Motor Protection Types - Short #192

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 16 April 2024

⏱️ 10 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast episode, Bryan talks about motor protection types, including overloads.

The most common overload we see in residential HVAC is a built-in thermal overload, which is usually a bimetallic disk that flexes in response to heat (such as from a locked condition, electrical problem, or simply running hot) and opens the circuit. The two metals have different expansion and contraction rates, which causes the flexing; they will return to their original position once the motor cools down. In some cases, these can fail when they open and close too often; they are not designed for switching duty. Many circuit breakers have a similar thermal design and may be prone to nuisance tripping in the summer.

A lot of commercial motors rely on external overloads; some are even built into the electrical box rather than the compressor. These external magnetic overloads are often integrated into the contactor, which turns the motor on and off; this type of contactor is called a starter. These starters may have adjustable overload settings based on current, not just temperature (which may also respond to nuisance sources of heat and require a cooldown period). Some circuit breakers also trip magnetically and are less likely to be affected by temperature.

Thermistor-based overloads usually consist of a PTC (positive temperature coefficient) resistor; as temperature goes up, resistance goes up, which can take a motor winding out of the circuit. NTCs are in separate parallel circuits with relays; as the resistance decreases, it pulls in a coil that opens the circuit.

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Transcript

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0:13.2	Hey, this is the H.V. A. A.C. School Podcast. I'm Brian, and I'm still sick. This is like the longest sickness ever. Anyway, you're not worried about that. Today we're going to be talking about motor protection. Overloads, overload types, because it actually does
0:17.8	matter and it is the sort of thing that you're going to need to think about,
0:20.7	especially if you're used to working on one type and then you go to a
0:23.4	different type and you're going to notice that there's some variation.
0:25.8	So before we do that, we're going to thank our great brand partners.
0:30.0	Fieldpiece, fieldpiece.com. And specifically the SR-47 wireless refrigerant scale, the new
0:37.2	heavyweight champion of digital scales. Refrigeration technologies at Refriger tech.com.
0:44.1	Carrier and Carrier.com
0:46.1	Navac at navac Global.com
0:49.6	All right, so the most common type of overload that you are used to if you work in
0:56.7	residential is an overload that is built into a motor your condenser fan motors your
1:02.4	blower motors and most specifically your compressors, and that
1:06.0	overload is a thermal overload. So it's a bimetallic disc, generally speaking, usually pretty small,
1:12.1	and they're usually positioned in the windings directly
1:15.8	behind the common terminal on a PSC type motor.
1:19.2	Now that's again, the PSC type motor, so you're not talking aboutter driven, you're not talking about ECM, this is just the basic type of motor that we've worked on for years. And what happens is when that motor overheats due to a locked condition or some electrical problem or just regular old overheating
1:39.0	Maybe that motor is running hot because it's low on charge in the case of a compressor or it's in an environment that it wasn't designed for.
1:46.7	Maybe it's running too hot because it's just in a super high ambient condition that it was never designed to be in.
1:51.5	It's going to open that circuit and shut off that motor.
1:56.2	So that is a thermal overload. That is a bi-metallic disc, bi-metal disc.
2:01.5	And that bi-metallic overload is very very simple it just has two different types of
2:05.0	metals that's the bimetal part plastered onto each other and so they have different expansion
	...

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