Week-7 Challenge: DC Motor Control

AIM: Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge', Comment on the armature current shoot-up from the scope results.

BJT: A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching. two semiconductor types, n-type and p-type, which are regions in a single crystal of material.

H-Bridge Model:

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(Forward)or Q2 and Q3(Reverse).

• When Q1 and Q4 are fired, a positive voltage is applied to the motor which is forward Motoring mode 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 which is Reverse Motoring Mode and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

Procedure:

• Open the MATLAB and In the Help documentation type Speed Control of a DC motor using BJT –H Bridge Results tab opens, and then select the model and click Open Model.
• The model is opened in a Simulink workspace.
• Run the model as it is and observe the graphs of the armature current.
• Now, at the Right part of the model, there’s a Pulse generator block, click on it and run the model.
• Check the outputs. Follow the same for 75% and 50% just to check if the armature voltage is decreased sharply after half of the simulation time.

Scope result: Here is the result of operation of H-Bridge, which shows Speed, Armature Current & Load torque.

for Duty Cycle: For 500 Hz

For Duty Cycle of 50% Results Are-

As, power of after 50% of supply, current goes nevative suddenly and then comes to zero due to motoring inertia & Inductance of motor. We can conclude by duty cycle decreament (from 75-50%) Armature current is can be reduce.

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

Simulink Model:

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, 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

Simulation:

• Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).
• Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.
• The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
• 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.

Result:

H-bridge model.	4Q-Chopper DC Drive
Stability of the voltage and current supply is lower compared to 4-Quadrant.	Stability of voltage and current is higher as 4-Quadrant is used.
Regenerative braking is not possible in BJT.	Regenerative braking is possible in four quadrant.
BJT is used in robotics and electro-mechanical devices to control DC motor speed and direction.	It is widely used in automobile sector to control DC motor speed and direction in Four quadrant.
The current values are continuous in BJT.	The current values are chopped in Four Quadrants so that simulation takes less time.
To change the direction of the motor, in BJT the polarity of the current needs to be changed.	To change the direction of the motor, current signals are interrupted in four quadrants.
The BJT can movie only is two directions.	The main advantage of Four quadrant is that it operates in all the four quadrants.
BJT that operates at moderate power levels efficiency.	Four Quadrants provides better efficiency and operates at high power levels.

AIM: Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Two Quadrant operation: In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load.

Steps:

• When the chopper CH1 is on the current flows from source to load.
• The inductor stores energy.
• When Ch1 turn OFF 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.
• The load current will be negative when the CH2 is on. The current completes its path through diode D2.
• Both Choppers should not be turn on otherwise short circuited.

Simulink Model:

Block Used:

Mosfet, Diode, inductance, DC Voltage source, current & Voltage sensors, Pulse Gnerator, logical operator(NOT), powergui.

Scope Result:

For First Quadrant either mosfet 1 or D2 should be turn on and for second Quadrant mosfet 2 or D1 should be turn on. Pulse generator used for triggering the mosfet.

Pulse provided 5amplitude with sample rate of 0.1 and cycle of 50%, keeping all parameter as default. As Two-Quadrant condition here, also can be seen on current graph current is going positive to negative and voltage keeps positive satisfying the 2-Q operation.

AIM: Explain in a brief about operation of BLDC motor.

BLDC motor: Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors).Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor.

Construction: In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings 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.

Operation of BLDC Motor: BLDC motor works on the principle similar to that of a Brushed DC motor. 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.
• When the stator coils get a supply from 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.
• 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 which causes the motor to rotate.

Figure shows the control circuit of BLDC Motor. It is a 6 pole BLDC motor,the resultant of flux and magnetic force (Can be check by vector resultant) attract the rotor and by result of it motor rotates.