Thread: Please can someone explain pulleys to me

Originally Posted by Sam-Q

It was my understanding that all VFD's have a built in over current limit to protect themself. I have tripped mine a few times when I managed to stall them.

Few times is fine, but doing that repeated (especially on a cheap one) is not really the way they should routinely be used.

Now why would the current double when the speed does?

Same as when you use the accelerator on a motor vehicle, more energy is needed to make anything go faster.

Kintetic Energy is proportional to the square of the speed (KE = 1/2 m x v²) double the speed requires 4 time more energy

I am a bit confused here, the kinetic energy is the total amount of energy from it being in motion. Isn't this formula only valid for when it spins up and down then? Newtons first law of motion. So therefore the only extra current going through the motor if the speed is constant is from the extra bearing drag and the higher flow of the fan?

Originally Posted by Sam-Q

I am a bit confused here, the kinetic energy is the total amount of energy from it being in motion. Isn't this formula only valid for when it spins up and down then? Newtons first law of motion. So therefore the only extra current going through the motor if the speed is constant is from the extra bearing drag and the higher flow of the fan?

What you say is correct.
What I said was very much a simplification of what happens.

The frictional forces involved are not constant and generally increase non-linearly with speed. Most motors use a fan, the frictional load for which varies as much as with the cube of the speed, plus there's some funny business going inside the magnetic fields when the motor is driven at speeds outside (usually above) it's design limits. There is some technical detail supplied in the VFD entry on Wikipedia that describes some of this stuff and makes for interesting reading if you are that way inclined.

The frictional forces are usually secondary to final power requirements which depend very much on the types of loads applied to motors.
One example they give on Wikipedia is for a centrifugal pump which on a 30% reduction in speed requires 4 times less power.
Folks with VFDs on their dust extractors will have some idea of the serious increased in currents a motor connected to an impeller will draw on just going from 50 to 60Hz.

Originally Posted by BobL

It's great when the penny drops. Just keep playing around and eventually "CA-CHING!"

This is why free play (a nasty word amongst modern educators) is so effing valuable and a useful way to learn.

Oh so true Bob!!