Chapter 29, Problem 74A

(a)

To determine

The circuit which allows the motor to run in only one direction.

Answer to Problem 74A

In circuit A, the motor runs in one direction.

Explanation of Solution

Given:

The figure:

  

As per problem

Consider the circuits in given figure

In circuit A, the forward biased diode is in series with the motor, so the current flows through the motor directly.

In circuit B, the diode is in parallel with the motor, so the current id divided between the diode and the motor.

In circuit C, there are two series diode connected parallel to each side of the motor, hence again the current is divided and the motor does not runs in a single direction.

Hence only in circuit A, the motor runs in one direction.

Conclusion:

In circuit A, the motor runs in one direction.

(b)

To determine

The circuit which will cause a fuse to blow if the incorrect polarity is applied.

Answer to Problem 74A

The circuit B will cause the fuse to blow if the incorrect polarity is applied.

Explanation of Solution

Given:

The figure:

  

In circuit B, a fuse is connected in series with the parallel circuit of motor and diode. If incorrect polarity, that is, positive terminal is connected to the fuse directly, it will overload and blow.

Conclusion:

The circuit B will cause the fuse to blow if the incorrect polarity is applied.

(c)

To determine

The circuit that produces the correct direction of rotation irrespective of the polarity applied.

Answer to Problem 74A

The circuit C produces the correct direction of rotation irrespective of the polarity applied.

Explanation of Solution

Given:

The figure:

  

In circuit C, either of the diodes connected to the left side of the motor will be forward-biased irrespective of the polarity applied. Subsequently the diodes connected on the right side of the motor will be forward biased or reverse-biased depending on the biasing of the right hand side diodes. Thus s the motor will rotate in the correct direction irrespective e of the polarity.

Conclusion:

The circuit C produces the correct direction of rotation irrespective of the polarity applied.

(d)

To determine

The advantages and disadvantages of the three circuits.

Answer to Problem 74A

The circuit A has the advantage of simplicity, the motor runs in one direction only.

Its disadvantage is the diode voltage drop is y $0.7\text{V}$ which can affect the motor in low voltage circuits.

The circuit B has the advantage that the diode voltage y $0.7\text{V}$ is not wasted.

Its disadvantage is that the fuse may blow if polarity is incorrect.

The circuit C has the advantage that the motor runs in correct direction irrespective of the polarity.

Its disadvantage is that 1 $1.4\text{V}$ is wasted on the diodes on the sides of the motor.

Explanation of Solution

Given:

The figure:

  

The circuit A has the advantage of simplicity, the motor runs in one direction only.

Its disadvantage is the diode voltage drop is $0.7\text{V}$ which can affect the motor in low voltage circuits.

The circuit B has the advantage that the diode voltage $0.7\text{V}$ is not wasted.

Its disadvantage is that the fuse may blow if polarity is incorrect.

The circuit C has the advantage that the motor runs in correct direction irrespective of the polarity.

Its disadvantage is that $1.4\text{V}$ is wasted on the diodes on the sides of the motor.