Week-7 Challenge: DC Motor Control

Q.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.

ANS:

AIM:

Modify the model such that the Armature Current doesnt shootup when the motor changes direction from forword to reverse.

BJT-H BRIDGE:

The bipolar junction transistor(BJT) used for power switching applications. When it is conducting a forword voltage is developed between collector and emmiter(in the range of 1V).

So, IGBT block can be used to model the BJT device. The IGBT block does not simulate the gate current controlling the BJT. The switch is controlled by simulink signal(1/0). The DC motor uses the pre-set model(5hp 24v 1750rpm). It simulated a fan type load (where load torqueis proportional to square of speed).

Armature means voltage can be varried from 0-240V when the duty cycle is varried from 0-100%.

The H-Bridge consist of for BJT/Diode pair. Two transistors are switched simultaneously Q1 & Q4 or Q2 & Q3. The left load of the motor will be connected to the ground. Current starts flowing through the motor which energizes the motor in the forword direction and the motor shaft starts spinning.

When Q1 & Q3 awitched ON, a positive voltage applied to the motor and diode D2-D3 operate as freewheeling diodes when Q1 & Q4 are switch off.

When Q2& Q3 switched ON, a negative voltage applied to the motor and diode D1-D4 operates as frewheeling diodes when Q2 & Q3 are switch off.

In a bridge, you should never ever closed both Q1 & Q2 ( or Q3 & Q4) at the same time. If you did that you just have created a really low- resistance path between power and ground, effectively short circuting the power supply.

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

ANS:

PROCESS:

1) Open the MATLAB & in the help documentation type DC MOTOR USING BJT-H BRIDGE.

2) Result tab open then select the model and click open model.

3) The model is opened in simulink workspace.

4) Run the model with default parameters & get the results.

5) Change the parameter to the armature current does not shoot up when the motor changes direction forword to reverse.

6) Observed the result with default as well as changed parameter and comment on it.

Result with default values:

The result shows the speed of the motor, armature current,load torque with forword and reverse operating mode. As we see the current is starting of the operating mode that is forword and reverse operating mode current get shootup and this current we have to reduce.

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

ANS:

The shootup current which we have to reduce is can be done by reducing the firing angle of the transistor, so we have to reduce the parameter of the pulse generator, So we will decreases the pulse width by 50%.

Result with change the pulse width parameter:

After changing the parameter we can see at starting mode no shootup current.

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

ANS:

Compare the DC7 and H-Bridge:

Type –E chopper or the Fourth-Quadrant Chopper:

Type E or Four quadrant chopper consist of 4 semiconductor switches and 4 diodes are arrange in antiparallel. The 4 choppers are numbered according to which quadrant belong. Their operation will be in each quadrant and corrsponding chopper only be active in its quadrant.

First Quadrant:

During the first quadrant operation chopper CH1 & CH4 will be ON condition. Voltage and current is positive. The load voltage Vo is equal to the source voltage. Vs and load current io will beimg to flow. Vo and io will positive in first quadrant operation. If we want to reduce or decrease the output voltage of the DC motor then CH1 will be turned OFF. The positive current freewheel through CH4 and diode D2. So in first quadrant Forword Motoring happens.

Second Quadrant:

In this quadrant CH2 will be turned ON and other 3 will be turned OFF. When voltage applied to DC motor, EMF get generated. Current start flowing CH2-D4-L. Energy stored in the inductor(L). When CH2 will be OFF then current fed back to the source through ther diode D1 & D4. Here (E+Ldi/dt) is more than the source voltage (Vs), its called Forword Braking, The battery is charged from inductor stored energy.

Third Quadrant:

In this quadrant the voltage and current both are negative. The CH2 & CH3 will be ON. For this quadrant working polarity should be zero. As the CH3 is ON, Then load get connected to the Vs,Vo and io will be negative in this mode. If we want reduce or decrease the output voltage of the DC motor then CH3 will be turned OFF. The negative current freewheel through CH2 and diode D4, (CH2-D4-E-L), So in third quadrant Reverse Motoring happens.

Fourth Quadrant:

In this case CH4 will be turned ON and other 3 are turned OFF. When voltage applied to the DC motor, EMF get generated. Current starts flowing CH4-D2-E-L. Energy stored in the inductor(L). When CH4 will be OFF then current fed back to the source through the diode D2 & D3. The operation will be in fourth quadrant voltage is negative but load current is possitive. Its called Reverse Braking the battery is charged from inductor stored energy.

The Four-Quadrant Chopper DC Drive(DC7) block Model:-

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 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.

The following result is obtained after run above model:

Comparison:

SR.NO.	Four-Quadrant Chopper DC Drive (DC7)	H-bridge
1	The main difference is it operates in all four-quadrant operation which is forward motoring, forward braking, reverse motoring, reverse braking.	In this model, only two-quadrant operations are which is forward motoring and reverse motoring.
2	Hear chopper mechanism is used for four-quadrant operations.	Hear H-bridge is used for two-quadrant operations.
3	AC supply is used and converted to DC with the help of a rectifier.	Direct DC supply is used.
4	Regeneration can possible	Regeneration can not possible.
5	Speed control is possible	Speed control is not possible
6	We have to give reference speed.	Don't need reference speed.

Q.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 )

ANS:

Circuit diagram of two quadrant chopper:

Two quadrant operations as below:

Develop 2-quadrant chopper using simulink:

Model Construction:

Click on Simulink browser and open Library to search for component required to build C-vlass chopper as follows:

• Power GUI
• Pulse Generator
• Cureent Measurement
• Voltage measurement
• Diode
• MOSFET
• Logical Operator
• Series RLC
• Scope

1) Arrange the block as per circuit connection shown above in the model.

2) Double click on MOSFET & Diode block to disable(uptick show measurement port), and rotate the diode in upword direction as per our requirement( source should be facing in downword direction for MOSFET and Diode source should be in upword direction).

3) Open series RLC Block and change the branch type to(L) with default value 1 mill H, once done arrange according to circuit diagram.

4) Open logical operator and select NOT in Operation options, connects it input to the output of pulse generator block. The reason we use NOT gate to ensure both the switches does not conduct simultaneously if they do so supply will be short circuited.

5) We will be giving the tapping for the pulse gate terminal for 1 switch MOSFET1 and from NOT gate we are giving it to other switch MOSFET2. So if MOSFET1 is conducting MOSFET2 will not conduct because we are using the NOT gate.

6) Once done we will be measuring the voltage by connecting its 1st port at starting of inductor and other to DC2 supply which is connected adjacent.

7) And output is connected to 1st of the 3rd port of scope input, current measurement port is also connected to 2nd port of scope and 3rd port is taken from output of pulse generator.

8) Double click on main DC Voltage supplu and set amplitude(V) to 24V and for adjacent DC2 supply set amplitude as 12V.

9) Open pulse generator block set amplitude to 10 for better viewing, time period set to 1 sec, pulse width width is 50( because we 50% to be ON and 50% to be OFF) i.e. NOT gate is ON when 50% OFF.

The working of the model is giving below:

Type-C chopper is obtained by connecting Type-A and Type-B choppers in parallel.We will always get a positive output voltage Vo as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage Vo will be eqal to Vs.The direction of load current io will be reverse. The current io will be flowing towards the source and it will be positive regardless of 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 the chopper and FD operate togather as a Type A chopper in the first quadrant. In the second quadrant,the chopper and D2 will operate togather as Type B chopper

The average voltage will be always positive but the avg load current might be positive or negative. The power flow may be like the first quadrant operation i.e from source to load or from load to source as the second quadrant operation. The two choppers should not be turn on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring, this type of chopper configuration is used.

Let the voltage of the DC voltage source be 24volts.

Let us consider an series RLC branch block and select inductor L. Thus block work as a load.

To consider LE load i.e motor load, and DC voltage source of 12v is connected in series with inductor.

The pulse generator block is used to creat the gate pulse.

The NOT gate is used in the model so that the two MOSFET will not get closed at the same time otherwise short circuit will be occured. Due to use of NOT gate , when one MOSFET is on the other MOSFET will get OFF.

The amplitude of pulse generator is kept 10 so that we can clearely see the pulse at scope and let period be 1 sec and let pulse width be 50% .

OUTPUT:

The voltage waveform is not going less then zero,the maximum value of voltage is zero and maximum value is always in the positive direction.

However in current waveform,the current increases and decreases,current may be positive as well as negative.

Pulse amplitide is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

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

ANS:

Brushless DC electric motor also known as electronically commutated motor. A brushless DC motor is a permanent magnet synchronous electric motor which is driven by direct cureent(DC). Electrically and 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. BLDC motors are also referred as trapezoidal permanent magnet motors.

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 magnet (or field poles) rotates and current carrying conductors are fixed.

The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. 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.

This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

The main design difference between a brushes and brushless motor is replacement of mechanical commutator with an electrical 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 same frequency.

Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifire which is triggered by the commutating device like an optical encoder.

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.

The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.as shown in above fig.

STATOR:

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta However, most BLDC motors have three phase star connected stator.

Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.

The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.as shown in above fig.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred.

ROTOR:

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density. The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

HALL SENSORS:

Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.

Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

WORKING PRINCIPLE:

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 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 case 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 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.

Consider the figure below in which motor stator is excited based on different switching states. 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 causing motor to rotate.

Observe that 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 motor moves in a clockwise direction.

Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

ADVANTAGES OF BLDC MOTOR:

• It has no mechanical commutator and associated problems
• High efficiency 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
• Quite operation (or low noise) due to absence of brushes.

DISADVANTAGES OF BLDC MOTOR:

• These motors are costly
• Electronic controller required control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need of additional sensors.

APPLICATIONS OF BLDC MOTORS:

• Electric vehicle, hybrid vehicle and electric bycycles can be constructed.
• Industrial robots,CNS machine tools, and simple belt driven systems.
• Fan, pump, blowers with a low noise-inducing environment.

CONCLUSION:

• MATLAB Demo model named "Speed control of DC motor using BJT H-Bridge and the armature current shootup is stuided.
• "the four-quadrant chopper DC drive(dc7) block is also studied and compared with H-bridge model.
• 2-quadrant chopper is devloped using simulink model and obtained the resulys.
• Operation of BLDC motor is studied.