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Hello.

Please do tell me why in some induction machines there is that pull up torque(I mean the one that is when you start a machine you have the starting torque which decreases to a value known as pull up torque then increases again untill it reaches maximum then decreases as the speed furtherly increases). Thank you!
 

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Must be your poor voltage regulation in Romania.
I've read that in an American book I don't even know how to call the pull-down torque in romanian so...one would assume the pull up torque occurs in other countries as well other than Romania :). So that's why it appears? Voltage regulation? I don't think so.
 

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Possible it is do with the magnetic properties of the field and armature,
it's not a linear relationship,
maybe it takes time for the magnetic field to build and stabilise.
that then effects the impedance which would effect current.

:whistling2:
 

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Hello.

Please do tell me why in some induction machines there is that pull up torque(I mean the one that is when you start a machine you have the starting torque which decreases to a value known as pull up torque then increases again untill it reaches maximum then decreases as the speed furtherly increases). Thank you!
Most likely you are referring to CEMF that tends to make an induction motor draw more current upon start up until it becomes more "self regulating" in terms of current after it reaches normal rotation speed.
 

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The entire process of starting a motor from standstill is dynamic, meaning everything about the circuit, or rather the "equivalent circuit" is changing as the rotor accelerates. Not only is the mutual induction between stator and rotor changing with speed, but as that changes it affects the flux penetration of the winding conductors into their respective cores, which then changes the magnetics and thus the torque dynamically at the same time.

The "pull up torque" is just the name given to the torque produced at the nadir (lowest point) in those complex interactions. It's not the same on all motors, on some the flux penetration is less dynamic and their curve looks more flat. Those charts are just meant to indicate the general nature of the torque-speed relationship curve.
 
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