Week-7 Challenge: DC Motor Control

1a) Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge'

H Bridge is a simple electronic circuit which enables us to apply voltage to load in either direction. It is commonly used in robotics application to control DC Motors. By using H Bridge we can run DC Motor in clockwise or anticlockwise directions. This circuit is also used to produce alternating waveforms in inverters.

Basic H Bridge

Basic H Bridge Working

When switches S1 and S4 are switched on, motor runs in clockwise direction.

When S2 and S3 are switched on, motor runs in anticlockwise direction.

MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge'

Description:-

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).

Model Description:-

1.DC motor used the present 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%.

2.The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models).

3.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.

4.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.

above the graph we can see that ,0 to 0.5sec the currrent flow through diode D3 and from 0.5sec to 1sec,the current flows through Q3.

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.

the waveform for speed,armature current and load torque in shown in graph

Armature Current :-

At starting threre is sudden spike increases in current at 0 sec and 0.5sec also

Voltage changed the circuit and motor speed also changed. In this transition of positve to negative direction we observe the high surge in current this is due to the transient switching occuring the circuit.

1B) the armature current shoot-up from the scope results

At starting threre is sudden spike increases in current at 0 sec and 0.5sec also this transition of positve to negative direction we observe the high surge in current this is due to the transient switching occuring the circuit.

as threre is a already current flowing in the circuit and on changing of switches after 0.5sec the current in the circuit adds to the changed direction of current and we abserve a surge.

the surge in the current can be reduced by reducing the duty cycle.by changing the duty cycle from 75% to 50% the surge current is reduced.

pulse generator is reduced from the duty cycle then speed,current,load torque also reduced.

1C) ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.

The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

Working Principle of Four Quadrant Chopper:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will discuss its operation separately for each quadrant.

First Quadrant Operation:

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io. When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive.

Thus, both the output voltage vs and load current io are positive and hence, the operation of chopper is in first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.

When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read Step-up chopper for detailed analysis and better understanding.

For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, vo is assumed negative. Let us now see what is the status of load current io. It may be seen that io is flowing in the direction opposite to shown in the circuit diagram and hence negative.

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper..

The operation of a four quadrant chopper or Class-E chopper is summarized in the figure below.

Comparison of Four Quadrant Chopper and BJT Bridge:-

1.The Four Quadrant Chapper works in all four models

Forward Motoring

Forward Braking

Reverse Motoring

Reverse Braking

The BJT H bridge model works on Two modes

Forward Motoring

Reverse Motoring

2) Four Quadrant Chopper have high Switching Frequency.

H-Bridge have low switching Frequency

3)The Armature Current in Four Quadrant Chopper have a little spike at transition from one to another switch because of high switching frequency while bridge model have high spike of current.

4) The Four Quadrant Chopper is Generallyused in industrial application while H bridge model is used in robotics.

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

Description:-

The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with two-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantages of this drive, compared with other DC drives, are its implementation simplicity and that it can operate in two quadrants (forward motoring and reverse regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.

Two Quadrant Chopper Model Using Simulink

let us take two mosfet blocks and diodes blocks and connect the dc voltage source be 24 volts.and let us consider on RLC Series branch blockselect inductor work as a load and dc voltage source 12 volts is also connected in series with inductor.the pulse generator block is used to create gate pulse.and the not gate is used in the model so that 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 another mosfet will get off and when other mosfet is on the first mosfet will get off. the first mosfet will operate on positive direction and second mosfet will operate on negative direction. the amplitude of pulse generator is kept 1 so that we can clearly see the pulses at scope and let period 1 sec and pulse width 75%

the current increases and decreases positive side operate on 75% and neagtive side operate on 25% waveform is positve and negative direction.

the Voltage is lightly negative direction

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

Introduction

Brushless DC Motors or BLDC Motors have become a significant contributor of the modern drive technology. Their rapid gain in popularity has seen an increasing range of applications in the fields of Consumer Appliances, Automotive Industry, Industrial Automation, Chemical and Medical, Aerospace and Instrumentation.

Even though they have been used for drives and power generation for a long time, the sub kilowatt range, which has been dominated by Brushed DC Motors, has always been a grey area. But the modern power electronics and microprocessor technology has allowed the small Brushless DC Motors to thrive, both in terms price and performance.

What is a BLDC Motor?

A Brushless DC Motor is similar to a Brushed DC Motor but as the name suggests, a BLDC doesn’t use brushes for commutation but rather they are electronically commutated. In conventional Brushed DC Motors, the brushes are used to transmit the power to the rotor as they turn in a fixed magnetic field.

As mentioned earlier, a BLDC motor used electronic commutation and thus eliminates the mechanically torn brushes.

Construction of BLDC Motor

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. Keeping that in mind, 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 Motors are available in three configurations: single phase, two phase and three phase. Out of these, the three phase BLDC is the most common one.

The following image shows the cross-section of a BLDC Motor.

Stator

The structure of the stator of a BLDC Motor is similar to that of an induction motor. It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC are slightly different than that of the traditional induction motor.

Generally, most BLDC motors consists of three stator windings that are connected in star or ‘Y’ fashion (without a neutral point). Additionally, based on the coil interconnections, the stator windings are further divided into Trapezoidal and Sinusoidal Motors.

In a trapezoidal motor, both the drive current and the back EMF are in the shape of a trapezoid (sinusoidal shape in case of sinusoidal motors). Usually, 48 V (or less) rated motors are used in automotive and robotics (hybrid cars and robotic arms).

Rotor

The rotor part of the BLDC Motor is made up of permanent magnets (usually, rare earth alloy magnets like Neodymium (Nd), Samarium Cobalt (SmCo) and alloy of Neodymium, Ferrite and Boron (NdFeB)).

Based on the application, the number of poles can vary between two and eight with North (N) and South (S) poles placed alternately. The following image shows three different arrangements of the poles. In the first case, the magnets are placed on the outer periphery of the rotor.

The second configuration is called magnetic-embedded rotor, where rectangular permanent magnets are embedded into the core of the rotor. In the third case, the magnets are inserted into the iron core of the rotor.

Position Sensors (Hall Sensors)

Since there are no brushes in a BLDC Motor, the commutation is controlled electronically. In order to rotate the motor, the windings of the stator must be energized in a sequence and the position of the rotor (i.e. the North and South poles of the rotor) must be known to precisely energize a particular set of stator windings.

A Position Sensor, which is usually a Hall Sensor (that works on the principle of Hall Effect) is generally used to detect the position of the rotor and transform it into an electrical signal. Most BLDC Motors use three Hall Sensors that are embedded into the stator to sense the rotor’s position.

The output of the Hall Sensor will be either HIGH or LOW depending on whether the North or South pole of the rotor passes near it. By combining the results from the three sensors, the exact sequence of energizing can be determined.

Working Principle

Consider the following setup of three windings in the stator designated A, B and C. For the sake of understanding, let us replace the rotor with a single magnet.

We know that when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i.e. the poles of the generated magnet will depend on the direction of the current flowing through the coil.

Using this principle, if we supply current to the coil A so that it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with A.

If we now pass current through coils B and C one after the other (in that order), the rotor magnet will rotate in clock wise direction.

To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction. Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle.

For a complete 360 degree rotation of the rotor magnet, six possible combinations of the coils A, B and C are applicable and are shown in the following timing diagram.

.

Base on the above diagram, we can confirm that at any time, one phase is positive, one phase is negative and the third phase is idle (or floating). So, based on the inputs from the Hall Sensors, we have two switch the phases as per the above diagram.

Advantages of BLDC Motors

1. No wear and tear (due to absence of brushes)
2. High efficiency
3. Better speed vs torque characteristics
4. Long life
5. Less noise or noiseless operation
6. Significantly higher RPM

Disadvantages of Brushless Motor

1. These motors are costly
2. Electronic controller required control this motor is expensive
3. Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
4. Requires complex drive circuitry
5. Need of additional sensors

Applications of Brushless DC Motors

1. Single speed applications
2. Adjustable speed applications
3. Position control
4. Low noise applications
5. High speed applications