Discuss AC Single Phase Motor - Expected Amp Draw in the Central Heating Systems area at ElectriciansForums.net
ThanksWith no load, I.e. disconnected from the fan
at 100v 50hz I would expect to see in the region of 1350rpm
With this type of little AC motor they often just use a lower voltage to give a lower speed. The slip goes off the charts and the efficiency plummets but they don't care and can often get away with it because the motor windings impedance is sufficiently high to limit the high currents that would cause burn-out in a larger motor. Sometimes they use a PWM (chopper) circuit which is essentially the same as a lighting dimmer except it has a minimum value of maybe 30% that stops the motor going into complete stall. This method reduces the RMS value of the supply by changing the width of the waveform. Either way there's no one-size-fits-all formulas that can be easily used to calculate RPM at a given voltage or PWM setting, it very much depends on the individual construction of the motor and when the copper and iron losses start running away and falling off a cliff. Also most general electrical testers won't give accurate test results for a PWM output because it's usually higher frequency and not sinusoidal which adds to the problems.
Thanks for that. New motor goes in on Saturday so I will see what RPM and amp readings are with new motorAre you actually making some assumption here and leading your self down the wrong path, if the fan is vfd controlled it is possible the speed may have certain variable dependencies ie, the controls could be instructing the fan to run slow due to some kind of feed back sensor or even a manually operated control like high and low settings, many things could restrict the motor speed but been unaware of the system I can only make assumptions too, a fast high heat setting may warrant higher circulation speed, temp' feedback may change gas flow and respectively fan speed, just food for thought but like I said, I am not familiar at all with the system. If I am correct this opens multiple options of something else been wrong not related directly to the drive or motor if indeed there is a problem, the manual speed control could be just a trim pot to a master control been overriden for some reason.
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