Discussion in 'Electronic Design' started by Joerg, May 9, 2013.

1. ### JoergGuest

Folks,

Got a replacement motor for an evaporative cooler. They failed to mark
the direction of rotation so I hooked it up. The core became hot
quickly. So I measured the no-load current and at 120V it was right at
the rated 4A (yes, four amps!).

Now I know that shaded pole motors are inefficient, but are they really

2. ### Tauno VoipioGuest

Are you sure that you're not being misled by reactive current?

3. ### JoergGuest

Yes, there will be a large reactive part. But the motor is dissipating
tons of power without load. After less than one minute you cannot touch
the laminated core packet any longer without pain, too hot.

4. ### P E SchoenGuest

"Joerg" wrote in message
Try measuring the DC resistance of the winding. That will tell you the
wasted power dissipation at 4 amps. Does the motor actually run? Can you
lower the voltage to see where it stops running, and also to see if the
current draw is proportional to voltage or if it shoots up a lot at 120V. I
would expect only a couple hundred mA for a small fan motor (1/100 to 1/20
HP or 8 to 40W). Most of these motors are "impedance protected", so that the
current draw is low enough even at locked rotor to stay under a safe
temperature. It sounds like a bad motor or a mismatch of voltage and/or
frequency.

Paul

5. ### P E SchoenGuest

I see that the rated current is 4 amps, so that's a hefty shaded pole motor.
The evaporative cooler motors I found at Grainger were all split phase or
capacitor start, and about 1/2 HP. The biggest shaded pole motor I found was
1/20 HP with full load current 2A:
http://www.grainger.com/Grainger/DAYTON-HVAC-Motor-5K004?Pid=search.

The in-phase current should be about 750/20/120 = 312 mA. So it might draw
close to 2A whether loaded or not.

Paul

6. ### JoergGuest

It's this guy:

http://store.essickair.com/search/?query=110441-C&x=0&y=0&records=6

DC resistance on the high-speed tap is 4.28 ohms and on the low speed
tap 6.31 ohms.

7. ### Guest

Some motors have an "air over" rating (it might say "air over" or "AO"
on the rating plate), which means they expect to be in some kind of
airflow, usually from the fan they are driving. So the case temperature
might not be as hideous when the thing is actually installed. On the
other hand, if there is a squirrel cage wheel on each shaft, I'm not
sure how much airflow the motor will get.

I agree that getting about 93 watts (1/8 hp) of spin for about 480 watts
of electricity would be a pretty bad deal. Have you tried powering it
through a Kill-a-watt (or equal portable kWh meter) that can read both
watts and VA?

Matt Roberds

8. ### JamieGuest

Yup, they get warm regardless of load. They get a little warmer when
stalled.

so you can set the direction? Not sure if you have a motor of that type
but it's important that you only use one set of shaded poles when
picking a direction.

In the case where you don't have this option, you are kind out to pasture.

On a second note, they can be speed varied simply by voltage control.
Lots a slip is acceptable with these and for some apps, it's a perfect
fit.

Jamie

9. ### JoergGuest

Well, I installed it, running right now. We'll see

This one is an el cheapo version that they likely sell at a fat profit
margin, only three wires and ground.

On evap coolers you have to run them full bore most of the time anyhow.

10. ### P E SchoenGuest

"Joerg" wrote in message
So at 4 amps on the high speed tap there will be about 65 watts of heat, and
on the low speed tap it *should* draw less than the 4 amps. There are
several motors shown on the following page and it seems that yours may be
the 10441-C which is 1/17 HP or 44 watts. But the 10441-2 is a 1/3 HP (250W)
2 speed motor, where 65 watts no load would be acceptable with 75%
efficiency.

Paul

11. ### JoergGuest

Yes, on low it "only" draws about 2.5A. Which I consider a lot.

Mine is a 1/8HP. Not sure whether the calculations about horses and all
that are right because this cooler is rated at 2800cfm of air at high,
while many larger coolers have big 3/4HP or even 1HP and rate 5000cfm.
Somehow all that doesn't really compute. I think I have to get myself a
real wattmeter so see what it truly pulls under load.

What became hot was not the winding. I could touch it through the cage
after turning off and disconnecting. What got hot was the laminated
section which is openly accessible with this motor.

I just installed it and so far it works. Knocking on wood.