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

 

[rezeew]

Hello,

The torque speed characteristic for a constant frequency variable voltage supply and a variable frequency variable voltage supply will differ in terms of their shape and maximum torque output. In a constant frequency variable voltage supply, the torque-speed curve will be linear, meaning that the torque output will remain constant at all speeds. This is because the voltage supplied to the machine remains constant, and thus, the torque output will also remain constant. On the other hand, in a variable frequency variable voltage supply, the torque-speed curve will be non-linear, with the maximum torque output occurring at a certain speed. This is because the voltage and frequency supplied to the machine can be varied, allowing for a higher torque output at certain speeds.

The advantage of achieving a constant volts per hertz control is that it allows for better control of the motor's speed and torque output. This is because the voltage and frequency supplied to the motor are directly proportional, meaning that as the frequency is increased, the voltage will also increase, resulting in a constant volts per hertz ratio. This allows for more precise control over the motor's speed and torque, resulting in better performance and efficiency. Additionally, this control method also helps to reduce stress on the motor, prolonging its lifespan.