Newman 3ph motor help

Hi dc31k.

Genuinely didn't know such a thing existed. It strikes me that that would be exactly the thing which I required (and thought I had with my vfd!).

Any reason why I wouldn't go down this route anyone? Seems by far the simplest solution

Rich

In principle one of the 380 volt output VFD boxes ought to work OK but it would be prudent to buy a seriously oversize one. I'd go for 2 hp (or more) rather than matching the 1 hp motor specification. I'd also try to buy from a UK supplier, or at least address rather than direct from China as there is more chance of after sales service / technical support. Its very re-assuring if the supplier can say "Yep, we sold one that worked just fine on the same machine.".

Two potential issues.

First one is that Dahlander connected pole changing motors are notorious current hogs during run up, how much so depending on design details. No great issue with umpteen megawatts of National Grid doing the supply bit but VFD boxes and rotary converters have strictly limited current supply capability and may not have the drive capacity to bring the motor up to speed. For example the three speed 3 hp motors used on Holbrook and other older British machines are notorious for needing a 7.5 or even 10 hp phase converter to reliably bring them up to high speed!

Second one is that the 380 volt VFD boxes inevitably have a lower DC bus voltage than a proper 440 volt system along with lower full voltage current drive capability. So they can easily run out of drive on a demanding application. Buying oversize gets you more current drive capacity.

As ever its price / performance ratio issue.

These devices are fundamentally economy range units built to give a useful performance to the undemanding and impecunious user at an affordable price. I seriously doubt if any of these devices, when matched to motor size, are capable of delivering full torque and power at 50 Hz let alone higher speeds. Which much matters not a jot to folk like us who are pretty much never going to drive the ex-industrial 3 phase machine we usually hook them up to at anything like its full power. Even 1 hp can shift impressive amounts of metal!

I found out about the limitations of 380 volt VFD boxes the hard way around 15 - 20 years ago when I bought a so called 440 volt output 4 hp VFD in a very professional looking box to run my S&B 1024 VSL lathe. That machine has its 3 hp motor connected directly to the spindle via a Reeves type varispeed belt drive using expanding and contracting pulleys. The VFD proved incapable of driving the lathe at its maximum 2,500 rpm. At anything over 2,000 rpm the unit couldn't supply enough power to accelerate the empty spindle up to full speed

Internal investigation showed the nice new box to contain a second hand ABB industrial VFD with its input rectifier re-arranged to give a nominal 380 volt output in most likely the same manner as will be used on the modern imports. Reeves drives are notoriously mechanically inefficient. My drive probably sucked up something like 3/4 hp, maybe more, at top speed and the rather crudely modified VFD box was unable to provide the power needed to accelerate the machine against that much load. Careful probing around with a meter showed the drive voltage basically collapsed.

The supplier proved uncontactable so the box was sold, at considerable loss, into a less demanding application where it worked just fine for at least a decade.

I wised up on VFDs along with the difference between a fancy web-site and actual professional products after that!

Clive

The motor in his photo is badged 1/2 hp in one speed and 1/4 hp in the other.

Given the passage of considerable time and the ongoing march of technology, how much weight should we assign to your experience of 15-20 years ago?

Your observations on Holbrook and the like could be misleading insofar as we need to consider the amount of inertia the motor is being asked to overcome and the rolling resistance of the drivetrain. It is the difference between an American muscle car and a Lotus 7.

In addition, the VFD will offer a soft start (ramp up) capability, so the initial current demand will be considerably less.

So, I wanted to come back to the forum to add the solution I came to in the end.

First of all, thank you all for your thoughts and suggestions. It really did help me understand the problem and options for a solution, of whic there were clearly a few!

I ended up going with dc31k and ordering a boosting vfd. It arrived from china yesterday and I got it wired in today. It allows me to retain the 2 speed motor, so I can use the switch to go to high and low speed (though I accept that the VFd will also do that, and also the raglan has a nifty system of adjustable pulleys to alter speed, exactly like a CVT type transmission).

Anyway I bought this. https://a.aliexpress.com/_mtJ5IkI

Its labelled as an he200 vfd.

Cost was £61.71 delivered to my home. Instructions were in mandarin, but the seller emailed me a link to English instructions on request.

Easily wired up, and it performs perfectly fine. With the mill set to "high speed" on the switch selector, I do have to give the spindle a quick spin to get it going, otherwise it just buzzes. Once it gets started it accelerates well and has sufficient power for what I need.

Once again, thanks to all for your advice. I'd never have got to this point without your help.

DC31k

Sorry missed your post earlier.

The march of time and technological advancement makes very little difference here as we are dealing with fundamental motor physics.

The issue is how big are the storage capacitors and how much energy is available to run the motor up. Even with soft start there is no getting around the fact that a motor is near as dammit a dead short when turned on. It needs a certain current flow and voltage to get it moving. If too much energy is sucked out of the capacitors too quickly the applied voltage falls too low before the moron gets moving and it cannot start. Just sits there and buzzes. Basically acting as a seriously weird transformer.

My big tome on motors has whole chapters devoted to calculating the immediate inrush current and design methods to control it so that a motor will reliably start under load without upsetting the local supply. Very important if running off a generator. Star-Delta switching and avoiding multi-speed motors being the easy beginning before the maths goes to eye-watering, brain leaking out of your ears level. 1970's book so it assumes algebra, slide rule and mechanical calculator level calculation.

The VFD Rich has is clearly undersize in the storage capacitor department when it comes to meeting the inrush current on the high speed setting. Only having 380 volts doesn't help as, obviously, the lower voltage reduces the energy the capacitors can store. Which may well be enough to make the difference between not quite and just enough.

Large capacitors are expensive.

As ever with economy range you have to expect usually good enough engineering and specifications. Which is excellent until it isn't.

Buying a higher power unit basically gets you bigger capacitors and more oomph to get things going.

It's annoying that, apparently, no-one makes professional level versions of these step up VFD boxes where there are no hidden performance issues. You'd have thought there would be a big market in America as an alternative to expensive Phase Perfect boxes and the like.

In the previously mentioned Holbrook application the motor only drives the gearbox on start up so the load was not be great. But the inrush current is still very, very high. With my Smart and Brown the modified VFD just ran out of volts at high speed. The capacitors were clearly adequately sized as the machine started OK. In standard trim running 440 volts that VFD would have had no problems.

Clive

I have a reeves drive lathe with single speed 1hp motor, starting instructions are to always start on the lowest speed and I have found at speeds above 1/3 revs it will not start even with an extra 1/4hp (standard motor is 3/4hp)

On the subject of inrush currents, Tubal cain talks of up to 1200% of full load and my own expierience bears out similar figures eg 180w motor stalling a 2Kw generator and a 5Kw motor making a 20Kw generator struggle with a a DOL yet start an 11Kw motor easy with star delta start. The first had to overcome the flywheel load, the second starting with no load. The 5Kw motor would cause such a collapse of the voltage that one had to hold in the start button as the contactor made a noise like a machine gun as speed built up and voltage recovered.  There is NO substitute for POWER, plenty of it ! Noel.

Edited By noel shelley on 11/03/2023 12:05:36