Week-7 Challenge: DC Motor Control
Question 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.
BJT-H Bridge:
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 pre-set model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armatures 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.
When the positive gate signal is given the switches Q1 and Q4 are turn-on and the motor rotates in forward direction. D1and D4 are in off condition. During changing of the positive to negative time period the D1 and D4 are in on condition at that time the switch Q1 and Q4 are off condition.
whenthe negative gate signal is given the switch Q3 and Q2 are turn-on and the motor rotates in reverese direction and D3 and D4 are in off condition.During changing of the negative to positive time period the D2 and D3 are in on condition at that time the switch Q3 and Q4 are off condition.We cannot trigger the two switches of the same leg.It creates a low resistance path and switches gets damaged.
From the waveform we can see that the from 0 to 0.5 time the the IGBT current is zero and the diode current is on after the 0.5 to 1 rime the IGBT starts the current flow the diode current is zero.
Now we will take the output waveform of the load torque, armature current and speed.
From the above graph the speed is starts from zero and the reaches up to the maximum speed of (1000rpm) and decreases after 0.5 time and reaches to the (-1000rpm).
The armarture current is starts from 0 and increases up to the and decreases after immediately and reaches to the nearer to the zero value.when the speed decreases the armature current is also starts flow in the reverese direction.when the speed comes to the nearer value of (-1000 rpm) the armture curent is flow in neagative direction nearer to the zero value.At the starting of the armature current i had noticed a spike in the current flow and also in the negative direction of the current flow.
The load torque follows the speed waveform in the positive direction.when the negative direction it follows the armature current current and the speed waveform.
By doing these process the speed control of DC motor using BJT H-bridge model and the output waveforms are obtained.
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With PWM 75%
PWM is 50%
From the above the armature current shoot-up at the starting of the armature current a sudden rise in the waveform we had noticed .To reduce the sudden rise in the armature current we will the reduce the duty cycle of the pulse width modulation from (75% to 50%).
The difference between the above two graph armature current in the second graph we observe that there is no sudden rise at the starting of the armature current. If we further reduced the pulse width modulation the negative of the rise will get reduced.
From the above graph it is clear that if we reduced the duty cycle ratio than we obtain a smooth variation in the output graph. The above graph PWM is 50%. and the motor details are default of the model.The torque and speed also varies the smoothly.
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1C: Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Solution: Chopper: Chopper is a static device(switch) used to obtain the variavle dc voltage from a source of constant dc voltage. The output voltage is less than the input voltage than the chopper is called step-down chopper. If the output voltage is greater than the input voltage than the chopper is called step-up chopper.
Chopper offers greater efficiency ,faster response,lower maintainence,small size,smooth control. choppers are widely used in trolley-cars,battery-operated vehicles,traction-motor control.There are different types of the choppers are available according to the direction of output voltage and current.
Class A Type A: First quadrant chopper: The output voltage and current both must be positive.
Class B type B: The output of the voltage is positive and the current is negative.
Class C /class D:Two quadrant chopper: The output voltage is positive and current can be positive or negative( class-C) or the output current is positive and the voltage can be positive or negative(class-D).
Class-E,Four quadrant chopper: The output voltage and current both can be positive or negative.
| l.no | Four Quadrant chopper | H-bridge model |
| 1. | The motor can be operated in all the four different modes (forward motoring,reverse motoring ,forward braking and reverse braking) | In this model we have only two modes forward and reverese by changing the direction of motor. |
| 2. | In this the vehicle control is better | In this the vehicle control are less |
| 3. | Regenerative braking can be achieved | In this regenerative braking is not possible. |
| 4. | The signals given to the four quadrant operation are discrete. | Here the signals are continuous. |
| 5. | The four-quadrant chopper widely usedin the automobile application for the control of DC motor speed and direction. | The H-bridge is used in the robotics and electro-mechanical devices to control the speed and direction of the DC motor at low level speed. |
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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 )
Solution:
A dc‐to‐dc converter, also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators. Choppers are of two types: (1) Step‐down choppers, and (2) Step‐up choppers. In step‐down choppers, the output voltage will be less than the input voltage, whereas in step‐up choppers output voltage will be more than the input voltage.
A 2-quadrant chopper :
In the 2-quadrant chopper are two types i) Class-c and ii) Class-D.
This is class -c Chopper the output of the voltage is positive and the output of the current is positive and negative.Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on 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 and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.
The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.
Simulation of class-D type chopper:
Simulation Model is develpoed as below
Here Pulse is taken with amplitude 10 and duty cycle 50% and and pulse is taken for time 0.02 sec that is 50Hz.
supply voltage is taken as 24 V with Series RLC load and this are connected as D type chopper.
Results are obtaines as discussed,
The output of the current waveform is in positive direction and the voltage waveform is in postive and the negative half cycle.
This the two quadrant operation of a chopper type -D the current is positive and negative and voltage can be seen positive.
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Question 3: Explain in a brief about operation of BLDC motor.
A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor). A BLDC motor is also known as the permanent magnet synchronus motor(PMSM).There are two types of BLDC motors are present i) 3-phase trapeziodal BLDC motor.ii) Sinusidal type BLDC motor.Most of the BLDC motors are 3-phase type.
As shown in the above figure the brushless DC motor has only two basic parts: rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator. The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.
In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.
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.
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.
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