Chopper Controlled DC Machines - mpgirnari.in · •A chopper is a high speed ON/OFF semi-conductor switch. ... we can control output voltage. ... fixed dc voltage is converted into

Chopper Controlled DC Machines

By: M. P. Girnari(SSEC, Bhavnagar)

For more visit :-www.mpgirnari.in

For more visit:- www.mpgirnari.in 1

Outlines


• Introduction

• Principle of operation of Chopper

• Four quadrant Chopper circuit

• Chopper for Inversion

• Model of Chopper

• Input to the Chopper

• Other Chopper circuits

• Closed loop operation

Introduction to Chopper• What is Chopper ?

• The circuit that can be used to convert a fixed DC voltage to variable DC voltage through the use of semiconductor devices as a switches.

• Two types of DC-DC converters• AC link chopper

• DC chopper• Step-Up Chopper (Boost Converter)

• Step-Down Chopper (Buck Converter)

• Step Down-Up Chopper (Buck-Boost Converter)

• How fixed DC to variable DC ?• By varying ON & OFF time


Principle of operation of Chopper• A chopper is a high speed ON/OFF semi-conductor switch.

• It connects and disconnects the source to load for predefined time periods.

• Chopper is represented by switch inside rectangle, which may be turned ON or turned OFF as per need.• During 𝑇𝑂𝑁 → Chopper is ON Load voltage = Source voltage

• During 𝑇𝑂𝐹𝐹 → Chopper is OFF Load voltage = Zero

• By varying 𝑇𝑂𝑁 & 𝑇𝑂𝐹𝐹, we can control output voltage.

• In this way, fixed dc voltage is converted into variable dc voltage at load terminals.


Principle of operation of Chopper• Frequency of operation is given by

• 𝑓𝑐 =1

𝑇𝑂𝑁+𝑇𝑂𝐹𝐹=

1

𝑇

• Where, 𝑇 = 𝑡𝑜𝑡𝑎𝑙 𝑡𝑖𝑚𝑒 𝑝𝑒𝑟𝑖𝑜𝑑, 𝑇𝑂𝑁 = 𝑜𝑛 𝑡𝑖𝑚𝑒, 𝑇𝑂𝐹𝐹 = 𝑜𝑓𝑓 𝑡𝑖𝑚𝑒

• Duty cycle :• Duty cycle is defined as the ratio of on time period to total time period.

• 𝑑 =𝑇𝑂𝑁

𝑇=

𝑇𝑂𝑁

𝑇𝑂𝑁+𝑇𝑂𝐹𝐹

• The output voltage across the load during the ON time is equal to the difference between the source voltage and voltage drop across semiconductor switch.

• Suppose, the switch is ideal voltage drop across it is zero

• The average dc voltage across the load is given as

• 𝑉𝑑𝑐 =𝑇𝑂𝑁

𝑇∗ 𝑉𝑠 = 𝑑 ∗ 𝑉𝑠

• Thus, 𝑉𝑑𝑐 can be vary by varying the duty cycle.


How to vary duty cycle:• Duty cycle can be changed in two ways:

• Keep switching frequency = constant & varying the 𝑇𝑂𝑁• Varying the switching frequency & keep the 𝑇𝑂𝑁 = constant

• Control Strategies:• Time ratio control (TRC) method

• Constant frequency method

• Variable frequency method

• Current limit control method


Four quadrant Chopper circuit

• Consider the figure.

• Transistor is used as a switch.

• Each transistor has freewheeling diode across it and snubber circuit is used to limit voltage.

• The load may consist of a resistance, an inductance, and an induced emf.

• The supply source is DC source and capacitor is connected across it to maintain constant voltage.

• It is assumed that the switches used are ideal.


First quadrant operation• Voltage & Current Positive• Achieved by triggering 𝑇1and 𝑇2 together. 𝑉𝐿 = 𝑉𝑆.

• For 𝑉𝐿 𝑡𝑜 𝑏𝑒 𝑧𝑒𝑟𝑜 either 𝑇1 𝑜𝑟 𝑇2 can be turned off.

• Assume that:• 𝑇1 is turned off current will decreases in

power switches & inductance.• As current decreases in inductance Due to

rate of change in current a voltage is induced in inductor ∝

𝑑𝑖

𝑑𝑡.

• Polarity of which is opposite to the load-induced emf.

• So, it will forward biasing diode 𝐷4.• Now, 𝐷4 provides path for armature current

continuously during this time.• Circuit configuration of which is shown in the

second figure.


First quadrant operation• Current and voltage waveforms for

continuous and discrete conduction is shown in the figure.

• Here:• The induced emf of the load appears

across the load when the current is zero.

• The load voltage is therefore is a stepped waveform.

• The operation will be motoring in the clock-wise direction or forward motoring.


Second quadrant operation• 𝑉 → 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 & 𝐼 → 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒

• Assume that load emf’s is negative.

• 𝑇1 𝑜𝑟 𝑇2is conducting for given time:• The current in the inductive load has to continue to

flow until the energy in it is depleted to zero.

• Hence, the diodes 𝐷3𝑎𝑛𝑑 𝐷4 will take over, maintaining the load current in the same direction, but the load voltage will be negative.

• When 𝐷3𝑎𝑛𝑑 𝐷4 are conducting, the source receives power from the load.

• Circuit configuration is shown in the figure.

• The voltage and current waveforms are also shown in the figure.


Second quadrant operation• If the source can’t absorb this power, provision has

to be made to consume the power.

• In this case, the overcharge on the filter capacitor is periodically dumped into a resistor connected across the source by controlling the on-time of the transistor in series with a resistor.

• This form of recovering energy from the load is known as regenerative braking.• Regenerative braking is common in low-HP motor drives

• When 𝐼𝐿 is decreasing 𝑇2 𝑖𝑠 𝑡𝑢𝑟𝑛𝑒𝑑 𝑂𝑁.• Load will short circuited through 𝑇2 & 𝐷4.

• Result 𝐼𝐿 will increases.


Second quadrant operation• Turning OFF 𝑇2 pulse of current flowing into the

source via 𝐷3& 𝐷4.

• This operation allows the priming up of the current and a building up of the energy in the inductor from the load’s emf.

• Thus, enabling the transfer of energy from load to the source.

• Note:• It is also possible to transfer the energy from load to the

source even when 𝐸 < 𝑉𝑠.

• This feature is referred to as boost operation in dc-to-dc power supplies.


Third quadrant operation• Load voltage & current negative

• A negative emf source –E is assumed in the load.

• 𝑇3 & 𝑇4 switched ON current through the load will increases.

• If we turn OFF any of transistor short circuit the load & current will decreases.

• In this manner, the load current can be controlled within set limit.

• The circuit configuration is shown in the figure.


Third quadrant operation• The voltage & current waveform

for continuous & discontinuous current conduction mode is shown in the figure.

• Waveforms of first and third quadrant operation are similar. Just change in the sign of voltage and current.


Fourth quadrant operation• 𝑉 → 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 & 𝐼 → 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒

• The load emf source is assumed to be positive.

• To send energy to the dc source from the load, the armature current has to be established to flow from the right side to left side as shown in the figure.

• Assume that:• Machine has been operating in quadrant-I with a

positive current in armature. • Now, when brake command is received, the torque and

armature current command goes negative.• The armature current can be driven negative from its

positive value through zero.• Opening 𝑇1& 𝑇2will enable 𝐷3& 𝐷4 to allow current via

the source. Reducing the current magnitude to zero rapidly.


Fourth quadrant operation

• To establish a negative current 𝑇4 is turned ON.

• That will short circuit the load & making emfsource to build up current through 𝑇4& 𝐷2 .

• When current will reach desired peak value, 𝑇4 is turned OFF.

• When current falls below a lower limit, 𝑇4 is again turned ON, to build up the current.

• Thus, the average voltage across the load is positive and average load current is negative, which indicate that power is transferred from the load to the source.


Chopper for Inversion• The chopper can be used as a single phase inverter because

• It can operate in all the four quadrants

• It can handle leading or lagging reactive loads connected in series with source

• The frequency of output is depends on the rate at which the forward to reverse operation is take place in chopper.

• A multi-phase inverter can be designed by using a number of chopper.

• Where chopper can be used as a basic building block.


Model of Chopper• The chopper can be modeled as a first order lag with a gain of 𝐾𝑐ℎ.

• The transfer function of chopper is given by

• 𝐺 𝑠 =𝐾𝑟

1+𝑠𝑇

2

• Where,

• 𝐾𝑟 =𝑉𝑠

𝑉𝑐𝑚

• 𝑉𝑠 = 𝑠𝑜𝑢𝑟𝑐𝑒 𝑣oltage

• 𝑉𝑐𝑚 = 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑐𝑜𝑛𝑡𝑟𝑜𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

• As we increase chopping frequency T will decreases Transfer function will becomes a equal to 𝐾𝑟


Input to the Chopper• The input to the chopper is either

• A battery or • A rectified ac supply.

• Rectified ac means ac input is firstrectified by rectifier (bridge or center tapped ) and then by using capacitive filter the output voltage is kept at the constant value.

• Advantage:• near-unity power factor.

• Disadvantage:• It can not transfer power from the dc link

into ac mains.

• In that case, the re-generative energy has to be dumped in the braking resistor as shown in the figure. For more visit:- www.mpgirnari.in 19

Input to the Chopper


• The phase-controlled converter is connected anti-parallel to the diode bridge to handle the re-generative energy as shown in the figure.

• The phase-controlled converter have smaller rating as compared to power-converter bcz of small rms value of regenerative current.• The duration of regeneration is only a fraction of

motoring time for most loads.

• In order to match the polarity of 𝑉𝑠, the output dc voltage across the converter must to be reverse.

• To reverse the output dc voltage, we have to operate the regenerative converter at triggering angles greater than 90°.

Input to the Chopper


• The phase-controlled converter is enabledonly when 𝑉𝑠 > Vdc ± ∆𝑉 (allowable upper and lower magnitude from dc voltage obtained from ac source).

• The phase-controlled converter is disabledwhen 𝑉𝑠 > 1.35*V (V is line to line rmsvoltage), to prevent energy flow from source to dc link and from dc link to source via the phase controlled converter.

Other Chopper circuits• As all applications does not need four quadrant operation, the power circuit

is therefore simplified to accommodate only the necessary operation.

• Power devices are reduced for one and two-quadrant drives in order to save cost of the power converter.

• Other chopper circuits for reduced quadrant operation is shown in the figure below.


Closed loop operation• In this section we will discuss about:

• Speed controlled drive system

• Current controlled system1. Pulse Width Modulation (PWM) Controller

2. Hysteresis Controller

• Modeling of Current Controller


Speed controlled drive system


• It has two loops:

• Outer loop: Speed-control loop• Similar to phase controlled DC-motor

drive

• Inner loop: Current-control loop• Different from phase-controlled DC-

motor drive

• The closed loop speed controlled separately excited DC motor drive is shown in the figure.

Current controlled drive system


• As we know that torque is depends on the current for dc drive system, with inner current control loop alone, the motor drive system is a torque amplifier.

• Closed Loop :• The desired value of current is compared with actual value of armature current.

• Error between them is corrected by current controller.

• The output of the current controller is in conjunction with other constraints determines the base drive signals of chopper switches.

• Types of current controller:1. Pulse Width Modulation (PWM) Controller

2. Hysteresis controller

• The selection of the particular current controller has effect on the transient response, hence, the overall speed loop bandwidth indirectly.

Pulse Width Modulation Controller


• The desired value of current is compared with actual value of armature current.

• Current Error is fed into current controller.• Controller could be P, PI or PID.• PI controller is most commonly used controller.

• The current error is amplified through this controller and emerges as a control voltage 𝑉𝑐 .

• It is required to generate a proportional armature voltage from a fixed source through a chopper operation. Therefore, the control voltage is equivalent to the duty cycle of chopper.

• Realization:• Control voltage is compared with ramp signal to generate ON & OFF

times as shown in the figure.• ON signal : produced if the 𝑉𝑐 > 𝑟𝑎𝑚𝑝 𝑠𝑖𝑔𝑛𝑎𝑙• OFF signal : produced if the 𝑉𝑐 < 𝑟𝑎𝑚𝑝 𝑠𝑖𝑔𝑛𝑎𝑙

• The duration for which the control signal exceeds the ramp signal determines the duty cycle of the chopper.

• The ON & OFF time signals are combined with other control features such as• Interlock, Minimum On & OFF times & Quadrant selections etc.

Pulse Width Modulation Controller


• Instead of ramp signal for carrier waveform, a unidirectional sawtooth waveform could be used.• Advantage of using sawtooth waveform as a carrier is that it has

symmetry between the rising & falling sides of the waveform, unlike the ramp signal.

• Principle of operation:• The voltage applied to load is varied within one cycle of the carrier

signal as shown in the figure.

• Summary of switching logic:• 𝑖𝑎

∗ − 𝑖𝑎 ≥ carrier frequency → Sawtooth waveformmagnitude Tp = 1, Va = Vs.

• 𝑖𝑎∗ − 𝑖𝑎 < carrier frequency → Sawtooth waveformmagnitude Tp = 0, Va = 0.

• For fast response, the current error is amplified so that a small current error would activate the chopper control.

• The PWM controller has the advantage of smaller output ripple current for a given switching frequency as compared to the hysteresis current controller.

Why ? Hysteresis Control loop


• In order to control the duty cycle of the chopper ,the PWM controller acts once a cycle.

• The chopper is then acts as a variable voltage source with average current control.

• Instantaneous current control is not exercised in the PWM current controller.

• In between two consecutive switching, the current can be exceed the maximum limit.

• If PWM current controller is sampled and held once a switching cycle, then a current is controlled on an average but not on an instantaneous basis.

• The hysteresis controller overcomes this drawback by converting a voltage source into a fast acting current source.

• Then, the current is controlled within a narrow band of excursion from its desired value in the hysteresis controller.

Hysteresis Control loop


• The current is controlled within a narrow band of excursion from its desired value in the hysteresis controller.

• The hysteresis window determines the allowable or preset deviation of current, ∆𝑖.

• The desired current & actual current are shown in the figure with the hysteresis windows.

• Summary of Control Logic:• 𝑖𝑎 ≤ 𝑖𝑎

∗ − ∆𝑖, → 𝑠𝑒𝑡 → 𝑉𝑎 = 𝑉𝑠• 𝑖𝑎 > 𝑖𝑎

∗ + ∆𝑖, → 𝑟𝑒𝑠𝑒𝑡 → 𝑉𝑎 = 0

Realization of hysteresis controller


• The realization is shown in the figure.

• The window ∆𝑖, can either be set externally as a constant or can be made a fraction of armature current by programming.

• Disadvantage:• The chopping frequency is a variable unlike

constant in PWM converter, So, as switching frequency will increases switching losses will also be increases.

Modeling of current controller


• The current-error amplifier is modeled as a gain (only Proportional control) and mathematically it is given by• 𝐺𝑐 𝑠 = 𝐾𝑐

• The chopper can be modeled as a first order lag with gain given by

• 𝐺𝑟 𝑠 =𝐾𝑟

1+𝑠𝑇

2

• The PWM current controller has a delay of the half time period of the carrier waveform and has a gain that of chopper. Hence, the overall transfer function (cascade configuration) including chopper is given by

• 𝐺𝐶𝐺𝑟 𝑠 =𝐾𝑐𝐾𝑟

1+𝑠𝑇

2

• Where, 𝐾𝑐 = gain of the PWM controller (which depends on the gain of the current error amplifier)

• The hysteresis controller has instantaneous response, hence, the current loop can be approximated as a unity gain.

Comparison


GTU Previous Year Questions


• With waveform explain second-quadrant chopper operation.

• Explain PWM (pulse width modulation) current controller of chopper controlled DC drive.

• Draw circuit diagram and explain chopper circuit for motoring and regenerative control along with necessary characteristics.

• With the help of Block diagram explain Closed loop control for DC drive

Tutorial:-4 Chopper Controlled DC Motor Drive


1. What is chopper? Discuss about different types of chopper in detail.

2. Explain the operation of first-quadrant chopper with waveforms.

3. Explain the operation of second-quadrant chopper with waveforms.

4. Explain the operation of third-quadrant chopper with waveforms.

5. Explain the operation of fourth-quadrant chopper with waveforms.

6. Draw circuit diagram and explain chopper circuit for motoring andregenerative control along with necessary characteristics.

7. Explain Motoring and Braking operation of chopper controlled separatelyexcited dc Motor.

8. Explain PWM (pulse width modulation) current controller of choppercontrolled DC drive.

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Chopper Controlled DC Machines - mpgirnari.in · •A chopper is a high speed ON/OFF semi-conductor switch. ... we can control output voltage. ... fixed dc voltage is converted into