Week-7 Challenge: DC Motor Control
1A:
BJT uses two N type semiconductor devices and two P type semiconductor devices connected together forming H bridge with motor in between to control the speed and direction of DC motor.Diodes are connected in parallel with each transistor to allow passage of current in one direction.The H bridge consists of four BJT/Diode pairs which are simulated by IGBT models.Two transistors are switched simultaneously.Q1 & Q4 form one pair while Q2 & Q3 form another pair.When the Q1- Q4 pair is fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free wheeling diodes when Q1-Q4 pair is switched off.On the other hand, when Q2-Q3pair is fired, a negative voltage is applied to the motor and diodes D1- D4 operate as free wheeling diodes when Q2-Q3 pair is switched off.
1B:
From the scope result a sudden shoot up of armature current is observed at motor start up.Speed is observed to increase in a certain period of time.During this period armature current is positive and load torque also moves forward.The time period is provided by a Pulse Generator(step time).After this time is elapsed sudden shoot up is arrested and speed gets decreased and armature current is in negative terminal while torque acts in reverse direction.Sudden shoot up occurs because armature current is processed at positive and negative terminals.
1C:Table II attached herewith shows a comparison between the Four Quadrant Chopper DC Drive and BJT H Bridge Model.
2.Two Quadrant Chopper model has been developed using Simulink vide TwoQChop file enclosed.The model uses MOSFETs,diodes,NOT gate,Voltage Measurement,Current Measurement,a series RLC element and Scopes.Here two MOSFETs have been used.They are never fired simultaneouly since in the event of simultaneous firing chances of short circuiting will be there.This has been ensured using the NOT gate.The system operates in two modes Forward Motoring and Forward Braking which are basically the first and second quadrant.The scope result shows that the output voltage is always positive while output current can be positive or negative.
3.BLDC motors commutate electronically thereby eliminating the possibility of mechanical wear & tear of brushes.They are available in three configurations: single phase,two phase and three phase.Out of these the last version is the most popular.
Two main parts of BLDC moter are : stator & rotor.Stator is made of stacked steel laminations with axially cut slots for windings.Most BLDC motors consist of three stator windings which are connected in star or 'Y' fashion without a neutral point.Based on the coil interconnection stator windings can be trapezoidal or sinusoidal type.The rotor part of BLDC is made of a Permanent Magnet(usually rare earch alloy magnets).Based on the application the number of poles can vary rom 2 to 8.When rectangular magnets are embedded into the core of the rotor we have magnetic embedded rotor.Another type uses magnets inserted into the iron core of the rotor.
In order to rotate the rotor the stator windings must be energized in a sequence and the position of the rotor N and S poles must be known to precisely energize a particular set of stator windings.A position sensor(usually a Hall sensor)that works on the principle of Hall effect is generally used to detect the position of the rotor and convert it into the Electrical signal.Most BLDC motors use three Hall sensors embedded in the stator to sense rotor's position.The output of the Hall sensor will be either high or low depending on whether N or S pole of the rotor passes near it.By combining the results from the three sensors the exact sequence of energization can be determined.
We know that when a current is applied through a coil a magnetic field is generated and the poles of the generated magnet depends on the direction of current flowing through the coil.To improve efficiency opposite coils are wound using a single coil.This doubles the force of attraction.Further two coils are energized at the same time so that one coil will attract the magnet while the other will repel it.During this period the third will be idle.For complete 360 degree rotation of the rotor magnet six possible combinations of three coils will be applicable.At any particular instant of time one phase will be positive ,one phase will be negative and the third phase will be idle or floating.So based on the input from Hall sensor phases will be switched in step up or step down (stair type) sequence.The drive circuitry of BLDC motor is known as Electronic Speed Controller.It also has a MCU with Pulse Width Modulated output,six MOSFETs for the three phase of stator windings,feedback from the Hall sensors and some Power Supply related components.The MCU can be programmed to appropriately switch the MOSFETs based on the data input from the Hall sensors.BLDC used for automotive application are usually 48V or less rated.