DC Motors Flashcards
What’s a nested branch?
A parallel circuit within a circuit
If given a confusing diagram, how can you make it easier and draw it out?
Write an A following one line that’s the same; after crossing a load, make it B. And then see where all the letters line up and can see what’s parallel and series and what not.
If a load is down stream of another load, are the positive ends of both loads the same?
No they’re just positive relative to the load itself (ie more positive than the negative side of the load).
If current splits evenly in a parallel circuit, we know the resistance of both branches is.
The same.
Describe the left hand conductor rule
If current flows away from you (x or
Plus sign, rather than dot, which means away) then thumb shows current direction North Pole and fingers wrap the conductor, showing the direction of the magnetic flux field.
Describe the left hand coil rule
Fingers wrap coil in same direction (neg to pos) as current, thumb indicates North Pole
In what polarity does flux flow?
North to south externally. South to North internally.
Where does flux concentrate in a core? Why?
At the ends. Conductors around a coil induce flux, and flux concentrates at poles as it travels externally from one pole to another.
Characteristic of flux. Name 5.
1) travel North to south externally (this is direction; not polarity)
2) lines never cross each other
3) align parallel and repel side by side
4) pass through everything
5) concentrates into highly permeable paths (ie soft steel)
6) has eleasticity
7) always pull and tighten to smallest possible size
What’s the formula for Egenerated ina generator?
E gen is equal to flux of the field, times length of the conductor, times rpm.
What are the components of a generator?
1) field yolk (pathway for flux; houses other components and is big)
2) pole core (flux travels pole to pole via field yolk; eddy currents limited by laminations in pole core)
3) armature (rotates on a shaft and contains conductors)
4) rotor shaft
5) conductors
6) commutator (rotates with armature and shaft. Conductors laterally spun around armature go back to the commutator where a brush (1-2 pounds per sq inch pressure) attaches to soldered on pig tails a1 a2)
7) bearing
8) end bells (hold bearing and support armature).
If brushes are too loose, then what? What if they’re too tight?
1) arcing and pitting damages commutator
2) brushes break.
What’s the left hand gen rule?
1)index finger is direction of field flux
2) thumb is direction of conductor with respect to flux
3)middle fingers direction of current
What’s neutral plane?
Space where no voltage (and thus no current) is developed.
Where is ac converted into dc ina genarator?
At the commutator.
A generator creates ac before the commutator. T or f.
T.
Permanent magnet generator is called a
Magneto.
A magneto has fixed flux and field conductor length on the windings.
True.
The l in blv refers to armature or windings?
Windings.
What does the l refer to in blv?
The length of the conductor in the armature being cut by flux.
In a separately excited generator, what terminals are there that are not present in a magneto?
The f1 and f2 shunt windings; magneto only has a1 and a2 windings.
What can be changed in a separately excited generator in the blv formula?
The b aka the windings with a dc source that is separate applies to them (changed via the fixed rheostat in series with the dc windings).
Is there flux in a seperately excited generator?
Yea, but it’s current in the separate windings causing magnetization.
What is easy to vary in a self excitedbgenerator?
The field windings.
What are the terminals in a compound generator?
A1/a2; f1/f2; s1/s2
The shunt is parallel to the armature. T or f.
T.
What are the characteristics of a shunt generator?
-Small CM size wire
-low current
-high resistance relative to series
A shunt can be seperately and self excited. T or f.
T.
A series gen can be self and seperately excited. T or f.
F. Only self excited.
What are the characteristics of a series gen.
- low number of turns.
- large CM
- high current
-lower r than shunt.
A compound can only be self excited because of which winding?
Series.
How much of the flux is in the series coil of a compound generator?
20%.
What are the standard rules?
1) L2 must be negative
2) f2 must be negative
3) prime mover must spin cw
4) current flows through all components from higher terminal number to the lower terminal number.
Describe flashing the field.
In the anscence of residual flux, disconnect the series windings, attach it a dc source and give it rated current for 10-30 seconds.
In a self excited shunt, critical resistance is…
The absence of current flow when the rheostat in the field shunt is too high (start at minimum and raise slowly).
A field rheostat offers what protection to the field shunt
CB protection
Why is a short stunt used for a generator more often
A shorter shunt is better at proving more field stability in a generator (opposite for a motor).
What are the requirements for building voltage?
1) flux or residual flux
2) below critical ohm of field circuit
3) correct cw direction (otherwise flux vs voltage)
4) rated rpm
What is armature reaction?
Twisting of the flux field by the arm field (north to south flux interacted by the flux around the individual conductors in the armature).
Why is the motor effect necessary?
It is the field flux being “broken” through by cw rotation of the armature. It loads the prime mover in the absence of any other load. It provides counter torque and is known as the motor effect of a generator.
In a motor effect of a generator, what happens to the neutral plane?
The neutral plane shifts according to the direction of the armature as flux is bent.
More conductor loops in the armature does what to the ripple voltage?
It makes it smoother.
What is wave, leg, and frog leg?
Wave is the higher voltage, lower current output of a series wound coil. Surfing the wave.
Lap is the higher current, lower voltage of a parallel. Lap speed.
Compound is the middle choice.
How to mitigate the motor effect of a generator?
Via 1) shifting brushes to match neutral plane
2) interpole mitigation to keep flux in the middle (wound in series and is only active when current created by armature flows through it). Poles perpendicular to pole cores keeps flux more aligned
3) compensating windings allow flux to go straight.
If a generator has a short compound config with a short shunt at the load too, then use..
Overcurrent protection so the prime mover isn’t damaged due to overcurrent in the seperately excited shunt
In a seperately excited shunt, what happens to voltage under no load vs at full load
At full load, voltage of armature is lower due to cemf in armature of the resistance in the armature getting current.
In a self excited shunt, the graph looks linear or curved towards the downward point
Curved.
In a self excited series, what’s the graph?
The residual flux is lower and it curves to the saturation (going above base voltage)
In a compound self excited, over compound is
Determined via a diverter rheostat. It installs parallel to series windings. Start with the rheo at half resistance of series. Then raise r slowly so more current goes through series flux that boosts overall into over compound.
Under compound is simply when it bucks.
What’s the formula for force of a dc motor?
Force is equal to B L I.
What does a motor to?
Provide rotating force aka torque.
In what polarity are interpoles placed?
If the current in the conductor on left side by North Pole is going towards us, then the flux will be bent above it, and so want the South Pole at top, North at bottom
If the neutral plane shifts, what are the consequences?
Arcing and reduced output voltage in generator. Arcing and increase in speed in a motor (lower flux means less regulated speed).
In which direction does neutral plane shift in a motor?
Opposite to rotation.
If shunt current flow is in same direction as through armature, then it’s clockwise. T or f.
T.
How do you know if c or d?
If current flows same a direction through shunt and series, then c.
What constitutes a large motor?
Above 2 hp
Which motor is eligible for full voltage start? Wb reduced voltage start?
2hp and below.
Above 2hp
What’s armature reaction?
Twisting of the flux field by armature.
What causes counter torque?
The armature field bending the flux field. It’s the motor effect of a generator.
Move brushes to avoid arcing when neutral plane…
Shifts.
What’s the formula for force?
Force is B L I.
Where is cemf made?
At armature against flux field
What’s the formula for hp?
(Torque x rpm)/5252
What’s the right hand motor rule?
Index finger is direction of flux of input
Middle finger is the current direction
Thumb is direction of motor torque.
What’s standard rotation for a motor
Ccw
A rotary switch is used to switch ac and dc in a dc motor
T.
At the start, should we use fla for calculations or no?
Nope. Only when running.
What’s motor speed formula?
Rpm is E applied divided by flux field.
Vary e applied for big or small movements vs flux varying?
For big.
Why is current in the field necessary to start a motor?
It makes the magnetic field necessary to start the motor via voltage.
If shunt fields open, what’s it called? What’s the danger?
Field loss. Runway rpm if no flux as rpm goes sky high as No generator voltage giving counter v in motor.
How is runway rpm via field loss mitigated?
Via field loss protection in shunt by way of a field loss relay that open if no current in it (it’s in series with field shunt) and the coil then opens the motor circuit via contacts.
What’s the formula for speed regulation?
(RpmNL-RpmFL)/RpmFL
Which has lowest speed regulation? Why?
Shunt. In series, adding load varies the flux which varies the speed, as load increases in current give overall field a boost in flux. Not with shunt.
In a compound, why is runway not as hazardous?
The series helps limit runway by counteracting with an increase in flux as load increases.
Why is differential hookup a hazard in compound?
Series winding cancels flux out with field shunt, mimicking a shunt generator in runway as field flux is lowered with a load. It can even go negative. This can cause reversal of motor and runway in opposite direction.
How to test series polarity?
Put a jumper across the series winding at medium load, and if slight increase in speed, you know it’s cumulative as you’ve bypassed flux which mitigates speed.
A series motor has a coupling or no?
No. A shunt does though. But series has so much torque we don’t couple. Just a straight solid bar to load.
I’m a series, what can cause runway?
No load meaning no counter torque via cemf as no current flows and so no big flux made and so rpm is high.
Rank motors in terms of speed reg and torque.
Shunt, compound, and series (poorest speed reg)
Series, compound, shunt (poorest torque)
In a shunt, as load is put on, describe what happens..
Rpm goes down, cemf goes down, e net rises, current in arm rises, increasing flux, more torque, and rpm recovers.
In series, what happens to torque? And why?
Since field flux doesn’t stay steady, the torque changes by square of the armature change in current (as both arm flux and field flux are in series) so current rises influences both. This, graph is sharper.
What are drum switch positions?
Contacts 1-2,3-4, and 5-6 joined, all contacts open, and 1-3, 2-4, and 5-6.
Friction brake advantages and disadvantages.
It’s fail safe and has zero rpm hold.
It’s expensive and bulky and high maintenance.
Advantages and disadvantages of eddy current brakes. How does it work?
Has no friction parts and gives strong initial braking.
Needs friction brakes, braking effect reduces with rpm, and voltage loss gives no brakes.
It gives a counter magnetic force via dissipation of eddy currents (attached to a voltage source).
Disadvantages and advantages of a degenerative/regenerative brake.
Needs little extra equipment and is low
Maintenance (just a few contacts and a r in parallel to motor).
It’s not fail safe and needs a friction brake.
What brake uses generator effect of a motor?
The degenerative regenerative brake. When motor is turning by itself following pressing stop, it acts as a generator.
What’s the neutral
Plane?
The neutral plane is the position where the armature windings are moving parallel to the magnetic flux lines,
What does a slip ring produce in a dc gen?
An alternating dc.
What is rotational vs mechanical loss?
There are two types of losses: rotational (fixed) and variable loss. The rotational losses can be magnetic losses and mechanical losses. The magnetic losses are due to stray magnetic fields whereas mechanical losses are the friction and windage losses takin
Which type of dc motor has lowest speed regulation?
Shunt.
Armature reaction causes what effect in a dc motor?
Increase in speed.
What two factors are related to compound field strength?
Shunt and series flux.
Armature and core polarities are related to…
Direction of current
Four conditions for a self excited generator to build up to rates voltage output?
Rated speed.
Below critical r.
Residual flux.
Rotation.
Which generators can only reverse voltage output by switching direction of rotation?
Seperately excited shunt or magneto.
Motor effect in a loaded generator makes the prime mover…
Increase power.
Which generator has lowest percent e regulation?
Flat compound.
When a generator is started, how should field rheo be adjusted?
Start at lowest setting and increase slowly until rated v reached with a full load.
Difference between applied and generated voltage?
Applied voltage minus generated voltage gives e net.
Which motor has almost constant speed?
Shunt
If a shunt and series motors lines are switched, what happens to rotation?
Nothing.
Dynamic braking is
Regen or degen.
With a load, cemf decreases as…
The load means more r which means less I which means less cemf.
If f2 and s2 are both positive, then is the generator c or d?
It’s D.
If motor, it would be C
If a2 is negative and f1 is negative, then for a motor or gen, cw or ccw?
Cw.
Efficieny of a motor takes rotational and ir losses into account. T or f.
All resistances are added for ir loss.
True.
Cemf is same as egen.
True.
Epplied minus egen is equal to e net.
T.
Is torque in Hp?
Yes.
Power out is equal to power in minus…
All the losses.
Formula for hp?
Torque times rpm/(5252)
Generated voltage is all voltage drops added together. Applied voltage goes to the load. Applied voltage minus the voltage drops is the cemf.
T.
In a motor, e applied goes across the shunt field. Then minus the series from that if it’s in the way. E net is arm drop. Add for generated voltages in a gen.
T.
Torque is equal
To the ratio of change in current squared.
t.
In a motor, cemf is same as generated. If series is in the rung with the armature, then…
E applied is minus the series and armature drops to get generated.
If series in rung with motor, then e applied minus series gives the real applied
Don’t take shunt into account.
