VFD Motor combo for lathe

[Richard Griffiths 1Participant @richardgriffiths1]

Hi,

I have purchased a Drummond M type lathe, it came with a custom made headstock. I am looking to eventually convert it to a CNC lathe is my ultimate goal. I have completely stripped it down and I am currently cleaning all parts. It is missing a load of bits, due to the headstock alone I am not looking to restore it.

I have been looking at an English made 0.55kW VFD and motor.

I am a complete noob, so any advice on the following would be kindly appreciated.

1. Would the motor and VFD combo be adequate?

2. The VFD is a lot more expensive than Chinese models but does come with a 5yr warranty any recommendations?

[Richard Griffiths 1Participant @richardgriffiths1]

VFD Motor for lathe

[Martin ConnellyParticipant @martinconnelly55370]

If you are planning to use the variable speed feature on a VFD then get a higher power motor than the original. Torque drops off with reduced speed. If you are going to use it fixed at 50hz then match the original motor power or slightly higher if the exact match is not available.

Martin C

[not done it yetParticipant @notdoneityet]

1. Adequate.

2. I would suggest running the motor at elevated speed while maintaining the lathe at its design maximum (no details of this ‘custom made headstock&rsquo. That allows the power to be maintained down to lower motor speeds.

5 year warranties are fine – as long as all ‘provisos’ are followed. None of my chinese inverters have failed (yet) but none have been in operation for that long. I expect I could break them, or severely stress them, but have no intention of so doing. More money generally buys a VFD with more overload safety, and better components, etc but my lathe is never left running unattended, so it’s down to me not to overload it.

I have a Raglan with 0.75kW motor. I don't often change the mechanical variable speed these days as the variable speed from the VFD copes with most smallish diameter jobs. I expect the spindle (has tapered roller bearings) could accept more speed than the power feed gear train.🙂

Edited By not done it yet on 02/05/2020 21:38:51

[Richard Griffiths 1Participant @richardgriffiths1]

Thank you for the replies, I will get a slightly bigger motor and a cheaper VFD. I created an album of the headstock it in pieces at the moment and still needs to be cleaned up. I also need to make a gasket for the front. Seems to have two tapered bearings at the front and a bearing at the back.

I was thinking of replacing the v pulleys for a wide toothed pulley, and replacing the rear bearing for a sealed bearing any ideas if this seems a reasonable idea.

[SteviegtrParticipant @steviegtr]

Time for me to get shot again.

The vfd is a big yes as so many forum members have the ch—I–se ones & they have no torque at low frequency. The one you are looking at will probably have a feature called torque boost. Which both of mine have, Omron & Toshiba. On my lathe at 5hz you could tow a car. Mnnn ok not quite. I used a 1.5hp motor on the Myford & again getting ready for the bullets. But honestly buy small & wish you had gone bigger. On the milling machine it came with a 380watt 1/2hp motor. If I f-rt too hard it stalls.

Go bigger & just smile. I have a car with 575bhp, but never really use it but it's nice to have. The price difference is negligible. Ok I am stood at the stocks guy's.

Steve.

[Anonymous]

When running an induction motor from a simple VFD the torque does not decrease as the speed decreases. It will stay approximately constant. But since the speed is decreasing, at constant torque, the power decreases in proportion to the speed decrease. Simples!

Andrew

[Gary WoodingParticipant @garywooding25363]

Andrew is correct – below designed frequency (50 Hz, say), torque is approx constant, with power proportional to speed; above that frequency, power is approx. constant with torque inversely proportional to speed.

I've got 4 VFDs, all made by Mitsubishi and purchased 2nd hand from eBay. One on my lathe, two on my mill, and one on my drill-press. Been installed for 9 or 10 years with no trouble (so far).

Edited By Gary Wooding on 03/05/2020 12:00:02

[Mike PooleParticipant @mikepoole82104]

Consider that the alternative to a VFD is speed reduction by changing pulleys and the back gear if fitted. A VFD will maybe keep 100% torque down to 10Hz but you will be down to 1/5 power so Steve’s big motor is down to 1/3hp, by comparison the mechanical speed reduction has 500% torque and the full 1.5hp in Steve’s case. I would recommend a VFD but I would also consider some speed reduction with pulley changing this can mean you can have your cake and eat it. The normal motor fitted to a Myford is 3/4hp so doubling the motor size will double the torque which is nice to have at low speed but must be used with care at higher speeds or something could break. A VFD does offer mechanical protection by means of current limit so can be a bit kinder to the machine in a disaster.

Mike

[Henry BrownParticipant @henrybrown95529]

I would suggest you speak to Peter at Inverter Drive Supermarket. I have no connection with the company, just a very satisfied customer.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Richard Griffiths 1 on 02/05/2020 20:21:59:

…

I have been looking at an English made 0.55kW VFD…

…

2. The VFD is a lot more expensive than Chinese models but does come with a 5yr warranty any recommendations?

I'm afraid 'English Made' doesn't help. We need to know the Specification, what make and model number?

In the recent past cheap Chinese VFDs were simple-minded, but the cost of electronics drops like a stone. A modern Chinese VFD, even an inexpensive one, might well have low speed torque boost and other bells and whistles.

Difficult to assess the quality of a VFD's insides without opening one up. Can't be assumed that Chinese electronics are badly designed and made, though they might be! Many games consoles, laptops, mobile phones etc are made in China and they are as good as anyone else's. Think well-equipped modern factories rather than paddy farmers learning how to solder on the job!

English made implies Industrial use though, maybe rated for continuous operation. Might have advanced electronics, or not. Could come fitted with expensive RF filters – a good filter might be more expensive than the VFD itself. Don't know though – it's in the specification!

Next point, most of us don't want to plunge into messy, difficult details!

If you buy a sophisticated VFD, it will come with a no-holds barred manual, written for experts. The cheap units are aimed at the average Joe, and are likely to be pre-set with 'good enough' values : how well these match your particular motor is up for grabs, and they may not perform to the max at all speeds.

Motor performance can be improved. One obvious way is to tell the VFD exactly what to expect by entering all the motor plate information; the nominal RPM, frequency, volts, amps, and power. This may require the owner to dip deep into the manual! Some units have 'Wizards' that simplify the process, others expect the owner to wade through hundreds of parameter settings and techno-babble. Not for the faint hearted! On top of that, a sophisticated VFD may be able to apply extra techniques like 'vector control' to improve motor performance. In essence the current flowing in the motor is managed by varying the base frequency (from which the VFD synthesises the motor frequency), and/or the motor frequency and/or voltage to maintain current through the motor.

Current can be managed to produce constant torque (rarely wanted), or constant speed. For best results a number of different settings are applied across the motor's operating range. They can be set manually, or, the VFD might have a learning capability which measures the motor during a trial run and automatically optimises the settings. A second-hand VFD taught to use one motor probably won't perform at it's best if new owner connects a different motor. (He might still be happy though!)

But 'improving' is double edged. Perfectly possible to tune a motor and VFD for maximum low or high speed performance only to find the motor and VFD emit the dreaded magic smoke. Fundamentally, what can be done safely is limited by both the physical motor and by the electronics. Neither are miraculous.

For those reasons I see these VFDs being best used to provide sensible speed control within the machines normal belt or gearbox ranges. I don't see VFD's as being meant to eliminate mechanical torque conversion for going very slow or very fast. A simple VFD isn't the lazy man's alternative to belt changing, though it might work well enough for the job in hand.

In practice a simple VFD used with respect is a jolly good thing. They're not a trouble free way of slowing a 50Hz, 1440rpm induction motor down to 30 rpm!

Dave

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