Week-7 Challenge: DC Motor Control

Challenge Objectives

1. A. To Explain a MATLAB demo model named ‘Speed control of a DC motor using

BJT H-bridge’.

1. To Comment on the armature current shoot-up from the scope results.
2. To Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
3. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results.
4. Explain in a brief about operation of BLDC motor.

Answer #1

1. A. Explain a MATLAB demo model named “Speed control of a DC motor using BJT 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 or 0). The DC motor uses the present 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.

The model below Fig.1 shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.

Fig.1

Here a block called pulse generator is used to generate pulses of certain magnitude which are fed to this system constituting of four IGBT to create pulse width modulation.

1.B.Case 1. In this case, we have taken the default parameters and the simulation is such that the motor starts rotating in positive direction with the duty cycle of 75% with mean voltage of 180V. Scope shows the motor speed, armature current and load torque. Initially the block parameters for pulse generator are as follows Fg.2.

Fg.2

Result- When we run the simulation for 1st case with keeping the parameters as default we get the following Fg.3 result in scope.

Fg.3

Fg.4

Here fg.4 in the result above for the plot of current we can observe that at 0.5 sec, the armature voltage is suddenly reversed and the motor runs in the negative direction. So, in the first half cycle, we can see that the motor attains a speed of 1000 RPM and a load torque of 6 Nm. Whereas, for the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor does not shoot up.

Case 2. In this case, the simulation is such that the motor starts rotating in positive direction with the duty cycle of 50% with mean voltage of 120V. Scope shows the motor speed, armature current and load torque. The block parameters for pulse generator for this case are as follows on Fg.5.

Fg.5

Result- When we run the simulation for 2nd case we get the following result in scope.

Fg.6

Fg.7

Here in the result above for the plot of current we can observe that at 0.5 sec, the armature voltage is suddenly reversed and the motor runs in the negative direction. So, for 50% duty cycle, in the first half cycle, we can see that motor speed reduced to approximately 420 RPM and a load torque of 1.3 Nm respectively. Whereas, for the second half cycle the same values in the negative direction. Here the armature current of DC motor does not shoot up at 50%.

From the above observation we get to know that, as we reduce the pulse width (duty cycle), the motor speed and torque reduces and can attain zero where’s when we increase the value of pulse width gradually the speed of motor and torque also increases gradually to its maximum value.

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

Fg.8

Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'C bus' variable). Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown. The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed. 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.

Fg.9

Result

Based on the above simulation performed we get the following result on the scope:

• The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse width of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.
• The desired armature current is obtained from the average armature voltage.
• The armature current follows the current reference very well, with fast response time and small ripples.

Note-

• The power system has been discretized with a 1 us time step. The speed and current controllers use a 100 us and 20 us sampling time respectively in order to simulate a microcontroller control device.
• In order to reduce the number of points stored in the scope memory, a decimation factor of 25 is used. Some transitions may thus not appear on the scope. To view detailed simulation results, reduce the decimation factor to 1.
• A simplified version of the model using an average-value converter can be used by selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The time step can then be increased up to the smallest control system sample time value. This can be done by typing 'Ts = 20e-6' in the workspace in the case of this example. See also dc7_example_simplified model.

Comparison of Four Quadrant Chopper DC Drive(DC7) with BJT H bridge

• In four quadrant chopper DC drive model, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed across the terminals of the motor with respect to the battery, to change the direction of the motor rotation.
• In four quadrant chopper, the speed reference values is changed to control the speed of the motor whereas, in BJT H-bridge the speed of the motor is controlled by varying the with of the pulse in pulse generator.
• The four quadrant chopper DC drive operates in four mode i.e, forward motoring, forward regenerative breaking, reverse motoring and reverse regenerative braking whereas, BJT H- bridge only operates in two modes i.e, forward and reverse.
• PI (Proportional) controllers are used for speed and current control in DC7 and it is absent in BJT H- bridge.
• The four quadrant chopper uses ramps to avoid current shoot up and smooth working and stability.
• The disturbances and current shoot up is low in DC7 whereas for BJT the shoot up is high because of switching losses.
• Due to the high switching frequency in the four quadrant chopper (DC-DC converters), a low armature current is obtained.
• This why the overall performance and stability is good in Four Quadrant Chopper DC Drive as compared to BJT H- bridge model.
• Only disadvantages is due to its complexity as it consist of four switching devices.

Answer #2

Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results.

Chopper- A DC to DC converter, also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators.

Choppers are of two types: (1) Step down choppers, and (2) Step up choppers. In step down choppers, the output voltage will be less than the input voltage, whereas in step up choppers output voltage will be more than the input voltage.

Chopper-Fed DC Motor Drive- D.C motor is considered a SISO (Single Input and Single Output) system having torque/speed characteristics compatible with most mechanical loads. This makes a DC motor controllable over a wide range of speeds by proper adjustment of the terminal voltage

A simple chopper – fed DC motor drive is shown in Fig.10. The basic principle behind DC motor speed control is that the output speed of a DC motor can be varied by controlling armature voltage for speed below and up to rated speed keeping field voltage constant. The armature voltage can be controlled by controlling the duty cycle of the converter (here the converter used is a DC chopper)

Fg.10

Fg.11

The above model is the model of Type Chopper(Type A Two Quadrant Chopper). In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 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.

We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF 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. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

Answer #3

Explain in a brief about operation of BLDC motor.

A BLDC or Brushless DC motor is a permanent magnet synchronous electric motor which is driven by direct current and it comprises of electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing the phase current through it at appropriate synch time) instead of mechanical commutation system. BLDC motors are also referred as trapezoidal magnet motors.

Unlike conventional brushed type DC motor, where in the brushes makes the mechanical contact with commutator on the rotor so to form an electrical path between a DC electric source and rotor armature winding, BLDC employs electrical commutation with permanent magnet rotor and a stator with sequence of coils. In this motor, generally permanent magnet (or field poles) rotates and current carrying coil (conductors) are fixed. So, BLDCM are superior to brushed DC motor as it has the ability to operate at high speed, high efficiency and better dissipation.

Thy are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion system, robotics, computer peripherals and also for electrical power generation.

The layout of a DC brushless motor cn vary depending on weather it is an 'Outrunner' or 'Inrunner' style.

• Outrunner- In outunner type BLDCM the field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that requires high torque and where high speed of motor is not required.
• Inrunner- In inrunner type BLDCM the stator is a fixed drum in which the field magnet rotates inside, inside the rotor. This style is usually preferred for applications that requires less torque and is capable of spinning at high speed.

But the basic fundamental principle for both type of BLDC motor is same. The images for inrunner and outrunner type BLDC motor is given below.

Fg.12.Outer runner BLDC motor FG.13. Inner runner BLDC motor

BLDC motor works on the principle similar to that of a brushed DC motor which is based on 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

Construction of BLDC

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.

1. 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.

Similar to an Induction AC motor, the BLDC motor stator is made out of laminated steel stacked up to carry the windings. Windings in a stator can be arranged in two patterns; i.e. a star pattern (Y) or delta pattern (∆). The major difference between the two patterns is that the Y pattern gives high torque at low RPM and the ∆ pattern gives low torque at low RPM. This is because in the ∆ configuration, half of the voltage is applied across the winding that is not driven, thus increasing losses and, in turn, efficiency and torque.

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 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. For industrial applications and automation systems, 100 V or higher rating motors are used.

FG.14 Stator winding FG.15 Stator core construction

Steel laminations in the stator can be slotted or slot less as shown in Figure 2. A slot less core has lower inductance, thus it can run at very high speeds. Because of the absence of teeth in the lamination stack, requirements for the cogging torque also go down, thus making them an ideal fit for low speeds too (when permanent magnets on rotor and tooth on the stator align with each other then, because of the interaction between the two, an undesirable cogging torque develops and causes ripples in speed). The main disadvantage of a slot less core is higher cost because it requires more winding to compensate for the larger air gap.

Proper selection of the laminated steel and windings for the construction of stator are crucial to motor performance. An improper selection may lead to multiple problems during production, resulting in market delays and increased design costs.

FG.16

2. 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 of a typical BLDC motor is made out of permanent magnets. Depending upon the application requirements, the number of poles in the rotor may vary. Increasing the number of poles does give better torque but at the cost of reducing the maximum possible speed. Another rotor parameter that impacts the maximum torque is the material used for the construction of permanent magnet; the higher the flux density of the material, the higher the torque.

Ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). 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.

FG.17

3. 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 Principles and Operation

The basic principles for the working of a BLDC motor are the same as for a brushed DC motor; i.e., internal shaft position feedback. In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders. Note: Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor.

If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall-effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft.

In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three electrical windings are energized at a time as shown in figure 4.

In figure 18 (A), the GREEN winding labelled “001” is energized as the NORTH pole and the BLUE winding labelled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labelled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction shown in figure 4(B). This causes the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces torque because of the development of repulsion (Red winding – NORTH-NORTH alignment) and attraction forces (BLUE winding – NORTH-SOUTH alignment), which moves the rotor in the clockwise direction.

FG.18

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating. Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation; i.e., 6 unique combinations in which motor windings will be energized.

FG.19

Brushless Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID Controller to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.

FG.20 FG.21

Torque and Efficiency

From the study of electric motors, torque is a very important term. By definition, torque is the tendency of force to rotate an object about its axis.

Torque = Force x Distance---- Nm

Thus, to increase the torque, either force has to be increased – which requires stronger magnets or more current – or distance must be increased – for which bigger magnets will be required. Efficiency is critical for motor design because it determines the amount of power consumed. A higher efficiency motor will also require less material to generate the required torque.

Efficiency = (Output power/ Input power)x100

Where,

Output power = Torque x Angular velocity, and

Input power =Voltage x Current

Having understood the above provided equations, it becomes important to understand the speed vs. torque curve.

FG.22

From the FG.22

• With an increase in speed, the torque reduces (considering the input power is constant).
• Maximum power can be delivered when the speed is half of the “no load” speed and torque is half of the stall torque.

Conclusion:

1. The MATLAB demo model of ‘Speed control of a DC motor using BJT H-bridge’ has executed and explained the results and armature current flow also, and the Four-Quadrant Chopper DC Drive (DC7) block also executed.
2. Two Quadrant chopper also explained.
3. BLDC Motor construction Operation and Output also explained.