Short explanation of 3 Phase Motors
Most of the people in the electrical trade have a serious lack of knowledge regarding one of the most useful, and as far as I'm concerned, the simplest way of converting electrical energy back into mechanical energy1. This statement is not made lightly. After 21+ years in this trade, I am continually surprised by the skill levels of our hypothetical professional electricians. The title gave it away but I'm talking about the Three Phase Motor.
The standard Wye (Sometimes called "Star") wound motor consists of six (6) windings with nine (9) leads at the junction box. The Delta wound motor has the same number of windings with twelve (12) leads. Voltage, Frequency (Hertz), Frame type and Service factors change the construction of these motors but the basic operation remains the same.
I am almost willing to bet that the vast majority (> 75%) of the field personnel today could not wire a basic three phase motor if the name plate was missing. A larger percentage could not if the lead numbers were missing.
That still leaves a percentage that understand how a motor operates (not writing an editorial but my guess would be < 3%) and with voltage and frequency thresholds not withstanding, would be able to take a motor at any plant in the country and put it back on line.
The art of this is being able to understand what is connected to what and what should be connected to what under the given circumstances. The "is" are the windings themselves, they do not change. The "should" takes into consideration the voltage source and starter type (Wye/Delta, Part Winding, etc.) for the proper configuration of the windings.
The figures above are the two basic types of three phase motors. Note that the Wye (Fig. "A") has an internal winding connection for nine leads and the Delta (Fig. "B") does not, so all twelve leads are available.
Note also, that both the Wye and Delta configurations are wired for their High Voltage (Windings in series) to line voltage.
A pamphlet entitled "Getting to Know 3 Phase Motors" is available for all concerned parties. This pamphlet will show how to:
Remember what numbers correspond to which windings.
High and Low Voltage connections for both the Wye and Delta configurations.
How to obtain a different Voltage from a motor than the source Voltage for brake coils, etc.
A step by step explanation of how to take a motor with no lead numbers and bring it back to life.
To order "Getting to Know 3 Phase Motors" please send $10.00 (U.S.) (This is to cover printing, shipping and handling) to:
Lewis Electric, Attention: Publications Dept.
444 West Mowry Street, Homestead, Florida 33030
Or download our order form.
One Service Call will pay for it. -
Please allow 2-3 Weeks for delivery in the Continental United States, slightly longer for elsewhere. Thank You.
1 Even Nuclear plants use mechanical means to generate electricity. Mother Nature has slightly different ways.
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Copyright © 1998 by Lewis Electric
Last modified: July 07, 1999