208v to 240v Buck Booster

[EShumake]

120/208 incoming voltage
The dryer in question is 208/240 capable and has a 30amp rating.
I planned on installing a step up transformer, primary being 208v and secondary being 240v which should make the dryer heating element function better, right?
My question is about the amperage range, is 30amp the max the dryer would pull or would I be ok going 25 or 26 amp booster?
I had also read the conversion would change amperage or throw it off, anyone familiar with that, also, the booster will be very close to source and load so I shouldn’t have much loss in voltage due to distance. Thanks in advance for assistance!

 

[ptonsparky]

Leave it alone. Have your electrician put the proper cord on it.

 

[petersonra]

If you boost the voltage up to 240 it will indeed make more heat. Since it appears to be designed for this your electrician could likely figure out how to do so safely, but unless you really care about drying time it is probably best to use the voltage you have.

You would get about 30% more heat at 240 volts so if that matters maybe having your electrician install transformer(s) to kick the voltage up makes sense. But at 208 the heaters will last a lot longer, and that might well be a consideration.

To answer your amperage question would require more information than you have provided.

Part of the reason the forum is very leary of answering DIY type questions is often the DIYer does not know all the questions that need answering, and thus while the question can be correctly answered, other issues may need to be addressed.

Best bet is have your electrician look at it and give you advice based on looking at the whole picture.

 

[garbo]

A 240 volt dryer that pulled 20 amps on the heater element would use 4800 watts . If you were to power this with 208 volts would use 2,496 watts . 240÷20 amps = 12 OHMS. 208 Volts ÷ 12 OHMS = 17.33 amps. 17.33 × 208 volts = 3,605 watts. So @ 208 volts dryer would only produce 75.1% of the heat if run on 240 volts. Won a free lunch years ago when a coworker insisted that a 240 volt baseboard heater produces halve the heat if ran on 120 volts. Told him for all resistance loads if you halve the voltage you only produce a quarter of the heat. Know somebody that saved money years ago by purchasing an electric dryer that ran on 120 volts. He wanted me to check his electrical system but told him that yes at 120 volts it might well take over 3.5 hours to dry a load of clothes.

 

[petersonra]

Is this a three phase dryer or single phase?

 

[MD Automation]

Leave it alone.

Words of Wisdom right there !!!

There are several reasons to leave it be, imo. Assuming this is a standard residential type clothes dryer.

The heating elements in a dryer function much like the oven in an electric stove. Meaning, when you set the oven for 350, the elements heat up, reach that setpoint temp, then turn Off for a bit and then back On. Then Off, then On, lather rinse repeat. The On / Off duty cycle is what determines how hot the oven gets. There is no "350" setting for the elements themselves. They run 100 % Blazing or 0% Off, that's all they know. The mix of On and Off times determines the oven temp. Stretch the On cycle a little longer, you get 400 degrees. Run the Off cycle a bit longer, you get 275 degrees.

It's the same with your dryer. The heating element will be cycling On and Off in normal operation. True... at 240V (really more like 236V) they will reach a setpoint temp a little quicker, and in the early part of a cycle with high heat selected and really damp clothes the "ON" duty cycle might be 100% for a bit. But after that, once the clothes dry up a little and the elements start cycling, the 208V supply will work just as well as the 240V. It will just have a duty cycle that is "On" a bit longer compared to the 240V. The 240V setup will work "a little" better, but not as much as you might think.

In addition, if you use the typical one xfmr configuration, the boost auto-transformer setup will raise one of the L-N voltages. I don't recall exactly how much, but I think probably 130V or so. There is a 50-50 chance that the higher leg will power the drum motor, which in a normal dryer runs on 120V. This is not an absolute disaster, but I'd rather not do that if I could.

Lastly, dumb wasted magnetizing current from having the transformer energized 24/7.

Untold numbers of standard clothes dryers work just fine on 208 VAC.

 

[ruxton.stanislaw]

Told him for all resistance loads if you halve the voltage you only produce a quarter of the heat. Know somebody that saved money years ago by purchasing an electric dryer that ran on 120 volts. He wanted me to check his electrical system but told him that yes at 120 volts it might well take over 3.5 hours to dry a load of clothes.

That is for sure true. Not exactly to the point, but in South America there are quite washer-dryer combo units from all the big name brands that have a 2 kW washer and 1.5 kW dryer rating. It can take hours to dry a lot of things, but I quite like them in that it handles the changeover seamlessly in one cycle. One could buy two of them, to make up for the extra run time if necessary. Anyhow, most of the year in most regions, you can sun dry if you want to.

The dryer in question is 208/240 capable and has a 30amp rating.

My advice is to try out the 208 V performance first. You might find that it's not half bad. Unless you are made of money for this project, then go for broke and step it up to 252 V to get the most out of the element. Keep in mind that dryers often have 120 V to neutral components, so make sure the transformer is wired for the side that does not have the light, buzzer, timer etc. to avoid frying anything. It would also be a courtesy to label the outlet, of the special voltage configuration, so someone else using it down the road does not get an expensive surprise.

 

[tortuga]

Since its either an open wye 120/208 or split phase 120/240 would it not work (to produce the same wattage) for the manufacturer to connect two 120V 5 Ohm elements in series with the neutral in the middle?
L1----5Ω----N----5Ω----L2

 

[petersonra]

Since its either an open wye 120/208 or split phase 120/240 would it not work (to produce the same wattage) for the manufacturer to connect two 120V 5 Ohm elements in series with the neutral in the middle?
L1----5Ω----N----5Ω----L2

You would need to change the elements which might change the listing. Big mess.

 

[wwhitney]

You would need to change the elements which might change the listing. Big mess.

Sure, for a field modification. I think the question was "does any manufacturer use (2) 120V elements to provide equal power output on 208V and 240V?" As well as, if not, why not?

Cheers, Wayne

 

[tortuga]

You would need to change the elements which might change the listing. Big mess.

I was not suggesting anyone other than the manufacturer, more just wondering if the manufacturer chose two elements would it not produce the same heat on 208 as 240?

 

[wwhitney]

I was not suggesting anyone other than the manufacturer, more just wondering if the manufacturer did that would it not produce the same heat on 208 as 240?

Yes, it would.

Cheers, Wayne

 

[ruxton.stanislaw]

Since its either an open wye 120/208 or split phase 120/240 would it not work (to produce the same wattage) for the manufacturer to connect two 120V 5 Ohm elements in series with the neutral in the middle?
L1----5Ω----N----5Ω----L2

This is the ideal design and I have often wondered why they don't do that. Some suggestions I have heard are undersized neutrals, the extra time and materials and potentially more points of failure. Instead of switching one conductor to control the element, they would need to switch two. It makes it every so slightly more costly, but from a technical perspective, it is ideal.

 

[petersonra]

I suspect the answer is probably found in the fact that it is simpler to make one design that uses the same stuff then multiple versions.

If you put the heating elements on the 208/240 v lines you can use a single element. Less wiring, less controls, no neutral connection.

One line can run to neutral for the controls and drum motor without worrying too much about how it affects the rest of the wiring.

While these ideas sound like improvements, they complicate things. And if you make it so you get max heat at 208 v, then you can't run it at 240 v, so you need two versions of the appliance.

 

[ruxton.stanislaw]

While these ideas sound like improvements, they complicate things. And if you make it so you get max heat at 208 v, then you can't run it at 240 v, so you need two versions of the appliance.

I respectfully half agree. While it does complicate the design, it would still be interoperable at both voltages using two 120 V elements. With this design, there would effectively be very little or no load on a 240 V split phase system neutral (the same thing as if it were disconnected), whereas a 208 V system with 120 degrees phase shift would see the same current as the two heating element phases, to balance the loads.

 

[kwired]

If you installed the boost transformer, you would need to assure you don't get the 120 volt motor and controls on the side that is boosted or you will have more than 120 volts to those components.

 

[petersonra]

I respectfully half agree. While it does complicate the design, it would still be interoperable at both voltages using two 120 V elements. With this design, there would effectively be very little or no load on a 240 V split phase system neutral (the same thing as if it were disconnected), whereas a 208 V system with 120 degrees phase shift would see the same current as the two heating element phases, to balance the loads.

But then you have complicated the innards. I am pretty sure this has been hashed over by the designers of these things and they all ended up with the same basic design. There is likely a reason for that. Maybe has to do with losing one of the elements and then your low neutral load goes up a lot. Plus if one element fails, the owner might not notice if it continues to work at a reduced rate

 

[jim dungar]

I respectfully half agree. While it does complicate the design, it would still be interoperable at both voltages using two 120 V elements. With this design, there would effectively be very little or no load on a 240 V split phase system neutral (the same thing as if it were disconnected), whereas a 208 V system with 120 degrees phase shift would see the same current as the two heating element phases, to balance the loads.

But that design would only work in countries with 3wire systems.
Many manufacturers like to keep as much of their internals as common as possible when they build for the world market.

 

[ruxton.stanislaw]

But that design would only work in countries with 3wire systems.
Many manufacturers like to keep as much of their internals as common as possible when they build for the world market.

However, the dryer already has a lot of 120 V components and the neutral connector, so they could make the most of it. I am well travelled and in my experience, there is a distinctly different market and line of products for 220 V 50 Hz appliances with Euro or Schuko plugs. Different regulatory agencies, features, build quality etc.

But then you have complicated the innards. I am pretty sure this has been hashed over by the designers of these things and they all ended up with the same basic design. There is likely a reason for that. Maybe has to do with losing one of the elements and then your low neutral load goes up a lot. Plus if one element fails, the owner might not notice if it continues to work at a reduced rate

This is true, I agree it is more complicated. Hopefully the internal computer would realise one way or another (perhaps current or temperature sense) and throw an error code for the user.

 

[tortuga]

if you make it so you get max heat at 208 v, then you can't run it at 240 v, so you need two versions of the appliance.

How would it not not produce max heat at 240?
Were talking about a L1----5Ω----N----5Ω----L2