DC Motor Final Flashcards
What is the prime function of all DC motor starters ?
Insert resistance when motor starts and remove resistance as motor gets up to speed.
What type of protection does a three point starter provide?
No field or loss of field protection.
What type of compounding is used when constant voltage or speed is required?
Flat-Compounding
What type of manual starter provides a greater ability to control speed?
Four point starter
What components ride against the commutator segments and connect the armature to the external circuit of a DC generator?
Carbon graphite brushes
What is the purpose of a field winding in a DC machine
To Create north and south magnetic poles at the pole pieces
Once a DC motor is started and all the resistance is removed, what limits the current through the armature?
CEMF induced in the armature
What does the successful starting up of a self-excited DC machine depend on?
Residual magnetism in the pole pieces
How should the direction of rotation of a DC motor be reversed?
Interchanging the armature connections (A1 and A2)
what protective devices connected in series with the shunt field do most large compound motors have?
Field loss relay
What is an advantage of a separately excited DC machine?
Better speed and voltage regulation with added load.
If resistance is removed from the shunt field winding circuit, what will happen to the magnetic field?
It will increase in strength
If resistance is added to the shunt field winding circuit of DC shunt motor, what will happen to the motor speed?
It will speed up
What type of wound armatures have their windings connected in parallel
Lap-wound
What is the distortion, or bending, of the main magnetic field of a DC machine, caused by its interaction with the magnetic flux developed by the armature current called?
Armature reaction
What type of compounding occurs when a cumulative long-shunt compound motor’s full-load speed is greater than its no-load speed?
Over-Compounding
What type of compounding occurs when a differential short-shunt compound generators full-load voltage is less than its no-load voltage?
Under-Compounding
What is the function of a series field rheostat?
To control compounding
What is the commutator of a DC machine constructed of?
Copper bars with mica insulation between them
What is the position at which the armature winding’s are parallel to the lines of flux and there is no induced voltage called?
The neutral plane
What happens to the counter-EMF generated as load is added to a series motor?
CEMF decreases because speed decreased
What is the main disadvantages of a manual three point starter?
Loss of speed control
What is the best way to slow or stop a DC motor under normal operation?
Dynamic breaking with Mechanical breaking?
Will increasing the number of turns of wire in an armature coil of a DC generator cause the induced EMF to increase or decrease?
Increase
What is the function of the shunt field rheostat on a DC shunt motor?
Speed control
Is resistance and voltage directly or inversely proportional in a DC shunt generator?
Inversely Proportional
What two components are used to hold the brushes in place and maintain an even pressure?
Brush holders and Springs.
What components of a DC machine supports the shaft and allows it to rotate freely?
Bearings
What are the three factors that determine the strength of an electromagnet?
- Loops of wire
- Core material
- Amount of current
What is the component of a DC generator that converts AC to DC called?
Commutator
What is the force that causes a motor to rotate known as?
Lorentz Force
What type of compound motor has its field winding’s wound in such a manner that they aid each other in the production of magnetism
Cumulative compound motor
Will increasing the number of turns of wire in an armature coil of a DC motor cause the CEMF to increase?
Yes
Name two industrial processes in which a differential compound generator may be used.
- Welding
- Electroplating
When starting a DC motor, what percentage of FLA shall the starting current be limited to?
150%
What type of motor is used when constant or adjustable speed is extremely important?
Shunt Motor
In reference to the left-hand rule for generators, what do the thumb, forefinger and middle finger represent?
Thumb - Direction of motion of the armature.
Pointer Finger - Direction of main field.
Middle Finger - Direction of current flow.
What are the winding’s embedded in slots or holes in the main pole face called?
Compensating winding’s
What are two types of magnets?
- Permanent magnets
- Electromagnets
When a generator supplies current to a load, what is produced, which makes the armature assembly harder to turn?
Counter-Torque
What causes the shift of the neutral plane on a DC motor under load?
Armature Reaction
When a DC motor is first started why is the armature current so high?
Little resistance in armature winding’s and no generated CEMF.
Where are the flux lines most densely concentrated?
Near the poles
What is the insulation separating the copper segments of the commutator constructed of?
Mica
What are the four components that comprise an armature assembly?
- Shaft
- Iron-core
- Commutator
- Armature winding’s
When an electromagnet reaches a point where strength increases only slightly with an increase in current, it is known as a point of ?
Saturation
What three factors determine the output of a generator?
- Number of loops in armature windings
- Speed of rotation
- Strength of main magnetic field
What are three ways that armature reaction can be corrected?
- Rotating the brushes
- Interpoles with commutating winding’s
- Compensating winding’s
When proper commutation is taking place what color is the commutator?
Dark Chocolate
What happens when the torque of a DC series motor as the load is increased?
Torque Increases
If resistance is added to the shunt field winding circuit of a DC shunt generator, what will happen to the output voltage?
Voltage Decreases
Is the resistance of a shunt field winding higher or lower than the resistance of the series field winding’s?
Higher
What are the winding’s wound around the interpoles called?
Comulating Winding’s
What are the six components that comprise most DC machines?
- Enclosure
- Pole Pieces
- Field Winding’s
- Armature Assembly
- Brushes
- Terminals
Shunt field winding’s are wound with _______ turns of ________ gauge wire relative to the series field winding’s
Many, Small
Series field winding’s are wound with________ turns of _______ gauge wire relative to the series field winding’s
Few, Large
What type of DC machine converts mechanical input into DC electrical output?
Generator
What are three basic types of armature winding’s
- Lab-Wound (low voltage, high current)
- Wave-Wound (high voltage, low current)
- Frogleg-Wound (moderate voltage and current)
What are two types of bearings found in DC machines
- Sleeve Bearings
- Ball Bearings
Why is the pole shoe of every pole piece curved to conform to the surface of the armature assembly?
For a wider and more even flux distribution.
What are the three types of field winding’s used in DC machines?
- Series
- Shunt
- Compound
What are shunt field winding terminals labeled ?
F1 and F2
What is the mechanical used to turn a generator called?
Prime Mover
How can ripple effect be smoothed in the output of a DC generator?
Increase number of winding’s
What are three types of DC machine field structures?
- Permanent Magnet
- Separately Exited
- Self-Exited
When a DC generator contains both series and shunt field winding’s, what is it called?
Compound
