Why Space Heater for Induction Motors

Dear all,
Why a space heater is required for Industrial Induction Motors?
My ans. is to avoid condensation. ok. i agree. If so Why space heaters are not provided for lower kW motors (say 1.5, 1.1 kW etc) . Why only for higher kW motors space heaters are provided (say 132kw above). Even all motors are to be located at the same Ambient Temperatures ( say 5 Deg to 45 Deg Max). For the same ambient temperature at site, in my opinion all motors should have condensation heaters. Could any body can please explain the philosophy behind the basis of above ?..............
---Govind

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-------------- Why should space heaters be required for any induction motors?
It seems to me that cooling is a more important consideration in most situations. The problem with condensation implies that the motor is cooler than the (humid) ambient air around it. A somewhat unusual situation considering that a motor is a heat source.

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Years ago, I bought a couple of 500 HP weatherproof motors for a dust collection system and ordered space heaters for them because they were installed outdoors; I wanted them to stay dry if the motors were ever shut off, especially in damp weather. Another gadget you can use instead of a space heater is a winding heater that passes a few per cent of rated current as DC through the winding when the motor is otherwise de-energized; this heat keeps the windings warm.
I've seen space heaters as an option on smaller motors, too.
Bill

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My experience is that if the motor is located in a humid space and subjected to changes in ambient temperature (All whilst not energised) the heaters are a good precaution, particularly as the motor ages. An example would be an outside 'weather proof' pump motor that would be cold overnight and warmed by the sun. Also once again the problem of condensation arises if the unit only gets random or occasional use like transfer pumps or sump pumps etc.

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Hi all...,,,,,,,
I agree with all your answers i.e., 1. Heaters are energised when motor is in idle - while not running 2. Even though motor is placed outdoor or indoor 3. Irrespective of Enclosure whether WP or TEFC etc 4. Not only in humid place - also for place where humid condition expected to occur 5. I also agree with heating by means of winding itself by injecting lower voltage of DC / AC
Please note my question that i need to have the basis to FIX A kW rating of a motor i.e from what kW rating i have to provide space heater???????????. Some Electrical consultants are providing space heaters for motors of 75kW and above for a particular region for the same region another consultant advising to provide space heater for 110kW and above rating. Why this controversy. what is the basis please explain,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Govind

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My experience with space heaters (BTW, I've also seen them used in transformer enclosures) is a couple of different criteria. I would say the biggest one is type of service. If the motor is off a fair amount of time (either a 'standby' unit, or emergency unit), then space heaters are almost mandatory. Sitting for months, indoors or out, the insulation used in many such machines will absorb moisture from the air and that lowers the insulation resistance. Keeping the winding temperature just a few degrees above the surrounding ambient will dry out the unit and keep it dry.
I think another key question is the insulation class versus the service voltage. Small, fractional hp units tend to be lower voltage (up to 600V), while really *large* machines can be 4160 or 13.8kV. With the lower voltages, even an insulation resistance as low as 50kOhms results in not much leakage current or localized heating. But the same insulation resistance at the higher voltages can result in 50x or more localized heating which can rapidly lead to breakdown. So with small units, you start it up and it dries out while running without much damage. Large units, you start it up and the insulation breaks down in some 'hot spot' before it can dry out and you spend $$$ to rewind the unit.
Whether the winding insulation absorbed moisture and how much depends on a lot of things. Type of insulation, humidity (not so much RH but absolute humidity), how much temperature 'swings' up and down from night/day, and whether the windings are kept warmer then the surroundings.
For semi-longterm layup (major outages, overhauls, etc...), I've seen the use of simple, 40 or 60 watt light bulbs placed inside an inspection plate, or underneath the motor with a tarp draped over it. Then someone would check the bulb was lit once each day. It doesn't take a lot to protect the unit and it can save a bundle.
How big a unit before you need them? How 'rough' the service (humidity, temperature cycle, etc...)? I think that comes down to 'sound engineering judgment' and 'rules of thumb'.
When I was in the Navy, we used to have a lot of problems with salt-water. Just drying out a motor that was wet down with sea-water wasn't enough. After drying, just sitting open to the air, the salt would 'draw' moisture back into the insulation (salt is 'hygroscopic', absorbing water out of the air, check a salt-shaker on a humid day). So we had to first flush the motor with deionized water to flush out the salt, then dry it (either oven, warm air, or low current).
daestrom

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IMHO the use of heaters is governed by the duty and location of the motors not by the voltage or KW. As mentioned if the operating voltage is above medium then the heaters are at least good insurance. I think in practice the decisions are made by the design engineers, either at the product end or the site.

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Don't know what they do in the oil fields but my concern was that the WP motor frame lets air freely circulate around the windings. My scenario was a summer shutdown when the motor would be down for a couple of weeks, taking in whatever dew and rain happens during that time, then soaking the insulation and blowing up when powered up again. True, the manufacturer did tell me that the windings were hipotted under water - encapsulated. Maybe it was over compensation.
An almost entirely unrelated problem happened at the same plant some years before. They found that motors left shut off during a -35 C cold snap destroyed their bearings when restarted; the particular motors shrank enough under the cold condition to damage the bearings. Changing motor manufacturers was sufficient to solve the problem. But if you plan on leaving a motor switched off for weeks in cold weather, it's worth putting that into the specs.
Bill

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Thanks Bill, Daestrom and Rheilly You have all provided good and succinct answers.

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Thanks Bill, Daestrom and Rheilly and to Don Kelly
i came to understand that a space heater requirement for motor is basically depend on atmospheric condition. However i basically raised this question keeping in mind that if we go for higher kW motors cold start will be a problem that is motor will stall for more time during cold start.
ok. thanks for all.

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Winding heaters are common in outdoor motors but like you say, I remember them primarily in motors of 25 H.P. and above. This is probably because smaller motors are not looked upon as such a capital investment. The cost of the motor, and the effort required to outfit them is comparitively very low for smaller motors. Also, the loss of large motors is often a bigger event from the standpoint of what they are driving. Typically the largest motors in a pumping plant for example, are at the heart of the system, and are a major problem if they stop. Particularly if there is little, or no backup or redundancy in the design of the plant. Smaller motors are likely an "off-the-shelf" replacement if they fail, where a large motor might be quite a while in being re-wound etc.

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You raise some good points. Another thought to add, getting replacements and the lead time involved. Small, standard sized motors can be ordered from a variety of suppliers and shipped readily. Once you get into large units, replacements aren't available sitting in a warehouse, they have to be made-to-order. Or the damaged unit taken out and re-wound. This can take weeks of down-time.
daestrom
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