VFD's and Motor Starting Currents

[kwired]

a solid state SS does not rectify the voltage
it has a set of + and - switching devices which control voltage peak (and therefore current) to the motor

Knocking off peaks doesn't change the effective RMS the motor sees? Which will effect current and torque.

 

[Ingenieur]

Knocking off peaks doesn't change the effective RMS the motor sees? Which will effect current and torque.

RMS = peak/sqrt2
if you reduce peak, you reduce RMS
but I did not mean 'peak' figuratively

 

[kwired]

RMS = peak/sqrt2
if you reduce peak, you reduce RMS
but I did not mean 'peak' figuratively

Whatever you meant the motor is seeing reduced voltage compared to direct connection to the lines but still at the same frequency, so synchronous speed is unchanged, but lower voltage causes less current to flow which reduces torque and causes more slip.

With the VFD you are starting out with low synchronous speed but supplying the proper voltage to go with the frequency to get the torque needed to run at the reference speed.

Both result in "soft starting" compared to across the line starting, but do have differences in how well they may work with certain loads.

 

[RumRunner]

Whatever you meant the motor is seeing reduced voltage compared to direct connection to the lines but still at the same frequency, so synchronous speed is unchanged, but lower voltage causes less current to flow which reduces torque and causes more slip.

With the VFD you are starting out with low synchronous speed but supplying the proper voltage to go with the frequency to get the torque needed to run at the reference speed.

Both result in "soft starting" compared to across the line starting, but do have differences in how well they may work with certain loads.

Not always.
This maybe true in resistive load but not in inductive load. To say that the frequency always stay hand in hand with the (frequency source) defeats the principle on why VFDs are invented.

In inductive load like motors . . . . when the voltage is applied it resist the change in current. The current builds up more slowly than voltage. During this event it lags in time and phase.

On the other hand when voltage is applied to charge the capacitor, the voltage is proportional to the charge on it. The process is the opposite since the current is needed in order for the capacitor to achieve the charged state, therefore current must lead the voltage.

This charging event is important because the capacitors connected to the SCRs dictate the level of output delivered to the load.

 

[Jraef]

I think there is some sort of communications breakdown issue taking place here by discussing VFDs and Soft Starters in the same vein that is leading to odd statements.

When it comes to starting and controlling AC induction motors, VFDs and soft starters basically are apples and oranges. You can’t really compare them, they do different things and do so completely differently, resulting in different effects on the line and load.

Although it occasionally happens, VFDs are not used as a “soft starter” ONLY, because it would be a waste of money. You use a VFD when you NEED to change the speed. When trying to understand what effect it has on the line, you have to understand how motors work. Because the BFD controls ALL aspects of how the motor gets and used power, the power drawn from the line is infinitely controllable. If you cannot handle more than the motor FLA being drawn from the line, a VFD can accomplish that. It will do so at the expense of TIME, meaning if Current is limited to FLA, it may take 10 seconds, 10 minutes, 10 hours or 10 days to get to full speed, but the drive and motor are fine with that.

You conversely don’t use a soft starter as anything EXCEPT as a means to start the motor, and do so softly with a lower impact on the line by reducing starting current. Discussing what a soft starter MIGHT do to a running motor is pointless, and most of the time a soft starter is bypassed once the motor is at speed in order to avoid it cooking itself in its own box. But because a soft starter is NOT changing the frequency, the ratio of voltage and frequency that the motor is designed for is not being maintained. That then reduces the torque (aka “soft” start), which in turn reduces the current. So BECAUSE the current and torque is reduced, the motor takes longer to accelerate. By “reduced” current, it means reduced to less than the 600% of FLA you see in startup, so depending on settings, 250%, 350%, 450% etc. This means that you are STILL way over FLA, so as you soft start, you are still overloading the motor and your time to get to full speed is limited, typically no more than 30 seconds. Despite the fact that some soft starters will allow you to set the current to 100% or 50% of FLA, the motor will NOT accelerate to full speed at those settings. It’s only there as a marketing trick!

 

[kwired]

Not always.
This maybe true in resistive load but not in inductive load. To say that the frequency always stay hand in hand with the (frequency source) defeats the principle on why VFDs are invented.

In inductive load like motors . . . . when the voltage is applied it resist the change in current. The current builds up more slowly than voltage. During this event it lags in time and phase.

On the other hand when voltage is applied to charge the capacitor, the voltage is proportional to the charge on it. The process is the opposite since the current is needed in order for the capacitor to achieve the charged state, therefore current must lead the voltage.

This charging event is important because the capacitors connected to the SCRs dictate the level of output delivered to the load.

What you highlighted in your quote of my post - I was discussing soft starters at that point, they do not vary frequency at all, they ramp up voltage at whatever input frequency is.