Reversible electric motor - what am I looking at?

AI Thread Summary
The discussion revolves around a vintage reversible electric motor used for an automatic skylight, specifically focusing on the unidentified rectangular gray components secured with a zip tie. These components are identified as 27-ohm power resistors, likely used to limit the motor's maximum current and torque. Some participants suggest they may be fusible cement resistors, while others clarify the distinction between cement and cermet resistors. The conversation also touches on the potential role of a series diode in providing directional control of the torque limit. Overall, the thread emphasizes the importance of understanding these components for the motor's functionality.
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What are these components on my reversible motor and how is the whole thing supposed to work?
I have a reversible electric motor that operates an automatic skylight, vintage 1985. I'm trying to figure out how it is supposed to work, but I don't recognize the rectangular gray components that are secured to the motor case with a length of plastic zip-tie. What might they be and how would the whole thing work?
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They are 27 ohm power resistors, probably to limit maximum motor current = torque.
 
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Nugatory said:
I don't recognize the rectangular gray components that are secured to the motor case with a length of plastic zip-tie
They look like fusible cement resistors, ##27~\Omega,\ 9~\mathrm{W}## with ##5\%## tolerance.

Edit: @Baluncore beat me to it.
 
Wrichik Basu said:
They look like fusible cement resistors
Probably just a typo, but I believe you are referring to Cermet.

That is a compound of cer[/size]amic and met[/size]al, which combine to give characteristics of both.

Further info: https://www.google.com/search?hl=en&q=cermet

Cheers,
Tom
 
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Tom.G said:
Probably just a typo, ...
Maybe not.
I think you will find the higher power, low value resistors like those are wire wound on the inside, with a ceramic or cement case to reduce wire oxidation and fire hazard.

Cermet resistors are usually more expensive, with power 1 watt or less, higher resistance, and lower inductance.

Destructive testing would resolve the question.

The series diode is interesting. Does it provide directional control of the torque limit?
 
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Tom.G said:
Probably just a typo, but I believe you are referring to Cermet.

That is a compound of ceramic and metal, which combine to give characteristics of both.

Further info: https://www.google.com/search?hl=en&q=cermet
I meant cement. I bought a few of those some months back to help in my experiments. In fact, I had never heard of cermet resistors; thanks for the information.
 
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