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.