VFD Steady State Input Voltage Tolerance QUESTION 1

[ThePunisher]

I would like to know if I can operate the input of my VFD at 5~7.5% voltage drop. We have an electric dredge rated for 13.8 kV. The dredge has a small electrical room with 13.8 kV switchgear feeding a 13.8-4.16 kV VFD and a 13.8-0.6 kV stepdown auxiliary distribution transformer within the 13.8 kV switchgear.

We are relocating the source 72-13.8 kV substation which unfortunately make us extend the cable around 200 meters more (or less). As such, at the nominal primary tap at the 72-13.8 kV transformer, the voltage drop at the terminals of the dredge switchgear is around 6-7% nominal. To keep the same source transformer tap, can the VFD actually tolerate the voltage drop continuously? We have not verified if a VFD input isolation transformer is there with primary taps. For station transformer, we may adjust the primary tap connection.

Worst case, we will end up adjusting the 72-13.8 kV source transformer primary tap, as long as the resulting over-voltage at light loading (Dredge under maintenance) is within 104.5 nominal%.

Regards,

 

[FacEngrPE]

As a general question, VFD's can be tolerant of input voltage being contentiously low up to a point. To provide a useful answer more information is needed.
Expected voltage at the drive terminals under maximum power output, and some information about the drive and motor.
Fred

 

[itsmoked]

As long as the motor is still getting about the right voltage it won't start to draw excess current which the VFD would care about. If on the VFD display the output voltage is within 8% or so of the motor name plate (to account for the drop from the VFD to the motor) it should be okay...

But if there are taps available to correct it I'd go to the effort to use them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

But remember, maximum output voltage = maximum input voltage*. Then on a VFD, if you reduce the output voltage you will have to reduce the speed too, or suffer a loss of torque. You are also likely getting a voltage drop through the step-up transformer already, so this may exacerbate that.

* The only time this is not true is on an Active Front End Voltage Source Inverter type of VFD, they can typically boost the output voltage about 10% over the input voltage. Those are rare in MV applications though.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Jeff, correct me if I am mistaken;
I understood that it was more of a torque limit issue than a speed limit issue.
That lower input voltage restricted the maximum HP.
If the torque demand is less than full load, the motor can be increased in speed until the maximum torque available at the supplied voltage is reached.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!