Week-7 Challenge: DC Motor Control

1.A. Explanation of Matlab Demo model "Speed Control of DC Motor using H-Bridge ":

In the Matlab/Simulink documentation, we can search for the simulation model of "Speed Control of a DC motor using BJT H-bridge". It is used to generate a chopped voltage and to control the speed of the DC motor. This model can be opened in Simulink from the documentation itself. As the name suggests, it uses a BJT (Bipolar Junction Transistor) which is a type of transistor that uses both electrons and holes as charge carriers, hence the name "Bipolar". Other transistors such as FETs (Field Effect Transistors) are unipolar transistors meaning that they only use one type of charge carrier. BJTs are used for switching and amplifying applications. BJTs have three terrminals namely, the Emitter(E), the Base(B) and the Collector(C). Current flows in the direction of emitter and collector and this current is controlled by  the base. BJTs use semiconductors in their working and so there are two types of BJT:

i) n-p-n type.

ii) p-n-p type. 

Their circuit symbols are shown below.

i) p-n-p Transistor:

A p-n-p transistor is a bipolar juntion transistor constructed by sandwiching an n-type semiconductor between two p-type semiconductors. A p-n-p transistor has three terminals- a collector(C), emitter(E) and base(B). The p-n-p transistor behaves like two p-n junction diodes connected back to back. These back to back p-n junction diodes are known as the collector-base junction and base-emitter junction. Regarding the three terminals of the p-n-p transistor, the emitter is a region used to supply charge carriers to the collector via the base region. The collector region collects most of all charge carriers emitted from the emitter. The base region triggers and controls the amount of current flows through the emitter to collector. The n-type semiconductor will be the base of the transistor. The emitter terminal is more positive in relation to the collector and base terminals. The collector is connected to positive supply and emitter is connected to negative supply. The base terminal is supplied with voltage and it operates the transistor by switching between on or off states. When the base voltage is same as the emitter voltage, the transistor is in the off state. When the base voltage decreases with respect to the emitter voltage, the transistor is in the on state. 

ii) n-p-n Transistor:

An n-p-n transistor is the most commonly used bipolar junction transistor and is constucted by sandwiching a p-type semiconductor between two n-type semiconductors. The p-type semiconductor will be the base of the transistor. Construction wise, it is exactly the same as the p-n-p type the collector terminal is connected to the positive voltage supply and the emitter terminal is connected to the negative voltage supply. The base terminal is supplied with a voltage and controls the transistor being either in an on or off state. When the base voltage is same as the emitter voltage the transistor is in off state. When the base voltage increases with respect to the emitter voltage the transistor is in on state. 

BJT H-bridge:

An H-bridge is a simple circuit that lets us control a DC motor to go backward or forward. The concept of the h-bridge is as follows. A DC motor spins either backward or forward, depending on how you connect the plus and minus. The figure for the h-bridge for control of DC motor is given below in figure(i). From the figure(ii), if we close switch 1 and switch 4, we have supply Vcc connected to the plus side of the motor and the ground is connected to the minus side of the motor. The motor will start spinning or rotating in one direction. If instead as shown in figure(iii), we close switch 2 and switch 3, the supply Vcc is connected at the negative terminal of the motor and ground at the positive terminal of the motor. Here, the polarity of the motor is changed. Hence the motor spins or rotates in the opposite direction. The BJT  when used for power switching applications, operates as an IGBT (Insulated Gate Bipolar Transistor). When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between the collector and emitter. 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 model: 5Hp, 24 V, 1750 rpm. It simulates a fan type load. The armature voltage can be varied from 0 to 240V when the duty cycle which is specified in the pulse generator block is varied from 0% to 100%.

H-Bridge consists of four BJT-diode pairs, here BJTs are simulated by IGBT models. Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired or switched on, a positive voltage is applied to the motor and diodes D2 and D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired or switched on, a negative voltage is applied to the motor and diodes D1 and D4 operate as free-wheeling diodes when Q2 and Q3 are switched off. By default, the motor will start in the positive direction and has a preset duty cycle of 75% with a mean DC voltage of 180V. At t= 0.5sec (total simulation time= 1sec), the armature voltage will reverse and the motor will run in opposite direction. The block of the DC motor BJT control using H-bridge is ahown in figure(iv). The block has two values as a display of the output. One is the current output for the IGBT and the diode current and the other is the scope for the values of the DC motor. The values of DC motor include: Speed (in RPM), Armature Current (in A) and Load Torque (in Nm). The gate pulse for the IGBTs are provided by the pulse generator and it is provided in the model also. The parameters of the pulse generator is shown in figure(v).

1.B. Armature current shoot-up from the scope output of the above model:

After running the model for 1 second, the scope provides us the waveforms of the IGBT and diode currents and DC motor speed, armature current and load torque. The waveforms are shown in the attachment below. For the DC motor voltage of 300V the value of armature current shoots up drastically when the motor goes in reverse direction. The value of current goes up to 66.9A in reverse direction of the DC motor. When we increase the DC motor voltage(field) the value of current decreases as expected. The value goes down to about 65.5A when thee DC motor voltage(field) is increased. The voltage used here is 350V. Another way to restrict armature current is to change the duty cycle of the motor. We have changed the duty cycle to 70%, 65% upto 40% and observed the results. When the duty cycle is 40%, the result obtained was satisfactory. According to the result, and in the figure below, the value of armature current is not shooting up like before.

1. C. Four-Quadrant Chopper DC Drive (DC7) Block and Comparison with H-bridge model:

According to the question above, we have to know about the four-quadrant chopper DC drive also known as DC7 block. The figure below is the arrangement of a DC motor drive using a four-quadrant chopper DC drive. In this circuit, the 200Hp DC motor is separately excited with a constant 150V DC field voltage source. The armature voltage is provided by an IGBT converter controller by two PI regulators. The converter is fed by a 515V DC bus obtained by rectification of a 380V AC 50Hz 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 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 5kHz 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. Compared to other DC drives, the main advantage of using the four-quadrant chopper DC drive is that it can operate in all four quadrants. Each quadrants in its characteristics represent a different mode of operation. The first quadrant is forward motoring mode, the second quadrant is reverse braking mode, the third quadrant is reverse motoring mode and the fourth quadrant is forward braking mode. Another advantage is that it uses high frequency switching DC-DC converters resulting in a lower armature current ripple is occured than compared to thyristor based DC drives. The four-quadrant DC drive chopper is also known as Class-E chopper.

The circuit of a four-quadrant chopper basically consists of four semiconductor switches CH1 to CH4 as shown in circuit diagram below and four diodes D1 to D4. The four diodes are connected in anti-parallel.

First-Quadrant:

For first-quadrant operation, thyristor CH4 is kept on, thyristor CH3 is kept off and thyristor switch CH1 is operated. With CH1 , CH4 on, the load gets directly connected to the source and armature voltage and armature current begins to flow. Here both ouput voltage and output current are positive giving first quadrant operation. When CH1 is turned off, positive current freewheels through CH4, D2. In this manner, output voltage and current can be controlled in the first quadrant and motor rotates in the forward direction so it is called forward motoring.

Second-Quadrant:

Here, thyristor CH2 is operated and CH1, CH3 and CH4 are kept off. With CH4 on, reverse or negative current current flows through the load, CH2, D4 and E. During the operation time of CH2, the armature inductance L store energy during the time CH2 is on. When CH2 is turned off, current is fed back to source through diodes D1, D4. Note that here (E+(L*(di/dt))) is more than the source voltage Vo. As the supply voltage, Vs is positive and load current Io is negative, it is a second quadrant operation gives forward braking mode. Here, as the power is fed back from armature to source, this mode is called as forward regenerative mode.

Third-Quadrant:

For third quadrant operation, thyristor CH1 is kept off, CH2 is kept on and CH3 is operated, polarity of armature back emf (E) must be reversed for this quadrant operation. With thyristor CH3 is on, armature gets connected to source V, so that both Vo, Io are negative leading to third quadrant operation. When CH3 is turned off, negative current free wheels through CH2 and D4. In this manner only Vo and Io can be controlled in the third quadrant. Motor rotates in reverse direction and it is called Reverse motoring.

Fourth-Quadrant:

Here thyristor CH4 is operated and other devices kept off, back emf (E) must have its polarity reversed as in third quadrant operation. With CH4 on, positive current flows through CH4, D2, Load and E(armature). Armature inductance L store energy during the time CH4 is on. When CH4 is turned off, current is fed back to source through diodes D2, D3. Here armature voltage is negative, but Ia is positive, leading to the chopper drive operation in the fourth quadrant. Also power is fed back from armature to source. This is known as Reverse regenrative braking.

H-Bridge:

An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward. The figure of H-bridge is given below. From the circuit diagram, if we close switch 1 and 4, we have the supply Vcc connected to the positive terminal of the motor and ground is connected to the negative terminal of the motor. So, the motor rotates in forward motion or in positive direction. If we instead close switch 2 and 3, the supply is connected to the negative terminal of the motor and ground is connected to the positive terminal of the motor. Here, the supply is given at opposite terminals. Hence the motor rotates in opposite direction or in negative direction or reverse direction. 

Below table is the comparison of DC7 chopper and H-bridge DC drive:

Four-Quadrant DC7 Chopper Block	DC Motor BJT H-Bridge Block
This motor has a DC to AC converter that gives the advantage of varying voltage. The maximum voltage achieved here is 400V changing phases to 200V.	This block has a straight transistor configuration using BJTs. As opposing to the name, it uses two quadrant DC motor system for voltage conversion on a single  phase. It achieves a maximum voltage of 300-350V.
Armature current changes phases due to the four-quadrant system thus providing further versality to the motor.	Armatur current produced has lesser range as compared to the DC7 Chopper.
Having individual duty cycles gives the motor more range for load torque and power outputs to the EV.	The Motor having a single pulse generator gives much less range as compared to DC7 Chopper motor

2. Two-Quadrant Chopper:

Two-quadrant chopper is also called as Type-C. Type-C chopper is obtained by connecting Type-A and Type-B choppers in parallel. We will always get a positive output 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 equal to supply voltage Vs. The direction of load current Io will be reversed. The current Io 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 diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, thee 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 like the first quadrant operation that is 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 may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used. 

Two Quadrant Chopper Model Using Simulink:

Here, we use two Mosfet blocks and diode blocks and connect it as shown in the circuit diagram below. Here, we are using the Mosfets as the switch. The DC voltage source is connected as shown. The DC voltage we are applying is 24V. We have selected the RLC branch block and select inductor L this block works as a load. A DC voltage source of 12V is connected in series so that it act as motor load. The pulse generator block is used to create 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 occur. Due to use of not gate, when one mosfet is on, the other mosfet will get off and vice-versa. The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse width be 50%. 

Below attachment also provides the scope result of the circuit of two quadrant circuit. The voltage waveform is not going less than zero. The minimum value of voltage is zero and maximum valur is always in the positive direction. However, in current waveform, the current increases and decreases. The current may be positive as well as negative. The pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

3. 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) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors. The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), inrunners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel). The advantages of a brushless motor over brushed motors are high power-to-weight ratio, high speed, nearly instantaneous control of speed (rpm) and torque, high efficiency and low maintenance. Brushless motors find applications in such places as computer peripherals (disk drives, printers), hand-held power tools and vehicles ranging from model aircrafts to automobiles. In modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes by a direct-drive design. The figure below shows the arrangement of a BLDC motor. These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motor, 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. 

Working:

Before explaining the working of a brushless DC motor, it is better to know and understand the function of a brushed motor. In brushed motors, there aree permanent magnets on the outside and a spinning armature which contains electromagnet inside. These electromagnets create a magnetic field in the armature when the power is switched on and help to rotate the armature. The brushes change the polarity of the pole to keep the rotation on of the armature. The basic working principle of the brushed DC motor and for brushless DC motor are same, that is internal  shaft position feedback. Brushless DC motor has two main parts: rotor and the stator. the rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. 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 other words, BLDC motor works on the principle similar to that of a conventional DC motor, that is 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 of 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 is energized as north and south poles respectively. 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. For energizing the stator, there is a sequence and how do we know the sequence is that as the continuous rotation of the motor depends on the switching sequence around the coils. This is obtained by the hall sensors placed at the stator windings, which gives 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.   

The advantages of a BLDC motor are:

1) Brushless motors are more efficient as its velocity is determined by the frequency at which the current is supplied, not the voltage.

2) As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.

3) BLDC motor can operate at high speed under any conditions.

4) There is no sparking and much less noise during operation.

5) More electromagnets could be used on the stator for more precise control.

6) BLDC motors accelerate and decelerate easily as they are having low rotoe inertia. 

7) it is high performance motor that provides large torque per cubic inch over a vast speed range.

8) BLDC motors do not have brushes which make it more reliable, high life expectancies and maintenance free operation. 

9) There is no ionizing sparks from the commutator and electromagnetic interference is also reduced.

10) Such motors cooled by conduction and no air flow are required for inside cooling.

The disadvantages of a BLDC motor are:

1) BLDC motors cost motr than a brushed DC motor.

2) 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. 

3) Electronic controller required to control this motor is expensive.

4) Requires complex drive circuitry.

5) Need of additional sensors.