Bearing Failure Leads to Cooked Winding

I ran across a failed capacitor start motor on an air compressor recently. It is obvious that the start winding got barbecued (see picture).

A student asked me how I knew right off that it was the start winding.  Notice that the winding which is burned has smaller gauge wire than the winding that appears OK. The start winding in single phase motors is constructed of smaller gauge wire than the run winding and has fewer turns. It is tempting to call this an electrical failure after seeing the cooked winding. However, most motor failures can be traced back to bearing failures.

Disassembling the motor we saw that the rotor had been dragging – a sure sign of bearing failure. (see picture)

The lead end bearing was to blame in this case. You can see that the dragging all took place on the lead end of the motor. Taking a close look at the stator you can see where the rotor has been rubbing the stator. (see picture)

This can cause two types of winding failures: one where the rotor knocks some of the metal layers into the slots where the windings are, and another where the rotor stays magnetically locked down at startup, which is what I believe happened here. Even though the rotor turns easily by hand and there is no play that can be casually observed by hand, it is obvious the bearing was allowing the rotor to touch the stator. If this happens on startup, the reaction will be like two magnets with opposite polarity pulling together. The motor will lock down, draw high current, and heat up.

Motor Rotation

Many single phase motors can only turn in one direction. For example, pump motors and fan motors. Since the pumps and fans they operate only work in one direction, the motors that drive them re usually built for one direction.  This can pose a problem for service techs when replacing these motors. Often, service motors solve this problem by being reversible. However, OEM replacement motors are generally not reversible, so you must specify the correct motor rotation. To do this you need to understand the terminology that is used to describe motor direction.

There are only two possible rotations: clockwise and counter-clockwise. However, there are also two perspectives: looking at the shaft end of the motor or looking at the lead end (opposite the shaft end). A motor which turns clockwise looking at the shaft end is turning counter-clockwise when viewed from the lead end! The point is that just stating a direction is not good enough. You must also identify a perspective.

There are several names for the two possible perspectives. The most common are shaft end and lead end. The shaft end can also be called the output end, drives end, or pulley end. The lead end is sometimes referred to by placing “opposite” in front of whatever phrase is used to describe the shaft end; such as, “opposite drive end.”

Normally these descriptions are abbreviated, which tends to add to the confusion. Below is  list of some of the abbreviation used. The graphic above each group uses an arrow to show the rotation looking at the motor shaft.

CCWSE Counterclockwise shaft end
CCWOE Counterclockwise output end
CCWDE Counterclockwise drive end
CCWPE Counterclockwise pulley end
CWLE   Clockwise lead end


CWSE  Clockwise  shaft end
CWOE Clockwise output end
CWDE Clockwise drive end
CWPE Clockwise pulley end
CCWLE Counterclockwise lead end

Motor Frames by the Numbers

Two standards organizations publish motor frame standards: National Electrical Manufacturers  Association (NEMA) and the International Electrotechnical Commission  (IEC). Do you know why a NEMA Frame 56 motor is a 56 motor? I admit, until recently I did not. I just knew that if two motors have the same frame, they will mount in the same place. That does not necessarily mean that they are interchangeable, just that you can physically interchange them. However, I did not know what the 56 stood for in a 56 frame motor. It is really pretty simple. The number in a NEMA Motor frame designation describes the distance from the center of the shaft to the bottom of the mounting base in eighths of an inch. So a 56 frame has a distance from the center of the shaft to the bottom of the base of 56/8, or 7 inches. Of course, there are other dimensions that are set by frame size, but that particular dimension gives the frame its name. Maybe your machines use motors with IEC fame designations. The frame number still describes the distance from the shaft center to the base, but in millimeters. So an IEC 112 Frame motor has a distance from the shaft center to the bottom of the base of 112 millimeters. If you are measuring in centimeters that is 11.2 centimeters. As in the NEMA frame size, the IEC frame size standardizes many dimensions. Both standards have many dimensions that are described but not standardized. This means that manufacturer will report these dimensions, but they will not necessarily be identical from one motor to another. Baldor has a couple of good pdf reference pages which show the dimensions for each type of frame. You can find these at

NEMA  
IEC        

Turbo 200 Capacitors

NOTE: - This is a re-posting - I got Mona's e-mail wrong on the first one.
Have you ever had to replace a dual capacitor only to find you do not have the right one on the truck? There are so many different possible combinations it is nearly impossible to have every one you might need. Amrad Engineering makes capacitors here in the USA. They make replacement capacitors which have a wide range of capacitors built into a single part called the Turbo 200. With these, it is possible to replace most any dual capacitor you need. These capacitors are great for teaching students about series and parallel capacitors. Basically, the Turbo 200 is 6 capacitors connected together at the center post. Any two outside connections are essentially in series. Measuring the microfarad between any two outside terminals shows the effect of capacitors wired in series. Jumping any two outside terminals puts those two capacitors in parallel. Placing a jumper between any two outside terminals and measuring the microfarads between either outside terminal and the center terminal shows how capacitors connected in parallel work.  Amrad engineering is offering a free kit to all HVACR instructors. They want to send out a kit of their products with instructions to schools all over the country to promote their products. The stuff they sent me was fantastic. Here are a couple of pictures of the kit.

To get a kit for your school contact
Mona Tompkins Amrad Engineering
1-800-445-6033

mtompkins@yahoo.com

DOE Small Motor Efficiency Standards for 2015

You may have missed the news about required small motor efficiency minimum standards while you were focused on refrigerant changes; regional efficiency standards for air conditioners, furnaces, and heat pumps; and trying to stay ahead of the latest news on climate change. I know I did. I was looking at the Grainger web site the other day and up popped a bulletin regarding the Department of Energy requirement that certain small electric motors 3 horsepower and under meet a new minimum efficiency requirement beginning in March, 2015. I immediately thought “oh bother,” that would include most of the motors we deal with in residential HVAC. Then I looked up some details and breathed a bit easier.  Motors which are exempt include multispeed, enclosed, specific purpose, or special mounting bracket. That just exempted most of the motors in residential air conditioning equipment. If you deal with regular mount, general purpose, 42, 48 or 56 frame capacitor start or capacitor start-run motors, you WILL be affected. An example might be ventilation fan motors. Grainger was not saying replacement motors would not be available, just that they would now have run capacitors and would be longer due to the extra windings required to meet the new efficiencies. Basically, this DOE ruling makes what was once the premium, high efficiency motors the minimum motor design for certain motor types defined by NEMA. I found several links with discussion and details on this requirement. They are listed below. You might want to take a look at some of these links to determine if you work with any of the affected motors. That way you can develop a replacement strategy before you get to a job and find that you will not be able to replace the old motor with another just like it.

Baldor http://www.baldor.com/news/article.asp?id=1014

US Department of Energy  http://www1.eere.energy.gov/buildings/appliance_standards/product.aspx/productid/40

Weg Industries http://ecatalog.weg.net/files/wegnet/WEG-motor-global-energy-efficiency-legislations-2015-and-beyond-energyeffleg-training-english.pdf

Design News   http://www.designnews.com/author.asp?section_id=1365&doc_id=271471