Week-7 Challenge: DC Motor Control

Solution 1. :-

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

-- The above model is been exported from Matlab-Simulink Examples library.

-- We can observe, from the Simulink model that the electric circuit is arranged in the form which reassemble like letter 'H'. We can observe that four BJT(IBJT) transistor with Diode are used with DC Motor(fan load) thus completeing the circuit.

-- 'H' type of circuit operation is carried out in such way that only 2-switching element are operational whereas other 2-switching element are turned 'OFF', the switching element is operated from each side to avoide short-circuit in the circuit.

-- We can observe, that DC Motor block has 6-terminals i.e. one terminal is the Torque(TL)(input) two terminals are for fields(F+ F-) two terminals are for armature circuit(A+ A-) and the last one is m.

-- The IGBT block implements a semiconductor device controllable by the gate signal. The IGBT is simulated as a series combination of a resistor Ron, inductor Lon, and a DC voltage source Vf in series with a switch controlled by a logical signal (g > 0 or g = 0). Two terminals input & output are IBJT Current & IBJT Ampere.

-- The IGBT turns on when the collector-emitter voltage is positive and greater than Vf and a positive signal is applied at the gate input (g > 0). It turns off when the collector-emitter voltage is positive and a 0 signal is applied at the gate input (g = 0).The IGBT device is in the off state when the collector-emitter voltage is negative.

-- The diode is a semiconductor device that is controlled by its own voltage Vak and current Iak. When a diode is forward biased (Vak > 0), it starts to conduct with a small forward voltage Vf across it. It turns off when the current flow into the device becomes 0. When the diode is reverse biased (Vak < 0), it stays in the off state. Two terminals input & output are Diode Current & Diode Ampere.

-- 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.

-- We can also observe, that there are two scope blocks, one block outputs the current values for IBJT & Diode, whereas the other block outputs the value of Torque(Nm), armature(A), Speed w(rad/s).

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

-- As, per the parameter are set with duty cycle of 75%(Duty cycle 180V).

-- 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.

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

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 PI regulators, the converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source.

-- To limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

-- We can observe, when the simulation starts the IGBT 1&4, IGBT 2&3 operates very similarly the IGBT 1&4 starts to operate from 0.5 it reaches to a value of 0.8 approximately and remaining constant upto 2 seconds and from there on the value drops untill 0 the IGBT 1&4 operates in the positive quadrant,whereas the IGBT 2&3 operates in the negative quadrant i.e. the values starts from - 0.5 reaches upto 0.3 approximately and remains constant upto 2 seconds and from there falling upto -1 value.

-- The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

-- At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

-- At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

-- At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Output Results For H-Bridge Drive :

Comparsion Between Four-Quadrant Motor Drive & H-Bridge Drive

-- For, the above mentioned output results of both DC7 drive & H-bridge drive varies quite significantly.

-- The IGBT operation also varies, in both circuit...In the DC7 drive as the simulation starts IGBT 1&4 start to operates for a certain peroid of time as soon the step is provided the IGBT 2&3 start to operates thus providing a smooth operation for DC motor drive, this smooth operation is done by the use of PI controller i.e. PI Speed Regulator & PI Current Regulator, whereas in the H-Bridge drive the motor is controlled by PWM i.e. if the input signal has a value greater than the Enable threshold voltage parameter value, the H-Bridge block output is on and has a value equal to the value of the Output voltage amplitude parameter, if it has a value less than the Enable threshold voltage parameter value, the block maintains the load circuit using one of the following three Freewheeling mode options:

Via one semiconductor switch and one freewheeling diode

Via two freewheeling diodes

Via two semiconductor switches and one freewheeling diode

we can observe in the H-Bridge drive output the first few seconds the freewheel diode is under operation and onces the step is provided the IGBT & Diode starts to operate..

This may result in unsmooth operation of the drive, to provide smooth operation PWM frequency must be large enough of the thershold value...

Solution 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 )

-- Too, develop a Two-Quadrant Chopper we have imported 2-MOSEFT blocks, 2-Diode blocks which are connected in Parallel mode.

-- For, motor drive we have imported DC Machine with given specification 5HP-240V-1750RPM Field:300V.

-- For, switching between the quadrant a pulse generator is been imported and 50% duty cycle is been given, a step block is imported by step time 0.5.

-- We have also added a bus selector for output signal for Armature, Speed, MOSFET, and Diode .

-- A separate DC Voltage supply of 300V is been given for DC Machine.

-- All,the necessary connection are made and 'powergui' block is added to complete the Simulink modelling.

Output Results :-

-- The simulation is been performed for 1second and 50% duty cycle.

-- We can observe, after the 0.5 of step time there is sudden rise in the Armature Current as well as the Speed, we also observe that MOSEFT also starts to operate, a constant 300V of voltage is observed, the current flacuatation is observed acoording to the load.

Solution 3. :- Explain in a brief about operation of BLDC motor.

-- Before, knowning about the operation of BLDC motor, we will go through a short introduction of the same.

-- BLDC or Brushless DC motor are those motor which does not have brushes to be operated, rather than rotating permanent magnets.

-- 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.

-- Construction of BLDC Motor

BLDC motors are constructed in different physical configurations, which depends upon the stator windings, they are configured as single-phase, two-phase, or three-phase motors.

-- The construction of BLDC motor is similar to Induction & Brush DC motor, it has a Stator & a Rotor.

-- 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. The arrangement of winding can be 'Star' or 'Delta', each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot.

-- ROTOR: The BLDC motor incorporates permanent magnets in the rotor, the number of poles in the rotor vary from 2 to 8 poles with alternate noth or south poles according to its application. For the maximum torque to be achieved in the motor the flux density of material should be high enough to produce required magnetic flux density.

-- Some of the material used are alloys- Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB).

-- HALL SENSOR: Hall sensor provides the information to synchronize stator armature excitation with rotor position.

-- The commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor, the Hall Effect sensor embedded in stator senses the rotor position.

-- 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.

Working Principle and Operation of BLDC Motor

-- BLDC motor also works on the principle of Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force, due to which a magnet will experience an equal and opposite force.

-- In case of BLDC motor the stator is the stationery part whereas the permanent magnet on the rotor 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.

-- The 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.

-- We can 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), due to which the motor moves in a clockwise direction.

-- The 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.

Brushless DC Motor Drive

-- A simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

-- Hall-effect sensors are used for position and speed feedback, the electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

-- The switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application, the speed control unit implmented are with PID controllers to have precise control.

Advantages of BLDC Motor

-- It has high efficiency due to the use of permanent magnet rotor.

-- It can be operated at high speed even in loaded and unloaded conditions due to the absence of brushes that limits the speed.

-- It has long life as no inspection and maintenance is required for commutator system.

-- Quite operation (or low noise) due to absence of brushes.

Disadvantages of Brushless Motor

-- Electronic controller required control this motor is expensive.

-- It requries complex driver circuitry.