Ac motors, Electrical Engineering

Assignment Help:

  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'

 

 


Related Discussions:- Ac motors

Classification of feedback control systems by control action, Classificatio...

Classification of Feedback Control Systems by Control Action A more common means of describing industrial and process controllers is by the way in which the error signal E(s) i

Show the procedure twos complement subtraction, Q. Show the procedure twos ...

Q. Show the procedure twos complement subtraction? Two's complement subtraction is the binary addition of the minuend to the 2's complement of the subtrahend (adding a negative

Why 8085 processor is called an 8 bit processor, 8085 processor has 8 bit A...

8085 processor has 8 bit ALU (Arithmetic Logic Review). Likewise 8086 processor has 16 bit ALU.

Explain daa instruction, Explain DAA instruction. DAA: The DAA instruct...

Explain DAA instruction. DAA: The DAA instruction follows the instruction ADC or ADD to adjust the result in a BCD result. The DAA instruction performs only with the AL registe

Determine the dielectric constant of slab, Determine the Dielectric constan...

Determine the Dielectric constant of slab: An air capacitor contains two parallel plates 10 cm 2 in area and 0.5 cm apart. While a dielectric slab of area 10 cm2 and thicknes

Realization of digital filter design, why we are going to realization of di...

why we are going to realization of digital filter after finding the H(z).

Circuit realization of the srff, Q. You are to construct a modified truth t...

Q. You are to construct a modified truth table for the circuit realization of the SRFF shown in Figure. As indicated in the text, you guess an output and then go back ato check it

Pn diode, PN Diode The current-voltage characteristics are of major ...

PN Diode The current-voltage characteristics are of major concern in the learning of semiconductor devices with light entering like a third variable in optoelectronics devic

Ramp type dvm, use of ranging and attenuator circuit

use of ranging and attenuator circuit

Explain load flow analysis, Explain Load Flow Analysis The method commo...

Explain Load Flow Analysis The method commonly used for load flow analysis is the nodal analysis. The nodal analysis method is given in many books on circuit theory and is base

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd