Week-7 Challenge: DC Motor Control

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

• H-bridge used to generate a chopped voltage and to control the speed of a DC motor

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). 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 basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground
• Current starts flowing through the motor which energizes the motor in (let’s say) the forward direction and the motor shaft starts spinning

• If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards

• In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time.
• If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply
• This condition is called ‘shoot-through’ and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit

Speed control of a DC motor using BJT H-bridge model:

• Using Pulse generator block the signal input is divided into 4 inputs(P1,P2,P3,P4) which will be sent to 4 switches to operate them
• The fan load to the DC motor is created using square law block(where torque is directly proportional to square of the speed of motor)
• The torque output is again given as a feedback to the motor for closed loop control of speed & torque supplied by the motor

Speed, Armature Current, Load torque Plot:

• From the plot we can see that the speed of the motor(w) reverses its direction(becomes negative) after 0.55 seconds
• It is because the direction of current supplied to the motor is also reversed & the load torque also reverses it direction as the speed of the motor reverses

IGBT current and Diode current plot:

• Now we observe the IGBT & diode currents for Switch Q3 & Diode D3
• When Q3 & Q2 are conducting then the direction of motor rotation is reversed
• As from the plot we can see the current flows from battery to the Load(current is positive) & when the Switches Q1 & Q4(for forward rotation of motor) are switched off the diodes D2-D3 conduct as freewheeling diodes & in the above plot the diode current can be seen as positive till 0.5 seconds the switch Q3 is OFF & Diode D3 is ON

B)

• The armature current shoot-up is due to the sudden increase in voltage during the starting period
• We have set of duty cycle to 75%. This means 75% of the 240V input (180V) will be across the armature windings
• This can be easily changed by regulating the duty cycle.

Below are the results for 50% and 35%,

Conclusion:

• To reduce the shootup, we need to control the duty cycle gradually fro 0-100%
• This will in-effect gradually build the current flowing accross the armature.

C)

DC7 Connections:

• 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

Working:

• Here Four IGBT switches are used
• Here pulses are given from Pl current controller, that controles the pulses by comparing armature current measurement and current reference
• Current reference can be generated by using regulation switch in that torque referance and PI speed controller pulses,that internally compared by speed of motor and refrance speed Mainly speed controlled by current controller
• In this case Four quadrant operations are performed
• Forward motoring mode,forward regenerative breaking mode: Q1,Q4 are on and Q2,Q3 are in off
• Reverse motoring mode,reverse regenerative breaking mode: Q2,Q3 are on and Q1,Q4 are in off

Comparisons:

• In DC7 chopper model speed controlled by varying current pulses from Pl current controller and in H-brige model speed controlled by varying duty cycles voltages
• Here regenerative breaking in occring while in H-bridge model breaking is occured by changing polarity of current flow.dynamic breaking occurs in it
• In four quadrant DC chopper model smoothing inductance is used for smooth increase and decrease in current flow which inturns controlling the motor speed smoothly
• while in H-bridge it is not present
• Here the operation is discreat and in H-bridge model it is continuous
• Hence 4 quadrant DC choppers are used mostly in electric vehicles

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 taken as 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.

• Brushless DC motors are common in industrial applications across the world
• At the most basic level, there are brushed and brushless motors and there are DC and AC motors
• Brushless DC motors,do not contain brushes and use a DC current
• These types of motors are highly efficient in producing a large amount of torque over a vast speed range
• 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
• A Brushless DC motor has only two basic parts: the rotor and the stator
• The rotor is the rotating part and has rotor magnets whereas the stator is the stationary part and contains stator windings
• In BLDC permanent magnets are attached to 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

Working Principle and Operation of BLDC Motor

1. BLDC motor works on a principle similar to that of a conventional DC motor, i.e., the Faraday's law which states that whenever a current-carrying conductor placed in a magnetic field it experiences a force
2. As a consequence of reaction force, the magnet will experience an equal and opposite force
3. In the case BLDC motor, the current-carrying conductor is stationary while the permanent magnet moves
4. When the stator coils are electrically switched by a supply source, it becomes an electromagnet and starts producing the uniform field in the air gap
5. Though the source of supply is DC, switching makes to generate an AC voltage waveform with a trapezoidal shape
6. Due to the force of interaction between the electromagnet stator and permanent magnet rotor, the rotor continues to rotate
7. Consider the figure below in which the motor stator is excited based on different switching states
8. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles
9. The permanent magnet rotor with North and South poles align with stator poles causing the motor to rotate
10. Observe that the motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment)
11. In this way motor moves in a clockwise direction

Advantages of Brushless DC Motor

The advantages of a BLDC motor are:

1. Brushless motors are more efficient as their velocity is determined by the frequency at which current is supplied, not the voltage
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency
3. BLDC motor can operate at high speed under any condition
4. There is no sparking and much less noise during operation
5. More electromagnets could be used on the stator for more precise control
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia
7. It is a high-performance motor that provides large torque per cubic inch over a vast sped range
8. BLDC motors do not have brushes which make them more reliable, high life expectancies, and maintenance-free operation
9. There are no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
10. Such motors cooled by conduction and no airflow are required for inside cooling.

Disadvantages of Brushless DC Motors

1. BLDC motor cost more than a brushed DC motor
2. The limited high power could be supplied to the BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged