Newman 3ph motor help

[Clive FosterParticipant @clivefoster55965]

Posted by DC31k on 16/02/2023 15:58:58:

Posted by Clive Foster on 16/02/2023 13:13:51:

I'd go for 2 hp (or more) rather than matching the 1 hp motor specification.

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.

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

[Dave HalfordParticipant @davehalford22513]

Posted by rich webb on 10/03/2023 22:08:45:

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.

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)

[noel shelleyParticipant @noelshelley55608]

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

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