Week-7 Challenge: DC Motor Control

Answer 1 ;

A BJT is a bipolar juntion transistor having three terminals.When an input is applied between the two terminal an amplified or bigger size of the same signal is given as output between the other two terminals.In other words we can say that output can be controlled by the signal given at the base as input.It is capable of switching.BJT is a current controlled device.So it cannot be used in power circuits.so in power circuits IGBTs which are capable of handling more power is used.IGBTs are voltage devices but it is used to model because IGBTs and BJTs are similar in certain aspects.

Here the seperately excited dc motor is used as load.Mux and de mux are used to get the combination of bjts.Two mux are there in one mux two bjts are powered and the other two will be off by using constant zero block.Then two mux are connected to a switch where according to the value at its control input of switch(threshold here it is given as 0.5) any of the two mux is connected to the output of switch.The output of switch is connected to the demux which is connected to the four bjts.According to the switvh output corresponding bjts are triggered using a 500Hz pulse generator.When Q1 and Q4 are triggered motor terminals will be applied with positive voltage and diodes D2 and D3 act as free wheeling diodes when Q1 and Q4 are off and When Q2 and Q3 are on a negative voltage will be applied across the motor terminals and diodes D1 and D4 act as free wheeling diode.

b) here time period is 2milli second and duty cycle is 75%.here since pulse width is high armature current is also high.

when pulse width decreases armature current also decreases.

c)

Answer 3)

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctance motor.They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), inrunners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel).

The advantages of a brushless motor over brushed motors are high power-to-weight ratio, high speed, nearly instantaneous control of speed (rpm) and torque, high efficiency, and low maintenance. Brushless motors find applications in such places as computer peripherals (disk drives, printers), hand-held power tools, and vehicles ranging from model aircraft to automobiles. In modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes by a direct-drive design.

Disadvantages:

• The friction of the brushes sliding along the rotating commutator segments causes power losses that can be significant in a low power motor.
• The soft brush material wears down due to friction, creating dust, and eventually the brushes must be replaced. This makes commutated motors unsuitable for low particulate or sealed applications like hard disk motors, and for applications that require maintenance-free operation.
• The electrical resistance of the sliding brush contact causes a voltage drop in the motor circuit called brush drop which consumes energy.
• The repeated abrupt switching of the current through the inductance of the windings causes sparks at the commutator contacts, which is a fire hazard in explosive atmospheres and a source of electronic noise, which can cause electromagnetic interference in nearby microelectronic circuits.

Answer 2:

In the 2 quadrant operation current can flow from source to load and load to source as well.

when CH1 is turned on the load will get connected to the source voltage and current will start to flow from source to load.When the CH1 is turned of the inductor will change its polarity so as to maintain the flow of current and the free wheeling diode will become forward biased and current will flow and the voltage become zero.

When CH2 is turned on the load is connected across the negative voltage and the current will flow in opposite direction as that of first.When CH2 is turned off the diode D2 will get forward biased and will start conduction.

Here in circuit not gate is provided so that only one MOSFET will get active at a time.

Here duty cycle is given as 50 percent so MOSFET will conduct for 50 percent.