Week-7 Challenge: DC Motor Control
Q1.
Most DC motors have two terminals because they are series type (armature and field windings are connected in series) DC motors.
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 arrangement to get reversible supply is shown in right side of figure. Four switches are connected in between +Ve supply and ground and DC motor is connected in between two switches as shown. Such circuit arrangement is known as H-bridge because it looks like letter 'H' (H-bridge circuits are most widely used in DC motor drivers). Let us see how it gives reverse supply to motor.
If SWI and SW4 are pressed simultaneously then current will flow from +Ve-SW1-A-B-SW4-Gnd. So motor will rotate in one direction. Open (release) SWI and SW4 to stop motor. Now if SW2 and SW3 are pressed current will flow from +Ve-SW2-B-A-SW3-Gnd. So motor gets reverse supply and it will rotate in another direction.
(Caution: care has to be taken that SWI and SW3 or SW2 and SW4 must not be pressed simultaneously otherwise it will short circuit the supply)
The circuit replaces the switches with NPN type transistors. Diodes are given for the current to free wheel as the DC motor is a RLE load where the current cannot change direction instantaneously. We all know that transistor works as switch. For NPN transistor if we give enough +Ve input to base it will turn ON (Saturation region) and if we give 0 input it will be turned OFF (Cut off region). So in this circuit if Q1 and are turned ON simultaneously the motor will rotate forward and if Q2 and Q3 are turned ON then motor will rotate in reverse direction.
In order to vary the speed of the DC motor, We have to change the voltage applied to the DC motor. This could be achieved with the help of Pulse width modulation.
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
V₁ = [Ton / (To + Toff)]* V,
⇒ V₁ = 6-V, where & is the duty cycle.
The voltage 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 example below shows the simulink model of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.
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 QI 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.
Note: The DC motor used in the simulink model is a separately excited DC motor and NOT a DC series motor. That is the reason why a DC voltage of 300V is given to the field windings.
1.b)Comment on the armature current shoot-up from the scope results.
Simulation Output:
1.) Pulse Generator Parameters:
Period: 2 milli seconds
Pulse Width (% of period): 75%
Initially, there is a spike in armature current. This is because, in a DC motor,
Ia=(Va-Eb)/Ra where Ia is the armature current, v is the supply voltage, E is the back emf and R is the armature resistance.
At starting, Eb=0 as speed=0 at starting.
V Therefore Ia = V/Ra and Ra is a very small value. Hence is huge and that reflects in the graph.
Therefore when pulse width decreases, the voltage applied across the DC motor decreases and the armature current finally decreases.
1.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 (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) 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. 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 Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:
• Speed Controller (DC)
• Regulation Switch
• Current Controller (DC)
• Chopper
H bridge:
Only Two quadrants of operation are possible. They are Forward motoring and reverse motoring
Higher armature current ripple.
Less complex
Four Quadrant chopper DC drive:
It can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration)
Lower armature current ripple due to high switching
frequency of DC-DC coverters
More complex
Simulation is continuous and simulation time is more Discrete and Simulation time is less
Widely Used in Robotics
Widely used in Automobile
Qn. 2)Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
Type C chopper or Two Quadrant class A chopper:
Class-C or Type-C Chopper is a category of chopper which can operate in first as well as second quadrant. This basically means that, the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as Two Quadrant Class-A chopper.
Operation or Working of Class-C or type C
As we know that, a Class A and Class B chopper operates in first and second quadrant respectively. Therefore, if we connect both these types of chopper in parallel then it is possible to have chopper operation in first as well as second quadrant. In fact, Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper. Figure below shows the circuit diagram of this type of chopper.
We can notice that chopper CHI, 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 CHI and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.
Case 1: When CHI is switched ON/OFF
When chopper CHI is switched ON, source V, directly gets connected to the load and hence, load voltage V, 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 CHI 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 V, becomes zero. However, the lo continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V, and current I, are positive and hence 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 this current i, will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V, 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 V, in this time as the load is connected to the source through D2 during OFF time of CH2. Thus, 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. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation).
Simulink Output:
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.
Qn.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:
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.
Working:
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.