Week-7 Challenge: DC Motor Control

Answer1-Explain a MATLAB demo model named ‘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.

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.

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

1. for 0.1 second the armature current is positive and then becomes constant until 0.5 secs in the first half cycle.

2. In the second half cycle the current becomes negative and after 0.6 seconds becomes constant.

• In the above graph, IGBT current and Diode current is passed through Quadrant Q3 & D3 is shown.
• Q3 represents the IGBT current and D3 represents the Diode current.

• During 0.5 sec, D3 switch is ON (given according to the pulse generator). After 0.5 sec, D3 is OFF and IGBT current starts flowing while Q3 remains ON.

• The speed and armature current is obtained b the DC motor and Load torque is calculated by load torque proportional to the square of a speed.
• At starting, a sudden rise or shootup can be seen.
• The above graph shows that the speed is increased in certain period of time and at that time armature current is positive whereas load Torque also moves forward.

• After 0.5 sec, a sudden rise acn be seen and speed gets decreased, in that time armature current is negative terminal whereas Torque act as reverse action.
• Here a sudden rise appears while Armature current is processed at a positive and negative terminal.

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

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring, and Reverse braking.
A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy,supporting its motion. In braking mode, the machine works as a generator and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward andreverse directions, i.e., in motoring and braking operations.
The product of angular speed and torque is equal to the power developed by a motor. For the multi-quadrant operation of drives, the following conventions about the signs of torque and speed are used. When the motor is rotated in the forward
direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.
In loads involving up and down motions, the speed of the motor which causes upward motion is in forward motion. For reversible drives,forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.
The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.

• A 200HP Dc motor is seperately 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 obtained by the rectification of 380V AC source.
• A braking chopper has been used in order to limit the DC bus voltage.
• First regulator is a speed regulator followed by the current regulator.

• The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference chnges that could cause armature over current and destabilize the system.
• The curent regulatory controls the armature current.
• For proper system behavior, the instantaneous pulse valuse of IGBT devices 1 & 4 are opposite to the IGBT devices 2 & 3.
• This generates the average armture voltage needed to obtain the desired armature current. In order to limit the ampliture of the current oscillations, a smoothing inductance is placed in series with te armature circuit.

Output:-

Comparison between Four Quadrant Chopper DC Drive and BJT H- bridge Model-

A.In Four Quadrant Chopper DC Drive, Regenerative Braking is possible, which it’s not possible in BJT H-
bridge Model
B. In Four Quadrant Chopper DC Drive, there are total of 4 modes – Forward Motoring, Forward Braking,
Reverse Motoring and Reverse Braking, while In BJT H- bridge Model, there are only 2 modes –
Forward and Reverse Motoring
C. In Four Quadrant Chopper DC Drive, the direction of rotation of motor is changed according to the
input speed reference value, while in BJT H- bridge Model, the direction of rotation of motor is
changed by changing the polarity of motor
D. In Four Quadrant Chopper DC Drive, the motor is controlled by the speed reference value, while in BJT
H- bridge Model, it is controlled using the duty cycle
E. In Four Quadrant Chopper DC Drive, the overall performance and efficiency is much better compared
to BJT H- bridge Model.

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

Simulink model-

Blocks used:
Power GUI
Voltage measurement
Curent mesurement
Dc voltage source
MOSFET
Pulse generator
Diode
Series RLC band
Logical operator

SCOPE RESULT-

Results-

• The VOLTAGE waveform is always positive.
• Current is increasing and decreasing and it goes negative as well.
• The amplitude to pulse generator is given 10 v.
• When CH1 is ON supply voltage will appear at output voltage and when diode D1 is ON supply appear to output voltage is Vs, the supply voltage Vs will be appearing while CH1 ON and Diode D1 ON in output.

Hence we can conclude that voltage is always positive in the 2 quadrant operation of type A or C chopper. However current can either be positive or negative.

Answer 3- BLDC motor-

Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors). 

-Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes.

It uses an electronic closed loop 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

Construction of BLDC motor:

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

-The main design difference between a 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.

• In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy. 
• The main design difference between a 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 commutators. 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 “Out runner” style or “In-runner” 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 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 Principle of BLDC motor:

-BLDC motor works on the principle similar to that of a Brushed 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 aligns with stator poles which causes the motor to rotate.
• In brushless DC motors, an electronic servo system replaces the mechanical commutator contacts.

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 absence of brushes

Limitations:

• These motors are costly
• Electronic controller required control this motor is expensive

• Requires complex drive circuitry
• Need of additional sensors

Applications:

Brushless DC motors (BLDC) use for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc. 

• Computer hard drives and DVD/CD players

• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt driven systems
• Washing machines, compressors and dryers
• Fans, pumps and blowers.