Week-7 Challenge: DC Motor Control

Objective:

• Explanation on Matlab model 'Speed control of a Dc motor using BJT H-Bridge and on its output.
• To compare Four-Quadrant chopper DC drive (DC7) block with H-bridge model.
• To develop a simulink model for 2-quadrant chopper.
• Brief on BLDC motor.

Description:

Power electronics:

Power electronics is the branch of electrical engineering that deals with the processing of high voltages and currents to deliver power that supports a variety of needs. Power supply in one form is processed using power semiconductor switches and control mechanisms to another form, supplying a regulated and controlled power. While switched-mode power supplies are a common application of power electronics where power density, reliability, and efficiency are of prime importance, motor control is gearing up with more electrification in transportation systems. Precise control and efficiency are key characteristics for power control applications. The study of power electronics is thus multidisciplinary, involving semiconductor physics, electrical motors, mechanical actuators, electromagnetic devices, control systems, and so on. Advancements in power semiconductor devices have paved the path for newer devices such as silicon carbide, gallium nitride field effect transistors (FETs), and power diodes. These devices have superior characteristics in terms of wide band gap that allows for high-voltage operation, thermal management, and efficiency. This has resulted in widespread usage of power electronics even in noise-sensitive areas, replacing the lossy linear power supplies and voltage regulators. The main advantage of these devices is that they can withstand high voltage when compared to the silicon devices. Thus, the systems can be designed with high-voltage capabilities, which, in turn, reduces the current and improves efficiency, for the same power to be delivered.

Power electronic systems are used in a variety of applications, such as:

• Power Generation
• Power Transmission
• Power Distribution
• Power Control

In all these applications, the input voltages and currents are switched using power semiconductor devices to provide desired outputs. The construction of basic semiconductor devices such as diodes, FETs, and bipolar junction transistors (BJTs) are altered to withstand high voltages and currents. As a result, we have silicon-controlled thyristors (SCRs), power diodes, power metal oxide semiconductor field effect transistors (MOSFETs), power BJTs, insulated gate bipolar transistors (IGBTs), gate turn-off thyristors (GTOs), and so on. The device selection is based on the power levels, the switching frequency requirements, efficiency, and the nature of inputs and outputs. For instance, in an EV powertrain, the power handled is of the order of kW. In such applications, power MOSFETs which can withstand the high voltage and switch at higher frequencies are commonly used. In the case of power transmission, where the handled power is of the order of few megawatts, silicon-controlled rectifiers (SCRs) are used.

BJT:

A Bipolar Junction Transistor (also known as a BJT or BJT Transistor) is a three-terminal semiconductor device consisting of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector and the emitter. A BJT is a type of transistor that uses both electrons and holes as charge carriers.

A signal of small amplitude if applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process.

There are two types of bipolar junction transistors – NPN transistors and PNP transistors. A diagram of these two types of bipolar junction transistors is given below.

From the above figure, we can see that every BJT has three parts named emitter, base and collector. J_E and J_C represent the junction of emitter and junction of collector respectively. Now initially it is sufficient for us to know that emitter-based junction is forward biased and collector-base junctions are reverse biased. The next topic will describe the two types of these transistors.

NPN Bipolar Junction Transistor:

In an n-p-n bipolar transistor (or npn transistor) one p-type semiconductor resides between two n-type semiconductors the diagram below an n-p-n transistor is shown.

Now I_E, I_C is emitter current and collect current respectively and V_EB and V_CB are emitter-base voltage and collector-base voltage respectively. According to the convention if for the emitter, base and collector current I_E, I_B and I_C current goes into the transistor the sign of the current is taken as positive and if current goes out from the transistor, then the sign is taken as negative.

PNP Bipolar Junction Transistor:

Similarly for p-n-p bipolar junction transistor (or pnp transistor), an n-type semiconductor is sandwiched between two p-type semiconductors. The diagram of a p-n-p transistor is shown below

For p-n-p transistors, current enters into the transistor through the emitter terminal. Like any bipolar junction transistor, the emitter-base junction is forward biased and the collector-base junction is reverse biased.

IGBT:

The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device.

The IGBT Transistor takes the best parts of these two types of common transistors, the high input impedance and high switching speeds of a MOSFET with the low saturation voltage of a bipolar transistor, and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive.

             Typical IGBT

The Insulated Gate Bipolar Transistor, (IGBT) combines the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor, (hence the second part of its name).

The result of this hybrid combination is that the “IGBT Transistor” has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

IGBTs are mainly used in power electronics applications, such as inverters, converters and power supplies, were the demands of the solid-state switching device are not fully met by power bipolar and power MOSFETs. High-current and high-voltage bipolar are available, but their switching speeds are slow, while power MOSFETs may have higher switching speeds, but high-voltage and high-current devices are expensive and hard to achieve.

MOSFET:

                  N & P-channel MOSFET symbol.

MOSFET stands for metal-oxide-semiconductor field-effect transistor. It is a field-effect transistor with a MOS structure. Typically, the MOSFET is a three-terminal device with gate (G), drain (D) and source (S) terminals. Current conduction between drain (D) and source (S) is controlled by a voltage applied to the gate (G) terminal. MOSFETs compare favourably with bipolar transistors in terms of relatively high-speed and low-loss operation. There are P type and N type by channel polarity, and there is enhancement type with normally off (gate voltage 0 V off) and depletion type with normally on (deactivated with gate voltage 0 V) by control method.

Working Principle of MOSFET

The main principle of the MOSFET device is to be able to control the voltage and current flow between the source and drain terminals. It works almost like a switch and the functionality of the device is based on the MOS capacitor. The MOS capacitor is the main part of MOSFET. The semiconductor surface at the below oxide layer which is located between the source and drain terminal can be inverted from p-type to n-type by the application of either a positive or negative gate voltage respectively.  When we apply a repulsive force for the positive gate voltage, then the holes present beneath the oxide layer are pushed downward with the substrate.

The depletion region populated by the bound negative charges which are associated with the acceptor atoms. When electrons are reached, a channel is developed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of the positive voltage, if we apply a negative voltage, a hole channel will be formed under the oxide layer.

1(a). Speed control of a DC Motor using BJT H-Bridge:

Above is the Simulink model of ‘Speed control of a DC motor using BJT H-Bridge’ which comprises of following blocks,

Construction:

Pulse generator:

The Pulse Generator block generates square wave pulses at regular intervals. The block waveform parameters, Amplitude, Pulse Width, Period, and Phase delay, determine the shape of the output waveform.

IGBT:

The IGBT block implements a semiconductor device controllable by the gate signal. The IGBT is simulated as a series combination of a resistor, inductor, and a DC voltage source in series with a switch controlled by a logical signal (g > 0 or g = 0).

The IGBT turns on when the collector-emitter voltage is positive and greater than and a positive signal is applied at the gate input (g > 0). It turns off when the collector-emitter voltage is positive and a 0 signal is applied at the gate input (g = 0). The IGBT device is in the off state when the collector-emitter voltage is negative.

DC Machine:

A DC machine is an electromechanical energy alteration device. The working principle of a DC machine is when electric current flows through a coil within a magnetic field, and then the magnetic force generates a torque that rotates the dc motor. The DC machines are classified into two types such as DC generator as well as DC motor.

DC Machine

The main function of the DC generator is to convert mechanical power to DC electrical power, whereas a DC motor converts DC power to mechanical power. The AC motor is frequently used in industrial applications for altering electrical energy to mechanical energy. However, a DC motor is applicable where good speed regulation & an ample range of speeds are necessary like in electric-transaction systems.

Diode:

A Diode is a semiconductor device that essentially acts as a one-way switch for current. It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Diodes are the most common type of diode. These diodes begin conducting electricity only if a certain threshold voltage is present in the forward direction (i.e., the “low resistance” direction). The diode is said to be “forward biased” when conducting current in this direction. When connected within a circuit in the reverse direction (i.e., the “high resistance” direction), the diode is said to be “reverse biased”.

The diode is said to be “forward biased” when conducting current in this direction. When connected within a circuit in the reverse direction (i.e., the “high resistance” direction), the diode is said to be “reverse biased”.

A diode only blocks current in the reverse direction (i.e., when it is reverse biased) while the reverse voltage is within a specified range. Above this range, the reverse barrier breaks. The voltage at which this breakdown occurs is called the “reverse breakdown voltage”.

When the voltage of the circuit is higher than the reverse breakdown voltage, the diode is able to conduct electricity in the reverse direction (i.e. the “high resistance” direction). This is why in practice we say diodes have a high resistance in the reverse direction – not an infinite resistance.

A PN junction is the simplest form of the semiconductor diode. In ideal conditions, this PN junction behaves as a short circuit when it is forward biased, and as an open circuit when it is in the reverse biased.

Working:

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.

Output:

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.

Above snap shows 'Currents' shows currents flowing in BJT Q3 and diode D3.

The above snap shows motor speed, armature current and load torque.

1(b). Armature current shoot-up:

In the above snap we can see that there is sudden increase in armature current and then it remains almost constant and from t= 0.5 seconds the DC starts to operate in reverse direction then the nature of the graph will be in negative. For forward operation of DC motor Q1 & Q4 are turned ON & for reverse operation Q2 & Q3 will be turned ON.

The above output was obtained for the following pulse generator input,

Pulse width is taken as duty cycle for this model.

The same model was run for different duty cycle values and the following armature current nature were observed.

Case 1:

For duty cycle 50%

Case 2:

For duty cycle 35%

From the above two cases we can observe that with reduced duty cycle the sudden transition in armature current nature is reduced, hence there is smooth transition from forward to reverse operation.

1(c). Four quadrant chopper DC drive:

Below shown is the Simulink model of Four quadrant chopper DC drive (an example from MathWorks),

In the above 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 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.

Working principle of 4-quadrant chopper:

• First Quadrant

During the first quadrant operation the chopper CH4 will be on . Chopper CH3 will be off and CH1 will be operated.  AS the CH1 and CH4 is on the load voltage v₀ will be equal to the source voltage V_s  and the load current i₀  will begin to flow . v₀ and i₀ will be positive as the first quadrant operation is taking place. As soon as the chopper CH1 is turned off, the positive current freewheels through CH4  and the diode D2 .  The type E chopper acts as a step- down chopper in the first quadrant.

• Second Quadrant

In this case the chopper CH2 will be operational and the other three are kept off. As  CH2 is on  negative current will starts flowing through the inductor L . CH2 ,E and D4.  Energy is stored in the inductor L as the chopper CH2 is on. When CH2 is off the current will be fed back to the source through the diodes D1 and D4. Here (E+L.di/dt) will be more than the source voltage V_s  . In second quadrant the chopper will act as a step-up chopper as the power is fed back from load to source

• Third Quadrant

In third quadrant operation CH1 will be kept off , CH2 will be  on and CH3 is operated. For this quadrant working the polarity of the load should be reversed. As the chopper CH3 is on, the load gets connected to the source  V_s and  v₀ and i₀ will be negative  and the third quadrant operation will takes place. This chopper acts as a step-down chopper

• Fourth Quadrant

CH4 will be operated and CH1, CH2 and CH3 will be off. When the chopper CH4 is turned on positive current starts to flow through CH4, D2, E and the inductor L will store energy. As the CH4 is turned off the current is feedback to the source through the diodes D2 and D3, the operation will be in fourth quadrant as the load voltage is negative but the load current is positive. The chopper acts as a step-up chopper as the power is fed back from load to source.

Output:

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.

Comparison between H-bridge and Four quadrant chopper circuit:

H-bridge can operate in only two modes i.e., forward and reverse, but in Four quadrant chopper operations in all four quadrants can be operated.

H- bridge has low switching frequency compared to Four quadrant chopper

The variation in the armature current is high in four quadrant choppers due to high switching frequency and low in H-bridge due to low switching frequency.

The H-bridge circuits are used in small electronics applications and Four quadrant chopper is used for industrial applications.

2. 2-Quadrant chopper:

2-Quadrant or Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper. Snap below shows the circuit diagram of this type of chopper.

                       Circuit diagram for 2-quadrant chopper.

We can observe that chopper CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CH2, D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2.

Case-1: When CH1 is switched ON / OFF

When chopper CH1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V_o becomes zero. However, the i_o continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence operation of chopper is in first quadrant. In fact, this is the Class-B mode of operation.

Case-2: When CH2 is switched ON / OFF

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current i_o will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V_o is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is V_s in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e., operation of chopper is within second quadrant. In fact, this is the Class-B mode of operation.

From the above two cases, we can conclude the following points:

• The output voltage V_ois zero when chopper CH2 is ON or free-wheeling diode FD conducts.
• The output voltage V_ois equal to source voltage V_s when chopper CH1 is ON or diode D2 conducts.
• The load current flows in the direction shown in circuit diagram i.e.i_ois positive when CH1 is ON or FD conducts.
• The load current flows opposite to the direction shown in circuit diagram i.e.i_ois negative when CH2 is ON or D2 conducts.

The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.

Simulink model for 2-Quadrant chopper:

Above is the snap of 2-Quadrant chopper developed in Simulink canvas and the following blocks with appropriate input were used.

Construction and blocks used:

Pulse generator: It is used provide the control signal to the MOSFET power switches (i.e., ON & OFF). The following were the inputs given for pulse generator.

Logical operator:

‘NOT’ logical operator is used in order to provide the opposite signal to second power switch.

Power switch:

Two MOSFET power switches are used in this model to obtain the 2^nd quadrant chopper output.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device that is widely used for switching purposes and for the amplification of electronic signals in electronic devices.

Diodes:

Two diodes are used which allows the current to flow in only one direction. If the positive port of the diode is connected to positive terminal of the DC source, then it will be termed as forward biasing and reverse biasing will be vice-versa. In this model the diodes are forward biased.

Series RLC branch:

From series RLC branch only inductor (L) is activated with all default values.

DC voltage source:

Two voltage sources are used one, for the power switches with 48V and diodes another for inductor with 24V.

Measuring devices:

Output current and voltage from this model is measured and output is analysed.

Output:

We can observe in the above graph that with the switch turned off the current and voltage are reduced to some values but, the min value to which the voltage can be reduced is zero and current has reduced to negative value.

3. BLDC Motor:

A Brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes 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.

Operation:

Coil and Magnets arrangement in a BLDC Motor.

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 silicon-controlled rectifiers 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. 

Advantages:

• Overall efficiency of the motor will be increased.
• Less noise and
• Longer life span.

Disadvantages:

• Limited constant power range.
• Cost of these motors is more.
• The permanent magnet can be demagnetized by large reverse magnetomotive force and high temperature. The critical demagnetization force is different for each permanent magnet material. Care must be taken when cooling the motor, especially if the motor is compact.

Conclusion:

• Models 'Speed control of a DC motor using BJT H-Bridge ' & 'Four-Quadrant chopper DC drive (DC7)' were discussed with their respective outputs.
• Simulink model for 2-Quadrant chopper was developed.
• Exlanation on BLDC motor was done.