DC Motor Flashcards
When an electric generator is in operation. It is driven _____ and develops a _____, which in turn can send a current through a load resistance.
Mechanically
Voltage
The load on a generator constitutes those electrical devices that convert _____.
Electrical energy into other forms of energy
Examples of load form in generator.
- Electric lighting
- Electric furnaces
- Electric welding
- Electric motors
- Electric battery charging
The voltage of generator can be adjusted by changing the _____ or changing the _____
Speed
Strength of magnetic field
The voltage of a generator tends to _____ when the load changes in series and shunt generator.
Change
A load increase is always acompanied by drop in _____
Terminal voltage
In _____, the voltage may fall (under-compounded), rise (over-compounded) or remain constant (flat compounded) as load changes.
Compound generators
The speed of rotation of a generator is usually _____ since the speed of the mechanical prime mover is generally fixed by the governor controls.
Constant
Generators can be and frequently are operated in _____ with others to supply power to a common load
Parallel
When an electric motor is in operation, it is driven _____ and develops _____ which in turn can produce mechanical rotation.
Electrically
Torque
The load on a motor constitutes the force that tends to oppose the rotation.
Counter torque
Example of load of motors
- Fan blades
- Pumps
- Grinders
- Boring mills
- Crushers
- Excavator
- Elevators
- Turntables
- Churns drills
- Food mixer
The speed of rotation of a motor tends to changes as _____ varies.
Load
An increase in load caused the speed of a shunt motor to _____, that of a compound motor to _____, and that of a serieis motor to _____.
Drop slightly
Drop considerably
Drop very greatly
The speed of rotation of a DC motor can be changed by varying the _____ or changing the _____ across the armature terminals.
Strength of magnetic field
Voltage impressed
In motors, the impressed EMF across the motor terminals is usually substantially _____.
Constant
Motors usually operate as _____ to drive their individual loads.
Single independent units
Machine that converts mechanical energy to electrical energy by using the principle of Lorentz law.
Motor
A current-carrying conductor experiences a force when placed in a magnetic field.
Lorentz law
When electric current flows through a conductor, it increase a _____ around the conductor.
Magnetic field
This _____ interacts with an external magnetic field, by attracting or repelling the magnetism. As a result, the wire experience a force that causes it to jump.
Temporary magnetic field
What is the formula of Lorentz force?
F = BILsinΘ
A DC motor rotates as a result of _____ interacting with each other, the magnetic field produce by the _____ and the magnetic field produce by the _____.
Two magnetic field
Current-carrying armature
Field poles
The direction of the force (or motion of the conductor) can be conveniently obtained using _____.
Fleming’s left hand rule
Consist of a loop of wire placed so that it can be rotated in a stationary magnetic field.
Elementary DC motor
_____ is applied across the wire so that current flows through it. _____ connect the loop to an external circuit supply in order to pick up the applied EMF.
Voltage
Sliding contact (brushes)
Provide the magnetic field and STATOR.
Pole pieces
Loop of wire that rotates through the field and ROTOR.
Armature
Usually made of carbon with wires attached to them, ride against the commutator.
Brushes
External input voltage is applied _____ these brushes.
Across
The coil ends of the armature loops (there are two conductors per turn) are connected to rings. They rotate with the armature.
Split rings/commutator
The loop (armature) field is both ____ and _____ by the field from the field poles.
Attracted
Repelled
The north armature pole is repelled from the _____ and attracted to the right by the _____
North field pole
South field pole
The south armature pole is repelled from the _____ and attracted to the right by the _____
South field pole
North field pole
After the loop turned far enough so that the north pole is exactly opposite the south pole, this changes the direction of ______ flow through the armature loop and also changes the _____ of the armature field.
Current
Polarity
Formula of the power developed by a motor?
P=EcIa
P=ΘPNZIa/60a
P=2πTN/60
Because force is acting on the armature conductors, _____ is developed in the motor.
Torque
The torque developed its tendency to produce rotation depends upon two factors.
- Flux created by the main field poles
- Current flowing in the armature windings.
The load on the motor constitutes the force that tends to opposed the rotation.
Counter torque
While a DC motor is running, it acts somewhat like a _____.
DC generator
This _____ causes current to flow in the loop with a direction opposite that of the current caused by the armature supply voltage. This induced voltage is called _____ or _____.
Induced voltage
Counter EMF
Back EMF
The counter EMF cannot be _____ or _____ the voltage applied across the armature terminals. There is no way to avoid the counter EMF.
Equal
Greater than
At the instant DC motor is started the counter EMF is _____ because the armature is not revolving.
Zero
As the armature accelerates to full speed the value of counter EMF _____ to a value that causes the proper value of armature current to flow.
Rises
At the instant DC motor when counter EMF is zeros, the armature current would be _____ unless some resistance were added to the offset the lack of counter EMF.
Extremely high
During initial start-up when counter EMF is zero, as the motor is coming up to a speed a _____ must be inserted (in _____ with the armature circuit) to take place of counter EMF.
Resistance
Series
As the speed increases resistance may be _____ gradually because counter EMF rises.
Cut out
Start-up resistor is connected in _____ with the armature.
Series
Effect upon speed and armature current
Loading a motor
When a load is applied to a motor the natural tendency of the latter is to _____ because the opposition of motion (the counter torque) is increased)
Slow down
The counter EMF is ____ for the reason that counter EMF is proportional to speed.
Decreased
Reduction in speed result in an ______ armature current
Increased
Increased in armature current leads to an _____ in torque developed by the motor to drive the increased load.
Increased
The direction of rotation of a dc motor is depends on the direction of the _____ and direction of _____ in the armature.
Magnetic field
Current
The connection of either the armature or the field must be reversed but _____.
Not both
Double pole, Double throw
Forward 1 shorted 3 and 2 shorted 4
Reversed 5 shorted 3 and 6 shorted 4
The armature field distorts the flux between pole pieces, as a result the neutral plane is shifted _____ in the direction opposite that of rotation. This is different from a DC generator where the neutral plane is shifted _____ in the direction of rotation.
Backward
Forward
Three way to address the armature reaction in motor.
- Brush repositioning
- Compensating windings
- Interpoles
The brushes must be shifted to a new plane.
Brush repositioning
Same in motors as they are in generators. They carry the same current as in the armature.
Compensating windings
Inter poles in a motor are wound and placed so that each has the same magnetic polarity as the main pole _____ it, in the direction opposite of rotation. Unlike in generator the the main pole is _____ of it in the direction of rotation.
Behind
Ahead
If compensating windings or inter poles are installed it is not necessarily to ______ when there is an increase or decrease in load.
Shift the brushes
The brushes are located on the _____ neutral plane. They remain in that position for all conditions of load.
No load
The winding of a gramme-ring armature are place on an _____.
Iron ring
The gramme-ring armature is _____ used in modern DC motors.
Seldom
The winding on the inside of the ring are shielded from magnetic flux which causes this type of armature to be _____.
Inefficient
Each coil is connected to _____. Each coil is in ______ with every other coil.
Two commutator segment
Series
It is generally used in AC motor, it is identical to the drum winding discussed on DC generator.
Drum type armature
This length of each conductor is positioned _____ to the faces of the poles pieces.
Parallel
Two general classification of DC motor
- Separately excited DC motor
- Self excited DC motor
Field winding is energized from a source DC external to the machine called exciter. This type of DC motor is not normally used.
Separately excited DC motor
Field winding is energized by its own armature possible only if the field pole pieces have retained a slight amount of permanent magnetism called remanent/residual magnetism.
Self excited DC motor
Type of self excited DC motor
- Shunt wound DC motor
- Series wound DC motor
- Compound wound DC motor
Shunt field motor its field winding also called shunt field winding is connected _____ the armature
Across
Series field motor its field winding also called series field winding is connected _____ the armature
Series
In shunt motor, torque vs armature current is _____ all through out all load variables. Equal lines in heavy load and light load.
Linear
Shunt motor flux is _____ of load.
Independent
Under heavy loading the shunt motor will be _____ to develop sufficient torque and will stall.
Unable
In series motor, torques vs armature current is _____ at light load variations.
Parabolic
In series motor, torques vs armature current is _____ at heavy load variations.
Linear
Series motor flux is _____ of load.
Dependent
Series motor has _____ overload torque. (Difficult to stall) the speed will become slower but it’s ok.
High
In compound motor, torques vs armature current is _____ at light load variations.
Curve
In compound motor, torques vs armature current is _____ at heavy load variations.
Linear
Compound motor flux consists of ____ for shunt and _____ for series
Constant flux
Load changing flux
Best speed regulations from no load to full load. Speed has a very definite no-load value
Shunt motor
Worst speed regulation from no load to full load.
Series motor
Series motor tends to _____ at light load - it should never be used _____ or when the load is such that torque might drop approximately _____ of the full load torque.
Run away
Belt drive
15%
Speed varies considerably. Speed has a very definite no load value.
Compound motor
Speed regulation of a motor refers to the _____ that takes place when the load changes.
Speed change
T/F
The higher the percent of regulation the better the motor.
False
Higher > Lower
Shunt motors are generally regarded as _____ motors because their percent speed regulation is very small.
Constant speed
Compound motors are properly considered to be _____ motors because their percent speed regulation is comparatively high.
Variable speed
Series motor certainly has a _____ characteristic their percent speed regulation is very high.
Variable speed
If a change from no mechanical load to full load cause the speed to drop approximately _____ the motor is said to be of constant-speed type.
8% or less
Internal action happens to the motor due to charge in load.
Speed regulation
It is imposed action usually through an external adjustment for the purpose of increase or decreasing the speed.
Speed control
The speed of a DC motor is change by.
- Changing the current in the field
- Changing the current in the armature
Speed control can readily be done in one or more of three different ways.
- Inserting a field rheostat in series with shunt field circuit
- Inserting a resistance in series with the armature circuit
- Varying the voltage across the armature circuit
For a constant applied voltage as the resistance of the rheostat is increased, the amount of current through the field _____ according to ohm’s law.
Decreased
A decrease in field current causes field flux to _____
Decreased
Reduction in field strength _____ the counter EMF of the motor since fewer lines are being cut by the armature.
Reduces
Reduction of counter EMF an _____ in armature current
Increase
Increased in armature current leads to an _____ in torque developed by the motor.
Increased
Increase in speed will proportionately ______ counter EMF
Increased
The speed and counter EMF will then continue to increase until the armature current and torque are reduced to values just large enough to supply the load at a new _____
Constant speed