AC Single Phase Motor - Expected Amp Draw

[Ted123]

Hi Forum Members

I have a 4 pole single phase 240VAC 315 watt motor from a central heating heating system that I think is bad.

It's the room fan inside the squirrel cage that sucks / pushes the air past the heat exchanger

It seems to run but very slowly. I think it's bad.

That said, there is no short to ground and when I do an earth to active and earth to neutral there is no short.

Also the resistance measurements on the coil windings are within spec.

My question is around what typical amp draw should I see against the motor when it's running with the full 240VAC ?

My clamp meter is saying it's pulling 2.9 amps.

This seems too high to me and indicates it's gone bad but I don't understand why/ how the winding resistance measurements are still good.

It still rotates nicely by hand so no mechanical issues.

My rough thoughts is that

P = V X I

Power = 240 vac X 2.9 = 696 watts

So with motor rater at 315 watts it's more than double so that must show it's bad ???

 

[Ted123]

Also the name plate on the AC motor says it's 240 vac 315 watts but it's rated 3.6 amps, that doesn't make sense to me ??

Can someone explain if I can write off the motor based on the 2.9 amp draw ?

I've tested the 15uF capacitor and it tests good.

 

[James]

If the name plate says full load current is 3.6A at 240v
you are showing 2.9A at 240v
that seems to be about right for a motor that is not fully loaded.

remember that the rating of a motor (315w) is likely to be the output power at the shaft not the input power including all losses.

 

[Ted123]

Thanks for that.

Photo of nameplate attached.

If my calculation isn't correct P = V X I + (factoring motor losses)

What is the correct formulae to work out the Amperage draw ?

Is it then possible that something is degraded in the windings like an insulation issue causing the motor to run slow ?

Thanks

 

[James]

Have you measured the rpm?

 

[Ted123]

Have you measured the rpm?

No I don't have a tool to measure RPM.

That said, I can say at speed 1 ( out of 10) the PCB sends the motor 100 vac which is correct as per the service manual the squirrel wheel turns so slowly you can put your hand in the blade if you wanted. Probably 60 - 100 rpm absolute max. Sort of just turning over

 

[James]

Sorry, your original post didn’t mention it was running off a speed controller.

to check motor performance against nameplate, it needs to be directly connected to a supply equivalent to the nameplate.

speed controller may be changing the voltage and the frequency and will give misleading results.

 

[Ted123]

No I don't have a tool to measure RPM.

That said, I can say at speed 1 ( out of 10) the PCB sends the motor 100 vac which is correct as per the service manual the squirrel wheel turns so slowly you can put your hand in the blade if you wanted. Probably 60 - 100 rpm absolute max. Sort of just turning over

Sorry, your original post didn’t mention it was running off a speed controller.

to check motor performance against nameplate, it needs to be directly connected to a supply equivalent to the nameplate.

speed controller may be changing the voltage and the frequency and will give misleading results.

 

[Ted123]

It was running around 60 rpm (approximately) with the speed controller giving it 97 VAC. At 97 VAC it was also drawing 2.8 amps ( similar to my original post amp draw) which was done via bridging active mains straight to the motor (so full 240 VAC) which also drawed around 2.9 amps

 

[Ted123]

Sorry, your original post didn’t mention it was running off a speed controller.

to check motor performance against nameplate, it needs to be directly connected to a supply equivalent to the nameplate.

speed controller may be changing the voltage and the frequency and will give misleading results.

What's Your Frequency? | Groschopp

Can a motor that is rated for 60 Hz be run at 50 Hz? Issues can arise when running a motor at a different frequency than the name plate frequency.

www.groschopp.com

My understanding is that a 4 pole AC motor at 240 vac / 50 hz is controlled the speed by altering the frequency in Hertz.

Therefore at 97 VAC the frequency is approaching 24 hz.

What's Your Frequency? | Groschopp

Can a motor that is rated for 60 Hz be run at 50 Hz? Issues can arise when running a motor at a different frequency than the name plate frequency.

www.groschopp.com

Then the calculation is approximately I should have seen around (720 rpm) rather than the 60 to 100.

 

[James]

No, the frequency is not linked to the voltage.
you can have 100v at 25hz, it will run slower because the slip will be higher but it won’t necessarily be half its rated speed.

 

[Ted123]

Hi James, yes I see the point you are making. Would be able to give me a ballpark figure on what RPM I should expect to see at the initial start up voltage which is 97VAC RMS supplied to the motor given it's rated at 1,350 rpm ?

My understanding of VFD is their alter frequency to control the speed but I could be wrong.

Even a ballpark figure plus or minus 20 % would be helpful for me. I've had the PCB tested and it's sending the right amount of voltage at 97 VAC RMS to the motor at the start up fan speed.

I thought in the range of 500 - 600 RPM. It's a motor that goes in a squirrel cage blower fan in the ceiling in a ducted gas heating system that circulates the air through the house.

 

[James]

With no load, I.e. disconnected from the fan
at 100v 50hz I would expect to see in the region of 1350rpm

 

[Ted123]

With no load, I.e. disconnected from the fan
at 100v 50hz I would expect to see in the region of 1350rpm

Thanks

 

[Darkwood]

You confusion here is you are looking at wattage as an electrical value, it is not in this case, its a mechanical value of available torque off the shaft, it used to be in horse power which was less confusing but the world changed and made life harder.

 

[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