Week-7 Challenge: DC Motor Control
EV - 21 WEEK - 7
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge
The H-bridge is a typical DC motor control circuit, named after its resemblance to the letter H. The four triodes form the four vertical legs of the H, and the motor is the horizontal bar in the H.
An H-bridge is an electronic circuit that reverses the voltage/current at both ends of the load or output to which it is connected. These circuits are used in robots and other real-world applications for DC motor inversion control and speed control, stepper motor control (bipolar stepper motors must also contain two H-bridge motor controllers), most DC-AC converters in electrical energy conversion (such as inverters and inverters), some DC-DC converters (push-pull converters), and other power electronics devices.
H-bridge is a typical DC motor control circuit because its circuit shape resembles the letter H, so it is named with "H-bridge". 4 transistors form the 4 vertical legs of H, and the motor is the horizontal bar in H.
The H-bridge circuit can be built as discrete components or integrated into an integrated circuit and is often used in inverters (DC-AC conversion). Through the opening and closing of switches, DC power (from batteries, etc.) is inverted into AC power of a certain frequency or variable frequency, which is used to drive AC motors (asynchronous motors, etc.).
Here there are four switching components Q1, Q2, Q3, Q4, in addition to a DC motor M. D1, D2, D3, D4 are MOS-FET continuity diodes.
Working principle
The operating principle of a single-phase bridge inverter circuit as shown in the figure H-bridge inverter (single-phase)
Switch T1, T4 on, T2, T3 off: u0 = Ud.
Switch T1, T4 off, T2, T3 on: u0 = -Ud;
When switching switches T1, T4 and T2, T3 alternately at frequency fS, an alternating voltage waveform (square wave alternating positive and negative) is obtained at the load resistor R with period Ts=1/fS. In this way, the DC voltage E is turned into AC voltage uo. uo contains all harmonics and can be filtered by a filter if a sinusoidal voltage is desired.
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.
1(B). Comment on the armature current shoot-up from the scope results
Pulse generator parameters
period : 2 milli seconds
Pulse width (% of period) : 75%
initially there is a spike in armature current this is because in dc motor Ia = V -Eb / Ra
At starting Eb = 0 as speed = 0 therefore Ia = V / Ra Ia is huge and reflects in the graph
2) Period : 2 milliseconds
Pulse width ( % of Period ) : 50%
therefore the pulse width decreases the voltage applied across the DC motor decreases and the armature current finally
1(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 the four - quadrant dc supplied chopper (or DC-DC converter ) drive for dc motors this drive features closed - loop speed control with four quadrant operation the speed contol loop outputs the reference armature of the current of the machine using a PI current controller the chopper duty cycle corresponding to the commanded armature current is derived this duty cycle thus compared with a sawsooth carrier signal to obtain the required pwm signals for the chopper
The four quadrant chopper DC drive block uses these blocks from the electric drives /fundamental drive blocks library
Speed controller (DC)
Regulation switch
Current controller (DC)
Chopper
The above figure shows the schematic of the four quadrant Chopper DC drive
H-BRIDGE FOUR-QUADRANT CHOPPER DC DRIVE : -
only two quadrant of operations are possible they are forward motoring and reverse motoring it can operate in the all four quadrants ( forward motoring, reverse regenerations , reverse motoring,and forward regenerations )
higher armature current ripple
lower armature current due to higher switching frequency of DC-DC converters
less complex
simulations is continuous and simulation time is more
widely used in robotics and automobiles
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
Class-C or Type-C chopper is a category of chopper which can operate in first as well as second quadrant. This basically means that, the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as Two Quadrant Class-A chopper. In this article, we will discuss Class-C or Type-C chopper, its working principle with the help of circuit diagram and its application.
Operation / Working of Class-C or Type-C Chopper:
As we know that, a Class-A and Class-B chopper operates in first and second quadrant respectively. Therefore, if we connect both these types of chopper in parallel then it is possible to have chopper operation in first as well as second quadrant.
In fact, Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper. Figure below shows the circuit diagram of this type of chopper.
Carefully observe the circuit diagram. You will notice 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 operationwe should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.
Case-1: When CH1 is switched ON / OFF
When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo 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 Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage Vo and current Io 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 io will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage Vo 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 Vs 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 Vo is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
- The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.
- The load current flows in the direction shown in circuit diagram i.e.io is positive when CH1 is ON or FD conducts.
- The load current flows opposite to the direction shown in circuit diagram i.e.io is 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.
Caution:
Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.
Application:
Class-C or Type-C choppers are used for motoring and regenerative braking of dc motor
Hence the simulink model corresponding to the circuit diagram is built using basic simulink blocks the pulse generator used with the NOT gate to prevent the misfiring of the mosfet so they do not conduct at the same time which results in the short circuit
An amplitude of 10 is chosen so that we could see the pulses clearly
DC input voltage : 24v , output voltage : 12v
Hence we could see the voltage 12v hence the type c chopper simulated and verified
3. Explain in a brief about operation of 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.
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.
he 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. 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 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. For industrail applications and automation system, 100 V or higher rating motors are used.
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.
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
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.
