Week-7 Challenge: DC Motor Control

Aim:

     To study MATLAB demo model of"Speed Control of a DC Motor Using BJT H-bridge' and the 4-Quadrant Chopper DC drive (DC7) block' and study about BLDC motor and make a two-quadrant chopper by using simulink

Objective:               

1. 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.
4. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
5. Explain in a brief about operation of BLDC motor. 

 

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward

H-Bridge concept

Here’s the concept of the H-bridge:

                              

A DC motor spins either backward or forward, depending on how you connect the plus and the minus.

If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.

                           

If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.

                        

The H-Bridge circuit:

You can build an H-bridge with four transistors.

 

            

Since the transistor can be a switch, you’ll be able to make the motor spin in either direction by turning on and off the four transistors in the circuit above.

What Transistors To Use?

The transistors you choose must:

• Handle enough current
• Use PNP (or pmos) at the top
• Have a low voltage drop between collector and emitter

Current

The most important thing is that all the transistors can handle enough current for the motor. Otherwise it will burn out.

For example, if the motor draws 1 Ampere of current, you need transistors that can handle a minimum of 1 Ampere.

Protection diodes and PWM mode

A side-effect of how a motor works is that the motor will also generate electrical energy. When you disable the transistors to stop running the motor, this energy needs to be released on some way.

If you add diodes in the reverse direction for the transistors, you give a path for the current to take to release this energy. Without them, you risk that the voltage rises and damages your transistors.

               

 

MATLAB demo model named 'Speed control of a DC motor using BJT H-bridge is shown below

                                

Reliable, smooth, and fault free speed control of a Permanent Magnet (PM) DC motor using an H-bridge is an important need for many industrial applications such as robotics, automotive, and process industry to improve the overall efficiency and productivity. The reliability of H-bridge depends on the semiconductor switches used. MATLAB/Simulink environment was used for simulation experiments and the results demonstrate the stable operation of the motor in the events of faults while maintaining its speed

Model Description

• 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.
•                

From the above graph we can see that, for 0 to 0.5 sec, the current flows through diode D3 and from 0.5 sec to 1 sec, the current flows through Q3

• 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.
• 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

                       

• The waveform for speed, armature current and load torque is shown in above graphs
• From armature current graph, At starting there is sudden spike in current at 0 sec also and at 0.5 sec also
• The Voltage will be changed in the circuit, the speed of motor also changes.
• In this transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occuring in the circuit.
• As there is already current flowing in the circuit and on the switch after 0.5sec the current in the circuit adda to the changed direction of current and we observe a surge.

 

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

Armature current Shoot -up:

• DC motors take electrical power through direct current, and convert this energy into mechanical rotation. DC motors use magnetic fields that occur from the electrical currents generated, which powers the movement of a rotor fixed within the output shaft. DC motors are suitable for many applications including conveyors, turntables and others for which adjustable speed and constant or low-speed torque are required. They also work well in dynamic braking and reversing applications, which are common in many industrial machines.
• We know that as the armature rotates, a voltage is generated in its coils. In the case of a generator, the emf of rotation is called the Generated emf or Armature emf and is denoted as Er = Eg. In the case of a motor, the emf of rotation is known as Back emf or Counter emf and represented as Er = Eb.
  We can say that back electromotive force (back EMF, BEMF) is a voltage that appears in the opposite direction to current flow as a result of the motor's coils moving relative to a magnetic field. It is this voltage that serves as the principle of operation for a generator.
  Motor e.m.f. equation: Eb=V−IaRa">Eb=V−IaRa. At starting Eb=0,">Eb=0,, so Ia">Ia will be maximum. So we can say that the armature current in a D.C motor is maximum when the motor has just started.

 

• In the armature current graph, we can see that at starting there is sudden spike increase in current at 0 sec and at 0.5 sec also it means at transition from positive to negative direction of rotation we observe a high surge in current this is due to the transient switching occuring in the circuit
• As there is already current flowing in the circuit and on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge in current
• The surge in the current can be reduced by reducing the duty cycle.In the below results, the duty cycle selected as 75%

                                                

  

By changing the duty cycle from 75% to 50%, the armature current surge is reduced tremendously

                                                    

  

If we change duty cycle to 40% the armature current graph is shown below

 

Therfore the armature current spike can be reduced by changing its duty cycle to low as possible.

 

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

• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-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.
• 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.

                                       

 

Four-Quadrant Chopper DC Drive block:

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library

 

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

Type -C chopper or Two-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 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.

 

TWO QUADRANT CHOPPER MODEL USING SIMULINK:

                              

Let us take two mosfet blocks and diodes blocks and connect it as Shown in Circuit diagram.

The dc Voltage Source is Connected as Shown

Let the Voltage of dc Voltage Source be 24 Volts

                                            

In this we consider as series RLC branch block and select inductor L, thus blocks works as a load.

To consider LE load ie., motor load and dc voltage source of 12 volt is also connected in series with inductor.

                                           

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. The following diagram shows how each parameter affects the waveform.

The Pulse Generator block can emit scalar, vector, or matrix signals of any real data type. To emit a scalar signal, use scalars to specify the waveform parameters. To emit a vector or matrix signal, use vectors or matrices, respectively, to specify the waveform parameters. Each element of the waveform parameters affects the corresponding element of the output signal. For example, the first element of a vector amplitude parameter determines the amplitude of the first element of a vector output pulse. All the waveform parameters must have the same dimensions after scalar expansion. The data type of the output is the same as the data type of the Amplitude parameter.

The block output can be generated in time-based or sample-based modes, determined by the Pulse type parameter.

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

• Brushless DC motors (BLDC) have been a much focused area for numerous motor manufacturers as these motors are increasingly the preferred choice in many applications, especially in the field of motor control technology. BLDC motors are superior to brushed DC motors in many ways, such as ability to operate at high speeds, high efficiency, and better heat dissipation.

 

• They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also for electrical power generation. With the development of sensorless technology besides digital control, these motors become so effective in terms of total system cost, size and reliability.

 

What is a Brushless DC motor (BLDC)?

• 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.
• 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 SCR 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. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

Construction of BLDC Motor:

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.

 

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.
• 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.

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.

                                                                          

• Ferrites 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.

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 Principle and Operation of BLDC Motor

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 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 energized as North and South poles. 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.

                                                                        

• Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give 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.

Brushless DC 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.

 

Advantages of Brushless DC 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

 

Limitations of Brushless DC motor:

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors

 

Applications of Brushless DC 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.

Conclusion:

            From the MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’. and ‘The Four-Quadrant Chopper DC Drive (DC7) block’. and we made a two-quadrant chopper using simulink and also briefly explain the BLDC motor.