Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
As shown in figure there are two terminals ‘A’ and ‘B’ of DC motor. Now if we connect terminal A with +Ve supply and terminal B with –Ve supply or ground the current will flow from motor from A to B and motor will rotate in one direction – say clockwise (CW) or forward direction. Now as shown in second figure we change the supply terminals. Now B is connected with +Ve and A is connected to ground. The current will flow from motor from B to A and motor will rotate in other direction (counter clockwise – CCW or reverse).
The circuit replaces the switches with NPN type transistors. We all know that transistor works as switch. For NPN transistor if we give +Ve input to base it will turn ON and if we give 0 input it will be turned OFF.
So in this circuit if Q1 and Q4 are turned ON simultaneously the motor will rotate forward and if Q2 and Q3 are turned ON then motor will rotate reverse.
The DC motor speed varies as applied input voltage varies. As you increase applied input voltage the speed will increase and vice versa.
Pulse width modulation means varying the width (duty) of pulse. Width means ON time Ton of pulse. The average output voltage (Vdc or Vavg) is given by equation
Vdc = [Ton / (Ton+Toff)] × Vs
It directly depends upon Ton. As shown in above figure if duty is 50% the average output voltage Vdc is exactly the half of Vs. If duty is increased to 75%, Vdc also increases to 3/4th of Vs and if duty is decreased to 25%, Vdc reduces to 1/4th of Vs. Thus as pulse width varies the average output voltage varies. So we have to apply PWM to DC motor to vary its speed.
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.
B. Comment on the armature current shoot-up from the scope results.
1.)Pulse Generator Parameters:
Period 2 milli seconds
Pulse Width (% of period):75%
Initially there is a spike in armature current .
Ia=V-Eb/Ra where Ia is the armature current,V is the supply voltage ,Eb is the back emf and Ra is the armature Resistance
At start Eb=0 and speed=0 at starting
Therefore Ia =V/Ra and Ra is a very small value .Hence Ia is huge and that reflects in the graph.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
The Four Quadrant Chopper DC Drive block represents a four quadrant ,DC-supplied ,chopper 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 .This duty is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper .
H bridge Four Quadrant chopper DC drive
It has two quadrants of operation possible which It can operate in all four quadrants(Forward motoring,Reverse regeneration
are Forward motoring and Reverse motoring ,Reverse motoring and forward regeneration)
Higher armature curent ripple Lower armature current ripple due to high switching frequency of DC-DC Converters
Less complex More complex
Simulation is continuous and simulation time is more Discrete and simulation time is less
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 )
Type C chopper or Two Quadrant class A chopper:
Class C chopper is category of chopper which can operate in first well as second quadrant .This basically means that ,the power can either flow from source to load or load to source in this chopper.It is also know as Two Quadrant Class A chopper.
Working of Type-C chopper:
As we know that Class A and Class B chopper operates in first and second quadrant .Therefore ,if we connect these chopper in parallel then it is possible to have chopper operation in first and second quadrant .
Figure below shows the circuit diagram of this type of chopper.
Case 1:When CH1 is switched ON/OFF
When chopper CH1 is switched ON, source(Vs) directly gets connected to the load and hence voltage Vo is equal to source voltage.The direction of load curent is from source to load as shown in the figure .
When Ch1 is switched off ,the freewheeling 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 figure .
Hence the average output voltage Vo and current Io are positive and operation of chopper is in first quadrant.
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 the current would be opposite and hence it is assumed negative .Output voltage Vo is zero during this time .When CH2 is made off Diode D2 gets forward biased and hence the cureent will flow from load to source .The load current is always negative i.e operation of chopper is within second quadrant .
The average load voltage is always positive but the average load current may be positive or negative .Therfore power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation).
Realization using Simulink:
Result:
Hence we could see clearly that the voltage is always positive but the current becomes both positive and negative .Hence the Type C
chopper is simulated and verified
3. Explain in a brief about operation of BLDC motor.
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.
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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.
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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.