UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND ENGINEERING
ECE 201
CIRCUIT THEORY I
REVERSING THE DIRECTION OF A DC MOTOR
BACKGROUND
A permanent-magnet DC motor is designed to run in a particular direction by default. Consider
the simplified DC motor shown below. A permanent magnet Stator establishes magnetic lines of
flux directed from the North Pole to the South Pole. The Rotor has one turn of wire carrying a
current into the A terminal and out of the B terminal. The associated lines of flux are shown, and
flux “bunching” occurs at the top of conductor A and at the bottom of conductor B. This bunching
effect causes the net force on conductor A to be downward and the force on the conductor B to
be upward, establishing a counter-clockwise rotation.
In order to change the direction of rotation, we must either change the direction of the magnetic
field established by the permanent magnet (not easy to do), or change the polarity of the DC
voltage that is applied to the rotor. In this experiment, we will use a double-pole double-throw
relay to reverse the applied voltage.
PRELAB
Devise a scheme using a relay that employs a relay with double-pole double-throw contacts to
apply opposite-polarity voltages to the rotor of a small DC motor upon the throwing of a singlepole single-throw “DIRECTION” switch. Your design should also include a separate “ON/OFF”
switch in order to stop the motor before changing directions.
The relay will be energized when 12 Volts is applied to the coil. The motor supplied will require a
voltage other than 12 Volts to operate, so be sure that your design allows for a separate motor
supply.
As part of your design, provide an indication of the direction of rotation by illuminating -a Red LED when the motor is turning CLOCKWISE
a Green LED when the motor is turning COUNTERCLOCKWISE.
Don’t forget to document all of the design work in your lab notebook!
PROCEDURE/RESULTS
Construct your circuit and test to see if you have accomplished the design objectives. If
necessary, modify the design as needed. Record any and all modifications in the lab notebook.
Demonstrate your final circuit to the instructor or TA for their approval.
Each group should submit a detailed schematic of the original design as well as any modifications
that were required to complete the task. Comments about why the modifications were needed
should be included.
Be sure that each team member who contributed to the design and testing signs the submitted
document(s).