A single phase transformer has 200 turns on the primary, 50 turns on the secondary and 120V applied to the primary. What will be the secondary voltage?

Given:
Primary turns, $N_p = 200$
Secondary turns, $N_s = 50$
Primary voltage, $V_p = 120$ V
The transformer equation relating voltage and turns is:

$$\frac{V_s}{V_p} = \frac{N_s}{N_p}$$

$$\frac{V_s}{120 \text{ V}} = \frac{50}{200}$$ $$V_s = 120 \text{ V} \times \frac{50}{200}$$ $$V_s = 120 \text{ V} \times \frac{1}{4}$$ $$V_s = 30 \text{ V}$$

The secondary voltage is 30 V.

The correct option is A.

2. The lifting power of an electromagnet depends on
The lifting power of an electromagnet is directly proportional to the strength of the magnetic field it produces, which is quantified by its magnetic flux density.

The correct option is B) its magnetic flux density.

3. In a dc machine, the yoke
The yoke in a DC machine serves as the outer frame, providing mechanical support and protection for the internal components. It also forms part of the magnetic circuit, carrying the magnetic flux.

The correct option is B) offers mechanical protection to the machine.

4. Magnetic flux has as unit;
The SI unit for magnetic flux ($\Phi$) is the Weber (Wb).

The correct option is D) Weber.

5. The role of a commutator in a dc generator is to
In a DC generator, the armature coils induce an alternating electromotive force (AC EMF). The commutator acts as a mechanical rectifier, converting this induced AC EMF into a direct current (DC) EMF at the output terminals.

The correct option is D) convert the induced ac emf to dc emf.

6. The role of brushes in dc generators is that
Brushes are stationary contacts that make electrical connection with the rotating commutator segments. Their primary function is to collect the direct current from the commutator and transfer it to the external circuit.

The correct option is C) they collect current from the commutator.

7. The quantity which is greater when a machine is functioning as a motor is
When a DC machine functions as a motor, an external terminal voltage ($V_t$) is applied to its terminals. This voltage drives current through the armature. As the armature rotates, it generates a back electromotive force ($E_b$) that opposes the applied voltage. For current to flow into the motor and for it to operate, the applied terminal voltage must be greater than the back EMF ($V_t > E_b$).

The correct option is A) the terminal voltage.