DC Motor Speed Control using Electric Drives.pptx

This PPT explains the speed control methodologies of DC motors and how to implement these methods using solid state devices and circuits

1. UNIT 2 SOLID STATE DC MOTOR DRIVE 20EE020 ELECTRICAL DRIVES AND CONTROL D.Poornima , Assistant Professor ( Sr.Gr ), Department of EEE, Sri Ramakrishna Institute of Technology, Coimbatore

2. DC Motor Drives Used mainly for good speed regulation, frequent starting, braking and reversing. Provides a high torque, 400 % more than the rated continuous torque. Applications Rolling Mills, Paper Mills Mine Winders Hoists Machine Tools Traction Printing Presses Textile Mills Excavators Crane.

3. STARTING OF DC MOTORS A DC motor is energized by the commutation process through brushes. Maximum allowable starting current is determined by the current which can be safely carried out by the brushes without sparking. In general, the motors are designed that they can carry almost twice the rated current during starting conditions. Why does so much current flow through DC motors during the starting condition? When the motor is at standstill condition, there is only a small armature resistance present in the circuit so no back emf is generated. When the motor is started with full supply voltage across its terminal, there is a huge current flow through the motor, which may damage the motor because of heavy sparking across the commutators and a huge amount of heat is generated.

4. The speed of a DC motor drive is usually controlled by the variable resistance method, which can be also used to limit the starting current When the motor gains speed and the back emf increases, the resistances are cut out one by one from the circuit and therefore the current is kept within permissible limits.

5. Braking of DC Motors Controlled slowing or stopping of a motor and its load is important in many applications (e.g. cranes, traction on a slope to avoid excessive speed, etc.). Braking of DC Motors methods based on friction, electromechanical action, eddy-currents, etc. are independent of the motor Electric braking is better as they are faster and more efficient (absence of brake wear)all around. During braking, the motor is operated as a generator and the kinetic or gravitational potential energy (cranes or hoists) is dissipated in resistors (plugging) or returned to the supply (regenerative braking) There are mainly three types of braking of DC motors : Dynamic or rheostat braking Plugging or reverse voltage braking. Regenerative braking

6. Dynamic Braking or Rheostatic Braking For dynamic braking in a shunt motor, Armature is disconnected from the supply and a rheostat is connected across it. The field winding is left connected across the supply. Now armature is driven by the inertia and hence machine starts acting as a generator. The kinetic energy that’s stored in the motor gets converted into electrical energy that turns into heat and will dissipate at the rate of I 2 R. Braking effect is controlled by varying the resistance connected across the armature. For DC series motor, The motor is disconnected from the supply, field connections are reversed and a rheostat is connected in series. The field connections are reversed to make sure that the current through field winding will flow in the same direction as before.

7. Pluggin g or Reverse Current Braking In this method, armature connections are reversed So, applied voltage V and back emf E b start acting in the same direction and hence the total armature current exceeds. To limit this armature current a variable resistor is connected across the armature. For separately excited motor supply polarity is reversed for plugging. Plugging gives greater braking torque as compared to rheostatic braking. Considered inefficient because both the power supplied by the load and the source is wasted in resistances. This method is generally used in controlling elevators, machine tools, printing presses etc.,

8. Regenerative Braking Used where load has very high inertia ( e.g in electric trains). When applied voltage to the motor is reduced to less than back emf E b , armature current I a will get reversed, and hence armature torque is reversed. Speed falls. Machine will act as a DC generator, power will be returned to the line, this action is called as regeneration. Speed keeps falling, back emf E b also falls until it becomes lower than applied voltage and direction of armature current again becomes opposite to E b . Generated energy is supplied to the source. The following condition is to be satisfied for regenerative braking and negative I a Doesn’t actually stop the motor, but controls and lowers the speed when it goes above the no-load speed. Friction brake is required to stop the motor.  

9. Regenerative Braking…… Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Series motors need a reversal of either the field or the armature connections , so generally not used Achieved by increasing the field current or armature speed or reducing the supply voltage. Theoretically about 35% of the energy put into an automotive vehicle during typical urban traction is recoverable by regenerative braking . Exact value of the recoverable energy is a function of type of driving, the terrain, efficiency of the drive train, gear ratios in the drive/train, etc. The method needs a supply capable of accepting the generated power without undue rise of the terminal voltage.

10. Speed Control of DC Motors From the voltage equation of a motor Shows that the speed of a DC motor is directly proportional to the back emf Eb and inversely proportional to the flux φ  

11. Methods of Speed Control of DC Motor Speed Control of DC Motors can be done using 1 2 3

12. Armature Voltage Control Preferred due to its high efficiency, good transient response and speed regulation. Provides speed control only below the rated speed as the armature voltage cannot exceed the rated value.

13. Field Flux Control Used for controlling the speed above the rated value. Maximum speed of the motor is twice the rated speed, and in the special motor, it is six times the rated speed.

14. The maximum torque and power limitations of dc drives operating with armature voltage control and full field below rated speed and flux control at rated armature voltage above rated speed are shown In armature voltage control at full field, T ∞ Ia consequently, the maximum torque that the machine can deliver has a constant value. In the field control at rated armature voltage, Pm ∞ Ia (because E ≈ V = constant). Therefore, maximum power developed by the motor has a constant value..

15. Armature Resistance Control Speed is varied by wasting power in an external resistor connected in series with the armature. Mainly used in alternate load applications where the duration of low-speed operation forms only a small portion of the total running time. Armature voltage control has replaced this method in various applications.

16. Types of Armature Voltage Control Variable armature voltage for the starting, speed control, braking and reversal of DC drives can be obtained by When the supply is AC Ward Leonard Drives Transformer and Uncontrolled Rectifiers Controlled Rectifiers When the supply is DC Chopper Control

17. Ward Leonard Drives Known after the name of its inventor H. Ward Leonard (1891) Consists of a separately excited generator feeding the dc motor to be controlled. The generator is driven at a constant speed by an ac motor connected to 50 Hz ac mains. The ac driving motor may be an induction or a synchronous. DC motor voltage is controlled by adjusting the field current of the generator. When field winding voltage is smoothly varied in either direction, the motor terminal voltage and therefore, speed can be steplessly varied from full positive to full negative.

18. Regenerative Braking in Ward Leonard Drives Has the ability for regenerative braking down to very low motor speeds. Allows efficient operation of drive in all four quadrants of speed-torque plane. For regenerative braking, the output voltage of generator G is reduced below the induced voltage of motor M by decreasing the generator field current. This reverses the current flowing through the armatures of machines G and M. Then machine M works as a generator and G as a motor. Mechanical energy provided to machine M, is converted into electrical energy. Electrical energy supplied by Machine M is converted into mechanical energy by machine G. The ac motor, now works as a generator and feeds electrical enrgy to the ac source .

19. Field Control in Ward Leonard Drives Control of generator field is obtained by rheostats when low ratings are involved For higher power applications or for closed-loop control, the field is supplied by a power amplifier - consists of a controlled rectifier, chopper or transistor amplifier. For reversible drives, a power amplifier capable of supplying controlled field current in either direction is required. It may consists of a single-phase or three-phase dual converter, four quadrant chopper or four quadrant transistor amplifier . For single-direction operation, a power amplifier consisting of a half-controlled rectifier, step-down chopper or one quadrant transistor amplifier is used to reduce cost.

20. Advantages and Disadvantages of Ward Leonard Systems Has inherent regenerative braking capability Allows efficient four-quadrant operation. Can be employed for power factor improvement by using a synchronous motor High initial cost Low efficiency Requires more frequent maintenance Produces more noise Large weight and size Needs large floor area and foundation Advantages Disadvantages

21. Armature Voltage Control using Transformer An auto-transformer is used for lower power ratings A transformer with tappings (either on primary or on secondary) followed by an uncontrolled rectifier is used for higher power ratings A reactor is connected in the armature circuit to improve armature current waveform. Tap changing is done with the help of an on load tap changer

22. A mid-point auto-transformer is used to carry out on load tap changing. When on tap position 1, both the terminals of auto-transformer are connected together. For changing to tap 2, terminal ‘a’ is first connected to tap 2. Terminal is now disconnected from tap 1 and connected to ‘a’. This scheme is employed in 25 kV single phase 50 Hz ac traction

23. Features of Armature Voltage Control using Transformer Output voltage can be changed only in steps; Rectifier output voltage waveform does not change as the output voltage is reduced. A good power factor is maintained at the source and current harmonics introduced in the supply lines do not increase abnormally, like in the case of a controlled rectifier when motor voltage is reduced to a small value Because of the use of diode bridge, circuit is not capable of regeneration.

24. Controlled Rectifier Fed DC Drives Used to get variable dc voltage from an ac source of fixed voltage. Also known as Static Ward-Leonard drives. Commonly used Controlled Rectifier Fed DC Drives and quadrants in which they can operate on Va- Ia plane are As thyristors are capable of conducting current only in one direction, these rectifiers are capable of providing current only in one direction.

25. Single Phase Fully Controlled Rectifier Control of DC Motor The armature voltage is controlled by a full-converter Field supply is separately given Motor current is unidirectional due to the thyristors, but voltage can reverse direction. When armature current does not flow continuously - discontinuous conduction. When current flows continuously – continuous conduction  Two quadrant converter Limited to application up to 20HP

26. Discontinuous Conduction Mode Current starts flowing with the turn-on of thyristors T1 and T3 at ωt = α. Motor gets connected to the source and its terminal voltage equals Vs. The current, flows against both, E and the source voltage after ωt = π & falls to zero at β. Due to the absence of current, T1 and T3  turn-off. Motor terminal voltage is equal to induced voltage E. When thyristors T2 and T4 are fired at (π + α), the next cycle of the motor terminal voltage starts. .

27. In this mode, the drive operates in two intervals Duty interval ( α ≤ ω t ≤ β) when motor is connected to the source and V a  = V s . Zero current interval ( β ≤ ω t ≤ π + α) when i a  = 0 and V a  = E. Drive operation is described by the following equations:   V a  and l a  are respectively dc components of armature voltage and current The output voltage is given by The speed is given by

28. Continuous Conduction Mode A + ve current flows through the motor, and T2 and T4 are in conduction just before α. Gate pulses applied to thyristors T1 and T3 at α. Conduction of T1 and T3 reverse biases T2 and T4 and turns them off. Armature current is not perfect dc, so the motor torque fluctuates. Torque fluctuates at a frequency of 100 Hz, so motor inertia can filter out the fluctuations Gives a nearly constant speed and rippleless E. .

29. In this mode, the output voltage is given by The speed is given by

30. The ideal no-load operation is obtained when I a = 0. When both thyristor pairs (T1, T3) and (T2, T4) fail to fire, I a will be zero. This will happen when E > V s throughout the period for which tiring pulses are present. Therefore, when α < π/2, E should be greater or equal to Vm and when α > π/2, E should be greater or equal to Vm sin ωt . No load speeds are given by The ideal no-load speed when fed by a perfect direct voltage of rated value will then be (2Vm/πK). The maximum no-load speed with rectifier control is (π/2) times this value. Maximum average terminal voltage (2Vm/π) is chosen equal to the rated motor voltage.

31. Boundary between continuous and discontinuous conduction is when β = π + α, shown by dotted line The critical value of speed ω mc  at this boundary for a given α is, In Continuous Conduction, The speed-torque characteristics are parallel straight lines, whose slope depends on the armature circuit resistance R a . Speed Torque Curves For a given α, an increase in torque causes ω m and E to drop so that Ia  and T can increase. Average terminal voltage Va remains constant. The speed oscillation is less, ripple amplitude is smaller

32. In Discontinuous Conduction, For torques less than rated, a low power drive mainly operates in discontinuous conduction The effect of discontinuous conduction is to make speed regulation poor. Any increase in torque and accompanied increase in Ia causes β to increase and Va to drop. Consequently, speed drops by a larger amount. Speed Torque Curves

33. Operation in Quadrants I and IV The drive operates in quadrants I (forward motoring) and IV (reverse regenerative braking). Under the assumption of continuous conduction, dc output voltage of rectifier varies with α as as shown in Fig. 5.28(a)

34. Operation in Quadrant I When working in quadrant I, ω m is positive α ≤ 90° Polarities of V a and E are shown in Fig. 5.28(b). For positive I a this causes the rectifier to deliver power and the motor to consume it, thus giving forward motoring.

35. Operation in Quadrant IV Polarities of E, I a and V a for quadrant IV operation are shown in Fig. 5.28(c). When connected to an active load, E will reverse due to the reversal of ω m . I a is still in the same direction, the machine is working as a generator producing braking torque. Further due to α > 90°, V a is negative, suggesting that the rectifier now takes power from DC terminals and transfers it to AC mains. This operation of rectifier is called inversion and the rectifier is said to operate as an inverter . Since generated power is supplied to the source in this operation, it is regenerative braking .

36. Single Phase Half Controlled Rectifier Control of DC Motor The armature voltage is controlled by a half-controlled converter. Field supply is separately given. The output voltage and motor current are unidirectional. When armature current does not flow continuously - discontinuous conduction. When current flows continuously – continuous conduction  Single quadrant converter

37. In this mode, the drive operates in three intervals Duty interval ( α ≤ ω t ≤ π) when motor is connected to the source and V a  = V s . Substitution of ωt = π in this equation gives i a (π). Freewheeling interval (π ≤ ωt ≤ β) Operation is governed by the equation: Zero current interval ( β ≤ ω t ≤ π + α) when i a  = 0 and V a  = E. The output voltage of the converter is given by The speed is given by Discontinuous Conduction Mode

38. Continuous Conduction Mode A + ve current flows through the motor, and T2 and T4 are in conduction just before α. Gate pulses applied to thyristors T1 and T3 at α. Conduction of T1 and T3 reverse biases T2 and T4 and turns them off. Armature current is not perfect dc, so the motor torque fluctuates. Torque fluctuates at a frequency of 100 Hz, so motor inertia can filter out the fluctuations Gives a nearly constant speed and rippleless E. .

39. In this mode, the output voltage is given by The speed is given by Continuous Conduction Mode

40. Boundary between continuous and discontinuous conduction is when β = π + α, shown by dotted line The critical value of speed ω mc  at this boundary for a given α is, Speed Torque Curves

41. Operation in Quadrant I Drive operates in quadrant I only and should not be operated in quadrant IV. Figure 5.31(a) shows plot of Va with α for continuous conduction operation. When coupled to an active load, the motor speed can reverse, reversing E as shown in Fig. 5.31(b). As the current direction does not change, the machine now works as a generator producing braking torque. Since, rectifier voltage cannot reverse, generated energy cannot be transferred to ac source, and it is absorbed in the armature resistance.

42. Operation in Quadrant I Braking so obtained is the reverse voltage braking (plugging). Plugging is inefficient and causes a large current [ I a = (V a + E)/R a ] to flow through the rectifier and motor. The current cannot be regulated by adjustment of firing angle, it will damage the rectifier and motor. When the load is active, care should be taken to avoid such an operation. If such an operation cannot be avoided, a fully-controlled rectifier should be used. A Single-phase Controlled Rectifier Control is cheaper and gives a higher power factor compared to single-phase fully-controlled rectifier.

43. Three Phase Fully Controlled Rectifier Control of DC Motor Thyristors are fired in the sequence of their numbers with a phase difference of 60° by gate pulses of 120°duration. Each thyristor conducts for 120, and two thyristors conduct at a time O ne from upper group (odd numbered thyristors) and the other from lower group (even numbered thyristors) applying respective line voltage to the motor.

44. Continuous Conduction Mode Motor terminal voltage and current waveforms for continuous conduction are shown in Figs. (b) and (c) for motoring and braking operations. Discontinuous conduction is neglected because it occurs is a narrow region of its operation. M otor terminal voltage cycle from α + π/3 to α + 2π/3 .

45. In this mode, the output voltage is given by The speed is given by

46. When discontinuous conduction is ignored, speed-torque curves of Fig. 5.33 are obtained. The Va vs α curve has same nature as for single-phase case. D rive operates in quadrants I and IV. Speed Torque Curves

47. In Discontinuous Conduction, For torques less than rated, a low power drive mainly operates in discontinuous conduction The effect of discontinuous conduction is to make speed regulation poor. Any increase in torque and accompanied increase in Ia causes β to increase and Va to drop. Consequently, speed drops by a larger amount. Speed Torque Curves

48. Three Phase Half Controlled Rectifier Control of DC Motor The output voltage of the converter is given by The speed is given by

49. Operation in Quadrant I Drive operates in quadrant I only and should not be operated in quadrant IV. Figure 5.31(a) shows plot of Va with α for continuous conduction operation. When coupled to an active load, the motor speed can reverse, reversing E as shown in Fig. 5.31(b). As the current direction does not change, the machine now works as a generator producing braking torque. Since, rectifier voltage cannot reverse, generated energy cannot be transferred to ac source, and it is absorbed in the armature resistance.

50. Operation in Quadrant I Braking so obtained is the reverse voltage braking (plugging). Plugging is inefficient and causes a large current [ I a = (V a + E)/R a ] to flow through the rectifier and motor. The current cannot be regulated by adjustment of firing angle, it will damage the rectifier and motor. When the load is active, care should be taken to avoid such an operation. If such an operation cannot be avoided, a fully-controlled rectifier should be used. A Single-phase Controlled Rectifier Control is cheaper and gives a higher power factor compared to single-phase fully-controlled rectifier.

51. Multi-quadrant operation with regenerative braking is considered. In such drives, current control is always provided to limit the current value during transient operations. Three schemes are used Single Phase Fully Controlled Rectifier with Reversing Switch Dual Converter Single Phase Fully Controlled Rectifier in armature with Field Current Reversal Multi Quadrant Operation of Converter fed DC Motor

52. Dual Converter Control of DC Separately Excited Motor Consists of two fully-controlled rectifiers connected in anti-parallel across the armature. For power ratings upto around 10 kW, single-phase fully-controlled rectifiers are used. For higher ratings, three-phase fully controlled rectifiers are employed. Rectifier A, provides positive motor current and voltage in either direction and allows motor control in quadrants I & IV Rectifier B provides motor control in quadrants III and II, it gives negative motor current and voltage in either direction.

53. There are two methods of control for the Dual Converter Control of DC Separately Excited Motor: In simultaneous control both rectifiers are controlled together. To avoid DC circulating current between rectifiers, they are operated to produce the same DC voltage across the motor terminals. Thus AC current circulates due to the difference between the instantaneous output voltages of the two rectifiers. Inductors L1 and L2 are added to reduce ac circulating current. Also known as circulating current control. Inductors are chosen to allow a circulating current of 30% of the full load current. Eliminates discontinuous conduction, and gives good speed regulation in the complete range of the drive.

54. When operating in quadrant I, rectifier A will be rectifying (0 < α A  < 90°) and rectifier B will be inverting (90° < α B  < 180°). For speed reversal α A  is increased and α B  is decreased The motor back emf exceeds magnitudes of V A  and V B. The armature current shifts to rectifier B and the motor operates in quadrant II. The current control loop adjusts the firing angle α B  continuously to brake the motor at the maximum allowable current from initial speed to zero speed and then accelerates to the desired speed in the reverse direction. As α B  is changed, α A  is also changed The inductances L 1  and L 2  increase the weight, volume, cost and reversal time. The circulating current increases the losses. A sudden drop in source voltage can cause a large current to flow through the rectifier working as the inverter, blowing its thyristors. Speed reversal In simultaneous control

55. In a non-simultaneous or non-circulating current control method , one rectifier is controlled at a time. No circulating current flows and inductors L 1  and L 2  are not required. Eliminates losses associated with circulating current and weight and volume associated with inductors. Discontinuous conduction occurs at light loads and control is rather complex

56. When operating in quadrant I rectifier A will be supplying the motor and rectifier B will not be operating. Firing angle of rectifier A is set at the highest value. The rectifier works as an inverter and forces the armature current to zero. After zero current is sensed, a dead time of 2 to 10 ms is provided to ensure the turn-off of all thyristors of rectifier A. Firing pulses are withdrawn from rectifier A and transferred to rectifier B. The firing angle α B  is set initially at the highest value. The current control loop adjusts the firing angle α B  continuously to brake the motor at the maximum allowable current from initial speed to zero speed and then accelerates to the desired speed in the reverse direction. The dead time, and therefore, the reversal time can be reduced by employing methods that can sense the current zero accurately. When this is done non-simultaneous control provides faster response than simultaneous control. Because of this and the advantages stated above non-simultaneous control is widely used. The speed reversal in non simultaneous method

57. Chopper Fed DC Drive

58. DC-DC Converters/Choppers A static device used to obtain a variable DC voltage from a constant DC voltage Variable voltage supply is used in electric tractions, trolley cars, golf carts, electric vehicles, SMPS, PV cell based power generation etc. Offers greater efficiency, smooth control, lower maintenance and smaller size Choppers are classified into Step down choppers Step up choppers If the output voltage of a chopper is less than the input voltage it is called step down chopper If the output voltage of a chopper is greater than the input voltage it is called step up chopper D.Poornima,AP ( Sr.Gr )/EEE,SRIT 64

59. Features of DC-DC Converters Input is fixed DC voltage Output is regulated by chopping the supply voltage Chopping is done by switching ON and OFF any of the switch When thyristor is ON, supply voltage appears across the load When thyristor is OFF, voltage across the load is zero Thyristors , power BJT, power MosFET , IGBTs are used Thyristors are for higher power applications D.Poornima,AP(Sr.Gr)/EEE,SRIT 65

60. Principle of Operation D.Poornima,AP ( Sr.Gr )/EEE,SRIT 66

61. D.Poornima,AP(Sr.Gr)/EEE,SRIT 67

62. Control Strategies of a Chopper Output voltage of a chopper is directly proportional to the duty cycle. Can be controlled using Time ratio control Current limit control D.Poornima,AP(Sr.Gr)/EEE,SRIT 68

63. Time Ratio Control D.Poornima,AP(Sr.Gr)/EEE,SRIT 69 Can be achieved in two ways On period of the waveform is controlled keeping the total time period constant – called Pulse Width Modulation Either T on or T off is kept constant and the total time period T is varied- called Frequency Modulation PULSE WIDTH MODULATION (Constant Frequency System) Total time period is kept constant, but the ‘On Time’ or the ‘OFF Time’ is varied. Thus duty cycle ratio can be varied. The ON time or the ‘pulse width’ is getting changed in this method - popularly known as Pulse width modulation .

64. D.Poornima,AP(Sr.Gr)/EEE,SRIT 70

65. D.Poornima,AP(Sr.Gr)/EEE,SRIT 71 FREQUENCY MODULATION (Variable Frequency System) Total time period is varied while keeping either of ‘On Time’ or ‘OFF time’ as constant. Total time period gets changed - frequency also changes accordingly Known as Frequency Modulation Control.

66. D.Poornima,AP(Sr.Gr)/EEE,SRIT 72

67. D.Poornima,AP(Sr.Gr)/EEE,SRIT 73

68. D.Poornima,AP(Sr.Gr)/EEE,SRIT 74 Disadvantages of FM over PWM In FM, chopping frequency must be varied in wide range to control the output voltage –filter design is relatively difficult OFF time of the switch is more in FM, so load current may become discontinuous Due to the wide range frequency control, interference in radio lines may occur.

69. CURRENT LIMIT CONTROL D.Poornima,AP(Sr.Gr)/EEE,SRIT 75 The on time and off time of the switch is adjusted as to keep the load current constant Current is allowed to fluctuate or change only between 2 values i.e. maximum current (I max) and minimum current (I min). When the current is at minimum value, the chopper is switched ON. The current starts increasing, and when it reaches up to maximum value, the chopper is switched off allowing the current to fall back to minimum value.

70. Chopper Control of Separately Excited DC Motor: Motoring Control : Transistor Tr is operated periodically with period T and remains on for a duration ton. Choppers operate at a frequency that is high enough to ensure continuous conduction . Waveforms of motor terminal voltage Va and armature current ia  for continuous conduction are shown.

71. The operation is described by In this interval, the armature current increases from i a1  to i a2 . Motor is connected to the source and called  Duty Interval . At t = t on , Tr is turned off. Motor current freewheels through diode D F  and terminal voltage is zero during interval t on  ≤ t ≤ T. This interval, known as freewheeling interval , is described by Motor current decreases from i a2  to i a1  during this interval.

72. Ratio of duty interval t on  to chopper period T is called  duty ratio or duty cycle (δ).  The output voltage is given by The speed of the drive is given by

73. Regenerative Braking: Transistor T r  is operated periodically with a period T and an on-period of t on . An external inductance is added to increase the value of L a . When T r  is on, i a  increases from i a1  to i a2 . The mechanical energy converted into electrical by the motor, now working as a generator It partly increases the stored magnetic energy in the armature circuit Inductance and remainder is dissipated in armature resistance and transistor. 

74. When the transistor is turned off, the armature current flows through diode D and the source V and reduces from i a2  to i a1 . The stored electromagnetic energy and the energy supplied by the machine are fed to the source. Interval  0  ≤ t  ≤ t on  is called the energy storage interval and the interval t on  ≤ t ≤ T is called the duty interval . The output voltage is given by The speed of the drive is given by

75. Speed torque characteristics Combined Circuits

76. Four Quadrant Chopper Controlled DC Motor Drive Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking Forward motoring Reverse motoring  Reverse braking .

77. Quadrant I- Forward Motoring Mode CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. V o  = V s . Load current flows from source to load as shown by the direction of i o .

78. When CH1 is switched OFF, the load current free wheels through CH4 and D2. Both output voltage V s  and load current Io are positive and hence, the operation of the chopper is in the first quadrant. Chopper operates as a step-down chopper in this case.

79. Quadrant II – Forward Regenerative Braking Mode CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.

80. When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that ( E+Ldi /dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source.

81. Quadrant III – Reverse Motoring Mode To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. Polarity of emf E in load must be reversed to have third quadrant operation. 

82. For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. Polarity of load voltage Vo is opposite ,Hence, Vo is assumed negative. io is flowing in the direction opposite and hence negative.

83. Now, when CH3 is turned OFF, the negative load current freewheels through the CH2 and D4. In this manner, Vo and io both are negative

84. Quadrant IV – Reverse Regenerative Braking Mode CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation. When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. 

85. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Load voltage is negative but the load current is always positive. Power is fed back to the source from load and chopper acts as a step-up chopper.

86. Thanks !