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  1. #11
    Quote Originally Posted by Jonathan View Post
    If the spindle is stalled and drawing rated, or even higher, current from the VFD, that's actually requires very little power from the mains. One way to think of it is to recall that power is the product of torque and speed, so when the spindle is stalled the speed is zero so the mechanical power required to apply rated (or any) torque is also zero. From an electrical point of view, you still have to put current through the winding to get torque, but that current in this situation is only supplying the losses in the motor ... so the power, and thus current, drawn from the supply is low - I'd guess an amp or two at most.
    Ok well I'm not arguing here as I'll bow to your superior knowledge of electronics but what your saying if I drive the spindle into the bed it won't pull any more amps cos it's not spinning.? So why does the VFD have current limit setting and trip when spindle embeds it's self into material.? Surely that power that trips the VFD is being pulled form some where.?

  2. #12
    Quote Originally Posted by JAZZCNC View Post
    Ok well I'm not arguing here as I'll bow to your superior knowledge of electronics but what your saying if I drive the spindle into the bed it won't pull any more amps cos it's not spinning.? So why does the VFD have current limit setting and trip when spindle embeds it's self into material.? Surely that power that trips the VFD is being pulled form some where.?
    Not quite - the spindle will draw more current as the tool driven into the bed, but once the motor speed is very low, or it has stalled, the input power required for the VFD to get the motor to output a given torque is small, so the current drawn from the mains supply is also small. However, in getting to that point you will momentarily get a higher power demand, as the motor takes some time to slow down and the VFD tries to respond. The VFD will allow higher than rated current to be used for a short time, as sometimes you need that.

    The VFD does not trip on power measurement, only current. The VFD protection is done with two different current measurements:

    1. To protect the motor it will sense the current in one (or more) of the 3-phase wires to the motor, and shut down if this exceeds the motor peak rating.
    2. For the VFD to protect itself, it will sense (or calculate) the input current from the supply and shut down if this exceeds some limit.


    These two currents can be very different values - e.g. if operating at a low speed you could have 1 amp drawn from the supply but 10A line current in the motor, but with only a low voltage applied to the motor.

    One complication is whether the VFD is doing Vf control (like the cheap ones), or vector control. If vector controlled, then the torque and thus current is measured, calculated and set thousands of times a second, so if the motor is set to run at a constant speed and it suddenly experiences an increase in load torque, the VFD will output more current to try and get the speed back to the setpoint. If the load torque required to do this is too big, the current required will be too high so the VFD will trip and display an error.
    The difference with vf control is it's open loop - the current and speed aren't controlled, so if you apply a bigger load the motor just slows down and draws more current, whilst the VFD happily keeps applying the voltage and frequency you set. That's why the current limit is there - to stop the current increasing indefinitely with load.

    This concept also applies to stepper motors - notice how little power they draw when stationary, even when the driver is set to apply rated current? If you apply a torque to the stepper motor when stationary it'll still draw the same current.
    Old router build log here. New router build log here. Lathe build log here.
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