Week-7 Challenge: DC Motor Control
Objective-1(part-1):- Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
Explaination:- MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’ we can find in HELP option of MATLAB.
Model-
here actually the semiconductor switch is use is IGBT which is combination of MOSFET & BJT. the input feature of IGBT is of MOSFET & output is of BJT. IGBT & MOSFET both are voltage control devices but BJT is current controlled device.
so the elements used in this models are-
1) 4 nos. of IGBT- Implements an IGBT device in parallel with a series RC snubber circuit.In on-state the IGBT model has internal resistance (Ron) and inductance (Lon).For most applications, Lon should be set to zero.In off-state the IGBT model has infinite impedance.
2) 2 nos. of ideal DC voltage source- 240v is use for armature winding of DC motor & 300v is use for field winding to generate magnetic field.
3) DC machine( DC motor)- Implements a (wound-field or permanent magnet) DC machine.For the wound-field DC machine, access is provided to the field connections so that the machine can be used as a separately excited, shunt-connected or a series-connected DC machine.
4) 4 nos. of diode- Implements a diode in parallel with a series RC snubber circuit.In on-state the Diode model has an internal resistance (Ron) and inductance (Lon).For most applications the internal inductance should be set to zero.The Diode impedance is infinite in off-state mode
5) pulse generator- this is use to trigger IGBT to ON.
6) step block- this is use to control armature voltage of motor.
7)2 nos. of scope- one is use of showing IGBT current & diode current. other is for showing motor rpm, armature current & load torque.
8)3 nos. of bus selector block- This block accepts a bus as input which can be created from a Bus Creator, Bus Selector or a block that defines its output using a bus object. The left listbox shows the elements in the input bus. Use the Select button to select the output elements. The right listbox shows the selections. Use the Up, Down, or Remove button to reorder the selections. Check 'Output as virtual bus' to output a single bus.
9)2nos. of MUX- its multiplexer block which has no. of input & 1 output.
10) DEMUX- demultiplexer block which has 1 input & no of outputs.
11) switch block- Pass through input 1 when input 2 satisfies the selected criterion; otherwise, pass through input 3. The inputs are numbered top to bottom (or left to right). The first and third input ports are data ports, and the second input port is the control port. The criteria for control port 2 are u2 >= Threshold, u2 > Threshold or u2 ~= 0.
12) powergui block- this block is use to give continuous power supply to the circuit.
working- 240v dc is use for DC motor armature winding, 300v dc is use for field winding. as shown in model the 240v is not directly given to armature , its first connected to all four IGBT which is anti parallel with free wheeling diode. the switching of IGBT will control the motion & speed of DCmotor.
when swicth Q3 & Q2 will get on/closed the supply voltage will pass through Q3 - motor armature- Q2- -ve, we consider as forward motoring, the motor will start running till the switches are on , when Q2&3 gets off/open, motor stops & the inductive voltage spike is reduce by free wheel diode D3 & D2 , which can cause damage to switch.
when swicth Q1 & Q4 will get on/closed the supply voltage will pass through Q1 - motor armature- Q4- -ve, we consider as reverse motoring, the motor will start running in reverse direction till the switches are on , when Q1&4 gets off/open, motor stops & the inductive voltage spike is reduce by free wheel diode D1 & D4 .
the on/off of IGBT is contolled by gate voltage, when gate voltage is given the switch become on & supply will pass through collector to emitter. to control the gate voltage a triggering ckt is made in this model.
here we can see pulse generator, mux, demux, switch block, step block, constant & go to block.
pulse generator is use to give input signals to MUX, mux has 4 input signal due to 4 nos of IGBT , the input sequence is Q1,2,3,4 from top to bottom of MUX. for both mux, sequence is same. as we can see in above ckt MUX1 has input signal on Q3 & Q2 , MUX2 has input signal on Q1 & Q4, output is given to switch block then it will compare the input switching criteria with the armature voltage control . if the criteria is fullfill the switch will operate input port1 signal & the DEMUX will give the signal to p3 & p2. then motor will start in forward direction. if the criteria in switch block is not fullfil then input port2 will get select & DEMUX will give signal to p1& p4, motor will start in reverse direction.
we can see two scope blocks, one scope shows IGBT current & diode current, the measuring signal from IGBT & diode is given to bus selector block then to scope block. 2nd scope shows armature current, rpm & torque. the speed signal is taken from motor block m port which is in rad/sec, it is converted to rpm with a gain block as shown in model. the armature current feedback is also taken from m port of motor block.
Load Torque proportional to square of speed (Fan type) T=k*w^2
the above ckt shows the calculation of torque, we take input from speed in rad/sec then square it , then multiply it with the output of signum to get torque, signum block is use if speed is in negative then torque will be in -ve & if speed is in +ve then T is +ve. this torque value is given to scope & motor input fan load.
most important, a powergui block should be added to give continuous supply without is the model will not run.
in this way the model works.
the motor data is given 5hp, 240v, 1750rpm, field voltage 300v.
LINK- https://drive.google.com/file/d/1n28-mUNdTQ_u6lbxV1mQP0LEquNxuAND/view?usp=sharing
now lets run the simulation for 10 sec. we will get result like this
the above graph show rpm, armature current & load torque.
this graph show IGBT current & DIODE current.
if we run the simulation for 1 sec we can see result much clearly
Objective-1(part-2):- Comment on the armature current shoot-up from the scope results.
Answer-
as we can see from above, the armature current got shoot upto 40 amps at 0.02sec while starting because at that time back emf is 0. after 160mSEC it become constant nearly 10 amps, rpm & load torque also become constant at the same time for 0.5sec . then it suddenly goes to negative value because of transient switching, the IGBT gets off motor does not get braking, emf induce get zero & due to back emf induce -ve values get generated.
if we change the pulse width of pulse generator from 75 to 50% the armature current shoot up got reduce to constant value of 10 amps.
Objective-1(part-3):- Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Answer- search four quadrant chopper in help section of matlab, it look like this
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.
in 4 quadrant chopper, 4 switches & 4 diodes are use. this is also known as class E chopper. it works on 4 quatrant as shown above. the following switches are operated according to quadrant.
quadrant-1 : in this quadrant s1 & s4 are operated, the operation is known as forward motoring.
quadrant-2 : this qudrant operation is known as forward regenerative braking. where s2&s3 are off,
quadrant-3 : this qudrant operation is known as reverse motoring. where s2 & s3 are triggered.
quadrant-4 : in this quadrant s1 & s4 are kept off, the operation is known as reverse regenerative braking.
comparison-
4 quadrant chopper(DC7)-* with this motor can achive 4 operation forward motoring, reverse motoring, forward braking, reverse braking.
* regenerative braking can be done.
* signal are continuous & discrete.
* this ckt gives better effeciancy than h bridge.
* its speed controlled is varied by current pulses from pi current controller.
BJT H- bridge drive- * motor can only run in forward & reverse direction.
* regenerative braking is cannot be done.
* signals are continuous.
* speed is controlled by varrying duty cycle voltage.
Objective-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 )
Answer:- This two quadrant chopper 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 Vs . 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.
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.
model-
element use in this model- pulse generator, igbt, dc voltage source, logical operator, diode, series RLC branch, current measurement
, voltage measurement, scope, powergui.
arrange the elements as shown in the model, pulse generator is use to give gate signal to igbt, logical operator not gate is use to not operate both igbt at the same time otherwise short ckt will occur, when igbt1 & diode is on load current is +ve. when igbt & diode1 is on load current is -ve.
this chopper can be use for both speed control & regenerative braking.
here we have given the above parameters to pulse generator, supply voltage is take 240vdc, & back emf voltage 300vdc. then run the simulation for 10 sec.
LINK- https://drive.google.com/file/d/1t9ACujp_4wsNxpOywWxXY-qOUi9MfPQh/view?usp=sharing
result -
Objective-2:- Explain in a brief about operation of BLDC motor.
Answer- A Brushless DC Motor, BLDC accomplishes commutation electronically using rotor position feedback to determine when to switch the current. The BLDC motor is electrically commutated by power switches instead of brushes. The structure of Brushless DC Motor, BLDC is shown in figure below.
a BLDC has no brushes and commutator for having unidirectional torque rather integrated inverter / switching circuit is used to achieve unidirectional torque. That is why these motors are, sometimes, also referred as Electronically Commutated Motors.
Construction of a BLDC Motor:
There are three types of the BLDC motor:
- Single-phase
- Two-phase
- Three-phase.
Stator for each type has the same number of windings. The single-phase and three-phase motors are the most widely used. The simplified cross section of a single-phase and a three-phase BLDC motor is shown in figure below. The rotor has permanent magnets to form two magnetic pole pairs, and surrounds the stator, which has the windings.
Rotor:
A rotor consists of a shaft and a hub with permanent magnets arranged to form between two to eight pole pairs that alternate between north and south poles. Figure below shows cross sections of three kinds of magnets arrangements in a rotor.
A Brushless DC Motor Working
BLDC Motor operation is based on the attraction or repulsion between magnetic poles. Using the three-phase motor as shown in figure below, the process starts when current flows through one of the three stator windings and generates a magnetic pole that attracts the closest permanent magnet of opposite pole.
The rotor will move if the current shifts to an adjacent winding. Sequentially charging each winding will cause the rotor to follow in a rotating field. The torque in this example depends on the current amplitude and the number of turns on the stator windings, the strength and the size of the permanent magnets, the air gap between the rotor and the windings, and the length of the rotating arm.
Advantage of BLDC Motor:
Compared with a brushed DC motor or an induction motor, the BLDC motor has many advantages:
- Higher efficiency and reliability
- Lower acoustic noise
- Smaller and lighter
- Greater dynamic response
- Better speed versus torque characteristics
- Higher speed range
- Longer life