Week-7 Challenge: DC Motor Control

Q01. 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.

Objective -

• To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
• To comment on the armature current shoot-up from the scope results.
• To compare the ‘The Four-Quadrant Chopper DC Drive (DC7) block’ with the H-bridge model.

Theory -

Speed Control of a DC Motor using BJT H-Bridge -

• The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT.
• When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V).
• Therefore, the IGBT block can be used to model the BJT device.
• The IGBT block does not simulate the gate current controlling the BJT or IGBT.
• The switch is controlled by a Simulink signal (1/0).
• The DC motor uses the preset model (5 HP 24V 1750 rpm).
• It simulates a fan type load (where Load torque is proportional to square of speed).
• The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
• The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models).
• Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3.
• When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off.
• When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

Simulation -

• The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 240V).
• At t= 1.0 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.
• Scope shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Armature Current Shoot-up -

• At t = 1.0 sec, the armature voltage is reversed and the motor is made to run in reverse direction.
• Due to sudden reversal of voltage the motor armature draws heavy current in order to run the motor in reverse direction.
• This sudden increase in armature current is known as Armature Current Shoot-up.
• The armature current shoot-up can be reduced by decreasing the pulse width.

Four-Quadrant Chopper DC Drive (DC-7) Block -

• 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 behaviour, 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 between BJT H-Bridge and Four Quadrant Chopper -

BJT H-Bridge	Four Quadrant Chopper
It operates in two quadrants Forward Motoring Reverse Motoring To change rotational direction of motor polarity of current is changed. Armature current shoot-up is present due to low swithcing frequency. Simulation time taken is less.	It operates in four quadrants Forward Motoring Forward Braking Reverse Motoring Reverse Braking To change rotational direction of the motor siganls are interrupted. Armature current varies alittle due to high switching frequency, so armature current shoot-up is absent. Simulation time taken is more.

Q02. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Objective -

To develop a 2-quadrant chopper using simulink and explain its working.

Model -

The simulink model for the 2 quadrant chopper is shown below -

• For the motor load, a inductor is taken with the value of 1e^-3.
• A dc voltage source 1 of 30 V is provided for the circuit and a dc voltage source 2 of 12V with motor load.
• During first pulse, CH1 is on and the current flows from source to load.
• During this inductor stores senergy.
• When CH1 turns OFF, the FD starts conducting and direction of load current is reversed.
• When CH2 turns ON, the current will be flowing towards the source and the load current will be negative.
• The current completes its path through diode D2.

Results -

The input gate pulse, load current and load voltage is shown below -

Q03. Explain in a brief about operation of BLDC motor.

Objective -

To explain in brief about operation of Brushless DC Motor.

Theory -

Brushless DC Motor -

• A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor.
• It is a synchronous motor using a direct current (DC) electric power supply.
• It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows.
• The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor.
• This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

Working Principle -

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force.
• As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.
• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap.
• Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape.
• Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
• With the switching of windings as High and Low signals, corresponding winding energized as North and South poles.
• The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

Advantages of BLDC Motor over DC Motor -

• It has no mechanical commutator and associated problems.
• High efficiency due to the use of permanent magnet rotor.
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed.
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system.
• Higher dynamic response due to low inertia and carrying windings in the stator.
• Less electromagnetic interference.
• Quite operation (or low noise) due to absence of brushes.