DESIGN AND FABRICATION OF A CHOPPER
AND CONTROL OF DC MOTOR USING IT
Under the guidance of prof. K.B MOHANTY
SUBMITTED BY:-
PRAHLAD KUMAR SAHOO (107EE035)
NIRMAL KUMAR BARIK (107EE045)
CHOPPER:-
The chopper is a static device that converts fixed dc input voltage to a variable to a variable
output voltage directly. A chopper is a dc equivalent to an ac transformer having continuously
variable terms ratio. The power semiconductor devices used for a chopper circuit can be forced
commutated thyristors, power BJT, power MOSFET, GTO or IGBT. Like a transformer a chopper
can be used to step down or step up the fixed dc input voltage.
PRINCIPLE OF STEP DOWN CHOPPER OPERATION:-
A chopper is a high speed on/off semiconductor switch. It connects source to load and
disconnects the load from the source at a fast speed. In this manner a chopped load voltage is
obtained form a constant dc supply voltage.
During the period Ton chopper is on and load voltage is equal to source voltage Vs . during the
interval Toff chopper is off ,load current flows through the freewheeling diode FD. As a result,
load terminals are shortcircuited by FD and load voltage is therefore zero during T off. In this
manner, a chopped dc voltage is produced at the load terminal.
The load current (io) is continuos as shown in the figure. During Ton the load current rises
whereas during Toff load current decays. From the fig. shown above average load voltage (Vo) is
given by
Vo = {Ton/(Ton+Toff)}*Vs
= (Ton/T)*Vs
= α*Vs …………………………(1)
Where Tonon-time ; Toffoff-time
T=Ton+Toff=chopping period ; α = (Ton/T)=duty cycle
Thus load voltage can be controlled by varying duty cycle α. Eqn. (1) shows that load voltage is
independent of the load current. This eqn. (1) can also be written as
Vo=f*Ton*Vs
Where f=(1/T)=chopping frequency.
The various control strategies for varying duty cycle α are
1. Time ratio control (TRC) and
2. Current limit control.
TIME RATIO CONTROL:-
Time ration control is divided into two types
1. Constant frequency system:-
In this scheme the on-time Ton is varied but the chopping
frequency f (or chopping period T) is kept constant. Variation of Ton means adjustment of
pulse width. So this scheme is also known as pulse width modulation scheme. Ideally α
can be varied from 0 to unity. Therefore output voltage Vo can be varied between 0 and
source voltage Vs.
2. Variable frequency system:-
In this scheme, the chopping frequency (f), the chopping
period (T) is varied and either (i) on-time Ton is kept constant or (ii) off-time Toff is kept
constant. This method of controlling α is also called frequency modulation scheme.
Frequency modulation scheme has some disadvantages as compared to pulse width
modulation scheme.
CURRENT LIMIT CONTROL:-
In this control strategy the on and off of chopper circuit is guided by the previous set value of
load current. These two set values are maximum load current Io max and minimum load current Io
min. When load current reaches the upper limit I o max, chopper is switched off. Now load current
freewheels and begins to decay exponentially. When it falls to lower limit I o min, chopper is
switched on and load current begins to rise. So the load current fluctuates between I o max and Io
min. Therefore it can’t be discontinuous.
STEP-UP CHOPPER:-
In this kind if chopper the average output voltage Vo is greater than the input voltage Vs.
Vo=Vs*(T/Toff)=Vs*{T/(T-Ton)}
=Vs*{1/(1-α)}
The principle of step up chopper can be employed for regenerative breaking of dc motors.
Usually step sown choppers are more common.
TYPES OF CHOPPER CIRCUITS:-
Power semiconductors used in chopper circuits are unidirectional devices. Polarities of output
voltage Vo and direction of output current Io are therefore restricted. However a chopper can
operate in any of the four quadrants by an appropriate arrangement of semiconductor devices.
According to the operation in any of the four quadrants chopper can be classified as follows:-
1. First quadrant or type A chopper
2. Second quadrant or type B chopper
3. Two quadrant or type C chopper (first quadrant and second quadrant)
4. Two quadrant or type D chopper (first quadrant and fourth qyadrant)
5. Four quadrant chopper or type E chopper.
MULTI PHASE CHOPPERS:-
A multi phase chopper is one that consists of two or more choppers connected in parallel.
When two choppers are connected in parallel, they are known as two phase choppers. Similarly
three choppers when connected in parallel will constitute a three phase chopper. A multi
phase chopper may be operated in two modes.
1. In phase operation mode
2. Phase shifted operation mode
In the in phase operation mode, all parallel connected choppers are on and off at the same
instant. In the phase shifted operation mode, different choppers are on and off at different
instants of time.
