Week-7 Challenge: DC Motor Control
QUESTION:
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.
Model:
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
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.
BLOCKS USED IN THIS MODEL
BATTERY
GAIN
DIODE
IGBT
MATH FUNCTION
STEP INPUT
SWITCH
SCOPE
GOTO
FROM
OUTPUT:
B. Comment on the armature current shoot-up from the scope results.
Pig: 4.8 (6) illustrates the conditione WR oll 4 and the second throwugh, 45% clockwise,
There are again two parallel paths, one through coil 1 and the second through coil 3. It may be
Ton That cola 1, 3 housed in slots 1, 3) have maximum emfs generated in them; because
shot colis are cutting the maximum flux. Coils 2, d are short-circuited by the brushes ant
these no currents, because c.m.fe, induced in these coils are zero. Currents in coll-sides
cortinine to slots 1, 3 in Fig: 48 (6), again establish armature m.m.f. that is directed
pertieni upward. Note that coil current in each parallel path is again 1,/2 as it is in
Fig. 4.3(a).
Pig: 1.3 c) depicts the condition when the rotor has turned through by further 45° from
the position of Fig. 4.3 (6). One parallel path is now through coils 4, 1 and the second paralle
path is through coils 3, 2. Coils in each parallel path carry again la/2 as it is in Fig. 4.3 (a) and
Pa. The armature m. m.f. is again directed vertically upward. Hence it can be concluded that
armature m.m.f. is always stationary in space and is directed along the interpolar or
quadrature axis, even though the armature rotates. For a motor, the electromagnetic torque is
diways acting in the clockwise direction, (Fig. 4.3) and the rotation continues. For a generator,
the armature must be driven in anticlockwise direction (Fig. 4.3), for the necessary energy
conversion from mechanical to electrical.
An examination of Fig. 4.3 reveals that just before the coils 2, 4 are short-circuited by
the brushes, these coils carry current 1_/2 (or in general La) in one direction. Soon after the
short-circuit is over, these two coils carry current - 1a/2 (or in general - I./a) in the reversed
direction as shown in Fig. 4.3 (c). Here a is the number of parallel paths and in Fig. 4.3, a is
equal to 2. Thus during the time of short-circuit, the currents in the short-circuited coils, in
general, must be reduced from la/a to zero and then built up to - 1/a. This process of current
reversal in the coils short-circuited by the brushes is called commutation and the time during
which it takes place is called commutation period. If the current variation with time is
uniform from I/a to -Ila as depicted in Fig. 4.4, a linear commutation is obtained.
Another important conclusion from
the
Commutation
foregoing analysis is that the magnitude of current
period
in the coils under each pole, remains constant and
equal to Ila, (Fig. 4.4). But the magnitude of
e.m.fs. induced in the coils under each pole is not
maximum e.m.f. induced in it, whereas the other
coils have their e.m.f. magnitudes proportional to
the flux density wave present there.
The coils short-circuited by the brushes
during commutation process must have their coil-
sides in the zero flux density region so that e.m.f.s
Time for travelling
induced in them are zero. In other words, the coils
about one pole pitch
undergoing commutation must have their coil sides
in the magnetic neutral axis of the field poles, i.e. in
Fig. 4.4. Variation of armature-coll current
the interpolar or quadrature axis. For example,
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
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.
Notes
1) 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.
2) 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.
3) 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.
Description
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= 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.
| Comparision of Four-Quadrant chopper DC7 block vs H-bridge model |
|
|
| Four-Quadrant Chopper can operates in all four modes (Forward Motoring, Forward Braking, Reverse Motoring and Reverse Braking) | H-bridge can operate only in two modes (Forward and Reverse). |
|
| In Four-Quadrant Chopper Two Pl regulator (Speed Regulator and Current Regulator) is used to obtain electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system | H-bridge model Pl regulator is not used. |
| Four-quadrant Chopper is electronic switch that is used to interrupt one signal under the control of another | H-bridge is a electronic circuit that switches the polarity of voltage applied to the load |
| Four-Quadrant Chopper circuit is used to refer numerous type of electronic switching devices and circuits used in power control and signal applications | H-bridge circuit are often used in Robotics and other applications to allow dc motor to run forward and backward |
| Four-Quadrant Chopper model gives better efficiency and high energy(power) | H-bridge lower efficiency and high energy |
| Four-Quadrant chopper high cost compare than H-bridge used for large application | H-bridge low cost compare to Four-Quadrant used for small application |
RESULT:
Hence the MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is explained
Comment on the armature current shoot-up from the scope results.
Under The difference between Four-Quadrant Chopper DC Drive (DC7) & H-bridge model explained
QUESTION-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 )
description
The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with two-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 advantages of this drive, compared with other DC drives, are its implementation simplicity and that it can operate in two quadrants (forward motoring and reverse 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, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.
regenerative braking operation takes place when S < (E/V) and transition from motoring ic
braking and vice versa occurs when 8 = (E/V). The above equations are similar to those obtainec
for chopper of Fig. (5.41), and therefore, given the same numbers.
Chopper for regenerative braking operation is shown in FiR. 5.42(4). Transistor T, is operated
periodically with a period T and on- period of fo. Waveforms of motor terminal voltage t,and
mature currently for continuous conduction are shown in Fie. 542/b). Usually an external
inductance is added to inc
rease the value of L. When 1, 15
on, L, increase from li to in The
Motor
(b)
Fig. 5.42
Regenerative braking of separately excited motor by chopper control
mechanical energy converted into electrical by the motor, now working as a generator, par
increases the stored magnette energy in armature circuit inductance and rethainder is dissipa
In armature resistance and transistor. When T. is turned off, armature current flows through dit
D and source V, and reduces from la tO far- The stored electromagnetic energy and ene
supplied by machine is fed to the source. The interval 0 S1 S % is now called energy stor
3. Explain in a brief about operation of BLDC motor.
Brushless DC Motor Working Principle and Applications
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 BDC. 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.
Result:Hence the BLDC Motor has breif discussion about operation of BLDC motor.