Week-7 Challenge: DC Motor Control

FOR QUESTION1:

A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

B. Comment on the armature current shoot-up from the scope results.

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

A)BJT H-Bridge:

the bi polar junction ttransistor (BJT) when it is used for power switching applications,operates as an IGBT.when it is conducting a forward voltage Vf is developed between collector snd emitter.therefore,the IGBT block can be used to model the BJT device.

The IGBT block does not simulate te gate current controlling the BJT or IGBT.The switch is controlled by a simulink signal.the DC motor uses the preset model.it simukates a fan type load (the torque load is proportional to square of speed).the armature mean voltage can be varied from 0-240V when the duty cycle (specified in the pulse generator block)is varied from0-100%

tThe H-bridge consists of 4 BJT/Diode pairs (BJT simulated by IGBT models).the 2 transistors are switched simultaneously:Q1&Q4 or Q2&Q3 . when Q1 &Q4 are fired,a positive voltage to TO the motor AND diodes D2-D3 operates as free wheeling diode.diodes when Q1&Q4 are switched off.when Q2&23 are fired ,a negative voltage is appiled to the motor and diodes D1-D4 operates as free wheeling diodes when Q2&Q4 are switched off.

B)CHOPPER:

A DC chopper is also known as DC-DC convereter.it is a statis device which is used to obtain a variable DC voltage from a constant DC voltage source.A DC-DC conveter is an electronic circuit or electromechanical device that conertes a source of direct current(DC) from one voltage level to another.power level range from very low small batteries to very high. choppers are of two types step-down chopper and step up chopper.

PROCESS:

1)Explain and run the MATLAB demo 'speed control of a DC motor using BJT H-bridge'.the armature shoot up from the scope results are to be obeserved.

• open the MATLAB and in that the help documentation type speed control of a motor using BJT H-Bridge.
• Results tab opens,and then select the model and click open model.

• the model is opened in a simulink workspace.
• run the model as it is and obesrve the graphs of the armature current.
• now,at the bottom part of the model,there is a pulse generator block,click on it and run the model,
• check the outputs follows are same for 75% &50% just to check if the armature voltage is decresed sharply after half of the smulation time.

RESULT FOR PULSE WIDTH IS 75%:

RESULTS FOR PULSE WIDTH 50%:

• AS pulse width decreses the step value of the armature current is decreses.

C). Four quadrant chopper DC drive block:

the 200HP DC motor is seperatly excited with a constant 150V DC field voltage source.the armature voltage is provided by an IGBT converter controlled by two PI regulators. the converter is fed by a 515V DC bus obtained by rectification of a 380V AC 50Hz voltage source, in order to limit the DC bus voltage during dyanamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator followed by a current regulator.the speed regulator outputs the armature current referance (in p.u) used by the current controller in orgdr to obtain the electromagnetic torque needed to reach the diserd speed.the speed referance change rate follows acceleration and deceleration ramps in order to avoid sudden referance changes that could cause armature over current amd destabilize the system. the current regulator controls the armature over current by computing the appropriate duty ratios of the 5khz pulses of the 4IGBT devices.for proper system behaviour ,the instantaneous pulse values of IGBT devices 1&4 are opposite to those of IGBT devices 2&3. it generates the avg armature voltage needed to obtain the desiredarmature current.in order to limit the amplitude of the current oscillations,a smoothing inductance is placed in series with the armature circuit.

• the stability of the voltage and current supply is higher in the 4Quadrant compared to BJT as the 4Quadrant uses the steps by step ramping strategy for the change in the motor direction whereas the stability is lost.
• Regenerative braking is possible in 4 quadrant but not in BJT.
• It is widely used in automobile sector to control DC motor speed and direction in 4 quadrant.whereas the BJT is used in robotics and electro mechanical devices to control the DC motor speed and direction.
• the current values are continous in BJT in 4 quadrant;it is chopped so that the simulation trakes less time
• to change the direction of the motor ,current signals are interruoted in 4 quadrant, whereas in BJT the polarity of the current needs to be changed.
• the main advantage of 4 quadrant is that it operates in all the 4 quadrant but BJT can move only in two directions.
• 4 quadrant provides better effeciency and operates at high power levels when compared to BJT that it operates at moderate power levels.
• 

FOR QUESTION 2:

2.TWO QUADRANT CHOPPER DC DRIVE BLOCK:

The 2 quadrant chopper DC drive block reprresents a two quadrant DC suppiled chopper(DC-DC PWM Converter) drive for DC motors. this drive features closed loop speed control with two quadrant operations.the speed control loop output referances armature current of the machine.using a PI controller ,the chopper duty cycle corres[onding to the commanded armature current is derived.this duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signal for the chopper.

The two quadrant chopper DC drive block uses these blocks from the electric drives/fundamental drive blocks library:

• spped controller(DC)
• Regulation switch
• Chopper

OUTPUT OF 2 quadrant dc chopper:

FOR QUESSTION3:

3. BLDC MOTOR:

A brush less dc motor (BLDC MOTOR) also known as an electronically commutated motor or synchronous DC motor using a direct current electric power supply.it uses an electronic closed looped controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet. A commutator-brushes arrangement helps in achieving unidirectional torque in a typical dc motor. Obviously, commutator and brush arrangement is eliminated in a brushless dc motor. Here, an integrated inverter/switching circuit is used to achieve unidirectional torque. That is why these motors are, sometimes, also referred as 'electronically commutated motors'.

The Permanent magnets are mounted on the rotor of a BLDC motor, and the stator is wound for a specific number of poles. Also, a control circuit is connected to the stator winding. Most of the times, the inverter/control circuit or controller is integrated into the stator assembly. This is the basic constructional difference between a brushless motor and a typical dc motor.

Stator windings of a BLDC motor are connected to a control circuit (an integrated switching circuit or inverter circuit). The control circuit energizes proper winding at the proper time, in a pattern which rotates around the stator. Permanent magnets on the rotor try to align with the energized electromagnets of the stator, and as soon as it aligns, the next electromagnets are energized. Thus, the rotor keeps running. The animations below will give you a clear idea of 'how a brushless DC motor works?'