Mark, like you, many moons have passed since I studied fluid flow dynamics. I have not altogether pleasant memories of trudging through Bernoulli's Equation while doing my homework.
Yes, what you say rings true. The variables are interdependent. You change one parameter and the dynamics of the whole system are altered.
John,
Where did you obtain your data??
There is no rating label on GE's non-PSC motors or their China produced PSC's. I can see that you would know that the impellers are the same or close as you say they are interchangeable. I doubt if the PSC motors operate at the same RPM as the non-PSC as there is not enough power output.
For a given pump, input power required increases as the square of the shaft speed. Even with the ineffeciencies, a 1.8 amp motor cannot compete with a 5.5 amp. As mentioned earlier, the maximum efficiency of a PSC over a non-PSC is 30 to 40 percent maximum.
Two other bits of data you mentioned that I am curious about.
"GE's shaded pole motor is only 30 percent efficient."
A 74 percent efficient motor is considered to be VERY inefficient. A typical induction motor runs in the 80 to 90 percent range, with the low 80's being considered an inefficient motor. I don't think a manufacturer could
make an electric motor that efficient if they tried.
I don't even think the little electric motor kits kids can buy, where they wind there own field and rotor could be that inefficient.
If that were true, GE's 5.5 amp motor, roughly 660 watts @ 120 volts would be producing 462 watts of waste heat energy. In other words, you would be having the heat, coming out from under your dishwasher, of four 100 watt light bulbs + a 60 watt bulb (actually more as light bulbs convert a portion of the energy to light). You would be getting pretty close to the rating of the dishwashers heating element that is used to heat the water and dry the dishes! These are often in the 500 to 600 watt range.
I think what you may have see, John, is that the starting torqure efficiency rating of a shaded pole motor. Starting torque of a tpical shaded pole motor is 40 to 50 percent of the full load torque.
I think GE did a good job of matching the motor to the task, as the starting load of the dishwasher is constant, and low. The exception where one might need more starting torque that what a shaded pole motor can supply, is the case where something has jammed the impeller or food disposer blade.
This is consistent with my obersvations as I have never had a GE straight inductiond motor stop in mid cycle. However, I have had cases where the motor would not turn, when the machine was first started, due to material jamming the impeller such as calcium chunks and drinking straws.
The last bit of data, that I am curious where you found it from, is:
"GE shaded pole motors is the most inefficient motor ever put in a dishwasher" How did you come by the data that supports this statement. Did you run a dynamometer test on the motor of every brand of dishwasher ever made?
Also, I have never dissasembled a GE motor to see if it was shaded pole or split-phase. So I am taking your word that it is shaded pole. Shaded pole motors are commonly used in record player turntables and low fractional horsepower applications.
Yes, I see drain pumps in newer washers and dishwasher are shaded pole, but they are usually under 2.00 amps. But, it does seem plausible, that older GE motors could be shaded pole, as you don't need high starting torque..and shaded pole motors are among the most reliable and long lasting off all motors as you have no starting windings, nor capacitors to fail.
Anyway, my personal opinion is that GE did a good job of matching the motor characteristic to the task required in their pre-Chinese PSC motored dishwashers.
