Hi Leg Delta

[sparky59]

I received a call today from a customer that has two 3-ton hvac rooftop units on a commercial building. After using the old units for 20 yrs. he had these new units installed last fall. Since then unit#1 just had the third blower motor burn out yesterday. Unit#2 has had one motor replaced. The electric service in the building is 120/240 delta hi leg. The hvac guy told the customer
that the problem was the hi leg was burning the motors up. I told the customer that phase A-B or B-C should both be 240 volts. He said they had checked it and it was 239 volts. I'm supposed to go by and check things tomorrow. Does anyone have any ideas why those motors are burning out so fast?

 

[iwire]

Check the actual running amps of the motor against the FLA of the motor.

There is a chance the new unit is trying to move to much air and is overloading the motor.

The fix could be as simple as closing down the suction side a little bit.

 

[brian john]

As Bob said and

1. Check starting and running amperage.
2. Check starting and running voltage.
3. Check voltage drop.
4. Visual inspection looking for anything out of the ordinary.
5. Do the units have phase protection?
6. Check for FOP under load across all devices.

 

[hillbilly1]

Could be a phase loss due to a loose connection some where, phase loss relays are cheap protection and easy to install. I know that surely they have checked the rotation. A grocery store had one of the condensor racks with all of the fans running backwards, the maintance guy had never noticed it before until I was checking rotation for a generator that I had just hooked up, all of the others were running correctly. Are the units rated for 240 volts? Most are rated for 208/240, but its worth checking the nameplate. Check the amp draw for the motors against the nameplate rating. All phases should be relativly close on the three phase motors, and equal on the single phase motors.

 

[sparky59]

thanks...i am adding all this to my list to check tomorrow.

 

[infinity]

The electric service in the building is 120/240 delta hi leg. The hvac guy told the customer
that the problem was the hi leg was burning the motors up.

The 3 phase, 4 wire Delta system should have nothing to do with this problem. The HVAC guy was mistaken or simply passing the buck. A-B, B-C & A-C should all be around 240 volts.

 

[eastjmg7]

The high leg should be B-Grounded Conductor being 208v

 

[beanland]

240V hitting 208V motor

240V hitting 208V motor

I ran into a similar. Old 208V motor running on 240V. Ran fine. The old motor had lots of copper and iron so it tolerate the 15% overvoltage. Newer motors are designed with no excess capacity. A 208V motor might tolerate 218V but not much more. You have to specity 208-240V 3-phase motors to get ones capable of running on 240V.