Week-7 Challenge: DC Motor Control
WEEK - 7 CHALLENGE: DC Motor Control
- 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.
Speed Control of a DC Motor Using BJT H-Bridge:
The following picture shows the simulation of H-bridge by BJT used to generate the chopper voltage to control the speed of the DC motor.
The BJT (bipolar junction transistor) is a current-controlled electronic device mainly employed for amplification and switching purposes. It comes with three terminals called emitter, base, and collector. The small current at the base side is used to control the large current at the remaining terminals.
So here in the speed control method of DC motors it is used as a power switching device to control the current and voltage flowing through the BJT’s which are formed as H bridge.Since it is dealing with high current and voltage , we are going for power switching semiconductors like BJT’s. H-bridge is formed for polarity changes (forward and reverse directions of the motor) according to the conduction of the diodes in the respective BJT’s.
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 the 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.
While analyzing the scope results for the Pulse width of 75% with load, speed and the armature current of the motor, we can see 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.
From the above figure, you can see that when the armature current drops in the negative direction for the given pulse width of 75%, the load and speed also get reduced and when the armature current is saturated, load and speed also get saturated.
The Four-Quadrant Chopper DC drive(DC7) block and comparing it with the H-Bridge Model:
The Four-Quadrant Chopper DC motor operation:
CHOPPER:
The DC Chopper or DC-to-DC Converter is a static power electronics device that converts fixed dc input directly into a variable dc output voltage.
The Four-Quadrant Chopper:
A four quadrant chopper is a chopper which can operate in all the four quadrants. The power can flow either from source to load or load to source in this chopper.
As you can see in the below image the 4 quadrants are classified as :
1.Forward motoring
2.Forward braking
3.Reverse motoring
4.Reverse braking
The following circuit diagram shows the typical 4 quadrant chopper
FIRST QUADRANT OPERATION:(FORWARD MOTORING)
For first quadrant operation, When both S1 & S4 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 vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io and inductor stores energy and the motor rotates in forward direction and hence it is forward motoring
When S1 is switched OFF, the load current frees the wheels through S4 and D2. During this period, the load voltage and current remains positive.Thus, both the output voltage vs and load current io are positive and hence, the operation of the chopper is in the first quadrant, the motor rotates still in forward direction and it is forward motoring.
SECOND QUADRANT OPERATION:(FORWARD BRAKING):
To obtain a second quadrant operation,S2 is operated while keeping theS1, S3 & S4 OFF. When S2 is ON, the DC source in the load drives current through S2, D4, E and L. Inductor L stores energy during the On period of S2, and the machine behaves as a generator, and it is forward braking.
When S2 is turned OFF, current is fed back to the source through D1, D4, the output voltage becomes positive and output current becomes negative and inductors release the energy through D1 and D4 and hence it is forward braking.
THIRD QUADRANT OPERATION:(REVERSE MOTORING):
The polarity of the back emf is reversed. The switch S3 operates, the switch S2 and S3 conducts. The output voltage and output current are negative and the power flows from source to load , the inductor stores energy and the motor runs in reverse direction and it is reverse motoring.
Now, when S3 is turned OFF, the negative load current frees the wheels through the S2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in the third quadrant, the motor rotates in the reverse direction and hence it is reverse motoring.
THIRD QUADRANT OPERATION:(REVERSE BRAKING):
To obtain the fourth quadrant operation, S4 is operated while keeping S1, S2 and S3 OFF and S4 and D2 conducts. The polarity of load emf E needs to be reversed in this case too, like the third quadrant operation.The machine behaves as a generator and inductor stores energy , the motor runs in the reverse direction and hence it is reverse braking.
When S4 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 the fourth quadrant. Here, power is fed back to the source from the load and hence it is reverse braking.
The operation of a four quadrant chopper is summarized in the figure below.(CH => S)
DC7 - Four-Quadrant Chopper 200 HP DC Drive:
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 overcurrent 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. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.
COMPARISON OF FOUR QUADRANT CHOPPER DC DRIVE AND H-BRIDGE:
- In four quadrant chopper DC drives, it distributes the equal current supply to all sectors and it contains four quadrants like forward motoring, forward braking , reverse motoring and reverse braking.whereas H-bridge does not have proper distribution of current which causes voltage fluctuations.
- In modern EV H-bridge, Four quadrant choppers are used in industrial applications.
- Efficiency is high in the chopper drive , whereas maintenance is less in the H-bridge.
- Regenerative braking is not possible in the H-bridge whereas it is possible in the chopper.
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)
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.
3.OPERATION OF BLDC MOTOR:
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.
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 magnets (or field poles) rotate and current carrying conductors are fixed.
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.
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 is 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 a 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 generates an AC voltage waveform with a trapezoidal shape. Due to the force of interaction between the electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
Consider the figure below in which the motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding windings are energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing the motor to rotate.
Observe that the 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 the motor moves in a clockwise direction.
The motor continuous rotation depends on the switching sequence around the coils. For that, Hall sensors give shaft position feedback to the electronic controller unit to know which stator coil should be energized.
As described above, the electronic controller circuit energizes appropriate motor winding by turning transistors or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of a MOSFET bridge (also called an inverter bridge), electronic controller, hall effect sensor and BLDC motor.
Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.
