Speed Current Relationship in Induction Motors

[bodoo23]

It is said that speed of an induction motor depends on its frequency and torque depends on current. But I will explain what i cant understand here.

Let’s think an induction motor runs with constant load and speed. When I increase its current (by varying voltage) but keeping the source frequency constant (by vfd) what happens ?

For me, when i increase current it will cause torque to increase , when torque increase and motor will start to accelerate aftet it gets high speed it will find another point source and load torque trades off each other. In this way I don’t change frequency but motor gets higher speed by increasing current.

Is this assumption correct ? Or do i miss something ?

 

[lfe]

Let’s think an induction motor runs with constant load and speed. When I increase its current (by varying voltage) but keeping the source frequency constant (by vfd) what happens ?

If you increase the voltage the speed will increase slightly and will be closer to the synchronism speed corresponding to the frequency but it will never exceed it.
The intensity is likely to go down.

 

[ivo.maenen]

It is said that speed of an induction motor depends on its frequency and torque depends on current. But I will explain what i cant understand here.

Let’s think an induction motor runs with constant load and speed. When I increase its current (by varying voltage) but keeping the source frequency constant (by vfd) what happens ?

For me, when i increase current it will cause torque to increase , when torque increase and motor will start to accelerate aftet it gets high speed it will find another point source and load torque trades off each other. In this way I don’t change frequency but motor gets higher speed by increasing current.

Is this assumption correct ? Or do i miss something ?

You miss something.

When you apply a 3-phase voltage, an magnetic field that turns with the synchronous speed will exist. This turning speed is
ns = 60*f / p with p : number off magnetic pole-pares

ns can be 3000, 1500, 1000, ect.

When the stator is still, this magnetic field will cross the rotor with a speed of e.g. 1500 rpm.
This induces a voltage in the contactor-bars and because there a short-circuit rings on both sides there will flow a current.

This induced current will give you the torque and the rotor will start to turn.
But when it turns it will have a certain asynchronous speed and thus the relative speed difference between magnetic field and rotor (the slip) will decrease.
And so there will be less voltage and current induced in the rotor...

With U/f-controlled drives the voltage does not only change in frequency but also in voltage (U/f = constant over a large part of the torque-speed-curve).

In doing so it keeps the torque constant untill the voltage can not be anymore higher when you increase the frequency. At that point the torque will decrease. This because the current will decrease and this finds it reason in the fact that:
- the voltage is constant
- the impedance will increase with the frequency: Xl = 2*pi*f