Week-7 Challenge: DC Motor Control

Aim:

• To run a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

1. Comment on the armature current shoot-up from the scope results.
2. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

• To Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to the article - Multiquadrant operation of the motor)
• To Explain in a brief the operation of the BLDC motor.

Solution1:

CASE 1: To run a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge

Speed control of a DC motor using BJT H-bridge

• This model consists of an H bridge with 4 BJT diode pairs. The input such as duty cycle and step time are given from pulse generator block, armature voltage control block
• This H bridge is made up of four switches Q1,Q2, Q3, Q4 and four diodes D1, D2, D3, D4. As IGBT can be used a switches which can be either in off or on position. When the motor is coasting it generates electricity
• 

Modes of operation:

• If switch Q1 and Q4 is closed, this will cause the current to flow from the source, through switch 1and then through the load, then through switch Q4, then go to the ground. This is known as forwarding motoring mode
• If switch Q2 and Q3 is closed, this will cause the current to flow from the source, through switch Q3 and then through the load, then through switch Q2, then go to the ground. This is known as reverse motoring mode
• If switches Q1, Q2, Q3, Q4 are closed, then both sides of the motor have no voltage and it is call braking mode
• If switch Q1,Q2 or Q3,Q4 is closed , D1, D2, D3, D4 will work as regenerating mode

Working:

• The dc motor use the present model(5HP 24V 1750rpm). It simulates a fan-type load. the armature mean voltage can be varied from 0-240v when the duty cycle is varied from 0-100%
• The H-bridge consist of four BJT/ Diode pair
• Two transistors are switched simultaneously such as Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are ON, a positive voltage is applied to the motor, and diodes D2, D3 operate as free wheeling diodes when Q2 and Q 3 are switched off
• When Q2,Q3 then negative voltage is applied to the motor and diodes D1-D4 operate as free wheeling diodes when Q2 and Q 3 are switch off
• The above graphs show the current passing through IGBT and diode. So for 0 to 0.5-sec current is flowing through the diode D3 and 0.5 to 1 sec the current is passing through Q3.

• The above plot shows that the beginning of the cycle there is a sudden peak is armature current the same repeat in the opposite direction when the motor rotates in opposite direction

CASE 2: B. Comment on the armature current shoot-up from the scope results.

• The default value of pulse width is 75. The pulse generator block is given below

The result is shown below

The pulse width decrease to 50 and then simulated

• From the result, we can see thet t=0.5 of the simulation the motor changes direction and the current shoot is reduced accordibly. Due to the reduced pulse width, sped of the motor and the load torque is also reduced

CASE 3: Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model

• The circuit of a four-quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4.
• The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

In the above circuit diagram, the choppers are numbered CH1 to CH4. For the first quadrant operation, CH1 is made ON, for the second quadrant operation CH2 is made ON, and so on. To better understand the working of the four quadrant chopper, we will discuss its operation separately for each quadrant.

First Quadrant Operation:

• For the first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that v_o= v_s. It may be noted that the load current flows from source to load as shown by the direction of i_o.
• When CH1 is switched OFF, the load current freewheels through CH4 and D2. During this period, the load voltage and current remains positive.
• Thus, both the output voltage v_s and load current i_o are positive, and hence, the operation of the chopper is in the first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

• To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.
• When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is the second quadrant operation of the chopper. Since the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read the step-up chopper for detailed analysis and better understanding.
• For the second quadrant operation, the load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

• To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram.
• If the direction is opposite to what is shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have a third quadrant operation.
• Circuit diagram of Class-E chopper for third quadrant operation is shown below.

Polarity of Load emf E reversed

• For the third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, the load gets connected to the source and hence load voltage is equal to source voltage.
• But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, v_o is assumed negative.
• Let us now see what is the status of load current i_o. It may be seen that i_ois flowing in the direction opposite to shown in the circuit diagram and hence negative.
• Now, when CH3 is turned OFF, the negative load current freewheels through the CH2 and D4. In this manner, v_oand i_o both are negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:

• To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.
• When CH4 is turned ON, positive current flows through CH4, D2, L, and E. inductance stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3.
• Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant.
• Here, power is fed back to the source from the load, and the chopper acts as a step-up chopper.

The operation of a four-quadrant chopper or Class-E chopper is summarized in the figure below.

• 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.
• 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 to obtain the electromagnetic torque needed to reach the desired speed.
• The speed reference change rate follows acceleration and deceleration ramps 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 behavior, 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. To limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

quadrant chopper	H- bridge
Regenerative braking is possible	Regenerative braking is not possible but dynamic braking can be achieved
The current values are chopped not continuous hence simulation takes less time	The current values are continuous
Widely used in automobiles to control DC motor speed and direction	Widley used in robotics and electromechanical device to control DC motor speed and direction
To change to direction of motor current signals are interrupted with one another	To change the direction of motor only the polarity of the current is changed

Solution 2: To Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

• Let us take two MOSFET block and a diode block and connect it as a circuit diagram.
• There is two dc voltage source and set to 24V,12V
• The pulse generator is usedto generate the pulseand the not gate to invert the signal.
• RLC branch circuit block and select inductor L. this block work as a load.

• The voltage waveform is not going less than zero. The minimum value of voltage is zero and maximum value is always in the positive direction
• The current waveform indicates the current increasing and decreasing. The current may be positive as well as negative

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

Brushless DC motor:

• Brushless DC electric motors are also known as electronically commutated motors (ECMs, EC motors).

• Primary efficiency is the most important feature of 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 of Brushless DC motor

• 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 brushed and brushless motors is the replacement of a 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.

• The Brushless motor does not have any current-carrying commutator. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.
• The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.
• Outrunner –The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.
• In a runner– The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style but is capable of spinning at very high rpm.
• Working Principle of Brushless DC motor
• BLDC motor works on the principle similar to that of 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 the source, it becomes an 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 aligns with stator poles which causes the motor to rotate.

Advantages of Brushless DC motor

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio 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
• Low noise due to absence of brushes

Limitations of Brushless DC motor

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need for additional sensors

Applications of Brushless DC motor

• 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 equipment, etc.
• 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.