DC Machines Flashcards
A DC machine that is rotated by a prime mover is
A Generator
A DC machine that is connected to an electrical supply is
A motor
For a DC generator, to generate voltage there must be
Flux, a conductor and relative motion between them (E=Blv)
What produces the flux on a generator?
Stationary windings when excited
Stationary windings when excited produce what for a generator?
The flux (B)
The windings on the rotating part (armature) provide what for a generator?
The length of conductors (L)
The length of conductors for a generator are provided by?
The windings on the armature
The prime mover for a generator provides?
The velocity of relative motion (V)
For a generator what provides the velocity of relative motion?
The prime mover
For a generator, what is the armature used for?
It is where AC is induced into
Where is AC induced into for a generator?
Armature
Often in DC machine, the armature is referred to as
The entire rotating assembly
List the components of the armature
Shaft Laminated stampings Coil windings (magnetic wire) Cell insulation (cellulose paper and wedges) Coil banding Bearings Fan Commutator
What are the laminated stampings for on the armature?
Reduce eddy currents
Cheap Construction
What is the commutator used for on the armature?
Rectifies the induced AC to DC
What is the commutator constructed of?
Insulated copper segments(bars)
Coils are connected to the segments
Mica (or synthetic) insulation between segments
(Generator) what is the purpose of the brushes?
To provide electrical contact to the rotating commutator
Brush construction (generator)
- usually rectangular (may be cylindrical)
- often have a “brush shunt” which is a flexible braided copper lead embedded in the brush to shunt current away from the brush spring
A shunt may also be called
Pigtail
What are the 4 main materials modern brushes are made from
- Carbon
- Graphite (softer than carbon and more slippery)
- Electrographite - graphatized carbon
- Copper or graphite and copper
What factors determine brush selection?
Voltage Speed Current Cost Duty Cycle
What brush material has a negative temp coefficient?
Carbon (semi-conductor) (Resistance goes down when pressure goes up)
What are the 2 types of field windings for generators?
Shunt Windings
Series Windings
Explain Shunt windings
Have many turns of small wire connected in parallel to the armature
Have values of 10s of ohms (ex. 40, 60, 100 etc.)
Explain Series windings?
Have few turns of large wire connected in series with the armature
Have very low values of ohms (ex. 0.08, 0.12 etc)
Why are windings preferred over permanent magnets?
The current through the windings can be varied which controls flux and output voltage
What are other windings that can be found on DC machines?
Interpoles (commutating windings)
Compensating Windings
Wat does adding more coils do for the windings?
Makes for a smoother and greater voltage
What is Armature Reaction?
Distortion of the main field flux caused by armature current
What negative effects does armature reaction have on the machine?
Sparking of the brushes
Reduction of overall flux in the poles
What is the purpose of interpoles in a generator?
to achieve good commutation (no sparks) by inducing a voltage into the coil undergoing commutation that is equal and opposite to the reactance voltage.
Interpole polarity is a …. ?
sign of things to come
what does the polarity of the interpole do?
Helps to strengthen the leading tip of the main field and reduce the effect of armature reaction
What can be used to counteract field distortion, excessive sparking and perhaps a fire ball around the commutator?
Compensating windings can be embedded in the pole faces and connected in series with the armature
What is a compound field?
When both shunt and field windings are used on each pole
When a compound field is connected so that their fluxes aid each other
Cummulative Compound
When a compound field is connected so that their fluxes oppose eachother?
Differential Compound
What are the conditions for build of voltage?
- Primer mover is turning
- Residual magnetism must be present
- Complete circuit between armature and field
- Resistance in the circuit is not higher than the “critical resistance”
- Proper connections are made for the direction of rotation.*
- The generator is not started with a big load (low R) connected.
If the prime mover direction is reversed and EMF is induced into the armature with the opposite polarity, this voltage will push a current through the field such that it will neutralize the residual flux and output voltage will be 0, what can this be corrected?
Reverse the armature leads and flash the field.
What are the power stages in a generator?
Input Power (HP x 746)
Losses (windage, friction) (hysteresis, eddy currents (iron losses))
Generated Power (Egen x Ia)
Losses (I2R armature, I2R fields (copper losses))
Output Power (Vterminal x Iload)
Input Power =
Output power + losses
Efficiency =
Power out divided by power in
Define percent voltage regulation
How good the generator maintains terminal voltage from no load to full load
What is the purpose of a diverter rheostat in a long shunt compound generator?
- Adjust current though series field
- High wattage
- Low resistance
- Adjusts the level of compounding.
What happens to the prime mover of a generator when electrical load is increased and why?
Tends to slow down because as the load current increases, the opposing flux increase and the primer mover has to work harder (motor effect)
What are the forces at work in a motor?
Flux and current
Explain the “right hand rule” for motors
Thumb - motion
Index Finger - Field/flux
Middle Finger - Current
Another name for “work” is?
Torque
Torque =
Force x distance in Nm (newton meters) or Ftlbs (pound feet)
The amount of force on the armature conductors is dependent on?
the strengths of the main field flux and the armature flux combined
Torque = Flux main field x Flux armature
Torque = Flux field x armature current
Horsepower is
the rate of doing work
HP = NT divided by 5252
What controls the amount of armature current in a motor?
Counter EMF or CEMF
What is CEMF?
an induced EMF that opposes the applied voltage
What can happen if the field is weakened or lost in an unloaded shunt motor
it can runaway above base speed (very hazardous)
When may a series motor be used?
For traction purposes (trams, trolleys, ski lifts, electric drills)
High Torque applications
For series motors torque is proportional to
Ia squared
Series motors have very poor
speed regulation
Most compound motors have protection to prevent?
Runaway if the shunt field is opened
What are the power stages in a motor?
Input Power - Vt x IL Losses - I2R fields, I2R armature (Cu losses), Hysteresis, eddy currents (Fe losses) Mechanical Power - Ecemf x Ia Losses - windage, friction Output Power - HP x 746
NEMA Standards - Rotation is viewed from?
The opposite drive end (lead end)
NEMA Standards - Generator standard rotation
Clockwise (CW)
NEMA Standards - Motor standard rotation
Counter Clockwise (CCW)
NEMA Standards - Standard Compounding
Cummulative
NEMA Standards - Positive Line Lead
L1
NEMA Standards - Current flow is from subscript
2 to 1
NEMA Standards - Rotational reversal
Interchange A1 and A2 leads
NEMA Standards - Differential compounding
Interchange S1 and S2
NEMA Standards - Shunt field current should always be
F2 to F1
Define magnetic field
the distribution of flux lines around a magnet
define flux density
quantity of flux lines per unit area (Tesla)
Define magnetic poles
the distinct region where flux lines concentrate
Ferro-magnetic materials
`those substances that exhibit the same magnetic properties as iron
Electromagnetism
the establishment of flux lines (magnetic field) resulting from the current flowing through the conductor or a coil
Magnetomotive Force
the strength which produces electromagnetism or ampere turns
Reluctance
the opposition to establishment of magnetic flux lines
Permeability
the ease which a material will become magnetized
BH Curve
a graph which illustrates the relationship between flux density and magnetizing force for electrical materials
Saturation
that max level of flux density where an increase in magnetizing force does not produce a significant increase in magnetic flux density
Residual Magnetism
the permanent magnetism retained by an electro magnet after the removal of a magnetizing force
Coercive Force
the magnetizing force required to eliminate residual (permanent magnetism)
List three fundamental laws of magnetism
- Magnetic line form closed loops
- Magnetic lines never cross each other
- Magnetic line stay short like elastic bands
To prevent permanent magnets from losing their magnetism, some precautions should be taken when storing them, what are they?
- Avoid mechanical shock
- High temperature
- AC fields
4, Use “keepers”
What is the “left hand rule” for electromagnetism for conductors and coils?
Conductors - Thumb points in direction of electron flow and fingers indicate field direction
Coils - Fingers indicate direction of electron flow and thumb points in field direction
List three factors that affect the field strength of an electromagnet?
- Magnitude of current flow
- Number of coil turns
- Reluctance of core material
When is an electromagnet said to be saturated?
When increasing magnetizing current does not appreciably strengthen field flux
What three conditions must be satisfied to induce any voltage?
- Magnetic field
- Length of conductor
- Relative motion of conductor
E=Blv
“left hand rule” for generators
Thumb - conductor motion
Index - field direction
Middle - current flow
Define the term “neutral plane”
Conductor position relative to the magnetic field where no voltage is induced within the conductor
What is a generic term for a generator?
Dynamo
What is the difference between a “separately-excited” and “self excited” DC generator?
Separately excited - field circuit is energized from external DC source
Self excited - induced armature voltage also energizes the field winding
Why is it necessary for self-excited DC generators to have “residual magnetism”
There will be no induced voltage to provide field strength and allow voltage build up for the machine
What is meant by the expression “flashing the field”
Application of external DC to the field circuit in order to establish residual magnetism within field iron
Generator Construction - End Shields
House bearings and support shaft, mechanical protection
Generator Construction - Frame
To house field structure and complete magnetic circuit
Generator Construction - Field Coil
Provides excitation
Generator Construction - Shaft
Allows rotation of armature
Generator Construction - Brush Holder
Supports and protects brush
Generator Construction - Brush
Provides armature connection to the load
Generator Construction - Field Pole (laminated)
Magnetic circuit for armature conductors
Generator Construction - Commutator
Mechanical rectifier (AC to DC)
Generator Construction - Armature Core
Magnetic circuit for armature conductors
Generator Construction - Armature Winding
Active conductor length
State one advantage of “self excited” versus “separately-excited” DC generators.
Self - no need for separate DC supply, cheaper
Separate - good voltage regulation, output for either direction of rotation, easier start up
Why is a “field rheostat” employed with a shunt generator?
Permits varying of field current to control terminal output voltage
What does a magnetization curve for a DC generator illustrate?
The build up of terminal voltage with field current
How does the “no load” terminal voltage of a DC shunt generator compare to its “full load” value?
Full load voltage is lower than no load due to armature IR drop, armature reaction, resultant field weakening
What is the simplest method of controlling output voltage of a shunt generator?
Varying the field excitation (field rheostat)
Why is it necessary to have the proper direction of rotation for a DC generator?
Generated armature voltage, for a given direction of rotation must excite field to aid residual magnetism of field poles
Why is a series DC generator very seldom used?
Since the field current is dependent on load current, terminal voltage varies with load, resulting in poor voltage regulation
With no load connected, what is the terminal output voltage expected from a series DC generator?
very low due to residual magnetism
What would distinguish if a compound generator was “cummulative” or “differential” as a load was applied?
Cummulative terminal voltage should rise (or constant) with load increase
Differential terminal voltage would drop with load increase
Explain “over compound” in compound generators
Full load voltage is higher than no load voltage
Explain “flat compound” in compound generators
Full load voltage is the same as no load
Explain “under compound” in compound generators
Full load voltage is lower than no load voltage
what is meant by the term voltage regulation?
compares change in terminal voltage between no load and full load
How are the affects of armature reaction minimized in DC generators?
Interpoles (commutating poles), Compensating windings, Brush shifting
What is the general “rule for interpole polarity” in DC generators?
is the same as the main field pole AHEAD in the direction of rotation
What fundamental principle of magnetism develops rotation for a DC motor?
Law of repulsion
What function does the commutator serve in a DC motor?
switches direction of current through the armature conductor (when under the opposite pole face) to maintain torque in the same direction of rotation
What are the two basic factors that determine the amount of torque that a DC motor will develop?
Main field strength
Armature current
T is proportionate to FLUX x Armature current
Why does increasing the mechanical load on a motors shaft cause an increase in armature current?
Armature slows down, less CEMF is developed, armature current increase to develop more torque to keep load turning at a slower r/min
What is meant by the term “base speed”?
rated motor speed with rated field and armature currents (full load)
Another name for a shunt motor?
constant speed motor
Why is a shunt motor typically referred to as a “constant speed” motor?
Very little difference in RPM between full load and no load (typically 5% change)
State what happens to the speed of a series motor as load is increase up to its rate value?
Motor speed drops drastically with increase in mechanical load
How could speed control of a series DC motor be accomplished?
Generally the amount of mechanical load control rpm, sometimes a diverter rheostat
Compare series and shunt DC motors for speed regulation and torque capability.
Shunt - very good speed regulation, moderate torque
Series - very poor speed regulation, extremely good torque
List two applications where DC series motors are commonly employed?
Hoists, cranes, elevators, ski lifts, drills
What hazards are associated with differential compounded DC motors?
May suddenly stall on increasing load and reverse direction (suicide machine)
When interpoles are used in a DC motor for commutation, what is the rule for their connected polarity?
moving in the direction of rotation, the interpoles have the same polarity as the main poles behind them
List three possible reasons for excessive brush sparking on DC machine?
Excessive load
Wrong interpole polarity
Brush mechanical troubles
Describe the appearance generally desirable on the commutator surface?
Chocolate brown
Why should emery cloth never be used to clean a commutator surface, or seat brushes?
Contains conductive particles
What should never be used to clean a commutator surface or seat brushes?
Emery cloth
What is meant by the term “undercutting” a commutator?
Cutting mica insulation between commutator bars to depth below the commutator surface
List two fundamental requirements of a DC motor starter?
Limit starting current
Provide running overload protection
Provide smooth acceleration
Why is the starting current for a DC motor so much higher than its full load value?
No CEMF, only armature resistance to limit in rush current
Should a shunt motor be started wit reduced or full field excitation?
Full field, to develop full starting torque
What would be the main disadvantage of a manual type DC motor starter?
No provision for remote operation, operator error
State the function of a motor starter?
to start and accelerate a motor
Why is it essential that switch and relay contacts be DC-rated?
B/C of the arcing that occurs when the current is interupted
Why does in rush current persist longer in large DC motors than in small Dc motors?
The inertia of the armature causes a slower rate of acceleration and a longer interval
Describe the basic construction of a drum controller.
Consists of a series of copper contacts mounted on a cylinder that is insulated from a central shaft
Name the two basic configurations of manual faceplate starters?
Three terminal
Four Terminal
Which faceplate starter provides no-field protection?
Three terminal
What faceplate starter allows speed control?
Four terminal
Name the six main parts of a DC contactor.
Electromagnetic coil Armature Moveable contacts Fixed contacts Spring assembly Blowout coil
Why is the holding current of a DC contactor much less then the pull in current?
For the pull in current, the instant the coil is first energized there is a large air gap that requires large current, once pulled in current decrease due to a decrease in reluctance
Give two methods that are used to prevent overheating of the coil of a DC contactor?
Inserting a resistor in series with the coil
Equipping the contactor with dual coils
The operation of a “blowout coil” is based upon what principle?
a principle of magnetism called the motor effect
What device is at the heart of the solid-state DC controller?
Silicon-controlled rectifier (SCR)
List three unwanted conditions that a large inrush current could cause.
Overheating of armature conductors
Excessive starting torque
IR volt drop in supply conductors
Define current-limit acceleration
Insertion and removal of external resistance to the armature circuit during the acceleration of motor from stand still to normal operating speed
As CEMF increases, armature current …. ?
decreases
In a CEMF controller, where is the coil of the accelerating relay connected?
Across armature leads A1 and A2
What control device is used to provide definite-time acceleration?
Timing relays
What type of timer operates by forcing a fluid through the orifice of a piston-cylinder assembly?
Dashpot
Why is it important that starting resistors not remain in circuit for too long?
They are rated for intermittent duty and can over heat
What principle does the capacitor-timing started use to provide time delay?
R-C constant of a resistor and capacitor
Why is field loss protection required?
For protection if the shunt field circuit opens it stops the motor from going an unsafe speed
Field loss protection is most commonly provided by using what?
FLR (Field Loss Relay)
The coil of a field loss relay is connected?
In series with the shunt field
Connecting a rheostat in series with the shunt field will enable a DC motor to operate…
Above base speed
Why are field windings are sometimes left energized ?
To keep machines warm
List 5 basic reasons that can cause excessive brush sparking?
Brush troubles - specifically wrong tension Commutator surface Open armature circuit Excessive Load Sudden load chnages
What are sometimes connected across the armature windings to reduce or prevent damage to the armature insulation due to the collapsing field during commutation?
Free wheeling diodes
When electromechanical braking is used with an electric motor, the braking system is operated by a
Solenoid
Braking systems used in electric motor applications are designed to
Fail-safe
How is the fail-safe in braking systems achieved?
the mechanical braking system is designed so that the brake is applied when the solenoid is de-energized
Define dynamic braking
a method of deceleration that involves reconnecting a DC motor to function as a DC generator
During dynamic braking of a DC motor, the armature conductors experience a force acting
opposite or counter
Why does the shunt field remain energized during dynamic braking?
Shunt field excitation is necessary for the motor to function as a generator during the braking period
Why are mechanical brakes often used in conjunction with dynamic braking?
Since counter-torque is proportionate to speed, the braking effect decrease as the motor slows down
What device is used to disconnect the shunt field after the dynamic braking operation?
Off-delay timing relay
What is the main disadvantage of dynamic braking?
Loss of heat dissipated by the braking resistor
Which braking system stores, or returns to the source, the energy released during the braking of a DC motor?
Regenerative Braking
What are the two fundamental steps in troubleshooting DC motor controls?
Visual Inspection
Circuit testing and measurement
The majority of faults in motor control involve ?
Loose connections
Failed components
What two mechanical conditions can be checked by manually operating a DC contactor?
Actuating device travels freely without binding or excessive friction
Travel distance provides proper operation of the contacts
What precautions must be taken before manually operating an electromechanical control device?
Control and power circuits must be tested to ensure they are both de-energized
What are the two main differences between DC and AC electromagnetic control devies?
Shading coil is not required in DC
Arcing is more serious in braking DC and requires use of blowout coils and greater air gaps
What device is often used to test for shorted coils on an armature?
Growler
A diverter rheostat, used with a DC generator, is connected
Across S1 and S2
During acceleration from a standstill, DC magnetic starters insert resistance
in series with the armature
The motor effect of a DC generator is what….
causes loading of the prime mover
The generator effect, in a DC motor, is what…
Keeps the armature current at a reasonable level
Large DC motors are usually started with…
solid state controls
If the interpole winding of a series DC motor opened while operating at rated load, the motor would?
Stop
The efficiency of a DC motor at no load is..
Less than at full load
A field acceleration relay is normally used to
limit armature current on inrush
What is an important part that must be check on brushes to maintain optimal operation?
Check for suitable pressure against the commutator surface (usually between 1 and 2 psi)
What are a couple key probable causes when there is excessive sparking at brushes when load is applied?
Insufficient brush pressure
Loose brushes
The generator effect, in a DC motor, is what?
Keeps the armature current at a reasonable level
The efficiency of a DC motor at no load is?
less than at full load
Where is the “field failure relay” normally located in a compound DC motor control circuit?
in series with the shunt field
In order to obtain “below base speed” control for DC motors, it is standard practice to
apply full voltage to the field and reduce voltage to the armature
EMF is actually produced in a generator by
conductors cutting flux
A separately excited shunt generator has a _______ in terminal voltage with an ______ in load.
Decrease
Increase
Flux distortion due to armature reaction causes the electrical neutral plane in a generator to
Move in the direction of rotation
Compensating winding are connected in ______ with the armature
series
A DC series generator has an ______ in voltage with an ______ in load up to normal load
Increase
Increase
When a given armature coil is being commutated, the brushes _____
Short circuit it
With an increase in load resistance, the output voltage of a series generator will?
Decrease
Armature reaction is counteracted by?
Compensating windings
The effect of DC motors drawing more current is best explained by the reduction of
CEMF
Below base speed control of a separately excited DC shunt motor is best done by inserting resistance _______
in series with the armature
Above base speed control of a separately excited DC shunt motor is best done by inserting resistance ______
in series with the field
