EET 272
Forward / Reverse Motor Control
Experiment – 5
Background
To change the direction that a motor is turning involves changing the direction of current flow in the
motor. For a DC motor this usually involves connecting the ammeter to the reverse polarity voltage.
For a 3-phase AC motor this involves swapping 2 of 3 input lines. This causes the magnets in the
stator (or rotor) to energize in a different order, or more correctly for the field to move around the
stator in the reverse direction.
For a motor that is capable of going in both the forward and reverse direction it is very important
that it is never commanded to go in both directions at once. This will cause power rails to short and
cause components to be overstressed and burned out. There are two basic methods for preventing
this condition. The first is mechanical interlocking. In this method, if the forward coil is energized,
the reverse coil is physically prevented from energizing. The 2nd method is electrical interlocking.
In this system, the control circuitry is designed such that both forward and reverse directions cannot
be energized at the same time. Electrical interlocking may or may not also involve physical
interlocking. Physical interlocking is very definitive but can only prevent one type of condition:
both contactors energized at once. Electrical interlocking is very adaptable. For example, electrical
interlocking can be used to require the motor to be off for 5s before it can be reversed. This gives
the motor a chance to slow down before putting it in reverse. This is a kinder, gentler way to reverse
a motor. Think about putting your car in reverse when you are driving. Electrical interlocking can
also mean that pushing the stop button is not necessary to reverse the motor.
Objective
●
Construct a Forward/Reverse control using Mechanical interlocking
●
Construct a Forward/Reverse control using Electrical interlocking
Farrell
1
02/17/14
EET 272
Forward / Reverse Motor Control
Experiment – 5
Planning and Preparation
For this lab we will be controlling a 3-phase squirrel cage motor. This motor should be wired in a Yconfiguration with no neutral.
1. Design a circuit that will will be used to control the direction of the squirrel cage motor. The
control circuit should contain 1 STOP pushbutton, 1 Forward pushbutton, and 1 Reverse
pushbutton. This circuit will use mechanical interlocking. You will have to push the stop
button before you can start the motor in the reverse direction.
a) Your circuit should include all applicable circuit protection.
•
Circuit breakers on the line voltage
•
Overload protection for the motor
b) When the forward pushbutton is pushed the motor should spin in the CW direction and
the green light should come on
•
If the motor is already turning this button does nothing
c) When the reverse pushbutton is pushed the motor should spin in the CCW direction and
the red light should come on
•
If the motor is already turning this button does nothing.
d) Be sure to indicate the mechanical interlocking in your schematic
2. Why does you circuit only need 1 overload protection device? Why should it not have a
protection device for the forward and reverse contactors?
Experiment
●
Build your circuit.
●
Demonstrate that your circuit works as expected.
Analysis
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Be able to explain how your circuit works.
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What happens when you push the forward button if the motor is turning in reverse?
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Why is interlocking important?
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What do you think would happen if you did not include the interlocking?
Farrell
2
02/17/14
EET 272
Forward / Reverse Motor Control
Experiment – 5
Additional Work
●
Design a Forward/Reverse circuit with electrical interlocking.
◦ Your circuit has the same requirements as previous section.
◦ Include Mechanical interlocking as backup.
●
Build and test your circuit
Analysis
●
What were some interesting things you learned in building this circuit? What caused
problems.
●
What happens when you push the forward button if the motor is turning in reverse?
●
How is this interlocking different than the purely mechanical?
Farrell
3
02/17/14