XPost: sci.electronics.design, sci.electronics.misc   
      
   On Mon, 14 Jul 2014 09:00:10 -0700, DaveC  wrote:   
      
   >I need to provide jog function on a machine (run the motor for a few seconds   
   >under load) without use of VFD**. It was done on this small printing press   
   >with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m   
   >changing to a 220 ³single phase² 5 HP motor.   
      
   Most likely the original three phase motor was a typical squirrel cage   
   AC induction motor, with three sets of coils 120 degrees apart running   
   at asynchronous speed (slightly below the synchronous speed 3600 rpm   
   or some submultiple). The new single phase motor is also an   
   asynchronous motor, the only difference is that there are only two   
   sets of windings 90 degrees apart. For unidirectional motors, the   
   other coil might be weaker "starter coil" and to get the required 90   
   phase shift a starter capacitor is used between the hot ends of the   
   coils. Sometimes the starter winding is disconnected.   
      
   As far as understand, in the US many single phase motors are   
   bidirectional with a fixed capacitor between equal power windings. To   
   reverse the direction, you just select which end of the capacitor you   
   couple the line voltage.   
      
   >Is there any inherent issue with starting an asynchronous motor under load?   
      
   Please remember that an induction motor can take about 6 times the   
   nominal current at startup. Traditionally big three phase motors have   
   been started with wye/delta starters, but these days soft starters   
   (triacs) are used.   
      
   >A   
   >3ph motor does this with ease, but can the same be said of an async motor?   
   >Should I be looking at a particular design of async motor?   
      
   Both are asynchronous induction motors, of course you have to check   
   that it can handle  full power in both directions.   
      
   >Also, reversing an asynch motor‹is this a straightforward thing to do?   
   With symmetrical windings, just select correct winding (or end of   
   starter capacitor).   
      
   >I'm   
   >thinking about bringing the directional wiring inside the motor's terminal   
   >box out to separate contactors. I'm also thinking about cross-connecting aux   
   >contacts in the contactors to eliminate possibility of actuating both   
   >simultaneously and some kind of timer relay to provide delay between forward   
   >and reverse actuation.   
      
   If you need to reverse the direction quickly, you need some load   
   (resistor) to dump the rotational energy), otherwise the current   
   demand would be much more than 6x relative to nominal current.   
      
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