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Single-phase motors do have an imbalance problem, but not from suppressed phases but rather from when the quadrature capacitor does not create a true 90° phase shift between the main and auxiliary coils. This, however, is not dealt with here.
Imbalance and motors is not the easiest thing to understand. Referring to the pages on Motor Failures and Imbalance, it can be seen 3-phase motors are particularly affected by differences in applied voltage (note: single-phase motors are not affected by imbalance). If there is one thing to remember, it is a motor is effectively nothing more than a rotating transformer. As we're dealing with three phases, the same magnetic summing as found in a transformer occurs in the motor i.e. the three phases, when summed, equal zero. Suppressed voltages therefore cause current imbalance, and it does not take much voltage imbalance to create a rather large current imbalance. The ratio is approximately 1:10 i.e. 1% difference in voltage causes about 10% difference in current (although much higher ratios have been experienced). There is one surprising attribute to all of this. The typical reason a suppressed phase is suppressed is because that phase has a higher current loading than the others. The fact the motor will not draw as much current on that phase as opposed to the other two will mean that should there be sufficient 3-phase motors on an unbalanced system, the system would actually become balanced (because all the motors would be trying to lift the voltage on the suppressed phase). Although, as you can well imagine, it is not the most desirable way to resolve the problem. The easiest analogy I can think of is think of three animals pulling a plough. If the one animal walks a little behind the other two, the one doing so has less burden on the yolk while the other two will have to take up the slack. If, however, the third tries to walk even slower then it will be dragged along by the other two (i.e. it has no choice but to stay with). In this explanation there was no explanation of "generation" (as was in the referred pages), but more "forces at play". If you can grasp this concept, then you will be able to grasp the effects at play in a motor - the negative force (that which drags the lagging animal along) is when the motor starts acting as a generator on the suppressed phase.
© 31.08.06 |
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