DC Electrics

Question 1

Power =

Answer 1

IV

or

(I squared) x R

Question 2

Voltmeters wired in … and have … resistance

Answer 2

parallel

high

Question 3

Secondary cells can be recharged by reversing the chemical process by applying a voltage of … in to the battery

Answer 3

112%

Question 4

positive plate in a lead acid battery is …

Answer 4

lead peroxide

(peroxide positive)

Question 5

negative plate in a lead acid battery is …

Answer 5

lead

Question 6

electrolyte in a lead acid battery is …

Answer 6

sulphuric acid

Question 7

off load voltage of a lead acid battery is …

on load voltage is …

Answer 7

2.2 volts per cell

2 volts per cell

Question 8

hydrometer reading for lead acid battery charge state

fully charged

low charge

fully discharged

Answer 8

1. 25 - 1.3
2. 2 - 1.24
3. 17

Question 9

NiCad battery positive plate is …

negative plate is …

electrolyte is …

Answer 9

nickel cadmium

iron

potassium hydroxide

Question 10

NiCad battery off load voltage and on load voltage is?

Answer 10

1. 3 volts
2. 2 volts

Question 11

total voltage and amp/hour rate for batteries in series?

Answer 11

Batteries connected in series give the same current but the total of all the voltages

Question 12

total voltage and amp/hour rate for batteries in parallel?

Answer 12

Batteries connected in parallel give the same voltage but the total of all amp/hours

Question 13

aircraft batteries normally connected in series or parallel?

Answer 13

Increased amp/hour rate (capacity) with lower voltage preferred so aircraft batteries are connected in parallel

Question 14

what is Fleming’s left hand rule?

Answer 14

way to find the direction in which the loop of wire moves in a magnetic field

Question 15

If the shunt field winding is in series with the motor it has a … torque because …

disadvatage is that …

Answer 15

high

all the current flow is through the field winding, creating a stronger field

they can easily over-speed

Question 16

shunt (parallel) field windings on a dc motor mean that the motor has a … starting torque because …

the advantage is that they …

Answer 16

low

only part of the current flows through the field winding

constant speed

Question 17

how to change direction of a dc motor?

Answer 17

either reverse current flow through the field winding or have a separate winding in the opposite direction (one CW and one CCW)

Question 18

what is flemings right hand rule?

Answer 18

RH rule for generighters

Question 19

is current or voltage induced in a wire moving through a magnetic field?

Answer 19

voltage

Question 20

The aim of a generator should be to provide constant output … irrespective of the output … and changes in drive speed

Answer 20

voltage

current

Question 21

biggest factor affecting output voltage of a dc generator?

Answer 21

current flow through the field windings

To keep a constant output voltage, we ideally need constant current through the field coils

Question 22

output voltage of a shunt wound dc generator … with an increase in load because …

Answer 22

decrease

As more electrical items are turned on, more paths are opened for the current to flow through and the main circuit resistance decreases. This in turn causes more current flow through the main circuit, decreasing the current flow through the field windings; and reducing field strength and therefore output voltage

Question 23

voltage output of series wound dc generators … with an increase in load because …

Answer 23

increases

The field coils are in series with the main circuit resistance so now as current increases, the field strength increases and voltage output increases

Question 24

how do compound wound dc generators combine the characteristics of series and parallel to produce a steadier output voltage?

Answer 24

As load increases and the shunt field and output decreases, the output in the series increases to compensate

Question 25

increasing the dc generator output voltage will cause an … flow through the voltage coil, which will then pull on the carbon disk pile to … its resistance, therefore … current flow through the field winding which is wired in … with the carbon disk pile, which will then cause generator output voltage to …

Answer 25

increased

increase

reducing

series

decrease

Question 26

how does a vibrating contact regulator work?

Answer 26

see image

Question 27

both carbon pile and vibrating regulators act to … generator output voltage, so failure of either regulator would cause out put voltage of the generator to …

Answer 27

reduce

increase

Question 28

dc generator paralleling and load sharing

Answer 28

When connected to the busbar the individual generators must have balanced output voltages or one will do more work than the other. An equalising circuit is added between the return lines to the generators

The equalising circuit depends on the voltage drop across the two resistors. If output current from both generators is equal, the drop across the resistors is equal

If the number 1 generator is taking more load than number 2, the return side of number 1 will be at a lower potential than number 2 causing a current flow through the equalising circuit. This will energise the voltage coil of number 1 and reduce voltage output of number 1, at the same time increasing output from 2 to restore the balance. In a steady state no current will flow through the equalising circuit

Question 29

A differential relay is fitted in a two-generator system to ensure …

Answer 29

that the voltages are almost equal before the generators are paralleled

Question 30

a reverse current relay will …

Answer 30

automatically open and disconnect the generator when output voltage drops to 27V and prevent reverse current flow

often incorporates an additional switch to allow the generator to be manually disconnected from the busbar and is referred to as the generator breaker or cut-out