Motor space heater requirement 1

[Mila2015]

Our specification indicates motors 20HP and above should be equipped with a space heater. I believe this has to do with a balance between price Vs condensation issues. My question is, how to determine when a motor space heater is required? I know we're trying to keep non-running motor temperatures above dew point...but what factors should be considered to determine this requirements? Do we need to consider historical area environment conditions for example?

 

[waross]

Any area where the temperature may rise significantly in a fairly short time is a candidate for heaters.



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

 

[jraef]

Or where the running and hot motor is turned off in an environment where there is cooler air and moisture. Dew point is variable with Relative Humidity.

There are devices called "Motor Winding Heaters" that trickle a current controlled low voltage DC into one winding of the motor when it is not running to use the motor windings themselves as the space heater.
Might be worth looking into. Motortronics makes them, as well as a small company called DNH in Los Angeles. If you want to "rollm your own", this article gives you the data on sizing the components.

https://acim.nidec.com/motors/usmotors/techdocs/profacts/heaters-tricklevoltage

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

The worst case is when the air temperature rises faster than the temperature of the iron and copper.
There is a time lag as the temperature of the internal iron lags behind a rising ambient temperature and dew point.
Once the dew point rises above the temperature of the internal iron you will have instant condensation, which will continue until the released latent heat of vaporization warms the iron up to the dew point.

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

 

[crshears]

"Space heater" is a misnomer; the intent is to warm either the motor windings à la method jraef described, or its iron by means of a direct contact strip heater that is in service whenever the motor is idle. I've been told the "space heating" approach is sub-optimal.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mila2015]

Thanks all! since it seems like we really can't predict what temperature will rise faster (motor or air), this issue exists in all cases, and affects every motor installed in non-temperature controlled conditions. In reality though, the majority of external installations I've seen (especially for smaller motors), space heaters are rarely used. Why do you think that is? Is it an issue of cost? like it's cheaper to replace a failed small motor than to install space heater circuit? Or condensation is not really an issue for some reason? I've worked in a major facility where space heaters weren't common, and don't recall having major issues that I knew off! It's maybe that current carrying conductors being insulated mitigate this issue?

 

[waross]

You don't get a lot of condensation inside a Totally Enclosed Fan Cooled motor.
The heaters are typically 240 Volt heaters energized with 120 Volts.
1/4 the Watts but they last forever.

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

 

[Hoxton123]

I think it is cost driven, and a need for a simple life. Remember that a space heater requires a separate power supply, from the motor and possibly a local isolator for this supply. The supply is normally switched automatically off when the motor is energised. Another layer of complexity.
Hazardous area location can lead to even more complexity...

Just a few thoughts:

Larger motors often have a higher capital cost and longer lead time.
Smaller motors can often be easily replaced with off the shelf items.

The selection of a rating criteria above which heaters are mandatory can be arbitrary, and dependent on local circumstances and experience.

The smaller the motor the lower the thermal mass, and the quicker it will heat up from ambient; larger motors will have more mass and be slower to heat.

Very often smaller motors are located indoors and may be subject to an effectively constant ambient temperature.

 

[Compositepro]

It is a matter of surface area to mass. Larger motors have much greater mass to surface area ratio, and thus the temperature of the motor will have a greater lag behind behind the ambient air temperature. It is not actually the rising air temperature that is the issue, it is the rising dew point, although the two are often correlated. Heaters are more important in climates with high relative humidity.