blown fan motors

[g-and-h_electric]

Hey guys need some help on this one.....

First off let me apologize to my late father and Mr. Cronkhite for the bad drawing.

I have a job with 2 rooftop condensers for a remote AC compressor. They are OVER 50 years old. I replaced 1 bad motor with a new one of comparable horsepower and did only the electrical repairs to get power from the "t" unilet to the motor (nothing else was in bad condition).

Today I get a call to go back to the job as our HVAC tech found a short on "C phase" I verified this, and went looking for it. I found a burned splice from the motor leads to the conductors back to the starter, causing a single phase condition at this motor only

So here is the question, seeing as both motors are fed from a common disconnect and starter, is it possible (and probable) that the failure at motor 2 caused a "domino effect" and thereby took out motor 1 ....... I am thinking the single phase condition blew the fuse, which created a "single phase" condition on the remaining moto, and this overheated the windings and caused them to short out.

I have attached a diagram of how things are currently wired.... I am thinking that either a second starter, disconnect, or both are needed to prevent this.

Any thoughts or ideas?????


Howard

 

[texie]

I agree. The motors are not properly protected for overload the way are connected. Each motor needs its own set of overloads.

 

[Eddie702]

I agree with @texie in general. Some oem HVAC motors have built in overloads. Probably not on this old equipment

 

[paulengr]

Single phasing is one reason. Or when one motor grounded it caused a large voltage unbalance which eventually destroyed motor 2. Take a look at this derating curve. It doesn’t take much of a fault close by to destroy a motor or start it down the path.

https://api.intechopen.com/media/chapter/40947/media/image18.png

 

[Jraef]

I agree with @texie in general. Some oem HVAC motors have built in overloads. Probably not on this old equipment

That's rare on 3 phase motors though. Not unheard of for small motors, but rare.

It was an illegal installation when you got there and if it were me, I would not have reconnected the new motor into an illegal setup, regardless of how long it had been there. Sizing of the OL heater was irrelevant in that you cannot have one OL relay for two motors.

As the last one to touch it, you might own it...

 

[kwired]

Does your motor starter only have two overload elements like you have on your drawing? That can possibly make things even worse when used for protecting multiple motors than if only protecting a single motor.

 

[g-and-h_electric]

Yes, the drawing is correct that the starter only has two overloads in it. I think it is a Square D, it is was designed that way . As to the rest of the "nasties"....... I didnt see those in their entirety until the whole thing went south. Right inside the back door is the main disconnect for the air conditioning system so I LOTO'd that and did my "magic" on the roof (which held up for over a month) After the incident was when I found all the problems.

I am planning the following corrective actions upon replacement of both motors... 1) properly sized fuses in the disconnect (25 amp fuses for 10 amps of motor seems a bit high, 2) separate starters for each motor, with at least 3 line break on open, and hopefully 3 line sense

As to last guy to work on it "owns" it ..... yeppers! Took a lot of heat from the shop until the problem was found , and you guys confirmed how it all went bad.

Thank you for all the info so far


Howard

 

[Jraef]

Yes, the drawing is correct that the starter only has two overloads in it. I think it is a Square D, it is was designed that way . ...

Just FYI, 2 OL heater elements on a 3 phase motor has been illegal since I think the 1971 code, so even if the original install was grandfathered, because it was installed prior to 1971, when the 2nd motor was added it should have been corrected then.