boost/buck transformer question

[stew]

120/208 volt system that requires a 240 volt circuit to a stack washer and dryer. there was a prevoiusly installed boost/buck of the proper size in the ceiling area that had been used for a tanning bed. I used the abandoned boost buck to bring the voltage up to 240 for the stack washer dryer unit which is 120/240. appliance guys say they are getting 120 volts on one leg and 150 on the other? I guess I have never taken that reading before so I dont know what I would get . It seems to me it would be 120 on both legs no? they get 240 acroos the legs. what the heck am I missing here? they say that the 150 volts is causing the washer to run hot. I am remote from this at the moment but they are going to swap the red/black and see if it makes a difference as they say the dryer wont care.

 

[jim dungar]

The appliance guys are correct.

It is a little complicated to explain in written words, but basically a L-L buck boost will raise one of the L-N voltages while leaving the other one unchanged.
In your case L1-L2 was 208V which you raised to 240V. But L1-N stayed at 120V while L2-N became 148V, due to the 120? phase difference between L1 and L2.


In general buck-boost transformers should almost never be used with simultaneous L-L and L-N loads.

 

[stew]

If I were to reconnect the transformer for the low voltage secondary output I should then get 220 vots secondary LL and about 130 volts on the hi leg?

 

[jim dungar]

If I were to reconnect the transformer for the low voltage secondary output I should then get 220 vots secondary LL and about 130 volts on the hi leg?

Yes.

BTW, swapping the red and black to the dryer, will effectively move their L-N loads from the 'high leg' to the 'normal leg'. This would definitely be my first option.

 

[stew]

the red black swithc was done and the washer is ok but with the hi voltage to the dryer the hi limit shut the dryer off after running a while. If I change the connection the 130 volts at the dryer should be ok i would think. Your opinion would be helpfull as I think you have agreed with me that this will probably work ok eh?

 

[stew]

and will I actually have 130 or some other derivative of the 120 deg shift.?

 

[jim dungar]

and will I actually have 130 or some other derivative of the 120 deg shift.?

It will be some derivative involving the actual boost voltage you get. It can be solved using basic trigonometry.
Effectively the formula is:
Vhn=SQRT{(Vll-104)?+(60)?}

where
Vll = Line to Line voltage
Vhn = high Line to Neutral voltage.
SQRT = square root

But, beware that this is a simple explanation. The final result will depend on how you have created the boosted voltage and if there is an internal 'bootleg' midpoint to ground or neutral connection in the appliance. There will be about a 60V difference between the center point of your boosted voltage and your 'real world' neutral/ground point, so you need to make sure these two points are never connected.

 

[stew]

as i understand the connection the washer requires 120 only so l to n only. the dryer requires 120/240 so it would have both the boosted l to l vlotage and a lto n voltage of maybe about 130 volts which I will check prior to running. not certain I understand what is meant by a bootleg n connection. there will be a panel neutral of course at the unit. sorry if I am dumb at this but it really has me puzzled.

 

[ptonsparky]

Remember your dryer motor only uses 120V so make sure you get it and the controls on the correct, or unchanged leg.

 

[stew]

I just figured out what you meant by a bootleg nuetral jim. thats the way this beast was hooked up before i got to it. they had only the 2 hots over to the j box that they pulled this circuit from so they used the bond in the box to go to the n connection at the unit. I changed the unit to 4 wire and ran a panel neutral and unbonded the n at the unit to make it code compliant in todays terms and installed a 4 wire pigtail. I will check the l to n voltage to be sure that it is 130 or lower after I reconnect the transformer. I would think that at 130 volts on the leg should allow this unit to operate ok even tho we will be a bit high.The other thing would be to just run it on 208/120 and forget about the transformer. thats the way this thing ran for several years. biggest problem was the length of time it took to dry a load. this unit is in a beauty salon/manicurist so uses lots of towels and hand towels.

 

[kwired]

If the only 240 volt load is the dryer heating element (very likley) I would just connect it directly to 120/208.

Only problem you may have is a little longer run time because of less heat than you would have at 240 volts.

 

[stew]

@kwire. that was the reason for adding the boost buck in the first place ,to get the voltage up a bit to get the dryer to perform better. unfortunatly I neglected to check the leg to n voltage at the time and did not realize there was 148 volts on one of the legs. I am gonna at least rty the 5% boost and check to see what i get. If its not too high on the highest leg I may try it. Live and learn in this business. Just dont like to learn the hard way is all.

 

[acrwc10]

@kwire. that was the reason for adding the boost buck in the first place ,to get the voltage up a bit to get the dryer to perform better. unfortunatly I neglected to check the leg to n voltage at the time and did not realize there was 148 volts on one of the legs. I am gonna at least rty the 5% boost and check to see what i get. If its not too high on the highest leg I may try it. Live and learn in this business. Just dont like to learn the hard way is all.

Is this stack unit a "Cord connected" unit? I would ditch the Buck Boost transformer on a "plug in" situation because you don't know if in the future they will replace the unit with a unit grabbing an internal 120volt connection by using the "Higher L-N" side. (it could be a potential fire hazard) I'm not sure if it is completely legal to feed a 120/240 4 wire receptacle from a buck boost transformer either, because it is expected that you will have a L-N connection on this type of outlet and you are supplying a much higher voltage than 125v on one side of the receptacle.

 

[kwired]

@kwire. that was the reason for adding the boost buck in the first place ,to get the voltage up a bit to get the dryer to perform better. unfortunatly I neglected to check the leg to n voltage at the time and did not realize there was 148 volts on one of the legs. I am gonna at least rty the 5% boost and check to see what i get. If its not too high on the highest leg I may try it. Live and learn in this business. Just dont like to learn the hard way is all.

Drying time will not be extended long enough to be that noticeable for most users. And like acrwc10 said - what happens when the unit is replaced and the other side needs to be the 120 volt side? What if 120 volts is used from both lines on a future unit? A full rated step up transformer is probably not worth the cost either for no more than what you will gain from using it.

 

[stew]

yes you are all correct. ditched the bost buck. reconnected for 120/208 and left it at that. they will just have to deal with lack of extra kw for the dryer. too many bad things can happen. even at the 5% boost i would have gotten about 132 or so volts on the high side. not comfortable with that so it is what it is now.

 

[acrwc10]

yes you are all correct. ditched the bost buck. reconnected for 120/208 and left it at that. they will just have to deal with lack of extra kw for the dryer. too many bad things can happen. even at the 5% boost i would have gotten about 132 or so volts on the high side. not comfortable with that so it is what it is now.

:thumbsup::thumbsup::thumbsup: Extra points for going down the right path.