Motor Space Heater voltage drop 7

[mikeengurs]

I have a 1187W motor space heater rated at 120V. I'm using 12AWG wire. The cable length is 450ft. I found a table indicating resistance for this size cable is 1.588 for 1000 ft, which means 0.7146 ohms for the 450ft. How is voltage drop calculated in this case, knowing that current will not be constant? Also, would less voltage cause any heater failure? I believe it's just gonna have less heat.

 

[dpc]

If there is just the resistance heating element at the motor end, it should not be a problem. You will have less heat than nominal, and the heater will last longer. But keep in mind that the watts output will vary with the square of the voltage. So if you have 90% voltage at the heating element, you only get 81% of the rated watts of heat.

You can calculate the nominal resistance of the heating element using the wattage rating and nominal voltage rating. That will allow you to model the circuit and calculate the current. It's an approximation because the resistance varies with temperature.

 

[mikeengurs]

Thanks dpc, that's what I thought. It'll be about little over 200 watts lost. Most likely this is resistive load, but how can I verify that without asking the vendor? All it says 1187 watts heater, at 120V, 1 phase.

 

[electricpete]

The space heater will be resistive. That's just the way it is.

Cables can be mixed, but as cable gets smaller, cable resistance tends to dominate over cable inductance. I think at 12 AWG you are probably safe to neglect cable inductance.

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(2B)+(2B)' ?

 

[waross]

That seem like a lot of heat for a motor. Are you sure that you are not replacing a 240 Volt rated heater? It is common practice to run 240 Volt heaters on 120 Volts for anti condensation service. Replacing a 240 Volt heater with a 120 Volt heater may result in motor damage from overheating and may damage control components which may be rated for the lower current (50%) of the reduced voltage. The motor does not have to be very warm, it just needs to be a few degrees above the dew point. Thermostats are not used on anti condensation heaters and may actually compromise protection. Condensations typically occurs with rising ambient temperatures and a thermostat may de-energize the heater when you need it the most.
Another issue is the cost of running the heater.
Running na anti condensation heater at 50% voltage results in 25% of the operating cost and a very robust heater.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I wouldn’t draw any conclusions about the required size of the space heater without knowing the size of the motor.

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(2B)+(2B)' ?

 

[mikeengurs]

Thanks everybody. Actually, the motor I'm concerned about is 900HP. The heaters are 672 watts (not 1187 as mentioned above). They are actually rated for both voltages 220 and 110 as seen in the attachement. is the 672 watts for the 220 or 110?

 

[petronila]

Hello Mike

I am with Waross The space heater could be replaced, and is Resistive, so the Space Heater watts: 1187, Voltage: 120, rated Current: 1187/120 = 9.89 amps,if you are ussing a 12 AWG this one is capable to carry 30 Amperes so is enought. The drop voltage in wire is 2 * 0.7146* 9.89= 14.13 volts so you willl apply to the heater: 105.87 Volts New Space Heater Watts: 9.89*105.87= 1047.05

The Space heater will work but the heat will be less.This is not a problem.

If you want a sharp calculation then use a 10 AWG wire or use the 12 awg and change the heater for 1000 Watts.

Regards

Carlos

 

[waross]

Hi Pete.
I have seen a few anti condensation applications.
I have observed that the location and mounting of the heater has more influence on the required size than the size of the motor.
Now that the size of the motor has been provided, 900 HP, I suspect that the heaters are mounted are not mounted in direct contact with the stator. That is a lot of heat to apply to a stator 24/7.
If the heaters must heat the stator indirectly, it will of course take a lot more heat.
For flexible sheet heaters or strip heaters mounted an inch or so below the stator, I usually find 200W or 300W. This is achieved by applying 120Volts to a 240 Volt rated heater of about 800W or 1200W.
I would be checking to see if the 120 Volts was originally applied to the 120V connection or the 240V terminals.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I have seen switch gear that needed heat to avoid malfunction during low temperatures. This was provided by a conventional heater application and controlled thermostatically.
Condensation often forms when the ambient temperature and the dew point rise faster than the thermal mass of the motor or switchgear and the dew point temperature rises above the lagging temperature of the large components. (The stator and rotor of generators and synchronous motors.)
For anti condensation protection of switch gear, a small heater energized at 50% voltage would be added to the freeze protection heaters.
As an example of the difference between freeze protection and anti condensation protection, remember that in the tropics condensation may form at temperatures above +80F.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikeengurs]

This is how I did my calculation for the 672 watts (assuming that's the rating for 120V). R=(V^2)/P = (120^2)/672 = 21.43 Ohm. I = 120/(2*0.7146+21.43) = 5.25A. Vdrop = 5.25*(2*0.7146) = 7.5V. So, the voltage will be 112.5V at the heater terminal.

I have asked the vendor if I should apply the 120V to the 220 or the 120 connection, and how much watts are needed to be applied 24/7. I will post the answer once I get it. Thanks for the help!

 

[waross]

Your calculation is close enough but may not be exact.
Use your voltage drop tables to determine the impedance of the cable. Add this to the heater resistance and calculate the current with the applied voltage. With the current calculated you may then calculate a more accurate value for the voltage drops.
Still not perfect.
For the ultimate solution you must know both the resistance and the inductive reactance of the cable. Add the resistance of the cable to the resistance of the heater and then use the inductive reactance to calculate the impedance of the total circuit, that is the heater/cable combination.
Given the range of copper resistivity depending on the purity and hardness and the change in inductance with spacing. Your first method is often acceptable for heater application. It is not exact but the error is small.
Sometimes it does matter, but probably not here today.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikeengurs]

Bill,
Thanks for your note. Actually, that's what i did. I added cable resistance obtained from a table to the heater resistance. But I multiplied the cable resistance by 2 to include the neutral return.

The vendor said, I should get 672 watts whether I'm connected to 220v or 110V terminal. I guess, they meant to apply the 120V to the 110V terminal to get a value close to 672 watts.

 

[waross]

OK follow the vendors instructions.
Personally I would have chosen a 2688 Watt heater and applied 50% of rated voltage for an output of 672 Watts and long life.
I have had failures but never burn-outs. Typically a
lead connection will burn off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I have a 1187W motor space heater rated at 120V. I'm using 12AWG wire. The cable length is 450ft. I found a table indicating resistance for this size cable is 1.588 for 1000 ft, which means 0.7146 ohms for the 450ft. How is voltage drop calculated in this case, knowing that current will not be constant? Also, would less voltage cause any heater failure? I believe it's just gonna have less heat.


....Thanks dpc, that's what I thought. It'll be about little over 200 watts lost. Most likely this is resistive load, but how can I verify that without asking the vendor? All it says 1187 watts heater, at 120V, 1 phase.

That seem like a lot of heat for a motor

I wouldn't draw any conclusions about the required size of the space heater without knowing the size of the motor.

Hi Pete.
I have seen a few anti condensation applications.
I have observed that the location and mounting of the heater has more influence on the required size than the size of the motor.
Now that the size of the motor has been provided, 900 HP, I suspect that the heaters are mounted are not mounted in direct contact with the stator. That is a lot of heat to apply to a stator 24/7.
If the heaters must heat the stator indirectly, it will of course take a lot more heat.
For flexible sheet heaters or strip heaters mounted an inch or so below the stator, I usually find 200W or 300W. This is achieved by applying 120Volts to a 240 Volt rated heater of about 800W or 1200W.

Are you suggesting motor size is irrelevant to selecting space heater size?
If so, I would respectfully disagree.


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(2B)+(2B)' ?

 

[waross]

No Pete. I am suggesting that the placement of the heater is much more important than the motor size. I haven't seen everything, but I have not seen 1000 + Watts applied for anti condensation protection.
I have seen quite a few heaters rated over 1000 Watts supplied with low voltage for about 1/4 of the nameplate output and long life.
It appears that the heater in question is intended to supply full output. So be it.
In the applications that I have seen with similar sized machines I would be worried about winding damage if the heaters were supplied with full rated voltage.
I expect several hundred Watts on a machine that size. I have seen 50 Watts applied to machines in the 20 KW to 50 KW size range.
Despite possibly mis-speaking, experience may count for something.
Come to think of it, isn't that what Eng-Tips is about, sharing experience. Is not part of that putting up warning flags when something seems out of the ordinary even though it turns out to be OK in the end.
It's probably time for both of us to take a coffee break.
See you later.
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I thought is was worth clarifying what I interpretted that you were first ignoring and then downplaying (the role of motor size in the discussion). As a wise man once said “Is not part of that putting up warning flags when something seems out of the ordinary”.

Sorry if somehow that came across in a bad way.

Moving on, I will state my personal opinion: Size of the motor is probably the most important single factor in selecting the “size” (operating watts) of the heater. As far as factors related to localized heating: it can be addressed by addressing the configuration of the heater in a number of ways you have mentioned: heaters operated at half voltage to reduce surface temperature, heaters standing off a safe distance from anything delicate or combustible, or just purchasing the large surface area heaters that have low max surface temperapture even when operated at their own nameplate voltage. But once this factor is addressed through selection of appropriate configuration (and however it is addressed), the heater sizing is still based on the size of the motor and other physical/geometric/environmental factors which affect the heat transfer problem of keeping the winding warm.

Just for general discussion.
Attached Slide 1 is one example selected because it is similar size motor to the one discussed here:
800hp, 900rpm, 4kv vertical outdoor open/wp2 induction motor.
Contains 6 heaters, each with rating of 240vac, 500W.
The heaters are hooked to ~120vac, so the operating watts is 6*(500/4) = 750 operating watts
We confirm by clamp-on periodically that the current is approx 6 amps, which corresponds to 720 watts at 120vac

Attached I have winding temperature traces (Yellow/Blow/Green) and ambient temperature in purple.
Daily variations in winding temperature are larger when running than while secured. This suggest that while secured, thermal time constant is longer than while running. Due to long thermal time constant of secured motor, it may not “keep up” during rapid temperature increases of warm front and therefore may go below ambient temp. In my neck of the woods, warm front is often humid front near 100% humidity where dewpoint temp ~ ambient temp. So if space heater allows motor winding temp to drop below ambient, then it is also below dewpoint. This illustrates the point “Condensation typically occurs with rising ambient temperatures”. Is it acceptable... probably a matter of opinion. The average winding temp is roughly 9F = 5C above ambient which I think was a target stated in an old IEEE standard.

Another case, slide 2.
3500 hp, 324rpm, 13.2kv vertical outdoor motor open/wp2.
24 space heaters with nameplate rating 240vac, 750 watts.
The heaters are connected in delta.. each delta leg has two parallels, with four series heaters per parallel.
Each space heater sees 480/4 = 120vac = half nameplate voltage, and has an output 750/4 = 187.5 watts. Total operating wattage: about 4,500 watts. It measures about 6 amps at 480vac, which confirms this operating wattage. It is even higher than one watt per horsepower.... but at 324rpm the physical size of this motor is much larger than we’d guess from the horespower (weight is about 50,000 pounds). Based on slide 2, this massive amount of space heaters does a better job at keeping motor temperature above ambient than the first motor. You might even argue we have more space heaters than we need. But at 13.2kv, I don’t mind the extra margin and it doesn’t hurt anything (other than space heater operating cost which is miniscule compared to cost of motor failure).


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(2B)+(2B)' ?

 

[waross]

I think that we are in agreement and just discussing different experience and details.
Thanks for sharing the graphs.
A couple of comments;
I am more concerned with the iron temperature than the winding temperature. I may be in error.
It seems that two windings are quite warm and one is very close to ambient. Am I reading this correctly? Is there a measurement glitch that is giving an erroneous low reading on one phase?
I am used to seeing the enclosures quite warm to the touch. (Didn't have the advantage of your instrumentation in the third world.)
1000 Watts in one heater would worry me. 1000 Watts total but distributed through several individual heaters would be closer to my comfort level.
I see that we are in agreement as to reduced voltage for anti condensation heaters.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I am more concerned with the iron temperature than the winding temperature.

With the motor shut down, I don’t think there is much difference between the two. Winding is in intimate contact with core iron in the slot.

It seems that two windings are quite warm and one is very close to ambient.

It wasn’t well explained. I tried to find a period of time where all motors were switched (either on to off or off to on). The high-winding-temperature condition is running, the low-winding temperature condition is shut down.

Am I reading this correctly? Is there a measurement glitch that is giving an erroneous low reading on one phase?

Only one winding temperature is displayed per motor (we have only one computer channel per motor... we manually read all 6 RTD’s with motor running and hook up the hottest one to the computer channel) . There is a glitch on the abmient temperature where it erratically jumps off-scale high or low. Also it looks like motor consistently reads a few degrees low on slide 1.

I agree with your comments. Good disussion.


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(2B)+(2B)' ?

 

[waross]

A little mutual respect goes a long way.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter