Posted by MarkB_MI on January 12, 2012 at 14:48:29 from (198.208.251.23):
In Reply to: follow up posted by teddy52food on January 12, 2012 at 09:19:37:
All other things being equal, the motor will pump EXACTLY the same amount of water and consume EXACTLY the same amount of energy at 120 as at 240. For all intents and purposes, the motor DOES NOT KNOW whether it's wired for 120 or 240. The current through its windings is IDENTICAL at either voltage. The only difference is whether the windings are connected in series or in parallel. If wired for 120, the motor draws twice as much current from its service as it does at 240. But the current through its windings is exactly the same.
Now, I did make the caveat of "all other things being equal". That is to say, the wiring that connects the motor to its service must be able to provide minimal voltage drop in either case. Typically there is significantly more voltage drop on a 120 circuit than a 240 circuit because the current is greater while the wire gauge is usually the same. As voltage drops, the motor draws more current to compensate, which further increases the voltage drop. But as long as the wire gauge of the 120 service is sufficiently big, voltage drop is minimal and there is no real difference to power consumption.
Two more comments:
First, the EFFICIENCY of the motor is measured AT THE MOTOR. Just because the motor is drawing more current doesn't make it less efficient. Now, power is lost due to voltage drop in the service, BUT YOU DON'T GET TO COUNT THAT AGAINST THE MOTOR'S EFFICIENCY. Overall efficiency, yes. Motor efficiency, no.
Also, the response of AC and DC motors to voltage drop is quite different. (Brush-type AC motors are actually DC motors, BTW.) A DC motor will run slower, draw less current and produce less power as voltage decreases. An AC motor will try to keep running at the same speed as voltage decreases, so it will draw more current and maintain nearly constant power. Note that the motor's output torque is determined by its LOAD, not by the motor, so the power will remain almost constant until the motor stalls out.
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