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Three Phase Question

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  • #31
    You said that the press will run for a short time before it trips out is the bypass valve working? I would think that if it runs for a short time and then dumps that there is a big load some where. Just my two cents


    • #32
      From what I am hearing you are popping the 50 amp breaker feeding the RPC. But before that happens the press starts and runs for 5 seconds. If the press can start then the RPC is at least close to adequate size. I would try a 60 amp breaker before I did anything else. I run my shop with a 20 hp RPC. One of the machines I run is a 15 hp Woodmizer sawmill. It has no problem starting the mill in gear or out. I occasionally start the mill in gear by mistake. I run the RPC off a 100 amp breaker. One other thing to look at is wire size and run length. Excessive run lengths or undersized wire will add resistance and pop the breaker sooner than it should.

      Try the 60 amp breaker first, I think it may solve your issue.


      • #33
        If the 50 amp breaker takes 5 seconds to trip, it is probably seeing 150-250 amps for that time, and unless there is a huge inertial or static load on the motor, it should be up to speed and drawing nominal current by then. A clamp-on ammeter would be a wise investment to see what is going on. You might also measure the currents on the three phases going to the press from the RPC, to see if one (or more) is drawing a lot more than the others, which would indicate an imbalance situation, or a defective phase in the motor.
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030


        • #34
          I would not be upsizing breakers without some in investigation as the real issue. Masking a problem is a great way to burn out a motor (either the idler or the target). BTW.. by NEC all these motors - idler and press should have OC protection - not just the source breaker.

          As I suggested before - just put a voltmeter on the t3 leg (t1/l1-t3, t2/l2-t3); that will tell you pretty much everything. In the absence of real data, it's pretty good bet the T3 is sagging, the motor isn't getting to its rated speed and is hogging current. It's a VERY common problem with undersized rpcs.

          Also... in an early post the OP mentioned that the idler has no capacitors. While there are a few motors that will produce close to the rated t3 voltage without corrective capacitors, it's unusual. The last two I made produced 212v and 192v respectively without correction (242v input). You should try to get within 5% at load. However... even 5% difference requires a significant derating of the target motor.
          Last edited by lakeside53; 11-19-2013, 09:12 PM.