Week-7 Challenge: DC Motor Control

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Here above fig. shows the speed control of DC motor using 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 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.

In that case 70% duty cycle is used for the pulse generator for the modulation. By using this duty cycle the armature current is shoots much higher value, is can be control by changing the duty cycle.

The step block has 0.5 step time interval for the modulation.

From the waveform i had notice that the 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 and the diode current is zero.

From the above graph the speed is starts from zero and the reaches up to the maximum speed of (950rpm) and decreases after 0.5 time and reaches to the (-950rpm).

The armarture current is starts from 0and increasesup to the (39) 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 (-950 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 spikes 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.

1. B. Comment on the armature current shoot-up from the scope results.

From the above the armature current increases at the starting and armature current a sudden rise in the waveform we had noticed . At thiss time the duty cycle is set at the 75% .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%).

After reducing the duty cycle the armature current is reduced slightly but not smooth also no sudden rise current observed. to smooth armature current flow we have to adjust the duty cycle again.

AT the above graph shows the no sudden rise in current and also smooth armautre currrent flow. It is obtained by reducing the duty cycle by 75% to 40% .

1.C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

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.

Type E chopper Quadrant Explaination:-

First Quadrant:-

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 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.

When CH1 is switched OFF, the load current freewheels through CH4 and D2. During this period, the load voltage and current remain 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. It may be noted that Class-E chopper operates as a step-down chopper in this case.

Second Quadrant:-

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the on period of CH2.

When CH2 is turned OFF, the current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is the second quadrant operation of the chopper. Since the current is fed back to the source, this simply means that load is transferring power to the source.

Third Quadrant:-

To obtain the third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

The polarity of Load emf E reversed

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, the load gets connected to the source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, vo is assumed negative. Let us now see what the status of the load is current io. It may be seen that io is flowing in the direction opposite to shown in the circuit diagram and hence negative.

Now, when CH3 is turned OFF, the negative load current freewheels through the CH2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in the third quadrant.

Foruth Quadrant:-

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON.

When CH4 is made OFF, the 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 the chopper acts as a step-up chopper.

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 over-current 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 behaviour, 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.

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 )

Two Quadrant 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 V_s. 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.

Using simulink The 2 Quadrant chopper developed shows in the below fig.

To develop this module

Click on the matlab .Click on the simulink model and click on the library browser.

2.Select the blocks of the powergui, Mosfet , diode,scope,inductor(load), pulse generator, dc supply ,voltage and current measurement.

3. The blocks of the simulink model can be arranged in a passion that the class -D type chopper as shown below

For this i have select the 10e3 load inductor and run the cycle for 1 sec.

Duty cycle is set at the 50% and amplitude is set at 10.

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 voltage can be positive and negative.

3.Explain in a brief about operation of BLDC motor.

Construction of Brushless DC motor

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armautue winding 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 commuter. 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.

• 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.

• 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 brished 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.