Generator size for vfd controlled 3 phase 5.5 kw motor

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Generator size for vfd controlled 3 phase 5.5 kw motor

This topic has 18 replies, 10 voices, and was last updated 2 April 2025 at 21:06 by John Haine.

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31 March 2025 at 11:56 #791237
PutneyChap
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@putneychap
Getting conflicting advise on the genset sizing. Its to power a 7.5hp 3 phase motor – which will have a VFD controller. Now this will be set to soft start – so I’m not expecting a high current inrush on power up, but I also read that that is not true – VFD take material current on start up. The ask is for a genset for occasional use – a few hours a week – for a lathe in my container workshop. Thoughts on this?
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31 March 2025 at 13:04 #791251
duncan webster 1
Participant
@duncanwebster1
VFD takes large inrush current whilst the capacitors charge up, but only for a very short time. Long enough to blow a normal MCB. If this is a problem you could arrange a 2 stage start up, stage 1 with some resistance in circuit to charge the caps, then short circuit the resistors for full power. The people who make wire wound power resistors will advise, you don’t need the full V^2/R value as the surge is very short term. However, the current drawn by a VFD is very peaky, which might upset the generator controller. That is above my pay grade
31 March 2025 at 13:19 #791252
John Haine
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@johnhaine32865
Presumably the generator is powered by a petrol or diesel engine – could it be started at low throttle with the alternator coupled up and just brought reasonably slowly to speed? Or just kept permanently coupled so the supply voltage ramps up over a second or so while the caps charge.
31 March 2025 at 15:39 #791279
DC31k
Participant
@dc31k
Hopefully it is clear from the above that the advice you were given is not conflicting. Both sides of the story are true, but each side relates to a different part of the system.

The soft start is on the output of the VFD, so the motor will not try to go from zero to 50 Hz in the blink of an eye, which is what causes the high current draw there.

The VFD itself has large capacitors in it, which will demand current if energised from full discharge.

If anything ‘blows’ an MCB, there is a serious problem. Hence, saying that the current may trip the generator’s MCB could be a more accurate description.
31 March 2025 at 20:10 #791334
Robert Atkinson 2
Participant
@robertatkinson2
This depends on the type of generator. For this application the most basic “builders” type will work well. A fancy one with complex regulation or an inverter is a waste of money and more likey to be upset by the VFD.
A basic VFD’s power input circuit just rectifies and smooths the AC input to DC. It has a poor power factor so you need to match the power rating, Watts, not the Volt-Amp rating (kVA). So for 7.5HP you would need at least 5.5 kW (about 7kVA) assuming you are not fully loading the motor. If fully loading you need to allow for the 80-90% efficency of the VFD so 6-7 kW (7.8 to 9 kVA).

Robert.
31 March 2025 at 21:10 #791347
Clive Steer
Participant
@clivesteer55943
A VFD with a power factor correction (PFC) front end module would be more tolerant of input voltage fluctuations as well improving the loading the VFD places on a generator. The PFC module would also manage any “inrush” issue when initially powering up but VFD’s with this feature are more expensive

Most hi Power VFD’s manage inrush by using a limiting resistor/thermistor between the input diode bridge and the DC link capacitors and a relay contact is used to bypass the resistor once the DC link voltage capacitors have charged up. This feature is not used on low power or low cost VFD’s because of cost.

Some VFD’s may cause the electrical protection devices of domestic property to trip through a combination of a residual current imbalance or momentary current overload.

I believe most VFD’s are classified as industrial devices and are supplied as components and are designed to comply with a totally different set of electrical safety and EMC requirement to those of domestic appliances. For instance a VFD may generate a significant amount of electrical noise back into the supply which may even exceed allowable industrial levels requiring additional filtering.

Clive S
31 March 2025 at 21:35 #791348
Julie Ann
Participant
@julieann
On 31 March 2025 at 21:10 Clive Steer Said:

I believe most VFD’s are classified as industrial devices and are supplied as components and are designed to comply with a totally different set of electrical safety and EMC requirement to those of domestic appliances.

That is correct. For a component, like a VFD, it is up to the installer to add the necessary items to meet the specified standards.

A rectifier/capacitor input stage inherently takes input current in short pulses leading to harmonic voltage distortion for which there are strict standards. A PFC will also help solve the voltage distortion problem, but at the very least a VFD should have an external, passive, filter to reduce voltage distortion, and prevent noise being injected back into the mains supply.

Julie
31 March 2025 at 21:47 #791349
John Haine
Participant
@johnhaine32865
Read the OP! This is a generator dedicated to the vfd. I don’t think noise getting back into it matters much.
31 March 2025 at 21:57 #791350
noel shelley
Participant
@noelshelley55608
First, forget cheap generators ! I had a 20 Kva diesel that would struggle to start an 7.5 Hp saw bench on A DOL starter but start a 14Hp motor on star delta starting. I would look at a 10 KVa as a safe minimum allowing for start up loading and to avoid disappointment. Fancy electronics may help but there is no substitute for rotating iron. Smallish generators need 2 or 3 times the output Hp to drive them – so you will be looking at about 15-20 Hp to drive it. You’ve been warned. Noel.
1 April 2025 at 08:12 #791375
Robert Atkinson 2
Participant
@robertatkinson2
A VFD with active power factor correction or voltage boost will present a worse load for the generator. This is because these are constant power / negative resistance devices. As the generator voltage sags under load they draw more current (to maintain power) causing the voltage to sag further. This can case instability of the generator contol.
Comparing a motor with DOL starter is not helpful. This is a completely different load. The soft-start / speed ramp feature of the VFD will manage the motor starting current.
VFD inrush current is a fairly minor issue for a simple generator.

Robert.
1 April 2025 at 08:16 #791376
Robert Atkinson 2
Participant
@robertatkinson2
PutneyChap,

What is the motor driving? This can make a big difference. A compressor or pump is worse than a typical machine tool.

Robert.
1 April 2025 at 08:42 #791378
Adrian R2
Participant
@adrianr2
Do you have a nearby hire place? If so you could ask their advice on sizing then actually try one out before purchase.

Also has the benefit that you get to listen to the noise it will make and decide whether that is acceptable – I bought a “super silent” online which was very much not so, if I had heard it run before buying I’d have carried on looking.
1 April 2025 at 10:38 #791386
noel shelley
Participant
@noelshelley55608
The OP states a lathe, at 7.5 Hp a bigish one ?

Robert my mention of a motor on a DOL was to illustrate the differing loads and starting characteristics. In the case of the DOL the voltage collapse would cause the contactor to drop out – only by holding the start button in for a second or 2 to enable the generator to pick up the load would the saw run. The generator rating was over twice that of the load but struggle to start it. In the case of the OP the lathe would be starting light, never the less only by trying will a certain answer be found. Noel.
2 April 2025 at 11:45 #791587
Stuart Smith 5
Participant
@stuartsmith5
The comments about VFD capacitor charging current made me decide to measure the one on my lathe.

It is a Warco Wm290v factory fitted with a VFD and 3 phase 2HP motor.

This chart shows the current when I switched the VFD on (approximately 6.4A) and then when I started the lathe motor.

For comparison, I started my circular saw which has a claimed 1200w motor. (starting current about 20A, ie about 4 times full load current)

The VFD charging current only lasted for about 50mS. (The time axis is in seconds)

Stuart
2 April 2025 at 12:20 #791595
Robert Atkinson 2
Participant
@robertatkinson2
Hi Stewart,

Your chart didn’t come through for some reason.

Your figures match what I would expect. Out of interest how did you make the measurements?

Robert.
2 April 2025 at 14:40 #791608
Stuart Smith 5
Participant
@stuartsmith5
The missing chart:
2 April 2025 at 14:47 #791612
Stuart Smith 5
Participant
@stuartsmith5
Robert

I used part of this project:

https://openenergymonitor.org/

I made it a while ago to monitor my electricity usage at home. It was interesting but after a while, I stopped using it. It uses an Arduino shield and an Arduino uno with a CT round the meter tails and a plug in transformer power supply for the voltage sensing. This connects to a Raspberry pi which takes the readings and runs a web server so you can view the information in a web browser.

For this measurement I just captured the output from the Arduino and put it in a spreadsheet to make the chart. It samples multiple times per half cycle and then calculates the values and outputs them via the serial port. For this chart I changed the default averaging to output the readings every cycle.

Stuart
2 April 2025 at 15:14 #791617
Stuart Smith 5
Participant
@stuartsmith5
Link to copy of spreadsheet:

https://www.model-engineer.co.uk/members/stuartsmith5/mediapress/energy-monitor/

Stuart

ps this doesn’t seem to be visible!

Does anyone know what to do?
2 April 2025 at 21:06 #791657
John Haine
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@johnhaine32865
Link takes one to your profile where the file appears in an album. Just another bizarre feature of this forum.
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