Week-7 Challenge: DC Motor Control

Q1.Ans

A.ANS

The MATLAB demo model "Speed control of DC motor using BJT H-bridge'' is as shown below;

>This Model shows the simulation of the H-Bridge used to generate a chooped volage and to controll the speed of a DC Motor.

>Model Discription

• 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 Free-wheeling Diodes are used to protect IGBT's from reverse bias inductive surges voltage due to motor field decay which results when transistor turn off.

>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.

>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 generator is set as default i.e 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.

B.ANS

>The output plot of the model are as shown below;

i.IGBT current and Diode Current Plot.

ii.Speed(w) in rpm, Armature current in Ampere and Torque in N-m plot

>As shown in output there is a peak overshoot (it is the largest error between reference input and the output during the transient period) in the armature current.

>Thise peak overshoot can increased and decreased with increase and decreased in % Duty Cycle this is due to the fact that :

• The larger pulse width produce higher resultant voltage.
• The smaller pulse width produce lower resultant voltage.

>Actually during transient period the voltage applied to the motor increases gradually( either +ve or -ve direction) and reached it peak value but it required some time to compared the reference voltage input and the actual volage output. When both the data are availabe the difference in both is going to be the error signal and the control system applied, corrected this error and bring the voltage at required voltage lavel.

>Due to this there is an armature current shoot up in the scope results during both the transient period.

>Note:When we increase or decrease the % Duty cycle the armature current also Increased and decreased due to which Speed is also going to Varry.

output with 50%duty cycle is shown below;

Output with 95% duty cycle is shown below;

C.ANS

The 4-Quadrant chooper DC Drive (DC7) or DC7 - Four-Quadrant Chopper 200 HP DC Drive simulink model is as shown below;

Model Discription:

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 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

>>About Chopper

• Chopper is a static device (switch) used to obtain the variavle dc voltage from a source of constant dc voltage.
• If he 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 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.

The Figure below shows Class E Chopper ckt;

Working

First quadrant

• When Chopper CH1 and Chopper CH2 are switched on, the load current flows through (+) V_dc– A – CH1 – LOAD – G – CH2 – K – V_dc ( – ).
• The output voltage and current both are positive therefore it is first quadrant operation.

Second quadrant

• When Chopper CH3 and chopper CH4 are switched off, the stored energy of inductor dissipates through G – LOAD – F –D1 – (+) V_dc – V_dc ( – ) – L – D2 – H – G.
• The output voltage is positive but output current is negative during that period therefore it is second quadrant operation.

Third quadrant

• When Chopper CH3 and Chopper CH4 are switched on, the load current flows through (+) V_dc – C – CH3 – G – LOAD – F – E – CH4 – V_dc ( – ).
• The output voltage and current both are negative therefore it is third quadrant operation.

Fourth quadrant

• When Chopper CH1 and Chopper CH2 are switched off, the stored energy of inductor dissipates through F – LOAD – G – H – D3 – D –(+)V_dc – V_dc( – ) – D4 – F.
• The output voltage is negative but output current is positive during this period therefore it is fourth quadrant Chopper operation.

• The DC supply is short circuited if chopper CH1 and CH4 or chopper CH2 and CH3 is simultaneously switched on.
• When the Chopper works in first and third quadrant, the output voltage is always less than the input voltage resulting it is step down type chopper.
• Similarly when the Chopper works in second and fourth quadrant, the output voltage is always greater than the input voltage resulting it is step up chopper.

>The Comparision are as below:

>>4-Quadrant DC Chopper DC drive >>H-Bridge DC drive

>The motor can be operated in all the four different modes(quadrant). >Only Two modes of Operation is Possible ( Forward motoring and Reverse Motoring).

(forward motoring, reverse motoring , forward braking and reverse braking)

>Regenerative braking is Possible. >Regenerative braking is may be Possible.

>AC to DC converter is employed in the Model. >DC supply is directly connected to H-Bridge

>The four-quadrant chopper widely used in the automobile application for the control >The H-bridge is used in the robotics and electro-mechanical devices to control the speed and

of DC motor speed and direction. direction of the DC motor at low level speed.

>In DC7 chopper model speed controlled by varying current pulses from Pl current controller. >In H-brige model speed controlled by varying duty cycles voltages.

>The armature current in 4 quadrant chopper have a little spike at transition period because of >H-bridge model have high spike of current.

high swithing frequency

Q2ANS

• The 2-quadrant chopper are two types i) Class-c and ii) Class-D.
• The figure Below shows Type C chopper
• 

• 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 i.e 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.

• The figure below shows Type-D chopper.
• 
• For the class-d chopper the output current is postive and the output voltage is positive and negative.
• 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 Type-C Chopper is Build in Simulink as shown Below:-

• The Connection is done as per the ckt diagram of Type C chopper.
• MOSFET are choosen as CHopper(i.e Switching Device)
• The Pulse Generator is used for to supply a Gate signal to the MOSFET.
• The Not logical operator is introduced to avoid the short ckt condition of two mosfet with Voltage Source.

The output is as shown below:-

• From the Waveform ( the upper is current and lower is voltage) we observe that the current is +ve and -ve both.
• And the voltage is only +ve.
• Hence it is satisfying the Type-C chopper Operation.

Q3.ANS

Brushless DC motor (BLDC)

• A BLDC Motor is a permanent magnet synchronous elctronic motor which is driven by direct current electricity and it accomplishes electrically controlled commutation system instead of a mechanically commutation system.
• BLDC Motors are aslo reffered as trapezoidal permanent magnet Motors.
• Unlike convectional brushed type DC motor, BLDC motor employs electrical commutation with permanent magnet rotor ans a stator with a sequence of coils.
• In this Motor Permanent magnet rotates and current carrying conductor are fixed in stator slots.
• The Armature coils are switched electronically by transistor or silicon controlled rectifier (i.e power electronic switchig devices) at 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 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.

Construction of BLDC motor

• 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.
• 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 phase star connected stator.

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

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.
• 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.

>Hall Sensors:

• Hall sensor provides the information to synchronize stator armature excitation with rotor position.
• Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary.
• So the Hall Effect sensor embedded in stator senses the rotor position.
• Most BLDC motors incorporate three Hall sensors which are embedded into the stator.
• Each sensor generates Low and High signals whenever the rotor poles pass near to it.
• The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

Working Principle and Operation of BLDC Motor:

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force.
• As a consequence of reaction force, the magnet will experience an equal and opposite force.
• In case of BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.
• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap.
• Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape.
• Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
• Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles.
• The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.
• Observe that 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). By this way motor moves in a clockwise direction.

• The motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.
• Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Advantages:-

• It has no mechanical commutator and associated problems.
• High efficiency 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.
• Quite operation (or low noise) due to absence of brushes.

Disadvantages:-

• Electronic controller required control this motor is expensive.
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors.
• Requires complex drive circuitry.
• Need of additional sensors.

Application:-

Brushless DC Motors (BLDC) are used 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. Some specific applications of BLDC motors are

• 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.