Question 1

Explain the operation of this “H-bridge” motor control circuit:

At any given moment, how many transistors are turned on and how many are turned off? Also, explain what would happen to the function of the circuit if resistor R1 failed open.

Question 2

Predict how the motor function in this circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Transistor Q1 fails open (collector-to-emitter):
Transistor Q2 fails open (collector-to-emitter):
Transistor Q3 fails open (collector-to-emitter):
Transistor Q4 fails open (collector-to-emitter):
Resistor R1 fails open:
Resistor R2 fails open:
Resistor R3 fails open:
Transistor Q3 fails shorted (collector-to-emitter):
Transistor Q4 fails shorted (collector-to-emitter):

For each of these conditions, explain why the resulting effects will occur.

Question 3

If the ends of a wire loop are attached to two half-circular metal strips, arranged so that the two strips almost form a complete circle, and those strips are contacted by two “brushes” which connect to opposite poles of a battery, which way will the wire loop rotate?

Question 4

When the switch closes, the ammeter will initially register a large amount of current, then the current will decay to a much lesser value over time as the motor speeds up:

In view of Ohm’s Law, where current is supposed to be a direct function of voltage and resistance (I = E/R), explain why this happens. After all, the motor’s winding resistance does not change as it spins, and the battery voltage is fairly constant. Why, then, does the current vary so greatly between initial start-up and full operating speed?

What do you think the ammeter will register after the motor has achieved full (no-load) speed, if a mechanical load is placed on the motor shaft, forcing it to slow down?

Question 5

A DC electric motor spinning at 4500 RPM draws 3 amps of current with 110 volts measured at its terminals. The resistance of the armature windings, measured with an ohmmeter when the motor is at rest, unpowered, is 2.45 ohms. How much counter-EMF is the motor generating at 4500 RPM?

How much “inrush” current will there be when the motor is initially powered up (armature speed = 0 RPM), once again assuming 110 volts at the terminals?

Question 6

Explain what a servo motor system is, in your own words.

Question 7

A common type of rotary encoder is one built to produce a quadrature output:

The two LED/phototransistor pairs are arranged in such a way that their pulse outputs are always 90o out of phase with each other. Quadrature output encoders are useful because they allow us to determine direction of motion as well as incremental position.

Building a quadrature direction detector circuit is easy, if you use a D-type flip-flop:

Analyze this circuit, and explain how it works.

Question 8

Radio-controlled toy cars, airplanes, and boats use small servo motors for positioning of the steering mechanisms, engine throttle position, and such. These servos have the motor, position sensor, and control electronics housed in the same plastic package, making them very compact.

Research the type(s) of control signals used to command these servo units. In other words, find out what sort of electronic signal they require to “command” them to go to certain positions. Then, suggest a circuit that could generate these signals.

Question 9

Determine all component voltage drops in this circuit when the motor is operating in the reverse direction. Be sure to explain how you performed all the analyses! Assume 0.7 volts as the standard forward voltage drop for a forward-biased PN junction, and 0.3 volts as the standard collector-to-emitter voltage drop for a saturated BJT.

Published under the terms and conditions of the Creative Commons Attribution License