large idler has lower impedance, so less voltage drop.

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    A 1/4" capacity shear would likely require a hefty motor. What does your motor tag list as full load amps? (Often listed as FLA)

    What is the breaker protecting? Phase converter?

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    First question is what breaker is tripping? 3 phase side or single phase side? What's the FLA of the shear? What size are the breakers? I guess my question is what makes you believe its an imbalance issue? Generally speaking phase converters are tuned to be balanced at a specific load.

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    Thanks for your replies. The breaker that is tripping is the 20 amp breakers on the shear. I believe the overload tripper is set at 13.5 amps. I don't have the FLA figure from the shear motor as I need to get out to the site and get it. Basically the RPC while running the shear without the pump connected to motor is measuring current at 4+, 6+ and 9+ amps on the 3 phases checked at output of RPC. I assumed that they should be more less equal. When the foot pedal is activated and loaded with pump, the amperage shoots up and trips breaker. I will be out there this weekend and check voltages. The WNY company is telling me that currents will be off and to check voltages instead as a better indicator of balance.

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    Hmmmmmm.......

    Motors run on current. Voltage "pushes" current through impedance. (yes, I know that power is the product of voltage and current at each instant, or, V*I * cos(phase angle), but the important factor is current that produces the magnetic field)

    If the currents are not balanced, then the motor is not drawing full power. If it were drawing full power, about 60 to 70% of current would be in-phase power-producing (resistive) current. AND the currents would be well balanced among the phases.

    My inclination when I see or hear of unbalanced currents is to disconnect ALL the "balance" capacitors, and see what the currents look like then. Balance caps can cause problems, they are not a cure-all, and more is not better..

    But there is also an issue if the connection of the pump causes the breaker to open. Either there is a wrong size breaker, OR the load is way too large. Breakers will usually hold double current for a considerable time, and double current is a substantial overload...so a breaker that pops right away may be seeing a VERY large current relative to its rating.

    That needs to be looked at FIRST. Something is clearly wrong, either wiring, a pump or relief valve problem, etc. A wiring issue on motor coils might cause an imbalance, although I'd expect more issues "unloaded" as well.

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