AC Single Phase Motor - Expected Amp Draw

Ted123 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

Have you measured the rpm?

Ted123 · Jul 16, 2023

James said:
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 · Jul 16, 2023

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 · Jul 16, 2023

Ted123 said:
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 said:
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 · Jul 16, 2023

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 · Jul 16, 2023

James said:
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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

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

Ted123 · Jul 16, 2023

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

Thanks

Darkwood · Jul 17, 2023

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.

AC Single Phase Motor - Expected Amp Draw

Ted123

Ted123

James

-

Ted123

Attachments

James

-

Ted123

James

-

Ted123

Ted123

Ted123

What's Your Frequency? | Groschopp

What's Your Frequency? | Groschopp

James

-

Ted123

James

-

Ted123

Darkwood

News and Offers from Sponsors

Similar threads

OFFICIAL SPONSORS

YOUR Unread Posts