ENERGY CONVERSION ONE (Course 25741)

1
ENERGY CONVERSION ONE (Course 25741)

• CHAPTER NINE .continued
• DC MOTORS AND GENERATORS

2
SPEED CONTROL of SHUNT DC MOTOR

• Effect of Armature motors resistance speed
  control on a shunt motors 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
• TmaxKfIA,max
• maximum torque is constant, regardless of speed
  

6
SPEED CONTROL of SHUNT DC MOTOR

• power o/p, PT.? ?maximum power of motor at any
  speed under armature voltage control is
  PmaxTmax ?
• 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 PT.?, 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 motors 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 O
  a field resistance of 41.67 O
• 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 motors speed if RF raised to 50 O
• (b) calculate plot speed of motor as a function
• of RF assuming a constant-current load
• SOLUTION
• Initial IA1 IL1-IF1126- 250/41.67120 A
• ? EA1VT-IA1RA250-120 x 0.03246.4 V
• RF increased to 50 O, ? IF2VT/RF250/505 A

11
SPEED CONTROL of SHUNT DC MOTOR-Example 3

• Magnetization
• curve

12
SPEED CONTROL of SHUNT DC MOTOR-Example 3

• EA2/EA1Kf2n2/Kf1n1, and since IA assumed
  constant ? EA1 EA2 ? 1f2n2/f1n1
• or n2 f1 / f2 n1
• Last Plot is EA versus IF , for a given speed
• EA directly proportional to flux ? on this
  curve
• EA2/EA1 f2/f1
  
• At IF5 A, EA0250 V, while at IF6 A,
• EA0268 V ? f2/f1 268/250 1.076
  
• New speed of motor
• n2 f1/f2 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 motors 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 VA250 V, IA120
  A, and n1103 r/min, while supplying a constant
  torque load.
• What will the speed of this motor be if VA is
  reduced to 200 V?
• SOLUTIONEAVT-IARA250-120x0.03246.4 V
• Since flux is constant
• EA2/EA1Kf2n2/Kf1n1n2/n1
• ? n2 EA2/EA1 n1
• Since torque is constant flux is constant ? IA
  is constant
• EA2200-120x0.03196.4 V?
• n2 EA2/EA1 x n1196.4/246.4 x 1103879 r/min

17
SPEED CONTROL of SHUNT DCEffect 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 fres EAKf? would drop
  with it
• cause an enormous increase in IA resulting
  Tind would be quite a bit higher than load
  torque on motor. Therefore motors 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 motors 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 DCEffect 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 motors 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 TindKfIA while flux in this machine
  directly proportional to IA (at least until metal
  saturates)
• Flux in machine can be given by fc IA
• Where c is constant of proportionality.
• ? TindKfIA K c IA2
  (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
• fc IA
  (2)
• VT EA IA (RA RS)
  (3)
• From (1) IAvTind /Kc EAKf?
• ? VT Kf? vTind /Kc (RA RS) (4)

26
TERMINAL CHARCATERISTIC SERIES DC MOTOR

• To eliminate flux from equation (4)
• IAf/c and TindK/c f2 ? fvc/K vTind
  (5)
• Substituting equation (5) in (4) and solving for
  speed
• VTK vc/K vTind ? vTind /Kc (RA RS)
• vc/K vTind ? VT - (RA RS) / Kc x vTind
• ? VT / Kc x 1/vTind - (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 RARS of 0.08 O. The series field
  consists of 25 turns per pole, with magnetization
  curve shown next

29
SERIES DC MOTOREXAMPLE-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 IA50 A
• EAVT-IA(RARS) 250 50 x 0.08 246 V
• since IAIF50 A, mmf25 x 501250 A.turns

31
SERIES DC MOTOR

• From magnetization curve at mmf 1250 A.turns ?
  EA080 V
• Speed can be found
• n EA/EA0 x n0246/80 x 1200 3690 r/min
• PconvEAIATind ? ?
• TindEAIA/?246 x50/3690x1/60x2p31.8 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 MOTORSPEED 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
• VTEAIA(RARS)
• Currents in compound motor are related by
• IAIL-IF
• IFVT/RF
• - Net mmf effective shunt field currnt in
  compound motor
• Fnet FF(,-) FSE-FAR
• IFIF(,-) NSE/NF IA FAR/NF
• () in equations associated with cumulatively
  compounded
• (-) associated with differentially compound
  motor

37
COMPOUND DC MOTORTorque-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 f
  constant) but lower than a series motor (whose
  entire f 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 MOTORTorque-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 motors characteristic
• A comparison of torque-speed characteristics of
  each of these types of machines shown next

39
COMPOUND DC MOTORTorque-Speed Characteristic

• (a) T-? curve of cumulatively compound,
  compared to series shunt motors with same
  full-load rating
• (b) T-? curve of cumulatively compound,
  compared to shunt motor with same no-load speed

40
COMPOUND DC MOTORTorque-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 MOTORTorque-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 MOTORTorque-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 machines 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 MOTORTorque-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 O.
  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 MOTORTorque-Speed Characteristic

• (a) what is the shunt current in this machine at
  no load?
• (b) if motor is cumulatively compounded, find its
  speed when IA200 A
• (c) if motor is differentially compounded, find
  its speed when IA200 A
• SOLUTION
• (a) At no load, IA0, so internal generated
  voltage equal VT 250 V. from Mag. Curve a
  IF5 A ? EA250 V at 1200 r/min ( IF5 A )

45
COMPOUND DC MOTORTorque-Speed Characteristic

• Compound dc motor of example 6

46
COMPOUND DC MOTORTorque-Speed Characteristic

• (b) when IA200 A flows in motor, machines
  internal voltage
• EAVT-IA(RARS)250-200x0.04242 V
• effective field current of cumulatively
  compounded motor is
• IFIFNSE/NF IA- FAR/NF 5 3/1000 x 2005.6A
• From mag. Curve, EA0262 V at n01200 r/min
• therefore motors speed will be
• n EA/EA0xn0242/262 x 1200 1108 r/min
• (c) If machine is differentially compounded,
• IFIF-NSE/NF IA- FAR/NF5 3/1000 x 2004.4 A

47
COMPOUND DC MOTORTorque-Speed Characteristic

• from mag. Curve
• EA0236 V at n01200 r/min ?
• nEA/EA0 x n0242/236 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 MOTORTorque-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
  EA0 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 O, full-load current
  less than 200 A, but current on starting is
• IAVT-EA/RA250-0/0.064167 A
• This current is over 20 times motors rated
  full-load current
• It is possible a motor severely damaged by such
  current