1. ENERGY CONVERSION ONE (Course 25741)
CHAPTER NINE ….continued
DC MOTORS AND GENERATORS

2. SPEED CONTROL of SHUNT DC MOTOR
Effect of Armature motor’s resistance speed control on a shunt motor’s torque-speed
Only used in applications in which:
- motor spends almost all its time operating at full speed or
- inexpensive to justify a better form of speed control

3. SPEED CONTROL of SHUNT DC MOTOR
In field resistance control, lower IF  higher its speed, & higher IF causes a decrease in speed
there is always a minimum achievable speed by IF control
Minimum speed occurs when IF has maximum permissible value
if motor operate at its rated terminal voltage, power, & IF then it will be running at rated speed
this is known also as : “base speed”
to achieve a reduction in this speed by IF control, require excessive IF that may burn up field windings

4. SPEED CONTROL of SHUNT DC MOTOR
In armature voltage control, lower armature voltage on separately excited motor, reduce its speed & higher armature voltage increase its speed
There is a maximum achievable speed, in maximum permissible armature voltage level
Armature voltage control would require excessive armature voltage, which may damage armature circuit
Armature voltage control works well for speeds below base speed & field current control works well for speeds above base speed
By combining two speed-control techniques in same motor, it is possible to get a range of speed variations of up to 40 to 1 or more
Shunt & S.E. motors have excellent speed control characteristics

5. SPEED CONTROL of SHUNT DC MOTOR
There is significant difference in torque & power limits on machine under two types of speed control
Limiting factor in either case is heating of armature conductors, which places an upper limit on magnitude of IA
For armature voltage control, flux in motor is constant, so maximum torque in motor is:
Tmax=KφIA,max
maximum torque is constant, regardless of speed

6. SPEED CONTROL of SHUNT DC MOTOR
power o/p, P=T.ω maximum power of motor at any speed under armature voltage control is: Pmax=Tmax ω
Thus maximum power out of motor is directly proportional to its operating speed under armature voltage control
on the other hand, while RF control used flux changes
& speed increase by decrease in flux
In order that IA do not exceed its limit, Tind must decrease as speed of motor increases

7. SPEED CONTROL of SHUNT DC MOTOR
since P=T.ω, & torque limit decreases as speed of motor increases:
- max. power out of dc motor under field current control is constant, while max. torque varies as reciprocal of motor’s speed
These shunt dc motor power & torque limitations for safe operation as a function of speed shown next

8. SPEED CONTROL of SHUNT DC MOTOR
Power & Torque limits as a function of speed for a shunt motor under VA & RF control

9. SPEED CONTROL of SHUNT DC MOTOR
Example 3:
figure, shows a 100 hp, 250 V, 1200 r/min shunt dc motor with an armature resistance of 0.03 Ω & a field resistance of Ω
Motor has compensating windings, so armature reaction can be ignored
Mechanical & core losses may be ignored
assumed to be driving a load with a line current of 126 A & an initial speed of 1103 r/min,
to simplify the problem assume armature current drawn by motor remains constant

10. SPEED CONTROL of SHUNT DC MOTOR-Example 3
(a) machine magnetization curve shown in next slide, what is motor’s speed if RF raised to 50 Ω
(b) calculate & plot speed of motor as a function
of RF assuming a constant-current load
SOLUTION
Initial IA1= IL1-IF1= /41.67=120 A
 EA1=VT-IA1RA= x 0.03=246.4 V
RF increased to 50 Ω,  IF2=VT/RF=250/50=5 A

11. SPEED CONTROL of SHUNT DC MOTOR-Example 3
Magnetization
curve

12. SPEED CONTROL of SHUNT DC MOTOR-Example 3
EA2/EA1=[K’φ2n2]/[K’φ1n1], and since IA assumed constant  EA1 = EA2  1=[φ2n2]/[φ1n1]
or n2= [φ1 / φ2] n1
Last Plot is EA versus IF , for a given speed
EA directly proportional to flux  on this curve:
EA2/EA1 =φ2/φ1
At IF=5 A, EA0=250 V, while at IF=6 A,
EA0=268 V  φ2/φ1= 268/250 =1.076
New speed of motor:
n2= φ1/φ2 n1=(1.076)(1103)=1187 r/min

13. SPEED CONTROL of SHUNT DC MOTOR-Example 3
A MATLAB M-file can be made to calculate speed of motor as a function of RF
plot of its speed versus RF shown below:

14. SPEED CONTROL of SHUNT DC MOTOR-Example 3
Note: assumption of constant IA not a good assumption for real loads
IA vary with speed in a fashion dependent on torque required by type of load attached to motor
these differences cause a motor’s speed –versus-RF curve slightly different than shown in last figure.

15. SPEED CONTROL of SHUNT DC MOTOR-Example 4
Motor in Example-3 now connected separately excited, as shown below:

16. SPEED CONTROL of SHUNT DC MOTOR-Example 4
Motor is initially running with VA=250 V, IA=120 A, and n=1103 r/min, while supplying a constant torque load.
What will the speed of this motor be if VA is reduced to 200 V?
SOLUTION:EA=VT-IARA= x0.03=246.4 V
Since flux is constant:
EA2/EA1=[K’φ2n2]/[K’φ1n1]=n2/n1
 n2= EA2/EA1 n1
Since torque is constant & flux is constant  IA is constant:
EA2= x0.03=196.4 V
n2= EA2/EA1 x n1=196.4/246.4 x 1103=879 r/min

17. SPEED CONTROL of SHUNT DC Effect of an Open Field Circuit
As shown speed increase as RF increased, what would happen if field circuit open while motor is running? The flux in machine would drop drastically, and reach φres & EA=Kφω would drop with it
cause an enormous increase in IA & resulting Tind would be quite a bit higher than load torque on motor. Therefore motor’s speed starts to rise & just keeps going up
Author Experience of undergraduate lab.
Where field cct. fused by mistake (instead of 3-A by a 0.3-A fuse) and when started after 3 s, suddenly a flash from fuse & motor’s speed skyrocketed. Someone turned main cct. Breaker off within a few seconds, but by that time tachometer pegged 4000 r/min, while motor rated 800 r/min needless to say every one present very badly scared
And learned to be most careful about field cct protection
A field loss relay normally included to disconnect motor from line in event of loss of field current

18. SPEED CONTROL of SHUNT DC Effect of an Open Field Circuit
Two other causes of field weakening:
(a) in shunt motors operating with light fields if A.R. effects severe enough, in case of an increase in load can weaken its flux and cause rise of speed until motor over-speed known as runaway
(b) motors operating with severe load changes & duty cycles, this flux weakening problem solved by installing compensating windings
Unfortunately compensating windings too expensive for use on ordinary run-of-the-mill motors
Solution: to use a turn or 2 turns of cumulative compounding to motor’s poles
As load increases mmf from series turns increases, which counteracts demagnetizing mmf of A.R.
A shunt motor equipped with just few series turns like this is called: stabilized shunt motor

19. PERMANENT-MAGNET DC MOTOR
A permanent magnet dc motor (PMDC) is a dc motor whose poles are made of permanent magnets.
PMDC motor offer a number of benefits compared with shunt dc motors in some applications
Advantage: Since these motors do not require an external field circuit, they do not have the field circuit copper losses. Because no field windings are required, they can be smaller than corresponding shunt dc motors

20. PERMANENT-MAGNET DC MOTOR
Disadvantages:
(a) Permanent magnets cannot produce as high flux density as an externally supplied shunt field
so a PMDC motor will have a lower induced torque per ampere of armature current than a shunt motor of the same size.
(b) PMDC motors run risk of demagnetization
due to A.R. effect which reduces overall net flux, also if IA
become very large there is a risk that its mmf demagnetize
poles, permanently reducing & reorienting residual flux
(c) A PMDC motor is basically the same machine as a shunt dc motor, except that flux of a PMDC motor is fixed. Therefore, it is not possible to control the speed of the PMDC motor by varying the field current or flux. The only methods of speed control available for a PMDC motor are armature voltage control and armature resistance control.

21. PERMANENT-MAGNET DC MOTOR
The magnetization curve of typical ferromagnetic material
Note: after a large magnetizing intensity H applied to core & removed, a residual flux Bres remains behind in core
Flux can be brought to zero if a coercive magnetizing intensity Hc is applied to core with opposite polarity
in this case, a relatively small value of it will demagnetize the core

22. PERMANENT-MAGNET DC MOTOR
(a)Typical ferromagnetic material & its Bres (b) suitable for P.M. (c) second quadrant rare earth magnets combine High residual flux and high coercive magnetizing intensity

23. SERIES DC MOTOR
A series DC motor is a dc motor whose field windings consist of relatively few turns connected in series with the armature circuit KVL for this motor is VT = EA + IA (RA + RS)

24. SERIES DC MOTOR
The Tind=KφIA while flux in this machine directly proportional to IA (at least until metal saturates)
Flux in machine can be given by: φ=c IA
Where c is constant of proportionality.
 Tind=KφIA = K c IA^ (1)
Torque in motor proportional to square of IA
As a result of this relationship, series motor gives more torque per ampere than any other dc motor
Therefore it is used in applications requiring very high torques
Examples: starter motors in cars, elevator motors, and tractor motors locomotives

25. TERMINAL CHARCATERISTIC SERIES DC MOTOR
As seen before an increase in flux cause a decrease in speed.
in series motor a sharply drooping torque-speed characteristic exist (since IA pass field winding)
Analysis is based on assumption of linear magnetization curve, & then effects of saturation considered in a graphical analysis
therefore:
φ=c IA (2)
VT = EA + IA (RA + RS) (3)
From (1) IA=√Tind /Kc & EA=Kφω
 VT = Kφω + √Tind /Kc (RA + RS) (4)

26. TERMINAL CHARCATERISTIC SERIES DC MOTOR
To eliminate flux from equation (4):
IA=φ/c and Tind=K/c φ^2  φ=√c/K √Tind (5)
Substituting equation (5) in (4) and solving for speed:
VT=K √c/K √Tind ω + √Tind /Kc (RA + RS)
√c/K √Tind ω= VT - (RA + RS) / [Kc] x √Tind
ω= VT / [Kc] x 1/√Tind - (RA + RS) / [Kc] (6)
Note: for unsaturated series motor; speed of motor varies as reciprocal of square root of Tind & its torque-speed characteristic shown next

27. TERMINAL CHARCATERISTIC SERIES DC MOTOR
Torque-speed characteristic of a series motor
One disadvantage can be seen from Eq.(6)
- when Tind goes to zero speed goes to infinity
- in practice torque can never go zero due to mechanical, core & stray losses that must be overcome,
however if no other load exist, can turn fast enough to seriously damage itself

28. TERMINAL CHARCATERISTIC SERIES DC MOTOR
Therefore; Never completely unload a series motor & never connect one to a load by a belt or other mechanism that could break
nonlinear analysis of a series dc motor with magnetic saturation effects, ignoring A.R. illustrated in EXAMPLE-5
Example 5:
consider the equivalent cct. of a series dc motor with a 250 V series dc motor having compensating windings, and atotal series resistance RA+RS of 0.08 Ω. The series field consists of 25 turns per pole, with magnetization curve shown next

29. SERIES DC MOTOR EXAMPLE-5
Magnetization Curve

30. SERIES DC MOTOR find speed & induced torque of this motor for
when its armature current is 50 A
(b) calculate & plot torque-speed characteristic for this motor
SOLUTION :
Pick points along operating curve & find torque & speed for each point
for IA=50 A
EA=VT-IA(RA+RS) =250 – 50 x 0.08 =246 V
since IA=IF=50 A, mmf=25 x 50=1250 A.turns

31. SERIES DC MOTOR
From magnetization curve at mmf =1250 A.turns  EA0=80 V
Speed can be found:
n= EA/EA0 x n0=246/80 x 1200= 3690 r/min
Pconv=EAIA=Tind ω 
Tind=EAIA/ω=[246 x50]/[3690x1/60x2π]=31.8 N.m.
(b) to calculate complete torque-speed characteristic, the same steps of (a) should be repeated for may values of IA, this can be done using a M-file of MATLAB

32. SERIES DC MOTOR SPEED CONTROL
Unlike shunt dc motor, there is only one efficient way to change speed of a series dc motor
Method is to change terminal voltage of motor
If terminal voltage is increased, first term in Eq. (6) increased, result in a higher speed for any given torque
speed of series dc motors can be controlled by insertion of a series resistor however is very wasteful of power & only used for very short time during start-up
Now with introduction of solid-state control, techniques available for variable terminal voltages

33. COMPOUND DC MOTOR
A compound dc motor is a motor with both a shunt & a series field
Such a motor shown below:
(a) long-shunt connection

34. COMPOUND DC MOTOR
(b) Compound dc motor with short-shunt connection

35. COMPOUND DC MOTOR
Current flowing into dot produces a positive mmf (same as in transformer)
If current flows into dots on both field coils, resulting mmfs add to produce a larger total mmf
This situation is known as cumulative compounding
If current flows into dot on one field coil & out of dot on other field coil resulting mmfs subtract
In previous (a)&(b) figures round dots correspond to cumulative compounding & squares corresponds to differential compounding

36. COMPOUND DC MOTOR KVl for the compound motor: VT=EA+IA(RA+RS)
Currents in compound motor are related by:
IA=IL-IF
IF=VT/RF
- Net mmf & effective shunt field currnt in compound motor:
Fnet =FF(+,-) FSE-FAR
IF*=IF(+,-) NSE/NF IA – FAR/NF
(+) in equations associated with cumulatively compounded
(-) associated with differentially compound motor

37. COMPOUND DC MOTOR Torque-Speed Characteristic
In cumulatively compound dc motor, a component of flux is constant & another one which is ~ to IA (& thus to its load)
 cumulatively compound motor has a higher starting torque than a shunt motor (whose φ constant) but lower than a series motor (whose entire φ ~ to IA )
Cumulatively compound motor combines best features of both shunt & series motors:
Like a series motor has extra torque for starting;
Like a shunt motor it does not overspeed at no load

38. COMPOUND DC MOTOR Torque-Speed Characteristic
At light load, series field has very small effect, so motor behaves approximately as a shunt dc motor
As load gets very large series flux becomes quite important & torque-speed curve begins to look like a series motor’s characteristic
A comparison of torque-speed characteristics of each of these types of machines shown next

39. COMPOUND DC MOTOR Torque-Speed Characteristic
(a) T-ω curve of cumulatively compound, compared to series & shunt motors with same full-load rating
(b) T-ω curve of cumulatively compound, compared to shunt motor with same no-load speed

40. COMPOUND DC MOTOR Torque-Speed Characteristic
Torque-Speed of Differentially Compound dc motor
In a differentially compounded dc motor, the shunt mmf and series mmf subtract from each other. This means that as the load on the motor increases, IA increases and the flux in the motor decreases.
But as the flux decreases, the speed of the motor increases. This speed increase causes another increase in load, which further increases IA, further decreasing the flux, and increasing the speed again

41. COMPOUND DC MOTOR Torque-Speed Characteristic
The result is that a differentially compounded motor is unstable and tends to runaway
This instability is much worse than that of a shunt motor with armature reaction. It is so bad that a differentially compounded motor is unsuitable for any application.

42. COMPOUND DC MOTOR Torque-Speed Characteristic
Differentially compounded motor is also impossible to start
At starting conditions, the armature current and the series field current are very high
Since the series flux subtracts from the shunt flux, the series field can actually reverse the magnetic polarity of the machine’s poles
The motor will typically remain still or turn slowly in the wrong direction while burning up, because of the excessive armature current

43. COMPOUND DC MOTOR Torque-Speed Characteristic
When this type of motor is to be started, its series field must be short-circuited, so that it behaves as an ordinary shunt motor during the starting period
Nonlinear Analysis of Compound dc Motors
Example 6: a 100 hp, 250 V compounded dc motor with compensating windings has an internal resistance, including series winding, of 0.04 Ω. There are 1000 turns per pole on shunt field & 3 turns per pole on series windings
The machine shown in next figure, & its magnetization curve shown also. At no load field resistor has been adjusted to make motor run at 1200 r/min. core, mechanical & stray losses negligible

44. COMPOUND DC MOTOR Torque-Speed Characteristic
(a) what is the shunt current in this machine at no load?
(b) if motor is cumulatively compounded, find its speed when IA=200 A
(c) if motor is differentially compounded, find its speed when IA=200 A
SOLUTION:
(a) At no load, IA=0, so internal generated voltage equal VT =250 V. & from Mag. Curve a IF=5 A  EA=250 V at 1200 r/min (& IF=5 A )

45. COMPOUND DC MOTOR Torque-Speed Characteristic
Compound dc motor of example 6:

46. COMPOUND DC MOTOR Torque-Speed Characteristic
(b) when IA=200 A flows in motor, machine’s internal voltage:
EA=VT-IA(RA+RS)= x0.04=242 V
effective field current of cumulatively compounded motor is:
IF*=IF+NSE/NF IA- FAR/NF =5 +3/1000 x 200=5.6A
From mag. Curve, EA0=262 V at n0=1200 r/min
therefore motor’s speed will be:
n =EA/EA0xn0=242/262 x 1200 = 1108 r/min
(c) If machine is differentially compounded,
IF*=IF-NSE/NF IA- FAR/NF=5 – 3/1000 x 200=4.4 A

47. COMPOUND DC MOTOR Torque-Speed Characteristic
from mag. Curve
EA0=236 V at n0=1200 r/min 
n=EA/EA0 x n0=242/236 x 1200 = 1230 r/min
Note:
speed of cumulatively compounded motor decreases with load, while speed of differentially compounded motor increases with load

48. COMPOUND DC MOTOR Torque-Speed Characteristic
Speed Control in Cumulatively Compounded DC Motor
Techniques available for control of speed in a cumulatively compounded dc motor are the same as those available for a shunt motor
1- change in field resistance
2- change armature voltage
3- change armature resistance
Differentially compounded dc motor could be controlled in a similar manner. Since differentially compounded motor almost never used, that fact hardly matters

49. DC MOTOR STARTERS
Equipments used for protection of dc motors, for the following reasons:
1- protect motor against damage due to short circuits in equipment
2- protect motor against damage from long-term overloads
3-protect motor against damage from excessive starting currents
4- provide a convenient manner in which to control the operating speed of motor

50. DC MOTOR PROBLEMS on STARTING
In order for a dc motor to function properly, it must be protected from physical damage during starting period
At starting conditions, motor is not turning & so EA=0 V
since internal resistance of a normal dc motor is very low compared to its size (3 to 6 percent per unit for Medium size motors) a very high current flows
Consider for example, 50 hp, 250 V motor of EXAMPLE 1, RA is 0.06 Ω, & full-load current less than 200 A, but current on starting is:
IA=[VT-EA]/RA=[250-0]/0.06=4167 A
This current is over 20 times motor’s rated full-load current
It is possible a motor severely damaged by such current