Service & Maintenance / Extra Edition

What to Do When the Wire Diagram Is Missing?

June 7, 2010
/ Print / Reprints /
ShareMore
/ Text Size+

Anyone whose job involves servicing electric motors has encountered the problem of a missing nameplate. Other articles in the Motor Doctor series have covered ways of determining the specifications of a motor lacking the nameplate, but what if you are trying to figure out how to wire that motor?

For some kinds of motors, principally motors with terminal-based connections, basic wiring is self evident. The terminal board itself usually has markings that indicate where line one and line two are to be connected. But what if you need to reverse that motor, use a different (but available) voltage setting, or have a motor that has nothing more than a bunch of color-coded or numbered leads coming out of it?

The colors or numbers themselves are often a clue, but they alone may not provide sufficient information. There is always the trial and error method, but I don’t recommend that because of the potential for destructive results. Instead, the Motor Doctor’s suggestion is to equip yourself with an ohmmeter (don’t settle for just a continuity tester) and learn to perform a few simple tests with it.

The first thing you’ll need to discover is whether you’re dealing with a three-phase motor. You may already know this from the application, but another giveaway is that the lead wires of most three-phase motors are single colors, not multiple colors, and usually identified with numbers. If, on the other hand, the motor diameter is less than seven inches and has a terminal board, it is most likely a single-phase motor.

For wiring a single-phase motor, the most important objective is to distinguish the starting circuit from the main winding. These two circuits are isolated from one another electrically if the lead wires are separated and not in contact with each other. Initially, the ohmmeter can be used to determine which wire belongs to which circuit as well as checking continuity between leads. You should be able to isolate into two groups any leads which have continuity with one another. The starting circuit is likely to isolate to two leads, the running circuit may have two or more leads that show continuity. If the running circuit has more than two leads, you will need to determine how those leads are to be used for voltage or speed changes.

You’ll need to use the ohmmeter as an ohmmeter and not as a continuity checker for the next step in the procedure. You’ll want to use the lowest ohm scale your meter offers, as the typical winding resistance in motors such as these is less than 100 ohms. If the motor is a permanent split-capacitor motor, you’re going to be looking for common and speed taps of the winding. Using the ohmmeter find the pair of wires that has the highest resistance as measured in ohms. This will give you your common and lowest speed tap. Using each of these two leads in turn, find the pair that gives you the second-highest resistance. This should provide you the common and second-lowest speed tap and should also allow you to isolate which of the two leads from the first test is the common.

In addition, note that the common lead in this type of motor is usually white or purple. If there are additional leads in the run winding group, continue to use the ohmmeter to test the now-identified common and additional leads. Descending resistance will give you ascending speeds.

All is not lost if you don’t have a diagram for a particular motor, at least not if you understand how to use an ohmmeter. As with any problem-solving exercise, the more tools you have at your disposal, the more effective you become in the field.

Publication date: 06/07/2010

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by Neil Simon

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

2014 MCAA Annual Convention

Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

Podcasts

NEWSmakers: Julian Scadden

Training is an ongoing process. Julian will discuss how you can generate maximum return on time and energy invested training by following a three part process. Listen to this podcast to get expert tips on training, tracking and follow up. 

More Podcasts

THE MAGAZINE

ACHRNEWS

NEWS 04-21-14 cover

2014 April 21

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe

SERVICE CALLS POLL

Which statement on service calls best applies to your business?
View Results Poll Archive

HVACR INDUSTRY STORE

plumbing-hvac.gif
2014 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research

 

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

DON'T MISS A THING

Magazine image
 
Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

STAY CONNECTED

facebook icontwitter iconyoutube iconLinkedIn i con