Changing a customer from delta to wye

[olly]

We occasionally run into situations where a customer is feed by an open delta transformers. The voltage is 120, 208, 240. (wild leg being 208) (240 would be A and C phase) (120 A to ground and C 120 to ground) We like to get delta banks off the system so we replace the delta bank with a three phase wye, 120/208.

From what I have been told, 99% of the time there is no issue doing this. That 99% of motors will run on 208 or 240 if single phase and a three phase motor would operate the same whether it is getting 120/208 three phase or 120,208,240 delta. I realize a motor will run hotter on 208 opposed to 240.

As a preventative measure we have an electrician check the customer's equipment, which entails them checking the motor and changing the windings if need be. Since that is such a tedious job I'm pretty sure the motors are never really checked. And I have never heard of anything burning up as a result of changing the configuration.

I've heard that harmonics can be effected but I'm not sure why?

Does anyone have any thoughts?

 

[jaggedben]

Well, as you said, what matters is whether they have any three-phase equipment that can only run properly on 240 and not 208. Around here, a lot of the high-leg delta's I've seen don't have any three-phase equipment left on the premises, so it's particularly moot. (Old industrial sites that have been converted to offices are common.)

I think it's probably not always that difficult to identify all the three-phase equipment and see what it is labeled for. Probably the biggest hit is in the efficiency of some appliances. It's less likely that 208 will damage 240V rated equipment than vice versa, although not impossible.

BUT, at some point there will be an exception that causes someone a big problem.

 

[Jraef]

As a gross general rule, it's true that MANY motors 10HP and under are now designed as "tri-voltage", meaning full HP at 208-230V or 460V input. So the LV connection is a compromised design wherein the 208V connections can tolerate down to 187V (208 - 10%) and 253V (230 +10%), but favored more toward the higher end. That's because they figure that in most places that will have 208V, the facilities are smaller so the voltage drop from service to motor is not going to test that lower limit too often. If you want to read more on that design philosophy, download the "Cowern Papers", written by a Baldor Engineer a few decades ago (when they were a real motor company and not a small division of a giant corporation). Read the section on AC Power Systems and Motors starting on page 53. http://www.baldor.com/mvc/DownloadCenter/Files/PR2525

But be careful with that as a general statement, because it usually does NOT apply to motors over 10HP.

 

[jim dungar]

But be careful with that as a general statement, because it usually does NOT apply to motors over 10HP.

And what about the older motors, like from when the delta service was first installed? Can 30 or 40 year old motors tolerate the lower 208V voltage like newer design ones can?

 

[retirede]

And what about the older motors, like from when the delta service was first installed? Can 30 or 40 year old motors tolerate the lower 208V voltage like newer design ones can?

It's really hard to make a blanket assumption. At the end of the day, it all comes down to winding temperature. The hotter it runs, the shorter the life. Shaft load, ambient temperature, voltage imbalance, starting frequency, as well as line voltage all come into play.

 

[kwired]

It's really hard to make a blanket assumption. At the end of the day, it all comes down to winding temperature. The hotter it runs, the shorter the life. Shaft load, ambient temperature, voltage imbalance, starting frequency, as well as line voltage all come into play.

Correct. Any motor that is already near fully loaded will draw even more current when you change the voltage to 208. If motor overloads are set correctly to begin with, those that are nearly fully loaded will likely start seeing overload tripping after the voltage change, if they are not reset according to new voltage. If not rated for 208-240 then they will still work, but will have a shorter life with the higher current.

Any 240 volt resistance loads (single or three phase) will see about a 25% decrease in wattage. Most of the time that won't be too big of a deal, but if the wattage were marginal to get the job done in the first place you will see problems keeping up with demands at the lower voltage.

 

[electrofelon]

We occasionally run into situations where a customer is feed by an open delta transformers. The voltage is 120, 208, 240. (wild leg being 208) (240 would be A and C phase) (120 A to ground and C 120 to ground) We like to get delta banks off the system so we replace the delta bank with a three phase wye, 120/208.

From what I have been told, 99% of the time there is no issue doing this. That 99% of motors will run on 208 or 240 if single phase and a three phase motor would operate the same whether it is getting 120/208 three phase or 120,208,240 delta. I realize a motor will run hotter on 208 opposed to 240.

As a preventative measure we have an electrician check the customer's equipment, which entails them checking the motor and changing the windings if need be. Since that is such a tedious job I'm pretty sure the motors are never really checked. And I have never heard of anything burning up as a result of changing the configuration.

I've heard that harmonics can be effected but I'm not sure why?

Does anyone have any thoughts?

I recently migrated some hvac equp from 240 to 208. The compressors were fine on either, but the hvac guy said he had to change a jumper for the controls.

 

[JFletcher]

We blew up a reefer truck changing it from 208V to 480V. Motor was rated straight 230V and not happy with the 208V and near 500' run of conductor. I would have just installed a buck/boost to change the 208V to 240V (3ph). Boss decides to make it 480V. He changed the motor taps correctly, forgot about the control xfmr. Smoke replacement kit ordered....

I recently migrated some hvac equp from 240 to 208. The compressors were fine on either, but the hvac guy said he had to change a jumper for the controls.

Saw a PTAC today with a dual voltage control xfmr: 208V and 240V input, 24V output. The jumper was on the 240V tap. It has been running fine on 208V for years. Most now just have a 208/240V tap and a 265/277V tap.