1. Eng. Mohammad Aqel
Faculty of Engineering & Information Technology
Al-AZHAR UNIVERSITY-GAZA
ITCE 5319: Programmable Logic Controllers (PLCs)
2nd Semester, 2007 / 2008
Bi-directional Control of DC Motor
Experiment # (2)
 Objectives

To understand how to control the DC motor in two directions.

To be familiar with relay’s applications.
 Equipments

Delta PLC Training Kit.

Banana Plug Wires
 Theory
There are several kinds of DC motors: stepper motors, servos, brushed/brush-less
motors.
Stepper Motors: The inputs of a stepper motor are signal pulses and the shaft of
stepper motor moves between discrete positions proportional to pulses. If the load
of the motor is not too great, open-loop control is usually used to control the
motor. Stepper motors, as shown in Figure 1, are used in disk drive head
positioning, plotters, and numerous other applications.
Figure 1: Stepper Motors.

2. Servo Motors: The input of a servo motor is a voltage value and the output shaft
of the servo motor is commanded to a particular angular position according to the
input voltage. Servo motors, as shown in Figure 2, are used in robots and in radio
control airplanes to control the position of wing flaps and similar devices.
Figure 2: Servo Motors.
Constant Speed DC Motors: The input of a DC motor is current/voltage and its
output is torque (speed). Figure 3 shows constant speed DC motors.
Figure 3: Constant Speed DC Motors.
DC Motor Theory
Direct Current electric motors operate under a basic principle of electricity:
interaction between two magnetic fields positioned at an angle from each other
will attract/repel resulting in movement. In the case of a DC electric motor, power
is provided to a stator field and an armature creating magnetic fields that are,
electrically, about 90 degrees from each other. The resulting attraction/repulsion
of the armature from the field generates a torque and the armature turns.
The basic components of a DC electric motor include:

Frame – Makes up the outer structure of the machine. It is used to mount most
of the other components of the motor.

Fields – Are coils mounted on field pole pieces that generate a stationary
magnetic field.
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3. 
Interpoles – Are coils that are placed between the field coils that generate a
field that is used to prevent excessive sparking of the brushes.

Endshields – Also called bearing housings, are used to house the brushes,
brush rigging, and to house the shaft bearings, holding the armature centered
in the frame.

Brush rigging – Holds and positions the brushes above the armature
commutator. Usually, a tension device is used to maintain a constant pressure
on the brushes.

Brushes – Are used to provide DC to the armature. The brushes ride on the
commutator.

Commutator – Consists of many copper bars that are separated by mica. Each
bar is connected to coils in the armature.

Armature – Is the rotating portion of the motor that contains coils.
There are two conditions necessary to produce a force on a conductor:

The conductor must be carrying current.

The conductor must be within a magnetic field.
There are many methods to control the speed of the DC motor and here are some
of them:

Varying the voltage applied to either the armature or field circuit.

Adding resistant in the armature circuit, but this method is in efficient and not
effective at light loads.

Adding resistant to the field circuit, and this is efficient method, but not
convenient nor as efficient as varying the terminal voltage.
The other field of DC motor controlling is controlling its direction, and this is
easy done by changing the polarities of its brushes, and this is the main scope of
this experiment. The direction of rotation of a DC motor may be reversed using
one of these methods:

Reversing the direction of the current through the field.

Reversing the direction of the current through the armature.
The industrial standard is to reverse the current through the armature.
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4.  Experimental Procedures
1- Connect the control and power circuit as shown in Figure 4.
2- Supply the circuit with a 12VDC.
Figure 4: Control and Power Circuit of DC Motor
When the NO push button onr (on right), which control the right direction of
motor, the current passes through the coil of the R1. Then the relay will change its
contact state, the normally open contact which is connected in parallel with the
onr button will be closed. So the current still passes through the coil of R1 (Sealin Circuit). As a result, the DC motor will turn right until the off push button is
pressed which will open the circuit.
When the NO push button onl (on left), which control the left direction of motor,
the current passes through the coil of the R2 and the motor will start to move in
the left direction due to the changing of the polarity on the coil of DC motor.
The following steps show and explain the idea of controlling the DC motor in two
directions and how the control circuit works:
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5. Step 1: After connecting the control and power circuits.
24 V DC
24 V DC
off
onr
R1
R1
R2
onl
R2
R2
M
R1
R1
R1
R2
R2
0
0
Step 2: At pressing the onr push button.
24 V DC
24 V DC
off
onr
R1
R1
onl
R2
R2
R2
M
+
R1
R1
R1
R2
0
R2
0
Step 3: After releasing the onr push button.
24 V DC
24 V DC
off
onr
R1
R1
onl
R2
R2
R2
R1
M
+
R1
R1
R2
0
0
4
R2

6. Step 4: At pressing the off push button
24 V DC
24 V DC
off
onr
R1
R1
R2
onl
R2
R2
M
R1
R1
R1
R2
0
R2
0
Step 5: At pressing the onl push button.
24 V DC
24 V DC
off
onr
R1
R1
onl
R2
R2
+
R2
M
-
R1
R1
R1
R2
0
R2
0
Step 6: After releasing the onl push button.
24 V DC
24 V DC
off
onr
R1
R1
onl
R2
R2
+
R2
R1
M
-
R1
R1
R2
0
0
5
R2