AC Single Phase Motor - Expected Amp Draw

[Marvo]

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.

 

[Ted123]

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.

This is a really good post and interesting information you have given.

I have bought a cheap non contact tachometer and I've bought a replacement motor and once I fit the motor in the gas heater I will test what speed RPM it spins. The motor is rated at 1,450 RPM (on the motor nameplate) at 240 VAC so I'm guessing at the start up voltage of 97 VAC RmS (approx. 20HZ) it will spin at the speed of approximately 545 RPM but I take your above point that there is slip and I don't really know how this is calculated.

I reckon the electric motor I have rather than having an electrical failure with the windings shorted to ground or insultation issue etc I reckon whilst it still spins by hand I bet there is some sort of mechanical issue, slight failure of bearings etc the reason it only spins at 60 - 100RPM at start up voltage of 97VAC which is fan speed 1 as per Seeley service manual.

My layman's understanding of how these VFD's work is that they have to maintain the ratio between AC voltage and cycle rate (HZ), i.e. to spin the fan slower on start up (97VAC on speed 1 as per the Seeley manufacture service guide) 97VAC means the frequency of the AC voltage oscillation slows to approximately 20.2 HZ.

i.e. 240 VAC 50 HZ motor rating - ratio is 4.8

so at at

97VAC (start up fan speed as displayed on controller and in service manual - ratio of 4.8

so 20.2 HZ

RPM governed by

RPM = (120 x 20.2HZ) / 4 pole electric motor (on motor name plate) = 606 RPM

factor in 10% motor efficiency losses

545 RPM

LEt me know if you think this is the wrong calculation

 

[Darkwood]

Are 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.

 

[Ted123]

Are 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.

Thanks for that. New motor goes in on Saturday so I will see what RPM and amp readings are with new motor