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Digital Phase converter...

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Matt Harrington12/12/2019 12:38:32
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Before I go any further, I am asking about a Digital phase converter rather than a VFD or similat inverter.

I currently use a rotary converter for my Hardinge lathe but (no matter where I place it) the noise is just a little too much for me so a while back I considered a Digital option from Drives Direct. Lathe itself is fairly quiet.

My question is whether a 5HP unit will be OK for the startup surge of the Hardinge (the main motor is 1.5HP) or has anyone installed one of these units? It is a big chunck of money to 'experiment' with and to go to the 7HP unit would mean I have lost most of my limbs in payment! Dave at Drives Direct has not shipped one to a Hardinge user and so can not advise.

Matt

Robert Atkinson 212/12/2019 12:57:43
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AFAIK the only difference between a phase converter and a VFD is the PC is non adjustable. Even if you use it at fixed speed a VFD would be a better choice for a single machine as it can "soft-start" so does not need to be over-sized.

What PC are you looking at (link) and why don't you want a VFD?

Robert G8RPI.

Matt Harrington12/12/2019 13:08:14
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Robert, they appear to be quite different. The Digital phase converter is 'plug & play' and it means I can change motor speed and operate all 4 3 phase motors on the lathe without issue. If I used VFD's I would need to completely re-wire the machine. I'm looking at their 5HP unit.

This is an example of the 5HP unit: **LINK**

Matt

SillyOldDuffer12/12/2019 13:55:02
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Posted by Matt Harrington on 12/12/2019 13:08:14:

Robert, they appear to be quite different. ...

Matt

I've been looking at various Digital Phase Converters on the web and none of them describe how they actually work. I'm with Robert: my guess is they're beefed up VFD's with better voltage regulation and load tolerance, ie electronics not discombobulated by multiple devices on the output. Price, weight and power output suggest industrial rather than hobby users.

I note the linked 5HP unit takes 240V single phase up to 415V 3-phase - it's a brute! The blurb says this unit will take a 7.5HP starting overload which is in the same range as a similar USA device claiming a 60% startup margin. So withstanding a 50% overload may be typical. Dunno if that helps -no idea what the start load of a Hardinge lathe is, or a cheap way of measuring it.

Dave

Matt Harrington12/12/2019 14:38:26
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Dave, you are so right - all cloaked in mystery!!

If I could find the surge on start up in high speed, then all would be easy to ascertain. Personally I'm sure the 50% overload will be fine but I thought I wouls try and find out if anyone else had gone through this process.

Matt

Clive Foster12/12/2019 14:56:08
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The sort of digital phase converters folk like us will encounter are basically normal VFD boxes with the input side converted to run in voltage doubler mode to give 440 V out from 220 V in. The important thing is that they have the short term overload capability to handle the start-up surge of bringing a motor up to speed when stationary when the VFD is already running at line frequency. They are also able to handle the back EMF and other nasties produced when a running motor is simply switched off rather than powered down by the VFD.

Given a suitable VFD design the overload capacity mostly comes from de-rating the device. Drives Direct say 150 % overload for 30 seconds so starting a 2 hp motor off load with their 5 hp unit is reasonable. Different story if its under load. I have one of their 10 hp boxes which handles everything in my workshop just fine but is clearly working hard to get my Smart & Brown 1024 lathe up and running as the machine doesn't have a clutch. 3 hp motor with direct belt drive to the spindle via Reeves type varispeed drive and flat belt. If you already have a motor running that helps with the start-up surge. Something I've frequently noticed if the shaper or saw is going when I start the lathe.

I'd not put anything with a single motor significantly over 3 hp on mine. It drives 9 1/2 hp total over 4 motors just fine tho'. I also have the sine wave filter box on the output which will also reduce the switch off nasties getting back to the box. All VFD boxes are hard on the capacitors. Voltage doubler converted ones are harder still. Mine is over 10 years old and really ought to be due for new capacitors, a £600 service job, but its still going strong.

I got mine as a temporary measure due to hiccups with getting a three phase supply installed. I have the supply now but finding an electrician to hook everything up seems impossible in my neck of the woods. As usual "nowt so permanent as a temporary job".

Would I go plug'n play again. Nope. Inverter prices being what they are now I'd just buy 10 brand name inverters and accept the re-wiring issues. I like the Eaton DE-1 series variable speed starters for this sort of thing. Bit of a pain to dig out the star point on the Pollard drill motor, gear drive so motor change not on. The P&W lathe would need the two speed motor changed out and I'd be sorry to see its splendidly steam-punk wartime Crabtree contactors go but overall cost effective.

Bottom line 10 hp box will do you just fine. 5 hp ought to but ....

Clive

 

Edited By Clive Foster on 12/12/2019 14:59:27

Andrew Johnston12/12/2019 15:16:13
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No mystery. It's basically a VFD, but digital in the sense that it uses vector control instead of an open loop v/f characteristic. While you may be able to run more than one motor they will all run at the same speed. So if you slow down the main spindle motor the coolant pump et al, will also slow down. The blurb specifically says you can't run electronics with the output unless you buy a sine wave converter. That's simply a lowpass filter that converts the PWM output of the VFD to a sine wave. Without the sine wave converter the output may well not drive contactors or any internal transformers for low voltage lighting or control circuitry. The VFD itself may be plug 'n' play but not in the sense that you can connect it to the 3-phase input to the machine tool and expect everything to work.

Andrew

Matt Harrington12/12/2019 15:26:47
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Posted by Andrew Johnston on 12/12/2019 15:16:13:

The VFD itself may be plug 'n' play but not in the sense that you can connect it to the 3-phase input to the machine tool and expect everything to work.

Andrew

Andrew, Dave frm Direct Drives says that I can operate it this way. (DRO's are powered separately)

Anyway, I'm glad you have a firm graps of electronics, unlike me!

Matt

Matt Harrington12/12/2019 15:31:09
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Posted by Clive Foster on 12/12/2019 14:56:08:

Bottom line 10 hp box will do you just fine. 5 hp ought to but ....

Clive

Edited By Clive Foster on 12/12/2019 14:59:27

Mmm, it;s the same reply most of the time - "ought to be OK but..."

The 10HP verion is eye watering in price and definitely a no no.

Matt

Clive Foster12/12/2019 15:45:08
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Posted by Andrew Johnston on 12/12/2019 15:16:13:

The VFD itself may be plug 'n' play but not in the sense that you can connect it to the 3-phase input to the machine tool and expect everything to work.

Andrew

Umm. Actually you generally can! Certainly I've found this so with mine, although that does have a sine wave filter box on the output.

Its all a bit brutal and pushing various envelopes tho'.

What most folk could really use is a properly engineered voltage doubling version of the Eaton DE-1 variable speed starter series to just replace the standard contactors. Maybe new start / stop buttons too. But I think there are regulations in the way.

Darn shame that none of the wind-power research into improving the basic Steimetz connection fundamental to rotary and static converters has filtered down into our world. And I don't mean a Phase Perfect. Never got the warm happy feeling that the folk who do such really understand the engineering in modern terms.

Getting back to Matts Hardinge the big stumbling block is the two speed spindle motor. So he is pretty much stuck with a voltage doubled system.

The diagram I found says 1.5 - 0.5 hp for the spindle motor and 0.1 hp for the coolant pump with 0.05 hp on the speed change. Pump and speed change motors only run when the main motor is running so a simple 3 hp voltage doubled VFD should be fine as the extra power is down in the noise. No need for plug and play. The feed motor is DC, 90 volts I presume, so that can either be given a separate transformer feed from the normal 220 single phase mains if the existing one lacks suitable taps or the whole control circuit launched into orbit and a proper modern board fitted.

Just how difficult is a main motor change?  A 6 pole 2 hp is around £200 and an Eaton starter maybe £150 so with sundries and futzing up'n running for £400 ish looks possible.

Clive

Edited By Clive Foster on 12/12/2019 15:48:55

Edited By Clive Foster on 12/12/2019 15:50:06

Matt Harrington12/12/2019 16:02:33
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Clive,

All interesting stuff.

The motors are, I believe, as follows:

The main motor is a dual speed 0.5hp/1.5hp 415V_ac 3 phase motor.
The speed change motor is a dual voltage (240V_ac/415V_ac) 1/20hp three phase motor.
The coolant pump motor is also a dual voltage (240V_ac/415V_ac) three phase motor. ( don't use mine)
The carriage feed motor is a 90V_dc motor.

The issue with the main motor (so I am being told) is that is is fully balanced and thus not a good idea to replace.

Matt

Brian Sweeting12/12/2019 16:05:28
414 forum posts
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If you want an inexpensive way to measure the inrush current of your lathe you can get an amp probe attachment for a multimeter from here **LINK**

Clive Foster12/12/2019 16:32:29
2157 forum posts
73 photos

Hi Matt

Tend to think the balanced motor thing is Hardinge gilding the lily a bit. Maybe could see the effect in certain circumstances of brand new, perfectly set up, machine under test but after many years and many years on the clock I doubt it. Modern motors from a decent brand are pretty well balanced anyway. I'd imagine a good Hardinge OEM motor has significant value to help defray the cost.

If it were mine I'd look very closely at changing the motor and fitting an Eaton DE-1 set-up for switch selected two speed drive. Its exactly the sort of thing the Eaton is built for after all. Another Eaton set for single speed drive would probably be the easy way do the coolant pump and speed change motor switching input wiring to delta for 240 V.

I personally would use the capacitor trick to persuade the coolant pump and speed change motors to run off single phase. Which I've done several time before on various low power devices.

Clive

Andrew Johnston12/12/2019 17:18:26
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Posted by Clive Foster on 12/12/2019 15:45:08:

Umm. Actually you generally can! Certainly I've found this so with mine, although that does have a sine wave filter box on the output.

Which is exactly what I said in the first place

Andrew

Clive Foster12/12/2019 17:58:40
2157 forum posts
73 photos

Andrew

I got my Digital Phase Converter used via E-Bay and initially ran it with no sine wave filter. Drove the Pollard drill, Bridgeport mill and Rapidor saw I had on a semi home brew rotary converter just fine. Originally bought the digital box because that rotary converter struggled with a Hydrovane 502 compressor and I didn't want to wait for utility three phase to be installed.

Having some electrical / electronic experience with VFD like devices I wasn't happy with the output side being directly exposed to switch off transients when using the normal machine control gear so so I got the sine wave filter box even though Drives Direct said it wasn't necessary.

So I've run mine both ways. Maybe I wasted several hundred £ on the sine wave filter, maybe not.

Clive

Maurice Taylor12/12/2019 18:53:55
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If the only problem with a rotary converter is noise ,why not put it in a waterproof container outside of workshop ?

Just seems easier and far cheaper to me, I’m probably wrong though.

Robert Atkinson 212/12/2019 18:55:02
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617 forum posts
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Using a 5 HP drive to power a 1.5 HP motor is just wasteful. The supplier is not giving good advice but why would they? Buy saying you need a bigger drive they are protecting themselves aginst failures bue to poorly rated components and making more money for themselves. £1500 is a hell of a lot for a "no-name" VFD just to save setting up.
Where are you located? Maybe I could help with a better solution.

Robert G8RPI.

Clive Foster12/12/2019 22:05:44
2157 forum posts
73 photos

Robert

My box is Teco branded, I think that is still the one Drives Direct use. Maybe not a top make but hardly no name and basic enough inside to handle the needed modifications.

Needing a nominally well oversize VFD as the basis for a "plug & play" phase converter is the equivalent of the usual "largest single motor is half nominal power" derating commonly found with rotary converters. For exactly the same reason. The box has to supply enough current when running a 50 hz to cover the much higher, short term, start currents needed to bring the motor from rest up to its nominal nameplate speed.

Unfortunately its harder with a VFD because the beasts are intrinsically made not to behave in the required fashion. Its supposed to see too much current when running as a fault condition and shut the box down before the magic smoke comes out of something. Its not programmed to recognise the large initial spike followed by rapidly decreasing current demand of a motor starting and running up to speed as safe. Also the capacitor bank has to be large enough to supply the current without its voltage dropping below what the inverter innards considers a safe level.

As I said previously, envelopes are being pushed and the VFD is doing things its not explicitly designed for. Perfectly satisfactory if you stick within the safe operating area.

Stupid thing is there is no electronic or electrical reason why a properly engineered, voltage doubling, variable speed start and run unit couldn't be made and supplied at a similar to an ordinary VFD. If the voltage doubler is correctly implemented its not going to be any nastier to the utility mains than the usual VFD. The standard VFD output devices are well up to the job. The voltage over each winding being the same whether delta or star. Only possible disadvantage is that the motor will take two or three seconds to run up rather than being "instant".

The Eaton box in my car lift brings the hydraulic pump up to speed in two seconds just fine.

Clive

Robert Atkinson 213/12/2019 07:23:05
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There is no difference electrically between a VFD and a "digital phase converter" (DPC). The DPC seems to have been "invented" to tap into a specific market with limited knowledge of power electronics. Their power ratings are disingenuous at best. If thery are going to rate a motor drive in horse power then it should be able to start up a motor of that horse power, not need over 300% overrating. This is purely down to the cost/quality of the components used in the converter. Full load continuous ratings are mainly based on thermal capacity, short term capability (in order of seconds) is mainly limited by cross sectional area of conductors and solid state devices. The thermal mass allows some overload before they overheat. If the particular unit will only start a 2hp motor then its a 2hp motor drive not a 5hp.
For the particular case of a machine with one large main motor and a number of smaller ones there is even a case for a "proper" VFD on the main drive motor, with the benefits of soft start, variable speed etc and a second, small VFD (or even DPC) set for fixed output for the auxiliary motors. The smaller motors could also be changed to single phase, torque smoothness is not a big issue for a coolant pump.
Yes there may be some re-wiring required but that is once, you save money and get the long-term benefits of a VFD on the main drive.

Robert G8RPI.

Robert G8RPI.

Clive Foster13/12/2019 10:13:44
2157 forum posts
73 photos

Robert

The drive in question is a 5 hp drive. It will drive 5 hp worth of motors. But it can only support the "from rest" start-up surge current of a 2 hp motor. So you can run two 2 hp and one 1 hp motor just fine.

No way round the need for over-rating. Fundamental to the physics of motors. My 5 kw generator won't start a 3 hp compressor and lets you know about it if you try a naked motor!

Same as rotary converters always having a maximum single motor size somewhat less than the nominal capacity. Transwave say about 2/3 rds max, Motorun say about 1/2 max. Transwave I've found a little optimistic for on load starting. We couldn't get a 3 hp compressor to start up and run reliably on a nominal 5 hp Transwave. Usually fine but sometimes it just wouldn't play.

Agreed that these devices were invented to fill a gap in the market. Being modifications of an existing device what can be achieved is limited by the specifications of the original device.

Bottom line is they do work well, they do what they say on the tin but they are expensive. I see there is a 5 hp one in a box on E-Bay right now for about half price.

The issue with the Hardinge motor is it being two speed. Two speed motors have a reputation for needing higher starting currents than their single speed brethren. Not always justified but some authorities advise that the safe procedure is to use the sum of the powers when working out the initial start-up load. So the 1.5 / 0.5 hp motor on the Hardinge should conservatively be treated as being 2 hp for start-up load calculations. So the 5 hp box ought to be fine. But, as I know to my cost, two speed motors aren't always entirely logical or reasonable beasts.

Clive

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