Ac motors, Electrical Engineering

  AC Motors

DC motors that have their magnetic fields produced by coils as described earlier can, in principle,  be powered from AC as well, since if the applied voltage reverses (as it does in ac), then the current  in both the rotor and stator coils will reverse together, leading to no change in the direction of the torque produced. This is not the case if the magnetic fields   are produced   by permanent magnets instead of coils. In practice, energising a motor designed for DC with AC may not be very efficient because the manufacturer may not have made the motor from materials that reduce eddy current losses, expecting this not to be important (which it isn't for DC).

 

The main distinction between a motor and a generator is that, in a motor the rotr back emf is less than the applied voltage by an amount  equal to  the voltage drop across the armature (and field coil if series connected) resistance whilst in a generator the back emf is larger than the supplied voltage for the same reason. If the stator windings of the AC alternator described above were energised by ac, then

 

the   machine   would   work   as   a   motor instead, provided the rotor was turning at exactly the right speed so that each time a rotor pole approached a stator winding, the field  direction  in  that  winding  was  such that a torque was produced in the direction of motion. For this to occur, the rotor must turn  at  the  same  speed  that  would  be needed to create the frequency of voltage being applied to the stator. For this reason, such a machine is called a 'synchronous motor'.

 

A brushless version is often preferred  in which the rotor field is produced by permanent magnets rather than a coil. The power   available   from   such   motors   is limited to <50kW.The main problem with this type of motor is running it up to the synchronous speed in the first place. If it is not very close to the synchronous speed, it will not turn at all. An auxiliary motor is required to achieve the synchronous speed, after which the main motor can take over. Because of these starting problems, it has limited use, mainly applications that do not require the motor to start and stop very often. More recently, this problem can be overcome by using modern power electronic drives that start the motor with very low frequency ac and gradually increase the frequency of the supply as the rotor speeds up, keeping the rotor close to synchronism. The motor is usually fitted with a rotor position sensing device  so  that  the  electronics  can  check that  the rotor is indeed  staying synchronous with the  applied  stator frequency.

 

To  avoid  such  problems,  a  very  widely used alternative form of synchronous motor is the 'induction motor'

 

 

Posted Date: 8/23/2012 2:28:17 AM | Location : United States







Related Discussions:- Ac motors, Assignment Help, Ask Question on Ac motors, Get Answer, Expert's Help, Ac motors Discussions

Write discussion on Ac motors
Your posts are moderated
Related Questions
Assembly Languages  ( Second Generation Language) To overcome  the difficulty of machine  language assembly  language  was  designed. After  facing the  difficulty of machine

winding turns of 415v step down transformer on 50%, 60% & 80% tapping

Q. Input offset voltage of operational amplifier? When both inputs are tied to ground, i.e., both differential-mode and common-mode inputs are zero, the output should be zero.

Ask questiochan #Minimum 100 words accepted# characteristics of transformer

Q. For the logic circuit of Figure, construct a truth table and obtain the minimum SOP expression.

Q. Consider two 1-C charges separated by 1min free space. Show that the force exerted on each is about one million tons.

What are Shift Registers - Sequential Logic Circuit? The Shift registers are a type of sequential logic circuit that mainly for storage of digital data and they are a group of

Operation of Zener Diode The Zener diode's operation depends upon the heavy doping of its p-n junction permitting electrons to tunnel from the valence band of the p-type mater

Discuss mode -2 (bi-directional mode) of 8255 (Programmable Peripheral Interface). Only permitted with port A. Bi-directional bus data used for interfacing two computers and GP

Operation of heterodyne wave analyzer with block diagram