Motor Space Heaters 1

[rinezry]

While reading through a client's spec recently, I came across the statement "anti-condensation heaters shall be rated at 240Vac but operated at 120Vac". Does anyone know how/where this statement came about and what are the reasons? Cheers.

 

[stardelta]

There may be 2 240v heaters connected in series, its not uncommon.

 

[edison123]

The space heaters are operated at lower voltages (typically 50%) to avoid high surface emission temperatures and to prevent red hot operation of the heaters. This is done to protect the windings. The heater output KW will reduce to 1/4th if they operate at 50% of rated voltage.

 

[rinezry]

Thanks for the info; is it stated in a standard (IEC, ANSI, API?) somewhere, or is it just a rule of thumb that people use?

 

[stardelta]

Why is there a need to drop the voltage to reduce the KW rating? Why not specify a heater with a lower rated output that will not effect the winding. I am looking at a brochure right now from a company that offers them in 11 different power ratings from 8 to 100 Watt at both 110 and 240 volt.

 

[ScottyUK]

stardelta,

Those ratings will probably assume an element temperature at or very close to the maximum that the element can sustain. This keeps cost down, but at the expense of running the element very hot to dissipate the power. A larger element can dissipate the same power but with a lower temperature rise. The method observed by rinezry keeps the power density down and thus keeps the temperature of the hottest part - the element - down too. This will extend heater life and prevent any reduce the risk of surface contaminants starting to burn.


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

[qutoe]Why is there a need to drop the voltage to reduce the KW rating? Why not specify a heater with a lower rated output that will not effect the winding.[/quote]
To further ScottyUK's response, it has to do with what is called "watt density". Lower wattage in a larger area of the heater strip means lower watts/sq.in., (or watts /sq.cm. for ScottyUK ;-) ) which gives you the lower surface temperature on the heater face itself. Just specifying a lower wattage heater at the same voltage ends up giving you a smaller heater element, so the watt density stays high.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[jraef]

Whoops, misspelled quote in the TGML tag! Ahhh, Mondays...
(I know it's Tuesday, but for us Americans, it is like a Monday because of the holiday).

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[itsmoked]

Haha Jraef.. Hung over again?!? <Just kidding.>

Why heaters at all? Why not PWM the windings these days?

 

[jraef]

Good point itsmoked (about the heater, not the hangover, an yes, that is a bit of an issue today ).

Rinezry,
If you already have a VFD, most new ones come with the ability to keep the motor warm from the inside by applying low current PWM or DC to the windings. this will heat the motor from the inside out, much more evenly than with space heaters, and it gets to the rotor bearings better as well. If you don't have a VFD, you can also get a separate electronic Motor Winding Heater that does essentially the same thing. Thye cost more than the space heaters as a component, but you don't need to run separate wires and conduit to the motor, so the installed cost is usually less. Motortronics makes one called the MWH Series.

http://www.motortronics.com/products/otpr.html

Allen Bradley used to make one as well, don't know if they still do.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[RRaghunath]

In India, the practice used to be 24V winding heating arrangement for 415V, 3phase motors rated 30kW and below(to prevent condensation). 24V single phase supply is connected to any two terminals of the three phase winding.

Now a days, the same is not popular any more. I don't really know the reasons. Currently, we specify no winding / space heating for motors rated 30kW and below . For above 30kW, it is space heaters rated 240V. I have not come across the stipulation that the space heater shall be rated 120V but connected to 240V. But, I think jraef is right, lower watt density is important in congested space inside the motor enclosure.

 

[weh3]

The heater operates off control power, which is 120 V, rather than line power. Your control power transformer is probably rated only rated 100 VA. It doesn't take much heat to prevent condensation.

William

 

[aolalde]

The whole idea is to keep the windings a few degrees above ambient temperature to avoid water condensation when the motor is idle.
The heaters must work properly when it is required, for that reason the probabilities of failure must be reduced to get satisfactory results. Watever is the most simple reliable way to keep the windings warm, that is the best solution.

 

[bacon4life]

When I had noticed we have some 240V heaters operating at 120V, I asked around and got several guesses (or answers?) including:

1 Simplified inventory management from only having one type of heater.
2 Simplified wiring changes when choosing to operate equipment at either voltage.
3 Prevent premature failure of strip heater due to operation at rated temperature.

Just specifying a lower wattage heater at the same voltage ends up giving you a smaller heater element, so the watt density stays high

Are strip heaters of proper wattage and heat density not available? It seems strange that the specifier would just count on not getting the equipment that is really needed.

 

[jraef]

Hmmm... why is this so difficult to understand?
OK, my final diatribe on this, I promise.

There is no need to make multiple versions of the same thing. It is well known in the process heater industry that if you want a lower watt density, just drop the voltage! I suppose that they could take a 240V 250W heater and put a label on it that says it is a 120V 62.5W heater to please the masses, but that's just a waste of time IMHO. Why not put a label on it that says it is a 60V 15.625W heater then too? Or a 480V 1000W heater? Its all the same unit.

Basic fact: an electric resistance heater is just a resistor built so that it doesn't burn up when power is applied to it continuously. Beyond that, "ratings" are just conveniences so that you don't need to get out your Ohm's wheel every time you want to know how much heat it will dissipate. The resistance is fixed, so the voltage you apply will determine the wattage of heat output. Since the surface are is also fixed, the watt density is controllable by changing the applied voltage. For convenience, the label on the heater says a wattage at a particular voltage so that you know where to start from, but beyond that it doesn't really matter. they could have just put a resistance value on the label and been done with it, but that is too confusing for the masses.

Re: my statement about this being the same as a 480V 1000W heater, higher voltages are sometimes a problem because of spacing of terminals, wire insulation rating etc. Many strip heaters are capable of having up to 600V applied however.

http://www.sdiohio.com/Pages/Technical Info.htm

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[aolalde]

Resistors used to heat electric motors are normally manufactured for other applications too.
Heat transfer factors will define the temperature rise of the resistive element.
The heat transfer performance is different if the resistive element is radiating heat free in air, submersed in a liquid or embedded in a metal body.
The heater manufacturer provides only the operating voltage and the watts produced at certain watt density or watts emanating from each squared inch of heated area of the resistor.
If the watt density is too high, the resistor or the heated material could be damaged. A heater with high watt density has lower life expectancies.
Basic thermodynamic principles must be understood to make a proper selection, other way duplicate the original motor manufacturer design.

 

[macmckim]

Another possible reason is if motor is used in Class 1, Division 2 area and engineer or end-user want to limit the space heater's sheath temperature to meet the T-Code rating that the motor will be operating in. Example T3C temperature (160 degree's C)

 

[mc5w]

Actually, a lot of strip heaters are rated for high temperature operation so that they will work well when heating metal plates or the underside of a corn cob meal parts dryer. ( A corn cob meal dryer works on the vibratory finisher principle and one machine that I encountered was design to also do vibratory finishing using steel shot. )

To get a lower temperature heater what you do is to operate the heater at 1/2 voltage and 1/4 heat. This can be done with a transformer or by connecting heaters in series.

In an artic climate you could run the heaters at full power for say 10 minutes to thaw out a motor that has been sitting in the cold for long periods of time. The higher watt density would not hurt the windings when the windings and bearings are below freezing.

Series parallel connection of heaters can also be done to effect 2 stage heating. For instance, a trailer with restroom facilities in it for a construction site needs very little heat between work breaks but lots of heat when the door is opened and closed a lot. For comfort you would want the heaters to have a long duty cycle so you would have them operate at 1/4 power series connection after the door closes. By switching off the water heater when the space heaters are at full power you can also do demand management just as long as the water heater has a big enough tank.

 

[acmotorengineer]

rinezry

the purpose of operating "space" type anti-condensation heaters at reduced voltage was a vain attempt to improve the abysmal life of cartridge type heaters. ( A wire encased in a refractory material ) Most of these heaters only lasted one year before they had to be replaced. A goodly number didn't survive the transport from the factory.

Around the 1970's most motor manufacturers switched to contact type, anti-condensation heaters ( originally designed and used in water beds ). These heaters were wrapped around the winding end turns. Since the heat transfer was so much better than heating the space around the winding, a much lower wattage was required. In addition the construction was, essentially, a wire wrapped by a silicon rubber insulation system. With the lower wattage and basicly a Class H insulation system, the life rating was/is better than the winding itself.

Thus the problem with short "space" heater life went away. Unfortunately the specification has remained -- with apparentley little understanding of where it came from.