Low Voltage Small Motors in Hazardous Area - Anti condensation Heaters

[NickParker]

We have a spec that says to provide low voltage motor as small as 5.5kW to be equipped with anti condensation heater and the motor starter schematic of this low motor does not have any thermistor relay to measure the heat in the windings that can cut off the anti condensation heater when the temperature in the windings exceed ambient temperature. For info, the motor is to be located at hazardous zone in an offshore FPSO.

Any precautions / relays in the starter schematic to be added / modified?

 

[bacon4life]

Is comparing a temperature differential between a winding thermistor and an ambient temperature a common approach? I had assumed the motor heaters usually were turned on whenever motor was turned off. I would be concerned whether some parts of the motor may cool off faster than where the thermistor is located.

Also, take a look at

https://www.eng-tips.com/viewthread.cfm?qid=474141.

Lots of cautions about using thermostatically controlled anti-condensation heaters. Condensation in the tropics still occurs under hot ambient conditions. Perhaps thermostatically controlled heaters would be OK if the thermostat is permanently set higher than highest possible ambient temperature. I have seen switchgear anticondensation heaters with the thermostats adjusted as low as possible by "helpful" people trying to save energy.

 

[nhcf]

NEC 501.125 lists the requirements for motor heaters in hazardous locations (if is a Class I location)

 

[Gr8blu]

NickParker
The motor manufacturer should be sizing the heater to bring the entire motor frame (including the windings) to roughly 5 C above the dew point. That means they have to know where the machine is going to be installed, so they can estimate the dew point.

Your bigger problem is going to be getting enough surface area on your heaters to keep THEIR surface temperature below the auto-ignition temperature of the local environment (presumably gaseous). For a T3 limit (200 C max surface temperature), it usually means operating the heaters at half voltage - which means four times as many heaters to get the same total thermal capability. In a small machine rating like 5.5 kW, you'll run out of physical space before you get anywhere near enough heater inside the machine enclosure/frame.

The windings are not going to be damaged by temperatures in excess of ambient conditions - provided the heater is not in direct contact with the winding surface. After all, the insulation system is presumably designed for either 155 C (Class F) or 180 C (Class H) total temperatures at the winding. Having it sit at 60 C because of a heater is not even going to make it break a sweat (pardon the pun).

All you really need to do is make sure the heater circuit is OPEN (e.g. OFF) before operating the machine under appreciable load.

If you do choose to go the thermostat-controlled route - be aware that certain code restrictions often result in building certain fail-safes into the control system. One of those might result in turning off the heater - and letting the winding suck up moisture to the point where it fails on startup.

Converting energy to motion for more than half a century

 

[waross]

Switchgear heaters. Switchgear may have thermostatically controlled heaters to avoid operating problems in cold climates.
These heaters are used to keep the temperature of the mechanism above the point where mis-operation may occur.
Cold weather heaters are a greater capacity than anti-condensation heaters.
I have seen switchgear with both thermostatically controlled heaters, and 24/7 connected anti-condensation heaters.
The issue with motors and generators and condensation is thermal lag.
In conditions of rising temperatures and dew points, the thermal lag of the core iron may lag behind the dew point even though the ambient temperature is above the dew point.

The motor manufacturer should be sizing the heater to bring the entire motor frame (including the windings) to roughly 5 C above the dew point. That means they have to know where the machine is going to be installed, so they can estimate the dew point.

I find that statement a little misleading on two points;
The dew point is dependent on both the ambient temperature and on the relative humidity. Estimating a widely variable effect may not be particularly valuable.
The heat added by the anti-condensation heater keeps the core above the ambient temperature. When the ambient temperature rises without the addition of moisture, the relative humidity drops.
The dew point rises when moisture is added to the local atmosphere.
The purpose of the anti-condensation heater is to combat the thermal lag of the core and keep the core temperature above the dew point when both the ambient temperature and the dew point are rising.
An example may help:
My greatest concern for condensation in our generators was a combination of a chilly night followed by a rapidly rising ambient temperature on a cloud free tropical morning, and a strong, moisture laden on-shore breeze.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!