CHAPTER 2 DET50063_SPEED CONTROL OF DCM_slide4

CHAPTER 2_slide 4

SPEED CONTROL OF DCMOTOR

DR FIZATUL AINI PATAKOR
https://scholar.google.com.my/citations?user=lnlf4VQAAAAJ&hl=en
https://www.researchgate.net/profile/Fizatul_Aini_Patakor/research

PRINCIPLES OF SOLID STATE CONTROL (THYRISTOR
CONTROLLED)

• Solid-state control is used for enhanced efficiency and for versatile
operation of electric drive systems.

• For dc machines, converters are often used in the armature circuit to
control the terminal voltage of the motor. In some cases, the
converter is also used to control the field voltage.

PRINCIPLES OF SOLID STATE CONTROL (CHOPPER)

There are two control strategies employed in DC chopper
o Time ratio control
o Current limit control

Time ratio control- value of TON/T is varied. This is effected in two ways-
variable frequency operation & constant frequency operation.
1. TON or TOFF is kept constant
2. TON or TOFF are varied

PRINCIPLES OF SOLID STATE CONTROL (CHOPPER)

Control Strategy of DC Chopper
• Current limit control- the chopper is switched ON and OFF so that

the current in the load is maintained between two limit.
• When the current exceed upper limit, the chopper switched OFF.
• When the current reached lower limit, the chopper switched ON.
• During the OFF period the load current freewheels and decreases

exponentially.

Time ratio control & Current limit control

Show time ratio controlled choppers for DC series motors

Average load Voltage is given by

V0 = Ton/ (Ton +Toff) * Vs = (Ton/T) V = A Vs
Ton : on -time
Toff : off- time
T = Ton +Toff= chopping period
A = Ton /T = duty cycle

So we know that the load voltage can be
controlled by varying the duty cycle A. equation
above shows that the load voltage is
independent of load current it can be also
written as

V0 = f. Ton .Vs
f= 1/T = chopping frequency

Show current limit controlled choppers for separately excited DC motor

Chopper For DC Motor Drives

• First Quadrant

Click to add text

Chopper For DC Motor Drives

• It is known as first-quadrant chopper
or type A chopper. When the chopper
is on, vin0 =thVeS as a result and the current
flows direction of the load. But
when the chopper is off ivn0 is zero but
Id0irecctoionntinuthersoutogh flow the same
freewheeling
the
diode FD, thus average value of
voltage and current assayshVo0 wanndinI0 will
be always positive the
graph.

• In type A chopper the power flow will
be always from source to the load. As
the average voltage V0 is less than the
dc input voltage Vs

• When chopper is ON, supply voltage V is connected across the
load.

• When chopper is OFF, vO = 0 and the load current continues to
flow in the same direction through the FWD.

• The average values of output voltage and current are always
positive.

• Class A Chopper is a step-down chopper in which power
always flows form source to load.

• It is used to control the speed of dc motor.

• The output current equations obtained in step down chopper

• with R-L load can be used to study the performance of Class A
Chopper

Chopper For DC Motor Drives

• second Quadrant

Chopper For DC Motor Drives

• In type B or second quadrant chopper
the load must always contain a dc
source E .

• When the chopper is on, dvr0iveiss zero
but the load voltage E the
current through the inductor L and
the chopper, L stores the energy
during the time Ton of the chopper .
•
When the chopper is off , tvh0e=s(oEu+rceL
. di/dt ) will be more than
voltage bVes . Because of this the diode
D2 will forward biased and begins
conducting and hence the power
starts flowing to the source.

Chopper For DC Motor Drives

• No matter the chopper is on or off the current I0 will be flowing out
of the load and is treated negative . Since VO is positive and the
current I0 is negative , the direction of power flow will be from load to
source.

• The load voltage V0 = (E+L .di/dt ) will be more than the voltage
Vs so the type B chopper is also known as a step up chopper .

• When chopper is ON, E drives a current through L and
R in a direction opposite to that shown in figure.

• During the ON period of the chopper, the inductance L
stores energy.

• When Chopper is OFF, diode D conducts, and part of
the energy stored in inductor L is returned to the
supply.

• Average output voltage is positive and average
output current is negative.

• In this chopper, power flows from load to source.

• Class B Chopper is used for regenerative braking of dc
motor.

• Class B Chopper is a step-up chopper.

Chopper For DC Motor Drives

• Two Quadrant operation

Chopper For DC Motor Drives

• Type C chopper is obtained by connecting type –
A and type –B choppers in parallel.

• WfreeewwihlleaellwinagydsigoedteaFDpoissiptirveeseonuttpauctrovsoslttahgeeloVa0da. s the

• When the chopper is on the freewheeling diode starts
conducting and the output voltage v0 will be equal to
Vs .

• TsTohhueercdeciruearcnrtedionnt itoiwf0 itlhl ebweloiplalodsibtceiuvrerefrlenogtwaiir0ndwgleisllstobtwheearrecdhvseorpspteheder.
is on or the FD conducts.

• The load current will be negative if the chopper is or
the diode D2 conducts. We can say the chopper and
FD operate together as type-A chopper in first
quadrant. In the second quadrant, the chopper and
D2 will operate together as type –B chopper.

• Class C Chopper is a combination of Class A and Class
B Choppers.

• For first quadrant operation, CH1 is ON or D2 conducts.

• For second quadrant operation, CH2 is ON or D1
conducts.

• When CH1 is ON, the load current is positive.

• The output voltage is equal to ‘V’ & the load receives
power from the source.

• When CH1 is turned OFF, energy stored in inductance L
forces current to flow through the diode D2 and the
output voltage is zero.

• Current continues to flow in positive direction.

• When CH2 is triggered, the voltage E forces current to

flow in opposite direction through L and CH2

• The output voltage is zero.

• On turning OFF CH2, the energy stored in the inductance
drives current through diode D1 and the supply Output voltage
is V, the input current becomes negative and power flows from
load to source.

• Average output voltage is positive
• Average output current can take both positive and negative

values.
• Choppers CH1 & CH2 should not be turned ON

simultaneously as it would result in short circuiting the supply.
• Class C Chopper can be used both for dc motor control and

regenerative braking of dc motor.
• Class C Chopper can be used as a step-up or step-down

chopper.

A. K. Gautam

Chopper For DC Motor Drives

• Two Quadrant operation

Chopper For DC Motor Drives

• When the two choppers are on the output voltage dvio0 dweilsl be eaqnudaDl 2towVisll . When
cvo0 n=d–uVcstinthge. two choppers will be off but both the D1 start

• tVim0 teheToanvweirlal gbee output voltage will be positive when the choppers turn-on the
more than the turn off time Toff its shown in the wave form below.

• As the diodes and choppers conduct current only in one direction the direction

• ppThooessiittpiivvoeew.etFhrrufolsomtwhtsehfefroorwmthasvqoeuufarocdrermatnoittlooisapdseeraetnaisotnhthaoetftathvyeepreaavDgeercavhgaoelpuvpeaeslruoeifsbooofbtVtha0ivni0seadn.d i0 is

• From the wave forms the Average value of output voltage is given by
V0= (Vs Ton-VsToff)/T = Vs.(Ton-Toff)/T

• n of load current will be always positive.

• Class D is a two quadrant chopper.
• When both CH1 and CH2 are triggered simultaneously, the

output voltage vO = V and output current flows through the
load.
• When CH1 and CH2 are turned OFF, the load current
continues to flow in the same direction through load, D1
and D2, due to the energy stored in the inductor L.
• Output voltage vO = - V
• Average load voltage is positive if chopper ON time is
more than the OFF time
• Average output voltage becomes negative if tON < tOFF .
• Hence the direction of load current is always positive but
load voltage can be positive or negative.

A. K. Gautam

Chopper For DC Motor Drives

• Four Quadrant operation

• Class E is a four quadrant chopper

• When CH1 and CH4 are triggered, output current
iO flows in positive direction through CH1 and
CH4 , and with output voltage vO = V.

• This gives the first quadrant operation.

• When both CH1 and CH4 are OFF, the energy
stored in the inductor L drives iO through D2 and
D3 in the same direction, but output voltage vO = -
V.

• Therefore the chopper operates in the fourth
quadrant.

• When CH2 and CH3 are triggered, the load current iO
flows in opposite direction & output voltage vO = -V.

• Since both iO and vO are negative, the chopper
operates in third quadrant.

• When both CH2 and CH3 are OFF, the load current iO
continues to flow in the same direction D1 andD4 and
the output voltage vO = V.

• Therefore the chopper operates in second quadrant as

vO is positive but iO is negative.

Chopper For DC Motor Drives

• Type E or the fourth quadrant chopper consists of four
semiconductor switches and four diodes arranged in antiparallel.

• The 4 choppers are numbered according to which quadrant they
belong. Their operation will be in each quadrant and the
corresponding chopper only be active in its quadrant.

Chopper For DC Motor Drives

• First Quadrant
• During the first quadrant operation the chopper CH4 will be on .

Chopper CH3 will be off and CH1 will be operated. AS the CH1 and
CH4 is on the load voltage v0 will be equal to the source voltage
Vs and the load current i0 will begin to flow . v0 and i0 will be positive
as the first quadrant operation is taking place. As soon as the chopper
CH1 is turned off, the positive current freewheels through CH4 and
the diode D2 . The type E chopper acts as a step- down chopper in
the first quadrant.

http://www.circuitstoday.com/types-of-chopper-circuits

Chopper For DC Motor Drives

• Second Quadrant
• In this case the chopper CH2 will be operational and the other three

are kept off. As CH2 is on negative current will starts flowing through
the inductor L . CH2 ,E and D4. Energy is stored in the inductor L as
the chopper CH2 is on.
• When CH2 is off the current will be fed back to the source through
the diodes D1 and D4. Here (E+L.di/dt) will be more than the source
voltage Vs . In second quadrant the chopper will act as a step-up
chopper as the power is fed back from load to source

http://www.circuitstoday.com/types-of-chopper-circuits

Chopper For DC Motor Drives

• Third Quadrant
• In third quadrant operation CH1 will be kept off , CH2 will be on and

CH3 is operated. For this quadrant working the polarity of the load
should be reversed.
• As the chopper CH3 is on, the load gets connected to the
source Vs and v0 and i0 will be negative and the third quadrant
operation will takes place. This chopper acts as a step-down chopper

http://www.circuitstoday.com/types-of-chopper-circuits

Chopper For DC Motor Drives

• Fourth Quadrant
• CH4 will be operated and CH1, CH2 and CH3 will be off. When the chopper

CH4 is turned on positive current starts to flow through CH4, D2 ,E and the
inductor L will store energy.
• As the CH4 is turned off the current is feedback to the source through the
diodes D2 and D3 , the operation will be in fourth quadrant as the load
voltage is negative but the load current is positive. The chopper acts as a
step up chopper as the power is fed back from load to source.

http://www.circuitstoday.com/types-of-chopper-circuits