Convection Oven rating

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Grouch1980

Grouch1980

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New York, NY
I attached the ratings for a convection oven, this is from the cutsheet.

My 2 questions are:
1) Is the 4000 watts rated for 208 volts, AND 220-240 volts? I don't have to derate the power rating by 25% (from 240 volts down to 208 volts), since this 4000 watt oven is rated for all voltages?
2) It has a nameplate value of 17 amps... I'm assuming though this is the amperage at the max voltage... 4000 watts / 240 volts is about 17 amps. If we don't have to derate the wattage, since the oven is rated for all voltages, is the amperage at 208 volts = 4000 watts / 208 volts = 19.2 amps?

Just making sure I'm reading it right.

Rating.jpg
 
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don_resqcapt19 said:
If the resistance is fixed the wattage changes by the square of the change in voltage.
(208/240)² = 0.75 so the wattage at 208 will be 4000 x 0.75 or 3000 watts.
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Makes sense.

Ok, so the 4000 watts on the cutsheet only applies to the 240 volt rating. As well as the 17 amps. (4000w / 240v = 17a)

They make it seem as if the 4000 watts applies to all the listed voltages.
 
You really don't know without seeing the actual datasheet.

Most heating elements are simple resistors, following the square law that @don_resqcapt19 gave.

But with electronic control you might be seeing constant current or constant power loads.

Jon
 
If it's a constant-resistance (resistive) load, the current will vary proportionately with applied voltage.

If it's a constant-power (inductive, i.e. motor) load, the current will vary inversely with applied voltage.

The current of a mixed load could vary anywhere above or below, depending on the proportion.
 
LarryFine said:
If it's a constant-resistance (resistive) load, the current will vary proportionately with applied voltage.

If it's a constant-power (inductive, i.e. motor) load, the current will vary inversely with applied voltage.

The current of a mixed load could vary anywhere above or below, depending on the proportion.
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I would expect that for motor loads, you'd have to adjust a tap setting on its windings, so it can operate at a lower voltage, and deliver the same mechanical power to the load. Is there a working principal of motors that makes them automatically do this, without adjusting taps on their windings?
 
Carultch said:
Is there a working principal of motors that makes them automatically do this, without adjusting taps on their windings?
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Yes. Motors tend to be constant-power loads. That's why a 208/230v motor has a higher current rating at 208v than at 230v. To wit:

1697999575676.jpeg 1697999607881.jpeg
 
Grouch1980 said:
Makes sense.

Ok, so the 4000 watts on the cutsheet only applies to the 240 volt rating. As well as the 17 amps. (4000w / 240v = 17a)

They make it seem as if the 4000 watts applies to all the listed voltages.
Click to expand...
Since a simple oven is primarily a resistive heater, the maximum power at 208V becomes 75% of its maximum power at 240V. Yes, you read that correctly, even though one might think it should be 87%. Volts directly determine amps, and the product of the two, will determine power, and thus the 87% is squared.

For this reason, ovens tend to take longer to preheat when running them in apartments on 208V supplies. The steady state operating power will still average to the same amount, since it is a thermostatically controlled heating element. It runs with a greater on-time during its cycling, and has a lower maximum temperature than it would have with a 240V supply. The line-to-neutral load inside it, will power the lights, controls, and if applicable, the fan motor, since these are more voltage sensitive.

You'd have to have engage additional heating elements in parallel to make it consume the full amount of power at 208V. A telltale sign of an oven that does this, is if it has different current ratings as a function of voltage.
 
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Carultch said:
I would expect that for motor loads, you'd have to adjust a tap setting on its windings, so it can operate at a lower voltage, and deliver the same mechanical power to the load. Is there a working principal of motors that makes them automatically do this, without adjusting taps on their windings?
Click to expand...

Over a sufficiently narrow voltage range, yes. ('Sufficiently' depends upon the specific design of the motor.)

As you reduce the voltage to the motor, various factors act to increase load current. For example in an induction motor, if you lower the applied voltage the magnetic field strength will go down, and slip will increase to maintain torque. The increased slip means greater current flow. Net result is that the motor presents an approximately constant power load even though the voltage is changing.

Another way to think of this is that the motor acts to convert electrical power into mechanical power. Over a sufficiently narrow voltage range, the _efficiency_ of the motor remains constant, so for constant output power the input power remains constant.

There are limits to this; the maximum available torque from a motor will vary as the square of the applied voltage, and if the applied voltage gets too high to motor saturates. So for large voltage changes you need to change motor connections or 'taps'.

A very common induction motor design will operate over say 200-240V, permitting operation with 208 or 240V supplies. However by changing taps the motor can be used at 480V (and presumably 400V as well, though this often isn't on the nameplate).

-Jon
 
winnie said:
A very common induction motor design will operate over say 200-240V, permitting operation with 208 or 240V supplies. However by changing taps the motor can be used at 480V (and presumably 400V as well, though this often isn't on the nameplate).
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The change from 240V to 480V is surely switching two internal motor coils from parallel to series, yes?

Whereas taps makes me think of changing the length of a coil by a much smaller fraction, like 5% or 10%. Are there commercially available motors that have taps in that sense, say to operate well at 240V vs 208V, rather than just being designed for some compromise intermediate voltage between 208V and 240V?

Cheers, Wayne
 
wwhitney said:
The change from 240V to 480V is surely switching two internal motor coils from parallel to series, yes?
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Yes. I almost mentioned this earlier, that "taps" was not the correct terminology.

That's where 9- and 12-lead 3ph motors come into play. That and wye/delta use.
 
Yes. I was (perhaps incorrectly) stretching the term 'taps' to include different gross reconnections of the coils in the motor.

I've never encountered a motor with separate 'taps' in the transformer primary sense in order to operate at slightly different voltages. I've only encountered motors which entire coils get reconnected in different patterns to get different voltages.

-Jon
 
ramsy said:
3kw my be true, but NEC 422.10 prohibits applying Ohms law to listed ratings.

4kw rating requires #12 branch conductor w/ 20A OCP, not #14 w/ 15A load per Ohms law.
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I never suggested that the actual current at 208 volts had anything to do with the require branch circuit rating.
The product standard should be changed to require both currents where an appliance like this has two voltage ratings.
 
don_resqcapt19 said:
The product standard should be changed to require both currents where an appliance like this has two voltage ratings
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Yes, non-standardized product spec may only show highest ratings.

For dual voltage appliances, I see most inductive-load nameplates show both current ratings, resistive-load nameplates show both kw ratings, but single voltage microwave nameplates show input kw & output kw ratings for an inductive-load appliance.
 
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