Motor current ratings when using Variable Frequency Drive (VFD)

[whitbuzben]

Dear Sirs,

I am having a little bit of difficulty understanding the datasheet, which is attached, for a 315kW BROOK Crompton Motor. Specifically, I could use some help understanding the significance of the current values. Is the 526.2 Amps (labeled Amps Ia @ 400V) the locked rotor current?

Is the 127 Amps (labeled N.L. Amps) the current draw at full load torque.?

If one was to use a Variable Frequency Drive(VFD) at which current level would one need to rate the Drive (and associated cabling)? Presumably, with the VFD one will not be drawing the full locked rotor current during start-up.

The motor is for use with a wood chipper. Would it be more common to use a soft starter than a VFD in such an application?
Any advice would be much appreciated. Thanks, Folks!

 

[djs]

526 amps is the full load rated amps. Use this value to size the VFD. 127 amps is the no load current. (No Load).

 

[ScottyUK]

If the chipper is a very high inertia load then the VFD will do a better job of accelerating it than the soft start and will put a lot less heat into the rotor.

 

[waross]

I would consider a soft starter with a bypass contactor. Also, a fairly sophisticated overload relay that can model the temperature of the motor. Wood chippers that I am familiar with may have a brutal load profile. There may be periods of light loading combined with frequent overloading. A relay that models the motor temperature will give better protection and fewer nuisance trips.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

While it's true that a VFD might actually be able to accelerate the chipper better than a soft starter (in terms of not needing high current), the problem I have with that concept is that there is often a very good reason to NOT vary the speed of a chipper. The blades are typically designed around a very specific and relatively narrow speed range. If you change that while operating, you end up damaging the blades. Besides, what is the intent behind slowing it down? If someone thinks it will save energy, they have been sold an "alternate fact" by some salesman. There will be NO energy savings. So if you are not going to vary the speed, a VFD is a very very expensive soft starter. If you just need to permanently CHANGE the speed, do that with pulleys.

Soft Starter + Bypass is the time honored and accepted way of accelerating a chipper. Buy a good quality soft starter, tell your supplier what you are using it for and have them acknowledge that they understand. You do NOT want the cheapest soft starter you can buy for this.


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

 

[LionelHutz]

For starting, a soft-starter with a bypass should work just fine.

A VFD that size will require a lot of airflow for cooling which means fans, and it's not hard to guess how quickly the fan filters will clog in that environment.

 

[whitbuzben]

Hi Guys,

Thank you for the responses. djs is it definitely the 526 Amps that should be used for sizing the VFD. Surely it will not draw anywhere near this if the startup is controlled.

Thanks ALL

 

[ScottyUK]

A soft starter will limit the current to pretty much any value you want, but below about 3x FLC (thanks Lionel ) the acceleration time will get unacceptably long on a high inertia load and you'll end up putting a lot of additional heat into the rotor. You're trading a loss of accelerating torque for reduction in starting current.

 

[jraef]

In my experience, anything less that 200% and a motor connected to any kind of load will not finish accelerating, at least not without going into the motor thermal damage curve. I don't know why some soft starter mfrs even bother have a setting lower than 200%... At one company I worked for we didn't, 200% FLA was our lowest setting. But occasionally a customer would buy a competitor's product because they could set it for 50% FLA, which they perceived as a "better" soft starter. I would challenge them to let me know how that worked out, the only one that ever responded did so just to tell me I was right.


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

 

[whitbuzben]

Great! It sounds like the consensus would be to use a soft starter. However, the chipper we are working with does have a very high inertia. Taking this into account it sounds like the VFD may still be an option despite the higher cost.

 

[LionelHutz]

ScottyUK - 3 x LRC? I think you mean 3 x FLA.

The lowest I have seen was 150% FLA current starting a bow thruster.

 

[ScottyUK]

Lionel - yes, thank you. I'll go back and edit it with a little note.

 

[Mahmoud-Abdullah]

I don't know why some soft starter mfrs even bother have a setting lower than 200%

May be the soft starter will not operate on full load at first of installation.

 

[whitbuzben]

Hi All,

One more question on this. Could someone clarify for me the meaning of LRC(DOL) and LRC(SD) on the attached Brook Crompton motor datasheet? I think LRC(DOL) is Locked Rotor Current direct online. I am not sure of the other. An explanation would be appreciated. What is the difference between the two?

Many Thanks

 

[waross]

LRC(SD) Locked Rotor Current (Star Delta)

Bill
--------------------
"Why not the best?"
Jimmy Carter