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 conductors’
voltage drop for both the rated operating and starting condition is calculated
to determine the appropriate size for the application.
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 conductors’ minimum ampacity for the low speed connection. This is because the low speed conductors’ 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
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.
