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A 3-phase AC induction motor has the following ratings:


rated power: 130kW

input power: 140kW

rated voltage: 3x400 V

rated current: 270A

supply freq: 50Hz

there is no soft-start


The motor will be used with the following variable frequency drive (VFD): http://www.clrwtr.com/PDF/Vacon/Vacon-100-Application-Guide.pdf

In this VFD case will the motor's startup current be more than 270A? The motor will sometimes run at its rated current. But my concern is: Will the motor drive more than 270A at start-up or will the VFD take care of that? Because Im afraid to damage the surrounding system with too high currents. Does VFD take care of the startup current problem?

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  • \$\begingroup\$ Potentially, starting current will be much higher than rated current. However a VFD will allow soft starting. VFDs are usually programmable so YOU specify the allowable current to IT rather than the other way round. Read its manual or consult with its maker's FAEs. \$\endgroup\$ – Brian Drummond Mar 18 '16 at 12:10
  • \$\begingroup\$ @BrianDrummond what is i/f in the manual?? is that inrush current? \$\endgroup\$ – user16307 Mar 18 '16 at 14:21
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Yes of course, you can limit the current in VFD. With VFD actualy you will have a soft start, the VFD is more sophisticated device than a soft starter. If the load is a fan (VFD for HVAC?) , then it's even more simple because the load is very small at low rpms, so at the start the all load is basicly the acceleration of rotor and blades inertia, you can set large ramp time - slow acceleration and there will be no problems.

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  • \$\begingroup\$ a fan. how bout rated 270A? can it be safe to drive? so u re saying startup current will be small ? \$\endgroup\$ – user16307 Mar 18 '16 at 11:41
  • \$\begingroup\$ so VFD will not also drive more than 270A from system? thanx \$\endgroup\$ – user16307 Mar 18 '16 at 11:41
  • \$\begingroup\$ What is your load? \$\endgroup\$ – Marko Buršič Mar 18 '16 at 11:43
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    \$\begingroup\$ The manual says that the start current is programmable. See page 50. The default is setting 80%. ------Did you read the manual??------ \$\endgroup\$ – Nedd Mar 18 '16 at 11:59
  • \$\begingroup\$ Max load will be less than nominal rated power of this motor. \$\endgroup\$ – user16307 Mar 18 '16 at 13:56
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The basic question has been answered by Marko Buršič. There should be no doubt that the VFD will very effectively limit the starting current. The input current to the VFD should never exceed 110% of the rated current. The starting current will be limited with the VFD set to the default settings as received from the factory except that you must enter the power supply voltage and the required information from the rating plate of the motor.

The I/f adjustments are concerned with "flying start." That is starting the VFD while the motor shaft is already turning for some reason, perhaps a power interruption or possibly air flow that due to a supply fan running prior to starting a return fan. If you don't understand that function and can't identify that you definitely need it, it might be better to disable it. If you feel you need it, leave the settings at the default values unless you can identify that you are having a problem with it. In that case, you may need some expert assistance. The use of that function should not raise a concern for excessive starting current.

Edit:

VFDs have a capacitor charging inrush current that occur only when power is first applied to the VFD. All VFDs must limit that inrush to prevent damage to the diodes and DC bus capacitors. Various methods including combinations of methods are used to limit the charging current. VFDs are normally not energized and de-energized very often and the motor is normally prevented from starting until capacitor charging is complete. The peak charging current is very brief and the magnitude is partly determined by the source impedance. It is rarely a concern for the user and VFD manufacturers don't usually publish information about it. AC and/or DC inductance that is often built into VFDs to limit harmonic current also helps to limit charging current.

The first item of the flying start parameters in the Vacon manual serves as a heading for the related parameters that include I/f. That and the context of the material provided about those parameters serve to explain what I/f is. Other manufacturers may explain that function differently and use different parameter names. Vacon may explain the various performance feature in a more general manual that applies to more than product.

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  • \$\begingroup\$ Is starting current same thing with the inrush current? When turn on a system at the beginning current makes transient peak. There is no info about how much will this current be in the data sheet. There is only quantative info about I/f case. Plus I spent 3 hours what does I/f mean in internet and no info. Why are they hiding these? \$\endgroup\$ – user16307 Mar 19 '16 at 11:18
  • \$\begingroup\$ The reason why you don't see inrush current specifications for motors driven by VFDs is because there is effectively no "inrush current" worth considering. Current to the motor is totally controlled by the VFD control algorithm, so whatever the VFD is told to allow going to the motor, is all the motor will draw. There is a small brief capacitor charging current surge, but even this is limited by the VFD circuit so as to be rendered insignificant. \$\endgroup\$ – JRaef Jun 12 '17 at 20:29

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