Heaters in AC Motors

There are, but usually only larger motors.

What is the statistics? As you have more population of AC Motors, you com across more AC motors with Space Heater too. And, as the population of the DC Motors is lesser, probably, the DC motors that you have come across had no heaters. Logically, both should have.

OK, you got me curious. Having lived essentially all my productive life in California, I'm not aware of heaters in any motors, DC or AC. I assume that is because heaters are not needed in our climate. Are they put there to prevent moisture intrusion when the motor is not running? Since copper is a better conductor when cold, I suppose the starting current could get excessive at some point. My ignorance could also be due to the size of motors I deal with; I don't think I've worked with anything larger than 25 or 30hp, and the vast majority have been 2hp or smaller.

Yes! It is to avoid moisture condensation inside the motor. Moisture can create tracking and subsequent insulation failure. Space Heaters would dry out the moisture. More than the worry about the excessive starting current - which, of course is only momentary - my main worry would be the prospective damage to the motor winding insulation. And, yes! As rightly noticed by you, they are inserted in motors beyond a certain HP - say, 50 HP - to be cost-effective.

In My limited experience with large HP motors "Heaters" were in the control panel and were acctually, I assumed, acted like a fuse. As I recall they were gaged in amps just like fuses. Am I wrong?

oilcan13

Heaters keep the bugs warm at night.

The heaters referred to are not for heating the motor. They are very small and placed in series with the supply lines of the motor in question. They are installed in the motor starter (the relay that acually turns the motor on and off). They keep closed one or more sets of contacts wired in series with the control circuit. As long as the motor is drawing it's rated current or less everything runs fine. If the motor is overloaded, the current draw increases creating more heat. An internal component of the heater then melts allowing the contacts to open thus dumping the control circiut. This, in turn, opens the starter contacts shutting off power to the motor and preventing a big (expensive) smoke show.

This is exactly what I was Thinking (reply#5)

oilcan13

<Insert standard rant about using vague terminology>

Thanks for clearing that up. had you been more clear at the outset the tangential nature of our group could have been avoided...

To answer your question, there are "heaters" in DC motors, but only if they are single speed directly controlled DC motors. That type of application however is virtually non-existent now, AC is much cheaper to deal with for fixed speed applications.

So what you are probably observing is that DC motors are primarily used in variable speed applications now, and that then implies there is a "DC Drive" that converts the AC line supply from the utility to DC. Once you have an electronic device like that, taking care of motor overload protection is easily done with the electronics, which is much much more accurate, reliable, repeatable and adaptable. So there is no reason to have "heaters" in modern DC drives. Older ones did have them, but also realize that in a 3 phase AC motor starter you had 3, in a DC controller you would have only needed one. You may not have realized it was there (if it was).