AC Motors And Machines Flashcards
1
Q
An alternator gives ac or dc?
A
It gives ac. It has slip rings and brushes. A commutator converts ac to dc, but this just gives ac.
2
Q
What are the 4 motors we look at?
A
SCIM, WRIM, synchronous, and split phase (single phase)
3
Q
Which motor can be a motor or alternator?
A
Synchronous. It’s hp output with a prime mover and ac without.
4
Q
What’s the secondary and primary in an ac machine?
A
Secondary is rotor windings. Primary is the stator windings.
5
Q
Why is there no poles in ac?
A
Because the mag field is changing constantly and shifting.
6
Q
How to determine polarity of stator windings?
A
Using left hand coil rule, thumb points North Pole (be wary of current in and current out). Do this for each phase.
7
Q
If A phase is positive 10 amps at 0, and positive 5 at 60, does its polarity change? Why?
A
No. It only changes polarity of stator windings if there’s a change from positive to negative or negative to positive.
At 60, B polarity does change though, and it shifts flux field by 60 degrees.
8
Q
In ac generator, switching any 2 leads does what?
A
It switches directions of the revolving mag field
9
Q
How do we classify poles?
A
We classify it by saying how many poles of each phase (ie 2 pole 3 phase is actually 6 poles)
10
Q
If the frequency is 60 hz, then what is the frequency of cycles per minute?
A
60 Hz x 60 seconds is 3600 cycles per minute.
11
Q
How is movement induced in the rotor?
A
The stator gets current. The magnetic field is formed. This changing magnetic field creates current in the rotor conductors. The conductors have their own flux. This flux interacts with the stator flux, and the repelling and attracting creates movement in a circular fashion as the conductor flux chases the stator flux to get to it (attraction) and away from it (repel).
12
Q
What’s the formula for synchronous speed?
A
(Frequency times 120)/(Number of poles per phase)
13
Q
Describe how the rotor chases the field?
A
The stator flux field is constantly changing and so it cuts the conductors of the motor. This induces a voltage. The induced voltage and subsequent current creates a flux field. This flux field attracts and repels the stator flux, chasing it.
14
Q
What are rotor bars made of?
A
Aluminum. Current runs through them and goes from one end plate to the other.
15
Q
What’s the formula for E of the rotor?
A
B l v with v being the field speed.
16
Q
Formula for slip percentage?
A
(Nsync-Nrotor)/(Nsync)
Using rpm’s.
17
Q
Nrotor plus the (Nsync times slip percentage) is equal to Nsync. T or f.
A
T
18
Q
Slip percentage is directly proportional to…
A
Rotor frequency.
Rotor frequency at 0 rpm is equal to supply frequency. Multiply that by slip percentage to get the new freq.
19
Q
When is there most current in the rotor?
A
At zero rpm.
20
Q
When is the optimal torque?
A
At 75% of rated rpm of a rotor. Or 25% slip.
21
Q
What percent slip is full
Load?
A
4%
22
Q
What percent slip is no load?
A
0.2%
23
Q
Why is 1800 rpm not realistic?
A
Must always have some slip for rotor to even move.
24
Q
E speed regulation formula?
A
(Nl minus Fl)/ Fl
25
What causes resistance in a ac machine?
Bars, end plates, cm size of wire, bar length.
26
What influences inductance amount?
The closer bars are to the surface of the rotor cover, more the flux is pinched, and with more pinch, there’s more inductance.
27
How does current, voltage output, and torque look for an ac motor graphed over a 100% slip to a 0% slip.
Current smoothly goes down and then suddenly dips.
Voltage is linear with slip decrease.
Torque has a pull up before the optimal at 25% slip and then a steep curved decline.
28
What does the power factor of an ac motor show?
The stator flux and rotor flux reacting (takes into account the rotor flux angle).
29
What are the 3 best motor criteria?
6-10x fla in rush current.
Torque is high.
Percent speed regulation is low.
30
What would a low resistance with high xl give?
Low in rush due to high impedance. Low starting torque due to bad PF angle.
Low percent speed regulation as current builds quick due to low Resistance.
31
If high r and low inductance.
Then low in rush due to high z. Strong torque as good pf. High percent speed regulation.
32
What about low z.
High inrush. Strong starting torque. And low percent speed regulation.
33
How does the secondary make the stator increase current with load?
As a load is added to secondary, means higher flux, means more interaction, and cemf lower in primary, and so current increases in rotor cancels out cemf in primary, allowing more primary current to flow in stator with a lower resistance.
34
What are the characteristics of a scim?
Simple design
Rugged
Low maintenance
Small size per HP
Good speed regulation
Low cost
Simple to control
35
Is the iron the rotor bars are contained in insulated or not?
Not.
36
How does a wrim change resistance to keep torque strong?
It keeps a 1:1 r to xl ratio via a field rheo.
37
Describe the wrim motor connections.
3 windings tied into a star point. Then, slip rings attach to the other ends. Each slip ring rotates on rotor. Stationary Brushes then attach to terminal leads going out of the motor which are also tied into a star.
38
What is secondary resistance vs internal?
Secondsry resistors are attached in each line after it comes out of the brushes, before the m1-m2-m3 terminals are tied together.
Internal is set.
Can vary the secondary to get good torque.
39
If you’re given a slip percentage and resistance, how can you get rpm and rotor fr?
The percentage times the stator rpm is the rotor rpm.
The supply frequency times slip
Percentage is the frequency.
Frequency times the full resistance is the new reisistance (or impedance).
40
How does a 9 lead wye connect?
It has two in series windings per phase. The ends connect internally in a wye. Then can connect 4-5-6 together and tie 1-7 etc to make a parallel low v. Or tie 4-7 direct to make a higher v wye.
(V is halved across each winding)
41
How are 6 leads connected in wye or delta?
Just normal, like in the lab.
Ie in delta t-4 joins to t-2.
42
How is a high and low v 9 lead delta connected? Know it’s delta Cz t3 in middle at bottom.
The t-2 is soldered internally
To t7 and 5. Then, join the 5-8 , 7-4, 6-9 for high v.
For low, the left side terminals are joined together, as are the top, and right (ie 2,4,8).
43
What’s the standard connection number system?
A is 1, b2, c3. Add 3 for other end. Add 3 again for a start of another winding in same phase. Add 3 again for end of those respective windings.
44
How is a 12 lead y connected
For low and high v?
4,5,6 are joined together, as are 10,11,12. But can join 4,5,6 and then just tie into 10,11,12 wirthout joining them together as it’s the same point. Then 1,7 and 2,8 and 3,9.
For high v, 4-7 in series etc. and tie 10,11,12.
45
How to connect a high and low v delta?
Connect the 4 most right point of each winding of the 2 phases involved with one another in a delta. (Ie 4-8-10-2) for a low E.
For high v, series it (4-7, 10-2, etc).
46
Why is the system
Voltage higher than motor voltage in some cases but it still
Works?
The rated motor voltage takes into account a 10% sway in either direction due to i square r losses.
47
If service factor of 1.15 or above, need what ol protection? Wb below?
125%. 115%.
48
Is current less in the ac motor at full load or no load?
At no load (0.6 x fla usually).
49
In terms of frequency, VFD’s are ____ rated.
Inverter.
50
A voltage rating can vary how many percent and still have the machine function as intended?
Plus or minus ten percent.
51
For the inrush current, should we use rated or add ten percent to rated first?
Use rated.
52
What is the SF of 1.15 or above showing in terms of OL protection required?
It means the OL protection must be 125% of fla. Otherwise, it’s 115%.
53
Pf angle is good or bad at startup?
Bad.
54
What does torque require?
Requires slip.
55
Is a synchronous motor induction or electromagnet?
The latter.
56
How many windings in a synchronous?
6,9, or 12.
57
Efficiency of a synchronous motor?
97-98%.
58
What are characteristics of a synch motor?
-operate at constant speed
- large loads at slow rpm’s
-mostly 20-20k HP
-rotor is wound with poles that require dc current (Wound around rotor poles which are salient).
-has external dc supplied via slip rings via f1 and f2 leads
-onboard exciter
59
Describe how a synchronous motor works
Ac is applied to the stator. The magnetic field moves (at 1800 usually). Energize dc supply to rotor. Now it’s at 100% slip still so no torque. Need 96-98% speed of the synch speed to function. (Called synching the motor). Use a pony motor to move rotor until it reaches that speed.
Have a field rheo in f1/f2 circuit as well as a synchro switch. Field rheo changes rotor flux via current increases and decreases.
Rotor travels at synch speed as rotor poles link magnetically to the stator (they don’t actually
Link but magnetically via flux, which btw, is never perfectly in line with the rotor due to slip).
60
If there’s no slip, what’s the freq?
Zero.
61
How are the rotor and stator poles linked?
Via flux linkages.
62
Do you energize the rotor before or after getting it to synch speed?
After.
63
Pull in torque is..
Torque needed for rotor and stator poles to link.
64
What’s amortisseur winding?
Physical windings linking rotor poles together (like rotor bars).
It dampens oscillations in rotor as it changes loads and speeds etx.
65
What angle is breaking out of synch?
60-70 degrees of flex stretching (aka pull out torque).
66
Does the stator move in a synch motor?
No , only the mag field
67
Should speed synching be done at NL or FL?
NL.
68
When does rotor oscillate without dampening effects in place?
When putting a full
Load on, it bounces until it reaches the speed needed.
69
Synch motor has how many leads when used as alternator?
9 or 12?
70
Purpose of field discharge resistor that’s place in parallel with dc supply?
To protect cct from high voltage and current created at startup in the rotor (applied e at stator stays the same Ofc).
71
What’s an onboard excited?
The field discharge resistor.
72
Does synch motor have a bridge?
Yea, to rectify ac into dc as it goes to supply dc to complete cct.
73
I line in a synch is equal to…
E net/ Z synch
74
Describe the graph of e net, cemf, and I line.
I line always lags e net by 90 degrees. Cemf is not at 90 degrees on the graph as in theory can’t have it 90 degrees from stator to rotor. 90 Ofc would be perfectly straight from rotor to stator.
75
What factors change the cemf angle?
1) more load, which means more torque angle, which means more lag in the current, which means sharper angle of cemf to the e line(aka applied)
2) lower the rheostat in series with f1/f2 (not onboard exciter) to add more rotor i which lowers primary cemf by cancelling it out more with more rotor flux.
3)
76
Why order an oversized synch motor?
It’ll give mechanical power to load and lower pf of entire system at supply by giving more i back to the supply, like a capacitor. This is done by adding a lot of cemf. More cemf means more current pushed back through the primary windings back to the supply.
77
What’s a synch condenser?
Synch motor without a shaft. It corrects pf.
78
In what way does synch motor act as a txr?
By correcting pf by having more cemf in primary which makes more current to supply other loads on same system.
79
How does split phase motor work?
The run winding (many turns, large? Diameter) and start winding connected in parallel series to l1 to n.
Run has high xl. Start has low xl and so higher r. This causes start amps to lead run amps.
This causes a phase shift in stator poles. Rotor turns within them.
80
What’s a centrifugal switch do?
Cuts out the start winding flux at 75% of rated rpm so it doesn’t interact with cemf and run flux.
81
If centrifugal switch doesn’t open or close, then…?
Burns out start winding.
Motor just growls.
82
4 different split phase configs and descriptions.
1) resistor start inductance run. Regular setup. 150-200 percent of full load rated torque at startup.
2) capacitor start inductance run. Cap in series with start. 450 percent startup torque.
3) capacitor start capacitor run. Run winding there. No start winding. Instead, has a run capacitor (bigger) and a smaller capacitor and a centrifugal switch in parallel series with it (start side). Runs with higher PF. 450%.
4) capacitor run. No start winding. Capacitor in place. Low starting torque and high pf.
83
How is a drum controller connected in 3 phase?
All lines connect as expected sideways across drum switch except for b. The line side of b connects to the right side terminal (if load is on the right). Load side connects to 3.
84
Cons of a WRIM?
Expensive to maintain.
85
To switch the split phase motor direction…
Switch start winding.
T1 to t4 is for run. T5-8 for start. T5 on t1 side means clockwise.
86
Reverse both start windings to reverse direction?
Yep.
87
For single voltage, does a split phase have two run’s or two start’s?
Two runs. Connect runs in parallel or in series (connect all in parallel or all in series for dual v too).
88
What is used for output in alternator?
The stator (9/12 leads).
89
Is a onboard exciter diff than a field discharge resistor?
Yep. (Ignore where i have said it’s the same).
90
Amortisseur winding takes out the need for what?
A pony motor.
91
What two things oppose rpm of a rotor?
Flywheels or speed control.
92
Benefits of flywheel or speed control?
Avoid freq dips and smoother freq output for the latter.
93
If rotor slip increases…
Then rpm goes down, pf goes down as angle increases
94
If torque is increase, I line
Is mainly for the power input
95
Locked Kva rating on nameplate.
Code.
96
Decrease excitation to lower…
Pf.
97
(Hp/sf) / (rated v times 1.05) is equal to
Fla.
98
Percent slip formula
Nsync-Nrpm/Nsync
99
Speed formula
Nl-fl/fl
100
How to use new hertz to get slip speed?
(New hz/old hz) times n sync.
Minus n sync by this to get new rpm.
101
For slip formulas, do we take efficiency into account?
Nope.
102
If rotor inductance or reactance is high at the start, fla is…?
Lower
103
Higher load, compared to no load, means more or less freq
Higher freq as more slip. (At 100 percent slip, full freq).
104
Is nl or nfl higher rpm?
Nl
105
Sync-fla speed gives
Speed of slip.
106
How to get Nsync?
F times 120/ number of poles.
(If not given, guess by 7200/whichever pole number gives closest to full voltage).
107
How to get Nsync?
F times 120/ number of poles.
(If not given, guess by 7200/whichever pole number gives closest to full voltage).
108
Reactance depends on frequency while resistance doesn’t. T or f.
T.
109
Is rotor made of magnetic steel or not.
It is.
110
Look into alternator connections, pf lagging or leading, and alternator info and connections.
After test.
