Week-7 Challenge: DC Motor Control

EV BATCH - 17

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

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

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

1A)

DC MOTOR USING BJT H-bridge :

• The matlab demo model of 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 IV). Therefore, the IGBT block can be used to model the BJT device .

• The IGBT block does not simulate the gate current controlling the BUT 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 02 and 03. When Ol and 04 are fired a positive voltage is applied to the motor and diodes D2 D3 operate as free-wheeling diodes when Q and Q4 are switched off. When Q2 and 03 are fired, a negative voltage is applied to the motor and diodes DI-D4 operate as free wheeling diodes when Q2 and 03 are switched off.

SIMULATION MODEL :

• Initially, we run MATLAB demo Speed control of a DC motor using BJT H bridge with all it's default values and different pulse width(duty cycle) 75%, 55%, and 45% .
• From this we get following three plot results.

OUTPUT OF IGBT AND DIODE CUURENT :

OUTPUT WITH DUTY CYCLE 75% :

OUTPUT WITH DUTY CYCLE 55% :

OUTPUT WITH DUTY CYCLE 45% :

1B)

• 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 three plots the value of this current shoot up is decreasing and the 3d plot has almost smooth 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.

1C) The Four-Quadrant Chopper DC Drive (DC7) block :

• 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.

SIMULATION MODEL :

OUTPUT :

COMPARISSION :

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2 . 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 )

ANS :

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

1 . FIRST QUADRANT OR TYPE A CHOPPER :

• This type of chopper is shown in the figure below.
• It is known as first-quadrant chopper or type A chopper.
• When the chopper is on, v_{0 =} V_S as a result and the current flows in the direction of the load.
• But when the chopper is off v₀ is zero but I₀ continues to flow in the same direction through the freewheeling diode FD, thus average value of voltage and current say V₀ and I₀ will be always positive as shown in the graph.
• In type A chopper the power flow will be always from source to the load. As the average voltage V₀ is less than the dc input voltage V_{s .}

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

5 . FOUR QUADRANT OR TYPE E CHOPPER :

• 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.

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,

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

ANS :

BLDC MOTOR :

Brushless DC electric motors also known as electronically commutated motors A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system BLDC motors are also referred as trapezoidal permanent magnet motors

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils in this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed .

The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles, Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current

This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges

In this motor, the permanent magnets attach to the rotor The current carrying conductors or armature winding 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

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.

The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor This motor has stator and rotor parts as like all other motors

The layout of a DC brushless motor can vary depending on whether it is in "Out runner" style or "Inrunner" 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.

stator

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phose star connected stator

Each winding is constructed with numerous interconnected colls, where one or more coils are placed in each slat in order to form qo even number of poles, each of these windings is distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors aro proferred.

ROTOR :

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.

ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

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.

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