Week-7 Challenge: DC Motor Control

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

• The H-bridge is an electronic circuit that looks like the letter H. An H-bridge is used to drive a load, such as a brushed DC motor, in both directions. And it controls the flow of current to a load.

• the model consists a motor(5HP 240V 1750rpm field:300V), which is connected to a 240V supply with four bipolar junction transistors(BJT) and diodes, which acts as full bridge The model is know as H-bridgebecause the connection of the BJT's and diodes are in such a way that switches the polarity of the voltage applied to the motor.
• The structure of H-bridgecircuit is

• When S1, S4 are connected the motor moves right
• When S2, S3 are connected the motor moves left.

Speed of motor:

• Initially increases to 0 to nearly 900 rpm upto 0.5 secs(Forward motoring action takes place).
• Then speed reduces to 0 rpm

Load torque of motor:

• Initially torque increase from 0 to 0.5 secs in forward motoring action.

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

• By changing the duty cycle from 75% to 25% , the current surge can be reduced tremendously.

• From the result it is see that, the current does not shoot up when the motor changes direction @ T=0.5s of the simulation. The speed of the motor and the load torque in of the motor are low.

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

• The Dc7-four quadrent chopper 200 HP DC drive model is opened by running the command (dc7_example) using the command window of MATLAB.

• The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.
• The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source.
• In order to limit the DC bus voltage during dynamic 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 reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.
• The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system.
• The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behavior
• , the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.
• This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

                   Comparison of The Four-Quadrant Chopper DC Drive (DC7) block with H-bridge

2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor)

• let the voltage of dc voltage source be 24 volts.
• let us consider an series RLC branch block and select inductor L . thus block works as a load.
• To consider LE load ie motor load , and dc voltage source of 12 volt is also connected in series with inductor.
• The pulse generator block is used to create gate pulse.
• the not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur .due to use of not gate , when one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off .
• The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse weidth be 50%

OBERVATION

• the voltage waveform is not going less than zero . the minimum value of voltage is zero and maximum value is always in the positive direction.
• however in current waveform , the current increases and decreases. the current may be positive as well as negative.
• the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

3. Explain in a brief about operation of BLDC motor.

• BLDC motor named as brushless dc motor having high effenciency than other types of motors.This motor is also known to be synchronous motor.
• Construction:
• Most of the motors are having brushes in their rotor as it transfers current into the rotor circuit.But here differently rotor is placed with permanent magnet.
• The stator coil is wounded with coils,they are stationary but producing variable DC voltages for rotating the permanent magnet as changing the magnetic field of stator poles continuously.
• The controlling of motor is by changing the magnitude and direction of current flowing through the stator windings

                Operation of BLDC motor:

• We can consider three winding trminal are comming out as U,V,W and their another ends are connected in single point as shown in below figure.The angle shift between two coils makes 120 degree.

• The above movement are acheived by continuous change in current direction over two poles of stator at single time.But this is slightly not continuous in their rotation.
• This makes motor vibration and noice will be produced.
• So, to attain smooth continuous rotation of rotor we have to control the current flow through all the three poles in same time.
• As shown below we can acheive continuous change in current flow in all stator poles at same time in 120 degree phase shift.
• As we have seen, we drive BLDC motors by continually changing the directionality of the flux produced by the coils. The permanent magnets on the rotor continually chase the shifting rotating magnetic field,
•