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

Explain mechanisms of propagation of radio communication, Explain mechanism...

Explain mechanisms of propagation of radio communication. Radio communication deals with electronic radiation of electromagnetic energy from one point to another through atmosp

Chemistry, write a short note on Scale

write a short note on Scale

CRO, what are the difference between digital and Analog storage oscilloscop...

what are the difference between digital and Analog storage oscilloscope ?

Types of field-effect transistors, Types of Field-Effect Transistors T...

Types of Field-Effect Transistors The channel of a FET (field-effect transistor) is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and so

D flip flop - introduction to microprocessors, D Flip Flop As we have ...

D Flip Flop As we have seen  in the SR flip flop  when the inputs  S= R  are applied the forbidden or indeterminate state occurs.  This state can destabilize the SR file flop.

Math, How you write this number in letters. 20,000.000.000

How you write this number in letters. 20,000.000.000

Determine supply current and reactive power, A single phase line has an imp...

A single phase line has an impedance of 8.4 + j11.2 Ω. The line feeds a load consisting of a resistor and an inductor connected in parallel as shown in Figure 1. The load is absorb

Resistance temperature sensor, A Platinum resistance temperature sensor has...

A Platinum resistance temperature sensor has a resistance of 120 ohm at 0°C and forms one arm of a Wheatstone bridge. At this temperature the bridge is balanced with each of the ot

Resume, who to write resume for job

who to write resume for job

RC phase shift oscillator, For the RC phase-shift oscillator if c=1pF and R...

For the RC phase-shift oscillator if c=1pF and R=10K determine the value of Rf

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