I am interfacing an air conditioning unit to a digital thermostat. It currently runs off manual controls. The a/c fan motor is a 3-speed, with a connection diagram like this:

BROWN---------------Capacitor

BRN/WHT-------------Capacitor

WHITE---------------Line (N)

BLACK-----0 HI

YELLOW----0 MED O-- Line (L)

RED-------0 LO

The current manual controls switch the live line connection between EITHER Hi, Med, or Lo to determine the motor speed.

The new thermostat has three relay outputs: Hi, Med, Lo. It is simple enough to interface these to the motor, but the problem is the three speed outputs are 'cummulative'.

That is, on low speed just the Lo output is active, on medium speed BOTH Lo and Med are active, and on High all three are active.

Given that the motor currently gets a Lo or a Med or a Hi connection, would it be detrimental to have two or three of these lines connected at the same time?

Thanks in anticipation of a reply - I have spent the morning researching this but cannot find an answer.

Chris

 

Are you sure you cannot connect the windings cumulatively? See this wiring scheme:

(Source: http://www.me.ua.edu/me360/spring05/Notes/Topic17-AC_Motor_Speed_and_Other_Motors_sv.pdf )

Would attaching high to all three speed windings be any different from attaching to high alone?

John

 

That's exactly the question I have myself.

It is easiest for me to wire it cummulatively - but will that be harmful?

Or do I have to get a bit more complicated to make sure only one speed connection is live at any time?

It's a 230V motor and when I connect 230V to the 'Lo' connection I get about 320V out of the 'Hi' connection (it may have been the other way around). This makes me think it would be bad to connect the 230V line to the 'Hi' that has 320V on it.

Chris

 

Ah, I think you have a point. Those speed windings will have induced emf, so it may not be a good idea to short them. I wasn't thinking clearly -- too much sun yesterday.

John

 

But just because the o/c leg 'floats' at 320V doesn't necessarily mean it's a bad thing to connect it to 230V.

Anyone definitively know the answer to this?

Thanks

Chris

 

So if you put 230v onto Hi, and u measure Low to Neutral, do you get 230v? or do you get a different voltage?

 

I'll take some proper voltage readings tomorrow, when I have access to the motor, and post them.

I've attached a picture of the motor rating plate. I Google'd the model number but didn't find anything. Also browsed the A O Smith website but couldn't see any tech info (or find a motor like this listed). The motor is in a Dometic 'Penguin' / 'Duo-Therm' air conditioner.

Chris

 

OK, here are the readings:

With 240V going in on Hi, Med is 312V and Lo 354V.

With 240 going in on Lo, Med is at 210V and Hi 156V.

This confirms the wiring is as in the diagram John attached. So connecting more than one line will put a short across all or part of the other windings. So it is a bad thing to do!

All seems obvious now it's been worked through, but hopefully this may help someone else out with a similar question in the future.

Incidentally I've now hooked the a/c unit up to the digital thermostat using c/o relays so the supply voltage is cascaded in such a way that only one input to the motor is live at any time. All works a treat.

Thanks for posting the diag John.

Chris

 

So if you put 230v onto Hi, and u measure Low to Neutral, do you get 230v? or do you get a different voltage?

240V in on Hi, Lo to Neutral measures 354V.

Just noticed I'm flipping between 230V and 240V. I'll explain. UK nominal voltage used to be 240V. Most actual supplies are round about 240V.

Some years ago to standardise with [most of] the rest Europe the nominal voltage was changed to 230V. This is the 'official' voltage I try to use.

However, the nominal voltage is +/- 5%, so 240V is within the spec. So the supply companies left the supply at 240V to save having to adjust all their transformer tappings. I'm so used to seeing 240V on my meter that I sometimes forget to say 230V.

Chris

 

Good stuff,once I new what voltages you wore getting on the other windings I could have told you what you figured out!

Glad it working!

Carlos Fandango