Portfolia Part B Flashcards
Name the major components of a rotating DC machine
Armature, frame, field coil
How many current paths does and 8 pole lap wound armature have
8 paths
The basic operating principle of any motor is
electrical to mechanical
When applying Flemings right hand rule for generators your thumb will point in the direction of
Motion of force
Describe the physical difference between the coils used for series-field and the coils used for shunt-field in a compound DC machine
Series - windings are very thick and less turns
Shunt- wires are thin with lots of turns
A 4-pole lap wound armature contains a total of 300 effective conductors. If the magnetic flux is 0.02 Webers per pole and the speed of rotation is 100RPM, what is the value of it’s generated voltage?
160V
What is the most likely cause of a DC generator failing to generate a voltage when rotated
There is no residual magnetism
2 methods used to determine losses in a D.C machine are by measurement and by calculation True or false
True
What is the amount of torque developed by a motor which exerts a force of 80 N over a perpendicular distance of 200mm?
16Nm
Why does the prime-mover for a DC generator have to work harder when the current drawn from the generator increases
It creates an opposing torque against the prime mover
When testing a 220V DC electrical motor for insulation resistance between coils on what setting would you set the instrument?
500V
Explain the purpose of a field Rheostat on a compound D.C motor
Allows you to change the resistance to control the motor
Describe the most common method used to regulate the output voltage of a DC generator
Rheostat - regulates current to the field coils
The methods used to create excitation in DC generators are
Separately excited, self excited
How will an over-compounded generator perform under load, compared to a level compounded generator
Level - will maintain constant voltage under full loads
Over- higher voltage is produced under full loads
What limits the current in a DC motor when it is running at normal operating speed
The opposing induced EMF
Why is it that a series motor should not be operated under no load conditions
It will cause damage to itself as it speeds up
A motor with an output of 5000 watts draws 25amps from 220volt supply. Calculate the motor efficiency
90.9%
When reversing the direction of a cumulative compound motor you must
Swap the polarity of the armature or change the connections
Name a method which can be used to reduce armature reaction
Compensating windings, inter-poles, brush shifts
Name 1 way to start a DC motor
Progressively lower the resistance in series with the armature
Which regulatory tool is used to ensure that Australians have efficient appliances and equipment
Minimum energy performance standards
A four pole DC motor has a lap wound armature of 30 coils, each with 20 conductors. If the flux per pole is 0.02 Wb and the armature current is 19 Amps, how much torque is produced?
36.3Nm
What is the most important safety factor associated with rotating machinery for electricians
No work shall be done on rotating machines if not locked out
What do fingers indicate in Flemings left hand motor rule
Thumb is motion, pointer finger is flux and centre finger is current
What electrical hand hel testing instrument can be used to test a DC armature for faults
Multimeter set to ohms
What is the basic operating principle of a generator
Mechanical to electrical
Which part of the DC generator produces the output voltage
The armature
State the difference between a generator and a motor in terms of energy conversion
Motor - electrical to mechanical
Generator - mechanical to electrical
Which electrical instrument can be used to compare the electrical measurements with nameplate details
A: voltmeter
B: watt-meter
C: Ammeter
D: all of the above
D
What is the purpose of a prime-mover when related to a generator?
To turn the armature
The voltage in a self excited generator may fail to build owing to
?
Four safety precautions that should be observed when starting generators are
Properly earthed, emergency stop, must be guarded, coil to coil test
A DC motor runs at 220v, draws a current of 22A, has a speed of 900 RPM and produces a torque of 50 Nm. calculate the output power
4712 watts
Which DC motor has wide applications, especially within the traction industry?
Series excited motor
What must DC motors have fitted to their windings to give better communications under all loads
A: Inter-poles
B: shunts
C: Rheostats
D: Permanent magnets
A
Which of the following creates losses in a DC machine?
A: Copper losses
B: Iron losses
C: Friction
D: all of the above
D
Does the speed characteristic of a shunt motor alter between no loads and full load?
Slightly on load
Which component of a DC machine requires regular inspection and maintenance
Brushes
