Week-7 Challenge: DC Motor Control

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
2. Comment on the armature current shoot-up from the scope results.
3. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Bipolar Junction Transistor H-bridge :

The H-bridge model consists of four Bipolar Junction Transistor(BJT) and four Flyback diodes that control the flow of current to the load.Using one current source, DC motor can drive in two directions by closing two switches.

If Switch 1 and 4 is closed, this will cause the current to flow from the source, through Switch 1 and then through the load, then through Switch 4, then down to the ground.This is known as Forward motion of the motor.

If Switch 2 and 3 is closed, this will cause the current to flow from the source, through Switch 3 and then through the load, then through Switch 2, then down to the ground.This is known as Reverse motion of the motor.

If Switch 1,3 or 2,4 is Closed, then both sides of the motor have same voltage and this is called as Brake mode.

If Switch 1,2 or 3,4 is closed, then it will cause a short and burn out the 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).

MODEL DISCRIPTION:

1) dc motor used is model is 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%.

2)The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models)

3)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.

4)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.

From the above plot, observed that '0s -0.5 s' Current flow Through 'Diode D3' and '0.5s - 1s' Current will flow through 'BJT Q3'.

The Ouput from the DC motor is Speed ("rad/s"is converterd into "rpm") and Armature Current(A) is connected with 'Scope'. where, Load Torque(T) is directly proportional to Square of Speed.

T∝ω^2

T=k⋅ω^2

Output from the Speed(ω) is multiplied with constant(k) gives Load Torque(T) and it is also connected with 'Scope'. 'Scope' shows motor speed, armature current and load torque.

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Currents' shows currents flowing in Bipolar Junction Transistor Q3 and diode D3. Adding of diodes in the Forward direction for the transistors, to give a path for the current to take to release this energy. Without diodes, it risk that the voltage rises and damages the transistors.

A Pulse Width Modulation (PWM) Signal is a method for generating an analog signal using a digital source. A PWM signal consists of two main components that define its behavior: a duty cycle and a frequency. By cycling a digital signal off and on at a fast enough rate, and with a certain duty cycle, the output will appear to behave like a constant voltage and current analog signal when providing power to devices.

Fig : Pulse Width Modulation (PWM)

The average value of Voltage (and Current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate.The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.

Here Duty Cycle is reffered as Pulse width. In Pulse generator block, decrease the Pulse width to stop the fluctuations of Armature Current during forward and reverse motion.

Decreasing the Pulse width and run the model to test the sope results until the armature current flow without any fluctuations by trial and error method.

Result :

Pulse-width modulation (PWM) is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts.PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

Here Pulse width is refered as Duty Cycle. Duty Cycle is directly proportional to Voltage. While Changing the Duty Cycle it changes the Voltage. By reducing the Duty cycle from 75% to 45%,means Average Voltage and current will be produced. There is no Sudden rise and drop occurs and constant current will be produced. Therefore armature current does not shoot up when motor changes direction from forward to reverse.

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

As the principle of Pluse Width Modulation (PWM)

The average value of Voltage (and Current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.

The peaks can be remove by reducing the duty cycle.

Initial Value:

Following plot show the output when the Pluse Width value is changed 50%:

As there is a peak when the motor is switching directions, Dutycycle is furture reduced to 40% and the following plot shows the output:

As the Duty cycle is reduced to 40% there is no sudden peak in the output.

1. The Four-Quadrant Chopper DC Drive (DC7) block’

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.

Controller Schematics :

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 armature current by computing the appropriate duty ratios of the 5kHz pulses of the four IGBT devices (Pulse Width Modulation).

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Comparing Four Quadrant Chopper and H- Bridge:

Four Quadrant Chopper characteristics:	H-BRIDGE characteristics :
1. Regenerative braking is possible.	1. Regenerative braking is not possible but dynamic braking can be achieved.
2. The current values are chopped not continuous hence simulation takes less time.	2. The Current values are continous.
3. Widley used in automobiles to control DC motor speed and direction.	3. Widley used in robotics and electro-mechanical devices to control DC motor speed and direction.
4. To change to direction of motor current signals are interrupted with one another.	4. To change the direction of motor only the polarity of the current is changed.

1. Four-Quadrant Chopper can operates in all four modes (Forward Motoring,Forward Braking, Reverse Motoring and Reverse Braking), but H-bridge can operate only in two modes (Forward and Reverse).
2. In Four-Quadrant Chopper Regenerative Braking is possible, but H-bridge Regenerative Braking is not possible.
3. In Four-Quadrant Chopper Two PI regulator (Speed Regulator and Current Regulator) is used to obtain electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system, but in H-bridge model PI regulator is not used.
4. H-bridge is a electronic circuit that switches the polarity of voltage applied to the load and Four-quadrant Chopper is electronic switch that is used to interupt one signal under the control of another.
5. H-bridge circuit are often used in Robotics and other applications to allow dc motor to run forward and backward.Four-Quadrant Chopper circuit is used to refer numerous type of electronic sitching devices and circuits used in power control and signal applications.
6. Four-Quadrant Chopper model gives better efficiency and high energy(power) storage than H-bridge model.

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:

two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V₀ 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.

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 these type of chopper configuration is used.

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

• Brushless DC electric motor also known as electronically commutated motors (ECMs, EC motors) are synchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor . — magnetic fields generated by the stator and rotor rotate at the same frequency — no slip • The stator consists of several coils which current is led through Creating a magnetic field that makes the rotor turns .

Construction of BLDC

• It consist of two parts mainly stator & rotor Stator

Stator

> Stator is made up of silicon steel stampings with slots.

> The slots are accomodated armature windings.

> This winding is wound with specified no.of poles(even number).

> This winding connected a dc supply through a power electronic switching circuits ( inverter circuits) .

Rotor

Rotor is made from permanent magnet

➢ no of poles on rotor is same as that of stator

➢ Rotor shaft carries a RPS (Rotor position sensor) and it provides information about the position of shaft at any instant to the controller which sends signal to the electronic commutator

➢ The electronic commutator function is same as that of mechanical commutator in DC motor

Working of BLDC motor:

The rotor and stator of BLDc motor is shown in figure. The stator has a coil arrangement, as illustrated; The internal winding of the rotor is illustrated in the Fig(core of the rotor is hidden here). The rotor has 3 coils,named A, B and C

Out of these 3 coils, only one coil is illustrated in the Fig for simplicity. By applying DC power to the coil, the coil will energize and become an electromagnet .

The operation of a BLDC is based on the simple force interaction between the permanent magnet and the electromagnet. In this condition, when the coil A is energized, the opposite poles of the rotor and stator are attracted to each other (The attractive force is shown in green arrow). As a result the rotor poles move near to the energized stator.

As the rotor nears coil A, coil B is energized. As the rotor nears coil B, coil C is energized. After that, coil A is energized with the opposite polarity . This process is repeated, and the rotor continues to rotate. The DC current required in the each coil is shown in the following graph.

Hall Sensors

When a magnetic field applied to a system with electric current a hall voltage Perpendicular to the field and to current is generated.This was discovered by Edwin Hall in 1879.

ADVANTAGES OF BLDC 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
  
  
  DISADVANTAGES OF BLDC MOTOR
• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors
  
  
  APPLICATION OF BLDC 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.