Why wattmeter shows negative reading during no load condition of 3phase ac induction motor?
We have some dodgy steam meters that show negative steam flow when the boiler is off (impossible). If it is a really small reading, then it is probably a measurement error. Chances are that the watt meter isn’t equally accurate across its measurement range. Phone the supplier and get some specs with regard to this. I’m only fishing here, but in my experience (water meters, steam meters etc, the accuracy changes with the flow). To my knowledge an ac induction motor will only act as a generator if you run it at negative slip. If you are running it at no load, then the slip will be very close to unity (the power draw should also be pretty low at this stage, hence the reason for the first paragraph). It is theoretically impossible for it to be generating any current. (I’m assuming that the watt meter is measuring the power draw of the motor and that it will give a negative reading when the motor starts pushing out current instead of drawing it, I hope this assumption is correct.) Added at
