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Sizing Supply Conductors for Two-Speed Single Winding Induction Motors

Timothy O'Hearn, P.E.


Course Outline

I. Introduction to two-speed, single winding applications.

II. Equation for stator current for a three-phase machine.

III. Stator winding connections for two-speed single winding motors.

IV. Sizing supply conductors.

V. Simplified schematic for a two-speed single winding variable torque induction motor.

VI. Course Summary.

This course includes a multiple choice quiz at the end.

Learning Objective

At the conclusion of this course, the student will:

Course Introduction

Two-speed single winding variable torque motors are commonly used in process facilities on air coolers. These motors have two horsepower ratings, two speed ratings, and two full-load current ratings. Because the motor has a single winding the speed change is accomplished by changing the wiring configuration and the number of stator circuits. These changes are described in NEMA MG1. The application is described including recommendations for sizing the supply conductors for avoiding application problems.

Course Content

The following discussion will establish the proper method for sizing the supply conductors for a three-phase induction motor that is a two-speed, single winding, variable torque application.

Two speed single winding variable torque motors are commonly used for air coolers in industrial processes. These applications require a limited amount of speed control but the application does not warrant adjustable speed drives.Two-speed single winding motors have only one winding for either of the two speeds. The motor has two horsepower ratings, two speed ratings, and two current ratings.The current in the line conductor of a three-phase machine is a function of the motor horsepower, line voltage, efficiency, and power factor.

††††

This equation indicates that the higher horsepower connection will have the higher current.The motor has rated current values for the high-speed winding (the higher horsepower rating) and the low speed winding (lower horsepower rating). For these two-speed single winding variable torque motors the low speed (lower rated rpm) is approximately one half of the high speed (higher rated rpm), and the low speed horsepower rating is one fourth of the horsepower rating.

The motor starter for this application must be specifically designed to accommodate the two-speed single winding configuration. The motor is supplied with six (6) stator winding connections (T leads), three for the low-speed winding and three for the high-speed winding. Changing the motor stator winding connection configuration from delta to wye or wye to delta or wye to wye and changing the circuits from multiple circuits to a single circuit accomplishes the speed change.These connections (motor T leads) are detailed in NEMA MG1.†† The following table is a representation of the information provided in the NEMA MG1:

Variable Torque Motors Two-Speed Single Winding Terminal Markings

 

Speed

 

†††††† L1††††††††† L2††††††††††††† L3

Insulate

Separately

 

Join

Low

†††††† T1††††††††† T2††††††††††††† T3

T4-T5-T6

...

High

†††††† T6††††††††† T4††††††††††††† T5

...

(T1,T2,T3)

The supply conductors for induction motors are basically sized using the following principals:

1)       The minimum required ampacity based on the motor full-load current is calculated.

2)       The conductor size, based on the actual insulation type and temperature rating, is selected from the NEC Tables.

3)       The conductor ratings are reduced for temperature effects and if the adjusted rating is too low than a larger conductor is selected.

4)       The short circuit protection is reviewed to determine that the maximum short circuit current for the rated interrupting time does not exceed the conductor rating.

5)       Conductor termination requirements are established.

6)       The conductorsí voltage drop for both the rated operating and starting condition is calculated to determine the appropriate size for the application.

The principal for determining the required ampacity for the supply conductors for two-speed single winding motors is the subject of this course.The other principals listed are beyond the scope of this course and are covered by the NEC (www.nfpa.com) or IEEE recommended practices (www.ieee.com).

For a two-speed single winding motor, the current for the high speed rating is used to calculate the minimum required ampacity of 125% (NEC Section 430-22) of the full-load current for the high speed supply conductors. However, the current provided by the motor manufacturer cannot be used to determine the supply conductorsí minimum ampacity for the low speed connection. This is because the low speed conductorsí load current is higher in the high-speed connection than in the low speed connection.

To better understand this application a simplified schematic is provided:

After reviewing the schematic it is evident that the low speed supply conductors carry current in the high-speed connection.The three phase currents will add together at the center of the wye at the two speed motor starter.The full-load current for the connection is split into two wye-connected stators.Because the high-speed current is typically more than twice the low speed current it is evident that the current in either of these two wye-connected stators is larger than the low speed current.

Course Summary

Two-speed single winding motors have only one winding for either of the two speeds.Caution should be utilized when selecting the current ratings for the supply conductors.For variable torque applications the current in the low speed conductors in high-speed operation is larger than in low-speed operation.For other motor applications such as constant torque or constant horsepower the connection configuration should be reviewed to determine the current levels in both the high-speed and low-speed configuration because the connections are different.

Recommendations:

Five specific application recommendations for two-speed single winding, variable torque applications are provided within the quiz section (available after the purchase). These recommendations provide the magnitude of the high speed current to allow for in the low speed conductors for sizing purposes. It also will include recommendation for simpler solutions.

References

1.      NEMA MG1-1993

2.    NFPA 70 National Electric Code

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz


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