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

 

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