Question about pilot motors for rotary converters

I'm currently building my own RPC with an 8KVA transformer and 7.5KW ABB

2860 RPM motor. My motor is 660 /415V so I have no choice but to start it in delta; check the rating plate to see if yours is 240 /415 or 415 /660V; I suspect the latter as the star voltage is about 1.6 times the delta voltage, and you should therefore start and run it delta. A big motor can draw 6-8 times the running current when starting, therefore a 4KW motor may pull 133 amps when starting! Even if it pulls only the rated current it will still draw 17A from a 240V supply. Motors need to be fed from a C rated MCB to avoid tripping, a 13A fuse is fast blow and will not stand up to this type of inrush current; I suggest you wire a dedicated circuit and fit a 16A C rated MCB with no other fuses downstream (as long as all the loads have overload relays fitted). The starting current, depends, of course, on the start capacitors you are using; what values do you have installed (I assume you do have a separate start capacitor switched manually or with a delay; I've fitted a small Omron timer relay that operated a contactor for starting). Straying slightly, has anyone found a source of 440V rated start capacitors? I have had to buy run capacitors as all the start caps I've found are rated at 260V (some 330V but limited values and availability) and 500uF of 440V run caps is both very bulky and VERY expensive. I've collected a list of useful websites on this subject; if you haven't already Googled the subject, I can post links to the more useful sites. Martin.

Hager and others do a motor-rated 'D' breaker, better still than a 'C' which is basically a thermal + magnetic unit. Legrand don't seem to advertise theirs much but they do make them, also Merlin-Gerin have the motor-rated types.

We use the 'D''s a lot on our bigger chargers, and they are 'much' better than 'C' types and no significant price difference that we can see.

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

My home-made 415V rotary converter uses a 4HP 2800RPM otor which is started and run in star. The motor is mounted vertically on top of an old 240V centre tapped isolation transformer craftily reconnected to run as an autotransformer for 415V. I have it sat on four Mini exhaust mount 'cotton reels'. It starts easily enough and I have a simple push button arrangement that puts extra capacitance in circuit just to start it faster. I just switch it on and then momentarily operate the push button. I had to fit a Type C circuit breaker to overcome the occasional tripping on startup. I built nyself a 3-phase voltmeter to measure the phase voltages so that i could get the runn capaciotrs correct. Each machine that it runs has its own set of run capaciotrs so that I don't have to do anything with the converter.

I use it tom power the Surface Grinder - an ancient Pallas - a Myford cylindrical grinder and an Alba 1A shaper.

My Centec 2B runs on 240V 3-phase via a 2.2kW inverter that gives me speed control as well and the Myford is planned to be converted to

3-phase 240V via an invertor when I can stop using it for long enough.

Dave wrote:

Your 4kW motor should be run in delta configuration to generate the lowest source impedance phantom phase.

If starting current is a problem, start it in star and switch to delta once it is up to speed. In any case a

13A fuse is not big enough for a fully loaded 4kW motor - you need a 30A circuit.

It still works if left in star but as a much lower powered pilot. It is easily discommoded by the peak starting current of the Holbrook high speed setting. At a pinch you could get round this by adding additional run capacitance but that's a non optimum kludge - delta is the right solution.

With 240v applied to a single star winding of a star connected 415v machine it can be run up to speed and it will act as a pretty poor 240v to "415v" three phase generator. The inverted commas round the 415v are because it will have a rather high source impedance and pretty severe unbalance.

A 415v in 415v out converter effectively uses about half its windings to generate a single phantom phase. A 240v to 415v converter uses one third of its windings to generate all three output phases. Unless it it is an extraordinarily efficient machine the output performance is lousy.

Jim

SNIP

For electrolytic AC rated start capacitors its perfectly OK to series connect equal value capacitors.

You don't even need equalising resistors. The voltage automatically divides in the inverse ratio of the capacitance. With equal values, half the volts appear on each and the overall series connected capacitance is half the marked value.

If anyone is desperate for run capacitors I've a small stock of 75 and 40 uF 440VAC items and assorted 250VAC caps. E-mail me if you need some.

Jim

... snipped

Unfortunately the rating plate just has 415 and then the delta symbol, I guess it's 415/660 and was used for high start torque.

I should have realised this, thanks for the reminder! I had wanted to make this vaguely portable (just in case), that's why I was thinking 13A. Has anyone experimented with series inductance or PWM techniques to slug the inrush?

I'm still experimenting with different values of C. I was surprised to find that a 30uF cap from Ducati gave different results to a 30uF cap from LCR - presumably down to ESR and parasitics. Do most people fit run caps between the synthesised phase and both "live" phases or just to one of them? I'm manually switching a small additional start cap but intend to design a control circuit to monitor the synthesised phase and switch in caps (probably using Triacs at zero-crossing) as appropriate, this should give self start and some degree of load regulation. Can anyone save me a lot of work and "round tuit" problems? Until I solve the "round tuit" problem and get this control circuit designed I need to somehow add additional run caps to the Holbrook switchgear so that appropriate caps are selected for each of the 3 speeds.

... snipped

Thanks! Dave S

Dave, I have a 4Kw 1425 dual would 220/440 motor going spare, nearly new.

I'm near Nottingham 2 miles from M1 J25/24 junction if that's any good? Either tonight or tomorrow I'll be going M1 - M25 J18. Leave here about

5.30pm

-- Regards,

John Stevenson Nottingham, England.

Thanks John, that is so tempting but it's a long way to Nottingham from Winchester. If anyone from this neck of the woods was going near your place or vice-versa it would be good to hear from them.

Dave S

Dave

I'm passing thru' John's next week and could collect it. I don't go near Winchester much but could get it to central London or even somewhere like the A3/M25 at junction 10. Is that any use?

Charles

A very kind offer Charles, I've sent you an email.

Dave S