Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
B. Comment on the armature current shoot-up from the scope results.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
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 )
3. Explain in a brief about operation of BLDC motor.
AIM :
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
B. Comment on the armature current shoot-up from the scope results.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
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 )
3. Explain in a brief about operation of BLDC motor.
1.Q: Speed control of a DC motor using BJT H-bridge
circuit
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.
simulation
The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 1.0 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.
and output
bty changing pulse value 75% to 60%
output
1.b
from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also means at transition from positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.
As there is already current flowing in the circuit & on changing of switches after 1 sec, the current in the circuit adds to the changed direction of current and we observe a surge in current.
the surge in the current can be reduced by reducing the duty cycle.
in above results , the duty cycle selected as 75%.
by changing the duty cycle from 75% to 60% , the current surge can be reduced tremendously.
thus we can find that , the armature curent spike is reduced by changing the duty cycle from 75% to 60%
1.c
‘The Four-Quadrant Chopper DC Drive (DC7) block
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, 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.
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.
The power system has been discretized with a 1 us time step. The speed and current controllers use a 100 us and 20 us sampling time respectively in order to simulate a microcontroller control device.
In order to reduce the number of points stored in the scope memory, a decimation factor of 25 is used. Some transitions may thus not appear on the scope. To view detailed simulation results, reduce the decimation factor to 1.
A simplified version of the model using an average-value converter can be used by selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The time step can then be increased up to the smallest control system sample time value. This can be done by typing 'Ts = 20e-6' in the workspace in the case of this example. See also dc7_example_simplified model.
output
The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases thecomplexity of the drive system.
Four Quadrant Operation of DC motor means that the machine operates in four quadrants. They are forward motoring, Forward braking, reverse motoring and reverse braking.
QUADRANT - I
When the motor draws power from supply, the voltage and current are in same direction.
Now the motor is said to be in motoring operation, its operation corresponds to the 1st quadrant.
In the 1st Quadrant power developed in positive and machine is working as motor supplying mechanical energy. The 1st quadrant operation is called Forward motoring.
QUADRANT - II
If there is a possibility of reversing the current direction keeping the voltage polarity same, the motor is said to be operating in 2nd quadrant.
This happens when regenerative braking is applied to motor.
During regenerative braking operation, reverse power flow will happen i.e power will be feedback to supply system. @nd quadrant operation is known as braking.
In this quadrant the direction of rotation is positive and the torque is negative and thus the machine operates as a generator developing a negative torque that opposes the motion.
The kinematic energy of the rotating part is available as electric energy which may be supplied back to mains. In dynamic braking energy may be supplied back to mains. In dynamic braking energy is dissipated in the resistance.
QUADRANT - III
If the voltage polarity and load current are reversed simultaneously, the motor operation will be in the opposite direction, but the power will be consumed by the load.
The operation is corresponding the 3rd Quadrant and even it is said to be in motoring operation. The 3rd quadrant operation is known as reverse motoring.
The motor works, in the reverse direction. Both the speed and torque have negative values while the power is positive.
QUADRANT - IV
If the voltage polarity is reversed maintain the current direction source to load the power is reversed and the operation corresponding to 4th quadrant.
Now the motor is said to be in reverse Regenerative operation.
In the 4th quadrant, the torque is positive and speed is negative.
This quadrant corresponds to braking in the reverse regenerative braking mode.
2.Q
OUTPUT
3.Q:
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 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 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.
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 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 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.
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 of additional sensors
Applications of Brushless DC motor
Brushless DC motors (BLDC) use 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 equipments 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.