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.
Ans (A):- The model ‘Speed control of a DC motor using BJT H-bridge’ is use for the simulation of an H-bridge to generate a chopped voltage and to control the speed of a DC motor.
Model :-
In this 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 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.
Consider Q1,Q2,Q3,Q4 as a switch:-
When Switch 1 and Switch 4 is closed, this will cause the current to flow from the source, through Switch 1 and then through the load, then through Switch 4, then down to the ground.This is known as Forward motion of the motor.
When Switch 2 and 3 is closed, this will cause the current to flow from the source, through Switch 3 and then through the load, then through Switch 2, then down to the ground.This is known as Reverse motion of the motor.
Ans (B) :-
Scope results analysis:-
a
Above scope results shows
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.
Here Pulse width is refered as Duty Cycle. Duty Cycle is directly proportional to Voltage. While Changing the Duty Cycle it changes the Voltage. By reducing the Duty cycle from 75% to 45%,means Average Voltage and current will be produced. There is no Sudden rise and drop occurs and constant current will be produced. Therefore armature current does not shoot up when motor changes direction from forward to reverse.
Ans (C) :- Comparison of H-Bridge and Dc7-Chopper Model:-
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H- Bridge |
DC7-Chopper |
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<!-- [if !supportLists]-->1 H-Bridge can operate the motor only in two modes Forward and Reverse. <!--[endif]-->H-Bridge regenerative braking is not possible. <!-- [if !supportLists]-->3 <!--[endif]-->H-Bridge model works in open loop, means there is no arrangement of feedback, which demands current according to armature requirement. . <!--[endif]-->H-Bridge is simple circuit and easy. |
<!--[endif]-->DC7 is a four-quadrant chopper which operates motor in all 4 quadrant. <!--[endif]-->DC7 chopper works as regeneration and regenerative braking with both Forward and reverse direction. <!--[endif]-->DC7 model works in close loop. It has the references for motor operation. <!--[endif]-->DC7 model has more complex because it has extra component to work on feedback. |
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
Quadrant operation of Chopper:-
The quadrant operation of chopper is basically the circuit configuration so adjusted to achieve the chopper voltage Vo and output current Io in all the four quadrants. A chopper can be operated in all four quadrants by employing required circuit configuration.
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.
Two Quadrant Chopper:-
Sometimes a chopper may be required to provide a two quadrant operation by retaining the current direction in both motoring and braking modes. It is basically a combination of first and second quadrant choppers, or known as Type-C chopper.
In two quadrant chopper you can use either First and second quadrant or First and fourth quadrant.
Now we are going to develop the First and second quadrant chopper. This is also called Class-C Or Type-C chopper.
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. 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.
In above diagram CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CH2, D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2.
Case-1: When CH1 is switched ON / OFF
When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.
When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage Vo and current Io are positive and hence operation of chopper is in first quadrant. In fact, this is the Class-B mode of operation.
Case-2: When CH2 is switched ON / OFF
When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current io will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage Vo is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant. In fact, this is the Class-B mode of operation.
From the above two cases, we can conclude the following points:
- The output voltage Vo is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
- The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.
- The load current flows in the direction shown in circuit diagram i.e.io is positive when CH1 is ON or FD conducts.
- The load current flows opposite to the direction shown in circuit diagram i.e.io is negative when CH2 is ON or D2 conducts.
The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation)
The operating region of Class-C or Type-C chopper is shown below by hatched area.
Caution:
Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.
Application:
Caution:
Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.
Application:
Class-C or Type-C choppers are used for motoring and regenerative breaking of DC Motors.
Model of Two quadrant chopper
In the model we used the mosfet as switch.
In the model we used the 100V dc voltage as power source and 50V dc for consider as BEMF of motor to see the output results for both quarant.
1st is as motoring and 2nd is as regeneratio
Output result of model
in which 1st is showing voltage 2nd is current and 3rd is switching pulse for mosfet.
The scope results shows that voltage is always positive and current may either positive or negative both in this type of operation.
3. Explain in a brief about operation of BLDC motor.
What is a Brushless DC motor (BLDC)?
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. BLDC motors are also referred as trapezoidal permanent magnet motors.
Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.
The armature coils are switched electronically by transistors or silicon controlled rectifiers at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.
This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.
Hall Sensors used for electronic commutation in BLDC motor
Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
Most 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. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.
Six step communication of bldc motor.
Working Principle and Operation of BLDC Motor
BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet 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.
Consider the above figure in which 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.
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). By this way motor moves in a clockwise direction.
Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that 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.