Week-7 Challenge: DC Motor Control

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

Answer: 'Speed control of a DC motor using BJT H-bridge.

H-bridge:In general an H-bridge is a rather simple circuit,containing four switching element,with the load at the center, in an H-like configuration. 

The basic operation of the H-bridge circuit diagram are given below.

    Motor rotates in Forward direction                              Motor rotates in reverese direction

When the positive gate signal is given the switches  Q1 and Q4 are turn-on  and  the motor rotates in forward direction.D1and D4 are in off condition. During changing of the positive to negative time period the D1 and D4 are in on condition at that time the switch Q1 and Q4 are off condition.  

whenthe negative gate signal is given the switch  Q3 and Q2 are turn-on and the motor rotates in reverese direction and D3 and D4 are in off condition.During changing of the negative to positive time period the D2 and D3 are in on condition at that time the switch Q3 and Q4 are off condition.We cannot  trigger the two switches of the same leg.It creates a low resistance path and switches gets damaged.

Simulink model:

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).

DC Motor used in the simulink model:

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.

Pulse width Modulation(PWM): Pulse width modulation(PWM) is a modulation technique that generates variable-width pulses to represent the amplitude of an analog input signal.The length of time that a pulse is in a given state (High/low)is the "width" of a pulse wave. Here the term comes the duty cycle which represents in percentage values. 

The Pulse gnerator given  by a pulse width  of 75% and period of 2e-3 seconds.The motor parameters of the torque and field voltage and armature voltages are taken as default values of the model.

Click on the run icon. the output waveforms of the diode current,IGBT current from the current scopes as shown below:

 From the waveform i had notice that the o to 0.5 time the the IGBT current is zero and the diode current is on   after the 0.5 to 1 rime the IGBT starts the current flow the diode current is zero. 

 Now we will take the output waveform of the load torque, armature current and speed.

 From the above graph the speed is starts from zero and the  reaches up to the maximum speed of (1000rpm) and decreases after 0.5 time and reaches to the (-1000rpm).

The armarture current is starts from o and increasesup to the (approx 45) and decreases after immediately and reaches to the nearer to the zero value.when the speed decreases the armature current is also starts flow in the reverese direction.when the speed comes to the nearer value of (-1000 rpm) the armture curent is flow in neagative direction nearer to the zero value.At the starting of the armature current  i had noticed a spike in the current flow and also in the negative direction of the current flow.

The load torque follows the speed waveform in the positive direction.when the negative direction it follows the armature current current and the speed waveform.

By doing these process the speed control of DC motor using BJT H-bridge model  and the output waveforms are obtained.

 1B. Comment on the armature current shoot-up from the scope results.

From the above the armature current shoot-up at the starting of the armature current  a sudden rise  in the waveform we had noticed .To reduce the sudden rise  in the armature current we will the reduce the duty cycle of the pulse width modulation from (75% to 50%).

The output of the waveform are shown below.

The difference between the above two graph armature current in the second graph we observe that there is no sudden rise at the starting of the armature current. If we further reduced the pulse width modulation the negative of the rise will get reduced.

 From the above graph it is clear that if we reduced the duty cycle ratio than we obtain a smooth variation in the output graph. The above graph PWM is 40%. and the motor details are default of the model.The torque and speed also varies the smoothly.

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

Chopper: Chopper is a static device(switch) used to obtain the variavle dc voltage from a source of constant dc voltage. The output voltage is less than the input voltage than  the chopper is called step-down chopper. If the output voltage is greater than the input voltage than the chopper is called step-up chopper.

Chopper offers greater efficiency ,faster response,lower maintainence,small size,smooth control. choppers are widely used in trolley-cars,battery-operated vehicles,traction-motor control.There are different types of the choppers are available according to the direction of output voltage and current.

Class A Type A: First quadrant chopper: The output voltage and current both must be positive.

Class B type B: The output of the voltage is positive and the current is negative.

Class C /class D:Two quadrant chopper: The output voltage is positive and current can be positive or negative( class-C) or the output current is positive and the voltage can be positive or negative(class-D).

Class-E,Four quadrant chopper: The output voltage and current both can be positive or negative.

Type E or the fourth quadrant chopper consists of four semiconductor switches and four diodes arranged in antiparallel.  The 4 choppers are numbered according to which quadrant they belong. Their operation will be in each quadrant and the corresponding chopper only be active in  its quadrant.

• First Quadrant

During the first quadrant operation the chopper CH4 will be on . Chopper CH3 will be off and CH1 will be operated.  AS the CH1 and CH4 is on the load voltage v₀ will be equal to the source voltage V_s  and the load current i₀  will begin to flow . v₀ and i₀ will be positive as the first quadrant operation is taking place. As soon as the chopper CH1 is turned off, the positive current freewheels through CH4  and the diode D2 .  The type E chopper acts as a step- down chopper in the first quadrant.

• Second Quadrant

In this case the chopper CH2 will be operational and the other three are kept off. As  CH2 is on  negative current will starts flowing through the inductor L . CH2 ,E and D4.  Energy is stored in the inductor L as the chopper CH2 is on. When CH2 is off the current will be fed back to the source through the diodes D1 and D4. Here (E+L.di/dt) will be more than the source voltage V_s  . In second quadrant the chopper will act as a step-up chopper as the power is fed back from load to source.

• Third Quadrant

In third quadrant operation CH1 will be kept off , CH2 will be  on and CH3 is operated. For this quadrant working the polarity of the load should be reversed. As the chopper CH3 is on, the load gets connected to the source  V_s and  v₀ and i₀ will be negative  and the third quadrant operation will takes place. This chopper acts as a step-down chopper.

• Fourth Quadrant

CH4 will be operated and CH1, CH2 and CH3 will be off. When the chopper CH4 is turned on positive current starts to flow through CH4, D2 ,E and the inductor L will store energy. As the CH4 is turned off the current is feedback to the source through the diodes D2 and D3 , the operation will be in fourth quadrant as the load voltage is negative but the load current is positive. The chopper acts as a step up chopper as the power is fed back from load to source.

The four quadrant chopper waveforms are shown below:

The simulink model of the DC7 block.

The detail description of the dc block . 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.

Comparision of Four quadrant chopper and 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 )

A 2-quadrant chopper :

   In the 2-quadrant chopper are two types i) Class-c and ii) Class-D.

This is class -c Chopper the output of the voltage is positive and the output of the current is positive and negative.Type C chopper is obtained by connecting type –A  and type –B choppers in parallel.  We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load.  When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s   . The direction of the load current i₀ will be reversed. The current i₀  will be flowing towards the source  and   it will be positive regardless the chopper is on or  the FD conducts.  The load current will be negative if the chopper is or the diode D2 conducts.  We can say the chopper and FD  operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation.  The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

For the class-d chopper the output current is postive and the output voltage is positive and negative.

The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage  v₀ will be equal to V_s  . When v₀ = – V_s  the two choppers will be off but both the diodes D1 and D2 will start conducting. V₀ the average output voltage will be positive when the choppers turn-on  the time T_on will be more than the turn off time T_off  its shown in the wave form below. As the diodes and choppers conduct current only in one direction the direction of load current will be always positive.

The waveforms of the positive and negative operations are given below:

Simulink model of the class-D type chopper:

1.Click on the matlab .Click on the simulink model and click on the library browser.

2.Select the blocks of the Mosfet, diode,scope,inductor(load), pulse generator, dc supply ,voltage and current         measurement.

3. The blocks of the simulink model can be arranged in a passion that the class -D type chopper as shown below

4.After assembly of the block.now we will assign the values of the pulse generator  and the DC supply voltage 24V

Now we will run the model for 1 sec of the sample.The output waveform of the current and voltage as shown below.For the load inductance value i had taken here is 5e-3.

 The output of the current waveform is in positive direction and the voltage waveform is in postive and the negative half cycle.

This the two quadrant operation of a chopper type -D the current is positive and voltage can be positive and negative.

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

A. A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor). A BLDC motor is also known as the permanent magnet synchronus motor(PMSM).There are two types of BLDC motors are present i) 3-phase trapeziodal BLDC motor.ii) Sinusidal type BLDC motor.Most of the BLDC motors are 3-phase type.

As shown in the above figure the brushless DC motor has only two basic parts: rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator. The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.

Construction of Brushless DC 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.