DOL motor starter and a VFD?

[Andrew TinsleyParticipant @andrewtinsley63637]

I have a mill powered by a 3 phase motor, It is turned on via a DOL starter with a thermal overload trip. It is powered from a genuine 3 phase supply.

I am considering inserting a 3 phase 415V I/P VFD between the starter and motor. This would allow some speed variation.

I consider a thermal trip to be a sensible precaution to protect the motor. However I am a trifle concerned that inserting the VFD in circuit will not allow the trip to function as it should. Obviously inserting the VFD before the thermal trip is not allowable.

Comments please?

Andrew.

[David JuppParticipant @davidjupp51506]

Any decent VFD will have an ‘ I squared t ‘ trip function, which will do something very similar.

[BazyleParticipant @bazyle]

Circuitry tends not to like sudden disconnects on input or output. While a 415 input VFD will be industrial oriented and so more robust if it doesn’t have specific instructions wrt such items that they ought to expect then enquire of the manufacturer.

[Andrew TinsleyParticipant @andrewtinsley63637]

David,

Thanks for the information. Never heard of that function before, although all of my VFDs are high end products, but it sounds as if it could be useful. I have some suspicion that it may be an overall power type of limiting function and not perhaps implemented on each phase of the output. Obviously need to investigate the function and find out what it actually does!

Thanks,

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

The VFD will protect the motor better than the DOL starters thermal trip. Don’t put the DOL or thermal trip at the input to the VFD.
Most VFDs will also support a temperature sensor input. Most modern motors have sensors bult in but you can add one to an older motor. Not as good as one buried in the windings but some additional protection.

Robert

[Clive FosterParticipant @clivefoster55965]

Further to Roberts comment a VFD replaces a DOL starter.

Indeed the Eaton Power XL DE series VFD’s were essentially lobotomised versions of a normal VFD. Sold as variable speed starters being pretty much drop in replacements for DOL boxes to add speed control. I liked them because once installed they are essentially proof against ill informed fingerpoken. All the usual settings were there but you needed a, quite expensive, plug in widget to change things. Unfortunately recently discontinued.

Having removed the DOL box I’d advise putting a lever operated disconnector in the input power line to the VFD. Put it somewhere where it can easily be got at and choose one whose on-off state is clearly visible. When I set my workshop up I found some compact affordable ones from Lewden, may well be discontinued by now tho’, with a yellow top cover behind a nice red handle whose position can clearly be seen from right across the workshop. Hopefully something similar can still be found. Unfortunately all plastic so vulnerable if hit, can’t have everything I suppose. No worry for me ‘cos Clives rule is such things go on the wall about 4 ft off the floor so accidental thumping is hard. I loathe the round knob disconnectors because the stars cannot be clearly seen until you get up close’n personal.

Naturally you will fit a button box remote from the VFD for the control functions so the electronics can go somewhere nicely protected with sufficient ventilation to keep things cool. Which can make viewing the speed display hard. Personally I prefer a calibrated knob to flashing digits. Nothing we get involved in is likely to need super accurate speed setting after all.

Clive

[Andrew TinsleyParticipant @andrewtinsley63637]

Thank you everyone for your interesting inputs.

Andrew.

[Clive FosterParticipant @clivefoster55965]

Further to my comments on high visibility lever isolators.

Looks like the Lewden ISO- ones I have can still be found on-line eg Power Discount list the 20 amp version  https://www.powerdiscount.co.uk/lewden-iso-20-isolator-20a-4-pole-ip56-59886-p.asp.

Come in -20,-30 and -63 variants denoting current capacity.

UK Electrical Supplies also have what appears to be an identical device in the Selectic Seal range eg SS20A-RSY. The URL is way too long to post!

Both around £20 a pop.

This is my set up behind my big P&W lathe. Perhaps a bit more of a stretch to reach than ideal but its safe and power on/off status is obvious.

Clive

 

[Andrew TinsleyParticipant @andrewtinsley63637]

I note Robert says do not put the DOL starter and thermal trip before the VFD. The DOL starter simply acts as a power switch to the VFD, in just the same way that any mains isolator would. After all there has to be a power switch to the VFD!

I appreciate this is rather academic if the VFD has a power / time function, which can be used, effectively a digital version of a thermal trip.

I still have a problem in determining if the power / time function is active on all three phases of the output. The manuals that I have, for my mainly redundant VFDs, are not explicit on this topic.

Thanks again,

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

The VFD needs an isolator on it’s input not a switch. The DOL (or any contactor is not reliable enough to use as an isolator. If one contact sticks the VFD will appear off but with lethal voltage on the input. The input should not be switched frequently. This stresses the unput circuitry.
Even if the VFD does not have a I2t protection even the basic current limiting will provide better protecton of the motor. A DOL is less energy efficent than an isolator.

Robert.

 

[Clive FosterParticipant @clivefoster55965]

Deleted reply as basically same as Roberts.

Clive

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