CLOSED LOOP SPEED CONTROL OF DC MOTOR WITH PWM TECHNIQUE
CLOSED LOOP SPEED CONTROL OF DC MOTOR WITH PWM TECHNIQUE.pptx (Size: 162.86 KB / Downloads: 149)
AREA OF INTERESTS
SPEED CONTROL METHODS
SPEED CONTROL USING PWM TECHNIQUE
Nowadays industries are increasingly demanding process automation in all sectors. Automation results into better quality, production and reduced costs.
The variable speed drives, which can control the speed of A.C/D.C motors are indispensable controlling elements in automation systems.
The variable speed drives, till a couple of decades back, had various limitations, such as poor efficiencies, larger space, lower speeds, etc.
However, the advent of power electronic devices such as power MOSFETs, IGBTs etc., and also with the introduction of micro-controllers with many features transformed the scene completely and today we have variable speed drive systems which are not only smaller in size but also very efficient, highly reliable and meeting all the stringent demands of the various industries of modern era.
Direct current (DC) motors have been used in variable speed drives for a long time.
The versatile control characteristics of DC motors can provide high starting torques which are required for traction drives.
Control over a wide speed range, both below and above the rated speed can be very easily achieved. The methods of speed control are simpler and less expensive than those of alternating current motors.
There are different techniques available for the speed control of DC motors.
The phase control method is widely adopted, but has certain limitations mainly, it generates harmonics on the power line and it also has got poor p.f when operated at lower speeds.
The second method is of PWM technique, which has got better advantages over the phase control.
In a Dc motor, the back emf depends on armature speed, governed as follows:
If the speed is high, back emf is high hence Ia decreases.
If the speed is low, more current flows which develops more torque resulting in acceleration.
Because of these two reasons, the speed of a Dc motor is regulated automatically.
In Dc motors, there are three types of motors.
Shunt motors, and
Separately excited Dc motors
c applications, because of their limitations.
Among all existing electric motors, the separately excited Dc machines has the best ability to fulfill the demands of adjustable drive systems, as its speed can be varied over a wide range through voltage and field flux control
There are many methods to obtain variable voltage for armature from Ac supply. The following are few important methods used for speed control of Dc machines:
Line commutated thyristor converters.
Uncontrolled bridge rectifier followed by chopper.
LINE COMMUTATED THYRISTOR FOR SPEED CONTROL:
The basic function of the phase-controlled converter is to convert an alternating input voltage to a controllable direct voltage.
In the operation of the converter, each thyristor turns on and conducts for a certain fraction of the Ac cycle time. Then the current is transferred (or commutated) to the next thyristor in the sequence. The turn-on of the thyristor is achieved simply by providing a gate pulse .
This commutation process is often referred to as natural commutation.
The converters, they themselves can be classified as:
Single phase converters.
Three phase converters.
Single phase converters:
Single-phase converters are normally used for motors of a capacity less than 10 Hp, usually up to 5Hp.
Three phase converters:
When the motor capacity is higher than 10 Hp, we generally selects the three-phase converters.
PWM is critical to modern digital motor controls. By adjusting the pulse-width, the speed of a motor can be efficiently controlled without the problems of line harmonics and also with improved P.F.
The general circuit diagram of a PWM based Dc motor speed control consists the Ac supply is converted to Dc by using an uncontrolled diode rectifier.
The rectifier output is filtered with a capacitor, so that a constant Dc is appearing at the input of the chopper.
This fixed Dc voltage can be converted to a variable average voltage on a load by placing a high-speed switch between the Dc source and the load.
The switches may be a static one such as SCR or power transistor, power MOSFET or IGBT. Depending on the power rating a power transistor, SCR, power MOSFET or IGBT may be used as a switch.
The L and C components form a filter and diode D acts as a free wheeling diode.
the average output voltage of the chopper is given by:
Average output voltage > V0 =ton / ton + toff . Vi
V0 = f ton - Vi
Where f is the switching frequency.
The switching frequency is a parameter which should be carefully selected at the time of design.
For high frequencies SCR can not be used because of the forced commutation circuit. If the frequency is low, the switching losses are minimum, but the filter components are like.
Thus, the switching frequency is a trade off between these two. For higher switching frequencies, as one cannot use SCR. Power MOSFETS or IGBTS can be thought of
PWM duty cycle control techniques enable greater efficiency of the DC motor .
PWM switching control methods improve speed control and reduce the power losses in the system.
The pulses reach the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily.
The main Disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference .
It can give speed below the full speed, not above. It cannot be used for fast controlling of speed.
Conveyor Belt carrying loads
Various motors requiring smooth speed control
DC motors of all range can be controlled, which are used for the production of materials Electric locomotives DC motor using precise job preparations
For better speed control applications dc motor can give a better performance than induction motor and ac motor.
PWM technique is the best method compared to other methods because in this method harmonics are eliminated. So that smooth speed control is possible.
The present project is practical one and high feasibility according to economic point of view, reliability & accuracy
It is programmable one therefore it can control various motors ranging small one to several hp motors.