Last visit: 7/26/2010
|Dear all ,
Here we will begin a series of ( Theory of operation of Electric drives (how does a drive work ? ) not How to use ?) .
I will do my best to
1-introduce the materials in clear sentences .
2- reference all info said
3- avoid the complex mathematical eqn. as possible
4- focus on the main concepts
I hope you will like it .I'll accept any modifications suggestoins of you.
here we go .
In order to talk about drives we should know thw concepts of 1- power electronics 2- machines . so first threads will be on them .
Induction Motor :-
The induction motor is by far the most widely used choice for development application in
industry and in the tertiary sector. Being both rugged and reliable, it is also the preferred choice
for the variable-speed drive applications. Low cost, high reliability, fairly high efficiency,
coupled with its ease of manufacture, makes it readily available in most parts of the world.
Figure 1.1 shows the typical constitution of a Squirrel-Cage Induction Motor, which is
composed by three sets of stator windings arranged around the stator core. There are no
electrical connections to the rotor, which means that there are no brushes, commutator or slip
rings to maintain and replace.(fig1.1 is attached )
Large induction motor can also have a wound rotor - Wound-
Rotor Induction Motor. As the name suggests, these motors feature insulated copper windings
in the rotor similar to those in the stator. The rotor windings are fed with power using slip rings
and brushes, and therefore this rotor is substantially more costly, presenting more maintenance
problems than squirrel-cage rotors. This type of induction motor was used in industrial
applications in which the starting current, torque, and speed need to be precisely controlled. In
new applications squirrel cage motors are by far the most widely used solution.
In the induction motor, a rotating magnetic field is created in the stator by AC currents carried
in stator windings. A three-phase voltage supply applied to the stator windings results in the
creation of a magnetic field that moves around the stator - a rotating magnetic field. The
moving magnetic field induces currents in the rotor conductors, in turn creating the rotor
magnetic field. Magnetic forces in the rotor tend to follow the stator magnetic field, creating a
motor torque. The speed of an induction motor is determined by the frequency of the powersupply, the motor number of poles, and to a smaller extent by the motor load. The speed
decreases a few percent (typically 1-3%) when the motor goes from no-load to full load
operation. Driven directly from the mains supply, induction motors have essentially a constant
speed. Therefore, to control the motor speed, without the use of external mechanical devices, it
is necessary to control the power supply frequency.
Many motor applications would benefit in terms of energy consumption and process
improvement, if the motor speed was modulated as a function of the process requirements.
Again to avoid math. eqn . we will mention the the ind. m/c char. curve directly (see fig1.2)
The key issue in speed control in the squirrel cage induction motor is the freq. of source
where the speed is varied according to ( V=120 *F / P) where v is the speed , F is the freq of source(our key issue ) and P is number of poles (cannot be controlled )
Electronic Variable Speed Drives can produce variable frequency, variable voltage waveforms. If these waveforms are applied to the stator windings there will be a shift of torque-speed curve,
maintaining a constant pull-out torque, and the same slope of the linear operation region of the
curve. In this way, the motor speed is going to be proportional to the applied frequency
generated by the VSD. see fig 1.3
The general configuration of most VSDs.
The three-phase, 50Hz alternated current (AC) supply is initially converted to direct current (DC), then filtered and finally, the DC/AC inverter converts the DC voltage to the variable voltage and variable frequency output applied to the motor. see fig1.4
The adjustment of the motor speed through the use of VSDs can lead to better process control,
less wear in the mechanical equipment, less acoustical noise, and significant energy savings.
However, VSDs can have some disadvantages such as electromagnetic interference (EMI)
generation, current harmonics introduction into the supply and the possible reduction of
efficiency and lifetime of old motors.
I hope these info. may add to you and were not boring.
'Electric Machinary fudamentals ' chapman
'Electric Machinary fudamentals ' robert macpherson and laramore
' induction notes ' Ewen Ritchie
Little drops of water , Little grains of sand , Make the mighty ocean and the pleasant land