Week-7 Challenge: DC Motor Control

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

ANS:

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

• The Bipolar Junction Transistor (BJT) operates as an IGBT when used for power switching applications.
• 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 the square of speed).
• The armature means 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 180V).
• At t= 0.5 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.

Current scope:-

The main scope:-

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

ANS:

• At t = 0.5 sec, the armature voltage is reversed and the motor is made to run in a reverse direction.
• Due to sudden reversal of voltage the motor armature draws heavy current to run the motor in a 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.

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

ANS:

• Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('The Initial States Setting' button and 'Cbus' variable).
• After simulation, the motor armature voltage and current, the four IGBT pulses, and the motor speed can be observed on the scope.
• The current and speed references are also shown.
• The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
• The speed reference is set at 500 rpm at t = 0 s and we can observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches a steady state around t = 1.3 s.
• The armature current follows the current reference very well, with a fast response time and small ripples (current ripple frequency is 5 kHz).
• At t = 2 s, speed reference drops to -1184 rpm.
• The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.
• At t = 2.2 s, the current reverses to produce a braking electromagnetic torque (dynamic braking mode).
• This causes the DC bus voltage to increase.
• At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative.
• The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s).
• At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Scope:-

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 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 between BJT H-Bridge and Four Quadrant Chopper: -

BJT H-Bridge	Four Quadrant Chopper
1)It operates in two quadrants Forward Motoring Reverse Motoring 2)To change the rotational direction of motor polarity of the current is changed. 3)Armature current shoot-up is present due to low switching frequency. 4)Simulation time taken is less.	1)It operates in four quadrants Forward Motoring Forward Braking Reverse Motoring Reverse Braking 2)To change the rotational direction of the motor signals are interrupted. 3)Armature current varies a little due to high switching frequency, so armature current shoot-up is absent. 4)Simulation time taken is more.

Question 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 ) ?

ANS:

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

Working:-

• In this 2 Quadrant model, we use MOSFET as a switch and two Diodes are connected to the MOSFET in parallel.
• We can give controle voltage 48v and inductive load voltage 24v.
• For the switching of MOSFETs gate pulse is given using pulse generator with parameters given in the figure below -

MOSFET:-

Diode:-

Inductive Load:-

Main scope:-

Result:-

• During the first pulse, CH1 is on and the current flows from source to load.
• During this inductor stores energy.
• When CH1 turns OFF, the FD starts conducting and the 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.

Question 3):- Explain in a brief about operation of BLDC motor?

Construction:-

In this motor, the permanent magnets are attached to the rotor. The current-carrying conductors or armature winding are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

The main design difference between brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutator. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in “Outrunner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In a runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

Working:-

BLDC motor works on the principle similar to that of a brushless dc motor. 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

When the stator coils get a supply from 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 which causes the motor to rotate.

Advantages:-

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio 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
• Low noise due to the absence of brushes

Limitation of BLDC motor:-

• These motors are costly
• Electronic controller required to control this motor is expensive
• Requires complex drive circuitry
• Need for additional sensors

NOTE:- model and screenshots attached(uploaded).