Induction Machines: Torque Speed Characteristics & Benefits

[billy fok]

< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

hello really need some help

i would like to know

how would the torque speed characteristic for a "constant frequency variable voltage supply" and 'variable frequency variable voltage supply"? the change on the torque speed graph.

what is the advantage of achieving a 'constant volts per herts' control??

hope to hear from you soon ...

is not homework at all..

 

[jim hardy]

how would the torque speed characteristic for a "constant frequency variable voltage supply"

You get a family of curves, one for each voltage.
They retain similar shape but torque is in proportion to square of voltage
ie at 90% volts you get 81% torque

'variable frequency variable voltage supply"?

Torque will still be a function of slip
and at lower frequency you'll have to have lower voltage
so you'll have to re-plot your torque-speed curves to reflect both the new lower synchronous speed and reduced voltage

so the curve will get less tall and squeeze to left as frequency decreases.

what is the advantage of achieving a 'constant volts per herts' control??

You do remember that volts is n dΦ/dt,
which means flux Φ is in proportion to ∫volts dt ?

At lower frequency your period of integration is longer, so flux is larger.
Volts per hertz control protects the iron against overflux, which could melt it.
Constant volts/hz gives constant flux.

 

[billy fok]

really thanks the reply.. i see light already

 

[jim hardy]

Thanks for the kind words,

a good motors textbook will have formulas
but of course one needs motor parameters
rotor resistance is a major player
and designers play tricks with rotor bar shape to make that a function of flux and slip.
With a wound rotor motor you can play your own external tricks

if you're in school see if your EE dep't has a motor lab you could take. That proved to be one of my mot valuable courses.

thanks again -

old jim