Week-7 Challenge: DC Motor Control

1. A. Explain a 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). 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.

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

Simulation

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 240V). 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.

OUTPUT OF IGBT AND DIODE CUURENT :

OUTPUT WITH DUTY CYCLE 75% :

OUTPUT OF IGBT AND DIODE CUURENT :

OUTPUT WITH DUTY CYCLE 50% :

The motor starts in the positive direction with a duty cycle of 50% (mean DC voltage of 240V). 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.

• From 1st plot we can see that the current plot has initially very high value and then becomes steady and after that shows same path in the - ve direction.
• In these two plots the value of this current shoot up is decreasing and the 2d plot has almost smooth zero current flow.
• To get steadier and smoother current floe we need to decrease and control the duty cycle.
• This shoot up happens because of the switching in the circuit that cause heat losses during switching. To avoid these losses, we use HEAT SINK material to dissipate the heat. From this the motor can change its direction of rotation easily and very smoothly.

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

• 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 Pl 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 pu) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed,

• The speed reference change rate fallows 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 puiso values of IGBT devices I 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.

OUTPUT :

Compare it with the H-bridge and ‘The Four-Quadrant Chopper DC Drive (DC7)

Two Quadrant Chopper

In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF 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. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

For discussing all quadrant operation of DC motor, we go for Type E or the four-quadrant chopper consisting 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 is active in its quadrant.

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 )

CHOPPER :

• In electronics, a chopper circuit is any of numerous types of electronic switching devices and circuits used in power control and signal applications.
• A chopper is a device that converts fixed DC input to a variable DC output voltage directly. Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another.
• In power electronics applications, since the switching element is either fully on or fully off, its losses are low and the circuit can provide high efficiency.
• However, the current supplied to the load is discontinuous and may require smoothing or a high switching frequency to avoid undesirable effects.
• In signal processing circuits, use of a chopper stabilizes a system against drift of electronic components; the original signal can be recovered after amplification or other processing by a synchronous demodulator that essentially un-does the "chopping" proces.

TYPES OF CHOPPER :

1 . FIRST QUADRANT OR TYPE A CHOPPER

2 . SECOND QUADRANT OR TYPE B CHOPPER

3 . THIRD QUADRANT

• TWO QUADRANT TYPE-A CHOPPER OR TYPE C CHOOPER
• TWO QUADRANT TYPE-B CHOPPER OR TYPE D CHOPPER

4 . FOUR QUADRANT OR TYPE E CHOPPER

2 . SECOND QUADRANT OR TYPE B CHOPPER :

• In type B or second quadrant chopper the load must always contain a dc source E .
• When the chopper is on, v₀ is zero but the load voltage E drives the current through the inductor L and the chopper, L stores the energy during the time T_on of the chopper .
• When the chopper is off , v₀ =( E+ L . di/dt ) will be more than the source voltage V_s .
• Because of this the diode D2 will be forward biased and begins conducting and hence the power starts flowing to the source.
• No matter the chopper is on or off the current I₀ will be flowing out of the load and is treated negative .
• Since V_O is positive and the current I₀ is negative , the direction of power flow will be from load to source.
• The load voltage V₀ = (E+L .di/dt ) will be more than the voltage V_s so the type B chopper is also known as a step up chopper .

3 . TWO QUADRANT TYPE-A CHOPPER OR TYPE C CHOOPER :

• 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.

4 . TWO QUADRANT TYPE-B CHOPPER OR TYPE D CHOPPER :

• The circuit diagram of the type D chopper is shown in the above figure below.
• 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 power flows from source to load as the average values of both v₀ and i₀ is positive.
• From the wave form it is seen that the average value of V₀ is positive thus the forth quadrant operation of type D chopper is obtained.

From the wave forms the Average value of output voltage is given by

V0= (V_s T_on-V_sT_off)/T = V_s.(T_on-T_off)/T

TWO QUADRANT TYPE-A CHOPPER OR TYPE C CHOOPER :

simulation blockes :

• pulse generatoe
• MOSFET
• NOT GATE
• DC voltage sorce
• Diode
• Current measurement
• Voltage measurement
• series RLC branch
• scope

simulation model :

EXPLANETION :

• First open the matlab and click on the simulink then we get the simulink blank model .
• Next go the library and search the required blocks i.e, pulse generator, MOSFET, NOT gate, DC voltage source , Diode , Current measurement, Voltage measurement, series RLC branch, Scope .
• Now connect the circuit as per circuit shown in above .
• Let the voltage of dc voltage source is 24 volts.
• Let us consider an series rlc branch and select inductor L. thus block works as a load.
• To consider LE load i.e, motor load, and dc voltage source of 12 volts is also connected in series with inductor.
• The pulse generator block is used to create gate pulse .

• the not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur due to use of not gate whan one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off

• The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse width be 50% .

• output :

• 
• the voltage waveform is not going less than zero. the minimum value of voltage is zero and maximum value is always in the positive direction.
• however in current waveform, the current increases and decreases.
• the current may be positive as well as negative. and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10,

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

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, as you may imagine, do not contain brushes and use a DC current.

It often helps to explain how a brushed DC motor works first, as they were used for some time before brushless DC motors were available. A brushed DC motor has permanent magnets on the outside of its structure, with a spinning armature on the inside. The permanent magnets, which are stationary on the outside, are called the stator. The armature, which rotates and contains an electromagnet, is called the rotor.

In a brushed DC motor, the rotor spins 180-degrees when an electric current is run to the armature. To go any further, the poles of the electromagnet must flip. The brushes, as the rotor spins, make contact with the stator, flipping the magnetic field and allowing the rotor to spin a full 360-degrees.

A brushless DC motor is essentially flipped inside out, eliminating the need for brushes to flip the electromagnetic field. In brushless DC motors, the permanent magnets are on the rotor, and the electromagnets are on the stator. A computer then charges the electromagnets in the stator to rotate the rotor a full 360-degrees.

WORKING OF BLDC MOTOR :

he underlying principles for the working of a BLDC motor are the same as for a brushed DC motor; Le, internal shaft position feedback In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor

If the direction of the magnetic field is reversed, the voltage developed will reverse as well For Hall effect sensors used in BLDC motors whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft

In a commutation system-one that is based on the position of the motor identified using feedback sensors-two of the three electrical windings are energized at a time as shown in figure 4

in figure 4 (A), the GREEN winding labeled (001 is energized as the NORTH pale and the BLUE winding labeled as '010" is energized as the SOUTH pole Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pale aligns with the RED winding labeled 100 in order to move the rotor, the RED and BLUE windings are driergized in the direction shown in figure 4(e) This causes the RED winding to become the NORTH pole and the BLUE winding to became the SOUTH pole. This shifting of the magnetic leid in the stator produces torque because of the deveopment of repulsion (Red winding NORTH NORTH alignment) and attraction forces (BLUE winding NORTH-SOUTH alignment) which moves the rotor in the clockwise direction.

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating. Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation, Lo, 6 unique combinations in which motor windings will be energized.

ADVANTAGES OF BLDC MOTOR :

• 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

DISADVANTAGES OF BLDC MOTOR :

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

APPLICATION OF BLDC MOTOR :

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

CONCLUSION:

1) MATLAB demo model named Speed control of a DC motor using BJTH bridge is studied and the the armature current shoot-up is studiod and The Four Quadrant Chopper DC Drive (DC7) block is also studied and Compare it with the H-bridge model.

2) 2 quadrant chopper using simulink model is made and the Various results obatined is studied

3)operation of BLDC motor is studied.