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
The aim of this project is to design, construct and build a motorised base driven by two small dc motors. The motor drives are two H-bridges. The switching devices in these bridges

Q. The flux-density distribution in the air gap of a 60-Hz, two-pole, salient-pole machine is sinusoidal, having an amplitude of 0.6 T. Calculate the instantaneous and rms value

Consider a colloidal suspension of latex particles confined between two plates. The plates are charged, so there is an electric potential difference V between them (as shown). The

Parity flag  - Registers If after  any arithmetical or logical operation if  number of  the accumulator  are even  parity  flag (P)  is set otherwise  reset.

Factors Contributing To High Technical Losses Power distribution systems in developing countries have faced a phenomenal and rapid growth within the last two decades. Although

Q. The response y(t) of a linear system to a unit-step excitation is y(t) = (4 - 10e -t + 8e -2t )u(t). (a) Find the system function. (b) Find the frequency at which the fo

Carry Fla Registers  The  size  of the accumulator is of 8 bit  if after  any arithmetical  or logical  operation 9 th   bit is generated i e  there is  carry beyond  D 7   bi

With a block diagram explain fiber optic communication system Ans: The block diagram of a fiber optic communication system is display below: The input signal is giv

Write explanatory notes on Hard disk drive controller. Hard disk drive controller: It converts instructions from software running upon the computer to the electrical signals

Define Electric current If an electric pressure or voltage is applied across any material there is a tendency for electrons to move in a certain direction. This movement of fre