voltage conversion for electric heater

[knjelectric]

I have a customer that bought a used commercial oven without telling me. He is
within his rights to do this. The problem is it is 3ph 208 v. His service is 3ph 240v.
This unit has a nameplate rating of 200 a. The 2 conveyor motors can be rewired to the higher voltage. My question is how do I compute the current draw at the higher voltage? If I use straight math and think the higher voltage the lower current it would be 173 a. But if I use ohms law I come up with app. 240 a. I have talked with a few good electricians and we are all scratching our heads. One gentleman
brought up the point that if you increase the voltage on a filiment (light bulb) you will shorten the life of the filiment or burn the filiment (resistor) out. We need a learned person to put this problem to bed. We all have grey hair and dont need any more. Thank you in advance. Jim KNJ Electric p.s. I have thought of a step down transformer also but still need to know the math to get an accurate draw on the service

 

[augie47]

On your heating elements your resistance is constant so you revert to ohms law. HOWEVER, connecting it would be a violation of 110.4. It might be a good candidate for a buck & boost transformer.

 

[knjelectric]

Thank you that is true. However, properly rewired equipment can be used at multiple voltages. What I am trying to determine is whether I can run this unit at a higher voltage safely and what the formula would be to allow me to determine that. I have heard of buck and boost transformers but don't know anything about them. Could you explain their operation please?

 

[Open Neutral]

With a fixed resistance, power is E^2/R. Your question is:

What is 208^2/R vs. 240^2/R?

Assume R is 1.0: 43,264 watts vs 57,600 watts; i.e. 1.33 times the output.

I suspect the oven will have a short lifetime....

While you could buy buck transformers; what would it cost to just replace the heating elements with 240V ones instead? You also save on wasted space & power loss in the xfmrs.

The only way to know is ask the manufacturer.

 

[iceworm]

Adding to the comments to augie and open

... What I am trying to determine is whether I can run this unit at a higher voltage safely and what the formula would be to allow me to determine that.

There is no formula to determine if one can operate equipment at a higher voltage that the nameplate. The ability to safely operate at a different voltage that the nameplate is a question of the equipment design.

If you are asking what happens if you connect heating elements rated for 3ph, 208V, 200A and connect to 3ph 240V - then the current goes up linear with the voltage I(240) = 200 X (240/208) = 231A The heating element power goes up with the square of the voltage. So if the elements put out P(208) = SQRT(3) X 208 X 200 = 72Kw. At 240V, P(240) = SQRT(3) X 240 X 231 = 96Kw. This assumes the motors are not part of the 200A.

This is a 1/3 increase in power. That is a lot. Everything is going to get a lot hotter.

... However, properly rewired equipment can be used at multiple voltages. ...

I don't see how one can rewire 208V oven heating elements for operation at 240V except for one possibility. If the elements are paralleled 120V, connected Wye, one might reconnect to series connected Delta for 3ph 240V. If you could do this, then the current to the heating elements would indeed go down. I(240V - Delta connected) = 200 X (208/240)=

If I use straight math and think the higher voltage the lower current it would be 173 a

Stay away from this concept. That applies to transformers - not to what you want to do. yes, it gives you the same number - but it is not the same concept.

The 2 conveyor motors can be rewired to the higher voltage.

Are the motors 3ph or 1ph? It doesn't matter much - just curious. In either case, I've never seen a motor with a 200V nameplate that has a tap or connection scheme for 240V. Yes, motors designed for 208v systems generally have a nameplate saying "200V". Now if the motors already say 230V, and have been running fine on 208V, then no reconnection necessary. They will run fine on 230V.

Then there is the issue of the controls. Are they 120V wired from one phase to a neutral or 208 wired phase to phase. If you are going to connect to 240V/120V Delta, it would be good to not the the wild leg and burn them up.

... I have heard of buck and boost transformers but don't know anything about them. Could you explain their operation please?

3ph, 240V to 208V, 200A, B-B are non-trivial. I can't imagine that you would purchase and install on the advice from some faceless wacko on the internet.

First, I'd call the mfg and see if they have any help . If that didn't work, then get your supplier to furnish appropriate B-B xfm. Maybe SQD, Hammond, Acme has a paper on sizing and connection.

ice

 

[gar]

130201-1151 EST

knjelectric:

A resistive heater is simply a resistor, and probably made from Nichrome wire. Over a wide temperature range most metal resistors have a fairly constant resistance with a positive temperature coefficient of resistance relative to temperature. An ordinary small 1500 W space heater has about a 10% increase in resistance from room temperature to its full power operating temperature, orange glow. As augie47 said you can assume resistance is constant for the two voltages you mentioned. Thus, power at the higher voltage compared to the lower voltage will be about (240/208)² = 1.33 times, temperature of the heating element will be much higher. Heater life will be shortened, and there might be fire problems.

If you have isolated ac voltage sources of the same frequency and phase, then you can add or subtract these voltages to obtain some other voltage. Suppose you have a 120 V wall supplied voltage with two terminals, hot and common. Connect a small 120 V to 10 V transformer primary to the 120 V. I will assume isolation between primary and secondary. Pick one of the secondary wires and connect it to the 120 V hot. Now between common and the unconnected wire of the transformer secondary there will be a voltage of 130 or 110 V depending upon the phasing. Switch the secondary leads and the voltage read will change to the other possibility. Or you could have interchanged the primary wires. The lower output voltage would be the buck mode, and the higher voltage the boost mode.


.

 

[Strathead]

Thank you that is true. However, properly rewired equipment can be used at multiple voltages. What I am trying to determine is whether I can run this unit at a higher voltage safely and what the formula would be to allow me to determine that. I have heard of buck and boost transformers but don't know anything about them. Could you explain their operation please?

You can find much more detailed information with a quick search of the web, but basically a "buck-boost" transformer is an autotransformer. It raises or lowers the voltage depending on how you hook it up. They are great for a constant load, but the amount of voltage difference is tied inversely to the amperage. They are basically sized by Higher voltage - lower voltage (or change in voltage) x the amperage of the circuit, so they don't carry the full load of the circuit and they can be smaller than an isolation transformer. Thereby cheaper.

 

[Jraef]

With regard to the heater elements getting hotter, that is technically true. But you are all leaving out the fact that this is an oven, there is likely a thermostat that controls the chamber heat. All it will mean is that the duty cycle of the heaters will be lower* (assuming simple contactor on-off control). In the long run the contactors may wear out faster (unless they are mercury, in which case you must consider replacing them) because they might cycle on and off more often. If there is an SCR / SSR power controller, you will never know the difference. Your circuit size however will indeed need to be based on the higher current, and that includes the ratings of the internal components, resistive controls are sometimes sized close to the bone.

The real concern to me is what iceworm said about the connection pattern of the elements. If designed for 208V wye and the elements are rated 120V connected in a Wye configuration with the Wye point connected to neutral, using that on a delta supply will be problematic. If the wye point of the elements are not tied to neutral, it doesn't matter, same if the elements are connected in delta.

I would also be concerned for the control voltage. In 208V machines, they often just tap one phase and neutral for 120V controls. If you have a 240V 3phase 4 wire system, you must ensure that the control voltage tap is on B phase of your supply only.

We really cannot address this all adequately from afar, it needs to be looked at with an eye on the internal oven schematic.

* Lower of higher? The elements will be hotter, so they will heat up the chamber faster and not be on for as long, thus I said lower, but I'm struggling with that concept. The shorter on time might mean that if the oven cools at the same rate, they may need to come on less often. But if the higher temperature of the elements creates more convection, they may cool faster, thus making the elements cycle more often, meaning a higher duty cycle. I'm not sure here..

 

[Open Neutral]

With regard to the heater elements getting hotter, that is technically true. But you are all leaving out the fact that this is an oven, there is likely a thermostat that controls the chamber heat. All it will mean is that the duty cycle of the heaters will be lower* (assuming simple contactor on-off control).

Do you think the heaters will survive even such intermittent operation at the higher current/power level?
And what about cold startup, where the duty cycle won't be brief?

Agree re: control voltages, etc.

 

[infinity]

I've been running a 1?, 208 volt heater on 240 volts in my garage intermittently for 10 years and it's still running. Even the fan motor has survived.

 

[Phil Corso]

knjelectric...

Jraef makes a good point regarding the connection of the heater. Especially if wye-connected! It is also also important to know if there is just one or multiple-ements per-phase.

1) If the oven is old are its elements balanced?

2) Do you know if there are 3-leads, or 4-leads connecting it to its supply?

Thus, I suggest you take a resistance reading between terminals!

Regards, Phil Corso

 

[augie47]

I've been running a 1?, 208 volt heater on 240 volts in my garage intermittently for 10 years and it's still running. Even the fan motor has survived.

Update

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[infinity]

Update

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View attachment 8041

LOL. :lol:

 

[knjelectric]

re: rewire oven

re: rewire oven

:slaphead:Thank you so much for your help. Knowing what you don't know is often more valuable that knowing what you think you know. This unit will not work for my customer and now I can explain why. I hope I can be of help to you some day. Jim

 

[kwired]

With regard to the heater elements getting hotter, that is technically true. But you are all leaving out the fact that this is an oven, there is likely a thermostat that controls the chamber heat. All it will mean is that the duty cycle of the heaters will be lower* (assuming simple contactor on-off control). In the long run the contactors may wear out faster (unless they are mercury, in which case you must consider replacing them) because they might cycle on and off more often. If there is an SCR / SSR power controller, you will never know the difference. Your circuit size however will indeed need to be based on the higher current, and that includes the ratings of the internal components, resistive controls are sometimes sized close to the bone.

The real concern to me is what iceworm said about the connection pattern of the elements. If designed for 208V wye and the elements are rated 120V connected in a Wye configuration with the Wye point connected to neutral, using that on a delta supply will be problematic. If the wye point of the elements are not tied to neutral, it doesn't matter, same if the elements are connected in delta.

I would also be concerned for the control voltage. In 208V machines, they often just tap one phase and neutral for 120V controls. If you have a 240V 3phase 4 wire system, you must ensure that the control voltage tap is on B phase of your supply only.

We really cannot address this all adequately from afar, it needs to be looked at with an eye on the internal oven schematic.

* Lower of higher? The elements will be hotter, so they will heat up the chamber faster and not be on for as long, thus I said lower, but I'm struggling with that concept. The shorter on time might mean that if the oven cools at the same rate, they may need to come on less often. But if the higher temperature of the elements creates more convection, they may cool faster, thus making the elements cycle more often, meaning a higher duty cycle. I'm not sure here..

True it will heat faster making duty cycles shorter, but the watt density of the element will be higher. Question is can the the element take this higher watt density with all other conditions remaining the same? If you improve the heat dissipation - maybe add a blower, or heat sinking material to the element, or something along those lines it possibly allows an element to last longer that otherwise would not take the added heat very well.