Is this furnace motor bad?

Reply

  #1  
Old 12-28-07, 09:15 PM
Member
Thread Starter
Join Date: Mar 2005
Posts: 49
Is this furnace motor bad?

How do I tell if a motor is bad? I pulled it out and there doesn't seem to be play in the bearings, it turns freely by hand. But, when power is applied, it has a hard time getting going. I have a multi meter. I'd like to check the resistance. What should the resistance be?

It's a 1979 Lennox.
 
Sponsored Links
  #2  
Old 12-28-07, 10:07 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Check to be sure that you have 120 volts being applied to the motor.

Is their a capacitor near the motor? If so, the capacitor aids in starting the motor and could be the problem.

If you push the motor to get it started will it come up to speed?
 
  #3  
Old 12-29-07, 11:28 AM
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 8,629
Originally Posted by gregp1962 View Post
What should the resistance be?
A relatively low number, but yet not 0, either. 0 would be a dead short. Every motor has some resistance. You can find that out by turning on your vacuum cleaner switch, unplugged, and hook meter to your plug ends at low scale ohms, and that will give you indication what I mean.
 
  #4  
Old 12-29-07, 03:51 PM
Member
Thread Starter
Join Date: Mar 2005
Posts: 49
There are 120 volts. But, no I don't see a capacitor mear the motor. How do I know if the motor is bad?
 
  #5  
Old 12-29-07, 04:39 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Originally Posted by gregp1962 View Post
There are 120 volts. But, no I don't see a capacitor mear the motor. How do I know if the motor is bad?
If it has 120 volts to it you need to use your experience and judgement to decide whether it starts properly and comes up to an acceptable speed .


Not all fan motors have start capacitors.
 
  #6  
Old 12-29-07, 04:44 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Originally Posted by ecman51` View Post
A relatively low number, but yet not 0, either. 0 would be a dead short. Every motor has some resistance. You can find that out by turning on your vacuum cleaner switch, unplugged, and hook meter to your plug ends at low scale ohms, and that will give you indication what I mean.
Of course you aren't going to have zero resistance, but resistance isn't what prevents a short on a motor or coil.



The coil creates inductive reactance when AC voltage is applied to the coil, which opposes the flow of current and acts like a resistance. Then there's capacitive reactance which does the same thing but is out of phase with inductive reactance.


Get inductive and capacitive reactance canceling each other out and you get zero reactance for a particular frequency, which is how a tuned cirvuit allows one frequency or radio station to be selected while excluding others.


That lesson courtesy of amateur radio license training forty years or so ago.






Seattle Pioneer
 
  #7  
Old 12-30-07, 07:46 AM
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 8,629
Originally Posted by SeattlePioneer View Post
Of course you aren't going to have zero resistance, but resistance isn't what prevents a short on a motor or coil.
But if 0 resistance is caused by winding short bridging windings to case, there is your dead short and 0 resistance. That is why I always ohms test between motor hot wire and the case to make sure there is not 0 resistance (0 resistance meaning 100 % continuity) between the two. Neither should there be any continuity between the motor's neutral wire and the case (on an a/c motor only). Should be done with wires disconnected to eliminate possible backfeed. Replaced shot motor successfully, due to this test, last year
.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes