Generator size for vfd controlled 3 phase 5.5 kw motor

[Stuart Smith 5Participant @stuartsmith5]

Your original question was about the size of generator if you used a vfd.

It sounds like you are now thinking of running it without a vfd. The starting current (for a short time) then could be 6 times the full load current so you would need a larger generator to cater for that. Also the power factor on starting is very poor.

Stuart

[noel shelleyParticipant @noelshelley55608]

The supply to the machine will be 415v 3 ph . It may be 3 wire or 4 wire depending on connections and equipment. The switching from high to low speed will be done by the switch gear in the machine. If you don’t have and understand the switch gear then you may find it difficult to sort out and get working.

Your questions make me think you don’t know much about electrical matters in which case I would suggest you seek the help and advice of a competent person, this is not any old electric – it’s 415 volt.  Noel.

[PutneyChapParticipant @putneychap]

I know domestic – but industrial 3 phase – not my area.  The genset will only supply my kit – of which this lathe is far and away the largest load.   My original question remains – how can I limit current inrush to charge capacitors etc on startup – I thought VFD might help – but it seems thats not true.   So I will hire a 10kw genset buiders type no inverters etc and connect the lathe to the supply (I am competent to do that).   I will then start on low speed – and measure the voltage drop – and see if it even starts.    I will then take it from there.    Its a workshop in a shipping container, hence – I need power (genset).

[PutneyChap]

On PutneyChap Said:

Tx Julie – so you think – from the plate its 4 pole 8 pole – and its STAR (series) supply all the time….

The part quoted above is correct; what followed isn’t.

In high speed range a given current will flow in each phase. That given current will produce a given torque. The torque times the rotation speed gives power; 7.5hp in this case in high speed range.

In low speed range the control electrics will ensure that the same current flows in each phase, so same torque as for the high speed range. But the speed will be halved due the windings associated with each phase being rearranged to double the number of poles. Same torque times half the speed is half the power; 3.75hp in low speed range as on the motor plate.

Julie

[noel shelleyParticipant @noelshelley55608]

Remember it may run in reverse depending on phase rotation. Noel.

[duncan webster 1Participant @duncanwebster1]

Having a vfd energised without a motor connected is reputed to blow the vfd. I wonder whether switching from high to low speed and vice versa has the same problem.

[Stuart Smith 5Participant @stuartsmith5]

 

”My original question remains – how can I limit current inrush to charge capacitors etc on startup – I thought VFD might help – but it seems thats not true.”

For a vfd, the initial current is very short and it’s value will vary and only occurs when the vfd is first powered up. It is the current required to charge the internal capacitors. You can also get a vfd with inrush limiting to reduce this.

For a directly connected motor, the starting current is up to 6 times its full load current and is of longer duration. It also occurs every time the motor is started. It is a lot more onerous than a vfd.

If you look at the chart I showed in my earlier post, you see the difference.

Stuart

 

[PutneyChapParticipant @putneychap]

So i am planning a vfd to the max power input all the poles.  Looking at the current inrush and associated voltage drop it looks manageable.   Any issues with using the star connection?   I dont see why there would be but. I will use the vfd to slow the speed and not bother with the half pole option.  Sounds like a plan?

[noel shelleyParticipant @noelshelley55608]

Star is 415v. Is your VFD star at 415v or Delta at 220V ? The first thing is to see what your VFD is ? Then start to think about how to power it. Noel.

[Robert Atkinson 2Participant @robertatkinson2]

A VFD output is not star or delta because it has no windings. It is closest to Delta because there is no Neutral connection. Thus the voltage is quoted phase to phase. This is the square root of the number of phases times the notional phase to neutral voltage. So 1.73 times for 3 Phase. Assuming a non-voltage multiplying VFD the output phase to phase voltage cnnot be higher than the input single phase voltage.Thus nominally in the UK 240V phase to phase compared to the nominal 415Vphase to phase of a UK 3 phase supply.
Note 415V is 1.73 times 240V.
The motor manufacturer has options on how to wind a 3 phase motor. Fundamentally there must be at least 3 windings (normally N x 3 because of the number of poles). For 415V they can have 415V windings and connect them in delta, basically a ring or they can have 240V windings and connect them in star with a common point (which may or may not be taken to a terminal block). Fortunatly for us, most motors of the sizes we use are wired with 240V windings and star connected. This is because the 240V winding has fewer turns of thicker wire which is easier to wind, more robust and thus cheaper.
We can then re-connect this type of motor as delta and 240V phase to phase to run of a 240V input VFD.
Note that a motor with 415V delta connected windings would require 718V to run full power if connected as star. Some motors are 415V delta. These were used with star-delta control gear to reduce inrush current. The motor was started in star and once turning switched to delta for full power.

For the OP issue using a VFD as a soft starter WILL reduce the size of generator required to run the motor.

Robert.

[PutneyChapParticipant @putneychap]

Well i will be buying a new vfd and generator. I know how vfd work. Rectify to dc. Maybe with a voltage step up then chop dc into variable frequency ac.  So I am not hearing star (series connection) is an issue for a vfd

[Robert Atkinson 2Participant @robertatkinson2]

Yep, not a problem. At this size of motor and generator a 3 phase 415V in 3 phase 415V out VFD is ideal.

[noel shelley]

On noel shelley Said:

Star is 415v. Is your VFD star at 415v or Delta at 220V ? The first thing is to see what your VFD is ? Then start to think about how to power it. Noel.

I fear it’s not that simple!

PutneyChap’s DSG has a whopping big motor originally powered by a hefty industrial 3-phase supply, not a generator and VFD.   Even with proper 3-phase, the machine’s heavy start-up current had to be managed by a controller that applied 3-phase to the windings in star until the motor was up to speed, and only then switched the windings to delta.  Contactors galore!   And not how star and delta are configured by Model Engineers applyng VFDs to small motors where in my experience the motor is always star or always delta, whereas PutneyChap’s big motor uses both!

Apart from star/delta starting being outside my experience two other confusions:

• How big is the motor?  The thread title says 5.5kW,  whilst the opening post says 7.5HP.   7.5HP is output, so input assuming 20% efficiency is of the order of 7kW, about 30A at 230v.   So I think putneychap is looking for a 400V 3-phase generator of at least that size, call it 8kW, and a 400V VFD rated to 7 or 8kW.  I suggest the generator and VFD  both be 3-phase, i.e. don’t convert single to 3-phase, so the VFD provides start and speed control, not phase conversion.   But read on!
• There’s a strong hint the motor is two speed, done by switching poles.  Is that the case?  If so, yet more contactors and complicated control wiring.  Worst of all VFDs are more-or-less unsuitable for driving 2-speed motors.

For the reasons explained by Stuart, I doubt VFD start-up current is a problem.   Unfortunately I fear the real issue is the power of the 400V motor, it being two-speed, and how complicated the existing wiring is likely to be.   As two speed makes it difficult to use a VFD maybe the answer is a simply a big generator, then cost is the only problem.  They start at about £6000 new.  Dunno about secondhand.

Otherwise, how about running the motor hard wired for delta only, limiting power to 3.75kW, and disconnecting the motor’s dual speed capability?  Once the motor is wired for single speed only, a VFD could manage.  Though underpowered the lathe should still be usable.

But I’m suggesting a simplified compromise in the absence of information.   Any chance of a circuit diagram and photos of the contactors, control switches, and wiring?   Might not help: gut feel is I’m out of my depth on this one.

Dave

 

[noel shelleyParticipant @noelshelley55608]

Dave has given the simplest and easiest answer A big enough genny, it need not be that big 20 or 25 Kva why use a VFD at all ? I made my own for under £500. 3Ph and neutral , you will be able to drive all your equipment. Noel.

[PutneyChapParticipant @putneychap]

So if i run a 20plus kw generator it will be idling for 90% if the time which drastically reduces the engine life .  Now maybe that equation works but I think a smaller genset capacity run 30% load with a vfd is a better total cost of ownership number.  I asked a couple of generator vendors and they both confirmed idling a genset is a no no for longevity

[Robert Atkinson 2Participant @robertatkinson2]

I’d missed that the OP has a two speed motor.

There is actually no issue running these off a VFD IF permanently wired to either high or low speed.
Wire it delta  / low speed and if use the VFD to get higher speed if required.

Robert.

[PutneyChapParticipant @putneychap]

Now home brew genset would be an interesting topic.  I have considered that. But 15kw 3 phase alternator are on ebay for £1000 plus.   So i think to make one for £500 would need either to have an engine and alternator already or to buy very well indeed.  That said i could buy a very well priced high hours genset and refurbish or replace the engine.   The control units are not very expensive

[noel shelleyParticipant @noelshelley55608]

Hi Putney Chap, I picked up a very little used 20 Kva 3Ph alternator For £110, it was PTO driven, I coupled it via it’s belt drive to a Peugeot 1905cc Diesel running at 2000 rpm. Used a frequency meter to set 50c/s It’s done 1000s of hours on a farm. Built in the first place to drive a 400A welder in a village where there is no 3 Ph. It also worked both the 24″ saw bench and the 11Kw hydraulic log splitter. Want to know more ? Send a PM. Noel.

[Peter BellParticipant @peterbell11509]

Maybe worth waiting to see what the lather actually has fitted as my DSG type 17 has a soft start, looks like factory fit, comprising 3 wire wound resistors and a bypass contactor?

[PutneyChapParticipant @putneychap]

Its very clear what a massive difference going through the VFD – verses direct – even though the saw is half the motor cpacity.   I assume the Warco was started with clutch out – eg no load.     This is why I have decided to go VFD to the motor – it means I can size the genset at say 10kw – not 20plus Kw – if I have to cope with full current inrush.   Its been confirmed to me by a couple of genrator pros that running a generator engine at tickover or under 20% load drastically reduces the engine life – so keeping the genset capacity down is a major goal – also they are cheaper ….

[duncan webster 1Participant @duncanwebster1]

Probably a silly question, but have you asked the electricity board what a proper supply would cost. It won’t be cheap, but 10 kW Kenny isn’t

[PutneyChapParticipant @putneychap]

Not a silly question.  This workshop is in a shipping container without mains electricity otherwise i would use mains

[duncan webster 1Participant @duncanwebster1]

Just because you haven’t got a supply now doesn’t mean you can’t have one. If there are any poles nearby a supply can be laid on, and it costs nothing to ask

[Stuart Smith 5Participant @stuartsmith5]

You could have a 3 phase mains supply even though it is a container. If there is 3phase nearby this may work out cheaper than a generator.

You would have to install a cubicle ( prefabricated ie fibreglass) or a brick built one. The supply and meter and your switchgear would be installed in that and then you can run an armoured cable to your container.

There are specific requirements for this type of installation (same as a caravan). You would need to install your own earth for instance.

I would suggest you approach your local DNO (electricity distribution network operator) for a price.

This website allows you to check who that is:

https://www.energynetworks.org/customers/find-my-network-operator  

Stuart

[Stuart Smith 5Participant @stuartsmith5]

About the test I did with the vfd on the Warco lathe . It doesn’t have a clutch so the test was to start it with the 6” three jaw chuck fitted from stop to about 1000rpm.

Stuart

[Author]

Posts