For electrical engineers: Does the current inc or dec when the voltage inc?

Question

I ve never understood this

I mean in motors for example,sometimes when u increase the applied voltage ,the current decreases but when u increase the voltage too much the current increases

So im puzzled between 2 laws"\

1) P=VxIx PF so neglecting the change in the PF and since the load on the motor is constant (load torque constant) then P is constant so if V increases I should decrease

But also

2) V=IxR ,and since the motor has a resistance so accoding to Ohms law when V increases I should increase too

So can someoen explain this please,,

I heard of the bell curve but i dont understand from where it came from and how the 2 equations i mentioned are apllied for the same motor

but for example in transformer,both laws are applied since P is constant then when V inc I dec + when u put the trans the primary does not see the actual resistance but the resistance multiplied by (n2/n1)2 so it makes sense

Answer 1

In a motor the applied voltage is opposed by the induced voltage caused by cutting magnetic lines of force.  At low voltages the motor will stall (not move, ot turn very slowly) especially if it has a constant load and the induced voltage will drop to zero leaving only the resistive and reactive impedance.  At this point the current will increase.  This is a very non-linear effect and is dependent on the motor design and the load.  When the voltage is increased the current will drop to a certain point.  The current will again increase as the motor supplies more power to the load.

Answer 2

Usually increases. But it depends on the load. If you have a reactive load, ie, inductive or capacitive, you can get anything, instantaneously. But even with a reactive load, if you measure the RMS voltage and RMS current, that is, average over a cycle, when you increase the voltage, the current goes up.

Transformers, when you increase the voltage, the current increases. You are mixing up the equations used for designing the transformer. But for any transformer, increase the voltage and the current increases, unless, as above, you have a very reactive load on the secondary.

Now, there are always exceptions. It's possible to configure an opamp so that it looks like a negative resistance, for example.