Week-7 Challenge: DC Motor Control

1. (a)Simulink model of a Speed control of 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 (1/0).
• The DC motor uses the preset model (5 HP 240V 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.

In the below we can see the simulink model of a Speed control of DC motor using BJT H bridge

here the DC motor parameter is of 5HP 240V 1750RPM Field:300V machine the speed of the motor is controlled by Pulse generator.

The pulse generator parameter are

After simulating the above model we can see the following observation.

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.

(b)Comment on the armature current shoot-up from the scope results.

• In the current output the amount of circuit acted is shown as the graphical representation during the period of duty cycle .The scope shows IGBT current and diode current passed in the quadrent Q3 and D3.
• The Q3 represents as IGBT current and D3 is represented as diode current in scope.
• The graph shows that when the switch Q3 is OFF on the starting stage of current flow and diode current is on that stage.
• During the 0.5sec of time D3 switch is ON that are given according to the pulse generator and after the time period of 0.5sec D3 switch is off and IGBT current is acted after 0.5sec and Q3 switch remain ON.

• the speed and armature current is obtained by the DC motor and load torque is calculated by load torque proportional to square of speed.
• In starting point there is a sudden shootup is acted.
• In this scope shows that speed is increased in certain period of time and at that time armature current is positive where as load torque also moves forward.
• After the time period of 0.5sec which is given in pulse generator sudden shoot up is acted and speed gets decreased in that time armature current is in negative terminal where as torque acts as reverse action.
• Here the sudden shootup is acted while Armature current is processed at a positive and negative terminal.

C. Comparison of The Four-Quadrant Chopper DC Drive (DC7) block with H bridge model

The Four-Quadrant Chopper	H bridge model
Four quadrant chopper can operate in all four modes (Forward motoring,Forward Braking,Reverse motoring,Reverse braking)	H bridge can operate in only two modes (Forward and Reverse)
In four quadrant chopper regenerative braking is possible	In H bridge regenerative braking is not possible.
In four quadrant chopper two PI regulator (Speed regulator and current regulator) is used to obtain electro magnetic torque needed to reach the desired speed and control the armature current and stabilize the system.	In H bridge PIregulator is not used.
Four quadrant chopper is electronic switch that is used to interrupt one signal under the control of another.	H bridge is a electronic circuit that switches the polarity of voltage applied to the load.
Four quadrant chopper circuit is used to refer numeroustype of electronic switching devices and circuit used in power control and signal appliction	H bridge circuit are often used in robotics and other application to allow dc motor to run forward and backward.
In four quadrant chopper Efficiency and power is more compared to H bridge	In H bridge Efficiency and power is less.

2. Two Quadrant chopper using simulink

Out put results

In the above we can see the pulse gerenerator output,Voltage measurement,Current measurement

I have taken the input voltage as 24v we can observe that in the voltage measurement due to freewheeling diode the voltage is going to negative and coming back to zero.

3. Brushless DC 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.

• 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 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.

working 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.
• 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

• 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.