Week-7 Challenge: DC Motor Control

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.
2. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to article – Multi quadrant operation of motor)
3. Explain in a brief about operation of BLDC motor.

AIM:

1. a. To Explain ‘Speed control of a DC motor using BJT H-bridge’. b. Comment on the armature current shoot-up from the scope results. c. Compare ‘The Four-Quadrant Chopper DC Drive (DC7) block’. with the H-bridge model.
2. To Develop a 2-quadrant chopper & explain the working of the same with the relevant results.
3. To Explain the operation of BLDC motor. 1. A.Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Speed control of a DC motor using BJT H-bridge:

In general, a H-bridge is a simple circuit, containing four switching element (BJT), with the load at the centre, which look like a H type construction.

The basic construction of the H- bridge circuit

It is commonly used in robotics application to control DC Motors. By using H Bridge we can run DC Motor in clockwise or anticlockwise directions. This circuit is also used to produce alternating waveforms in inverters.

Here when the positive signal is given, the switches Q1 and Q4 are to be turn on and the motor rotates in forward direction. when these two are on condition, Q2 and Q3 are in off condition to get the outputs without any damages or errors.

To get the negative signal, Q3 and Q2 are to be turn on, now the motor rotates in reverse direction. Here also when these two are on condition, Q1 and Q4 are in off condition to get the outputs without any damages or errors. We cannot trigger the two switches of the same leg like Q1 and Q2, Q3 and Q4. Because it crates low resistance path and may be the switches get damage and we won’t get the exact output.

Simulink model :

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).

Dc motor used in the Simulink model:

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 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.

This example shows the simulation of an H- bridge used to generate a chopped voltage and to control the speed of a dc motor

Gilbert Sybille (Hydro-Quebec)

Here when we observe that the input to the signal IGBT is coming through the P1, P2, P3, P4 at the gate terminal ‘g’ this ‘p’ input to the gate be came through the pulse generator by this pulse formed and it given to the bus bars and given to switch with respect to armature voltage (constant block), now the output is connected to the bus bar again which connect to the port’s ‘p’.

The pulse width modulation (PWM) is a modulation technique that generates variable width pulses to represent he amplitude of an analog signal. The length of the time that a pulse is in a given state (high/ low) is the width of a pulse wave. Here the term comes the duty cycle which represents in percentage values.

Here the IGBT’s are connected to the motor terminals as H, as shown above, and the terminals IGBT current, diode current, speed, load torque is connected to the scope to know the results as a graph model.

Now take the pulse generator given by the pulse width of 75% and period of 2e-3 seconds.

The motor parameters of the torque and field voltage and armature voltage are taken as default values of the model.

Now when we run the model we get the results as shown below.

After running the model when we see the results in IGBT and diode current they look like as above. When we observe the results, we can notice that from 0 to 0.5 the armature current is 0 after that it raise from 0 to 67 amp. When we observe the diode current it seem opposite to armature that it starts raise up to 0 to 0.5 from 0 to 38.5 amp. after that it reduce to 0, up to 1sec.

Now when we click on the another scope we get the results of speed, armature current and load torque as shown below.

When we see the results, we can observe that the speed raises from 0 to 950 rpm from 0 to 0.5, at start it start increase gradually when it reaches 950 at 0.15 sec it remains constant. After 0.5 to 0.7 sec, it starts decreasing from 950 to 0 at 0.55 sec and reduce gradually up to -950 rpm and it remains constant from 0.7 to 1 sec.

After that when we observe armature current it raise up to 40 and reduce 5 to 10 amp from 0 to 0.5 after that it reduce to -67 means negative direction from 0.5 to 0.6 and reaches to -0.5 to -10 amp and it continues up to 1.

After that if we take the load torque it seems same as the speed that, it reaches to 6 Nm. and it remains constant from 0 to 0.5. and it reduce to 0 and reduce to negative of 6 Nm from 0.55 to 0.7 and remains constant up to 1.

By doing this process the speed control of DC motor using BJT H- bridge model and the output waveforms are obtained.

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

From the above armature current shoot- up at the starting of the armature current a sudden raise in the wave form that we had noticed. To reduce the sudden raise in the armature current we have to reduce the duty cycle of the pulse width modulation i.e., is pulse generator, from (75% to 50%). As below.

The output of this model wave form is shown below.

The difference between the above two graph armature current in the second graph we observe that there is no sudden raise at the starting of the armature current. If we future reduced the pulse width modulation the negative of the rise will get reduced.

Next as the same reduce the pulse generator from 50% to 40% and the results after running the results are shown below in speed, armature current, load torque.

From the above graph it is clear that if we reduce the duty cycle ratio than we obtain a smooth variation in the output graph. The above graph PWM is 40%. And the motor details are default of the model. The torque and speed also varies the smoothly.

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

As we know that a chopper is a dc to dc converter circuit that it convert or vary the fixed dc into a variable dc voltage.

Here This example shows the DC7 four-quadrant chopper DC drive during speed regulation.

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. 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

• 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.
• The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.

Comparison of four quadrant chopper and H-bridge

Four Quadrant Chopper	H-Bridge
The motor can be operated in all the four different modes (forward motoring, reverse motoring, forward braking, and reverse braking)	I this model we have only two modes forwarded and reverse by changing the direction of motor
In this the vehicle control is better	In this the vehicle control are less
Regenerative braking can be achieved	In this regenerative braking is not possible
The signals given to the four-quadrant operation are discrete	Here the signals are continuous
The four-quadrant chopper widely used in 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
The stability of the voltage and current supply is higher in the four quadrants	The stability of the voltage and current supply is less in BJT
Regenerative braking is possible in four quadrants	Regenerative braking is not possible in BJT
It is widely used in automobile sector to control DC motor speed and direction in Four-Quadrant.	BJT is used in robotics and electro-mechanical devices to control DC motor speed and direction
In four-quadrants it is chopped so that the simulation takes less time	The current values are continuous in BJT
To change the direction of the motor, current signals are interrupted in four quadrants	In BJT the polarity of the current needs to be changed
The main advantage of four-quadrants is that it operates in all the four quadrants	BJT it can move only in two directions
Four- quadrants provides better efficiency and operates at high power levels	BJT operates at moderate power levels

2.Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to article –

Multi quadrant operation of motor

)

2-Quadrant chopper:

In the 2-quadrant chopper there are two types i) Class-C or type C and ii) Class-D or type D.

Type C or Class C or 2 Quadrant Type A chopper:

Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V_{0 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 V_s. 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.

Here, 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 this type of chopper configuration is used.

Type D or Class D or 2 Quadrant Type B chopper:

For the class-d chopper the output current is positive and the output voltage is positive and negative.

Two Quadrant Type B chopper or D Chopper Circuit

The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage v₀ will be equal to V_s. When v₀ = – V_s the two choppers will be off but both the diodes D1 and D2 will start conducting. V₀ the average output voltage will be positive when the choppers turn-on the time T_on will be more than the turn off time T_off it shown in the wave form below. As the diodes and choppers conduct current only in one direction the direction of load current will be always positive.

The waveforms of the positive and negative operations are given below.

The power flows from source to load as the average values of both v₀ and i₀ is positive. From the wave form it is seen that the average value of V₀ is positive thus the forth quadrant operation of type D chopper is obtained.

From the wave forms the Average value of output voltage is given by

V0= (V_s T_on-V_sT_off)/T = V_s. (T_on-T_off)/T

As from the MATLAB model ,The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with two-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.

Simulink model of the class-D type chopper:

1. Click on the MATLAB. Click on the Simulink model and then click on the library browser.
2. Select the blocks the Mosfet, diode, scope, Series RLC Branch (inductor), pulse generator, dc supply, voltage and current measurement power gui.
3. In Series RLC Branch, we have taken the Brach type is inductor
4. The blocks of the Simulink model can be arranged in a passion that the class-D type chopper as shown above.

The model is looks like as shown below.

5.After assembly of the block now we will assign the values of the pulse generator as below, and the DC supply voltage 24V.

Now we will run the model for 1 sec of the sample. The output waveform from the current and voltage measurements through the scope as shown below. For the load inductance value, I had taken here is 1e-3.

The results for the voltage and current looks as below.

The output of the current waveform is in positive direction and the voltage waveform is in positive and the negative half cycle.

Due to this we can say that in two quadrant operation of a chopper type-D the current is positive and voltage can be positive and negative as well.

The main advantages of this drive, compared with other DC drives, are its implementation simplicity and that it can operate in two quadrants (forward motoring and reverse regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.

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

Brushless DC motor (BLDC motor):

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor). It is also called as the permanent magnets synchronous motor (PMSM). These are two types of BLDC motors are present.

These types of motors are highly efficient in producing a large amount of torque over a vast speed range. 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.

Most of the BLDC motors are 3-phase type, they are

1. 3- phase trapezoidal BLDC motor
2. Sinusoidal type BLDC motor.

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 magnets whereas stator is the stationary part and contains stator winding. 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 attached to the rotor. The current-carrying conductors are armature windings are located on the stator. They use electrical commutation to convert electrical to 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 senesce that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor doesn’t 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.

Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two is only in designing, their working principles are same.

principles are same.

Inner Rotor Design:

In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.

Outer Rotor Design:

In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.

Advantages of a BLDC motor:

• Brushless motors are more efficient due to the use of permanent magnet motor.
• As brushes are absent, the mechanical energy loss due to friction is less.
• It can operate at high-speed under any condition.
• There is no sparking and much less noise during operation.
• More electromagnets could be used on the stator for more precise control.
• BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
• BLDC motors do not have brushes which make it more reliable, high life expectancies,
• maintenance free operation.
• There are no ionizing sparks from the commutator, and
• electromagnetic interference is also reduced.
• Such motors cooled by conduction and no air flow are required for inside cooling.
• Brushes can generate a lot of sparks, which can shorten the life of a brushed DC motor or cause it to completely burn out.

Disadvantages of a BLDC motor:

• BLDC motor cost more than a brushed DC motor.
• The limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.
• Additional sensors are required.
• More controls required due to expensive of motor
• it produces tiny vibrations during the low-speed rotation.

Applications of Brushless DC Motors (BLDC)

Brushless DC Motors (BLDC) are used 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 equipment’s, etc.

Some specific applications of BLDC motors are,

• Computer hard drives and DVD/CD players.
• Used in the car parts and wheel coupler from battery.
• 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.
• Robotics, bikes, drones etc.