Excitation of 65 Kva Alternator

[George]

I posted this on an Electrical Engineering Group, but no one knew there, - does anyone know here ?

I have had a very big 3 phase 415v 50 c/s alternator for years now and at last I need to put it into service.

It was manufactured by Mawdsley in the early 80s and when I shut it down, I disconnected the AVR too and kept this and the spare I held. I have checked with the Company and there is no way that they can help me with the excitation to fire it up. Before I go to all the trouble of connecting it up to a diesel engine and spin it up to 1500 rpm, can anyone please tell me a few things about it.

It has a separate excitation generator on the end of the shaft and I would imagine that this is still in circuit, as I have not been inside it yet. How does this set up actually work. does the alternator self excite and the AVR just keep the output within narrow and automatic limits? Will it produce 3 phase on it's own or is the AVR essential and in this case is it a standard wiring circuit or will it be unique to this alternator and AVR setup?

There is quite a lot of information on the makers plates on the two units, but no information on how to connect it all up. The AVR itself has a whole lot of wire connections available but nothing to tell me what goes where. I have spent hours looking for information on the web, but with no luck.

This is what I wrote to the manufacturer:

Thanks for any comments, It really is a super machine. a bit dirty now but that will be easy to rectify.

Regards George.

I have had a closer look at the big alternator - which is the one that I really would like to see working again. There are two units, the main alternator body which has the following information:

Mawdsley A/C Generator BS 2613?70 (there is a scratch across the number) ser. no. 4FA4KB. 501 KVA 65 ( I thought it was 85, but this is better really as my engines would be struggling at 85 KVA.) Volts 415. Amps 90. 3 Phase. conn. star. rpm 1500. Excitation volts 71. amps 18 pf. 0.8 enclosure SP?

Then on the excitation generator there is another label: Exciter BS 2613/70 No. 6R4N3155 rpm 1500 volts 71 amps 18 3 phase hz 50. Exc. volts 16.5, amps 1.8, ins. class F.

I have two identical voltage regulator boxes made by Emrec but with your Co. name on too. Type 4VR ser. no. 1197 (one of them)

30 VDC. 6 Amps.

I am not sure about this, but am I correct in assuming that the voltage regulator feeds into the exciter and this is internally fed into the main alternator? If so and if the 4VR is the one for this alternator, then all I have to do is wire it up and I do have the connections for the 400 Hz alternator to another voltage regulator 6VR I think they are,I have two of these too. I have a small number of explanatory sheets in how the 400 Hz machines work with their AVRs If you can confirm the 4VR is the one for the

65 KVR 50 HZ machine I think I will be OK.

Hoping this makes sense to you and that you can shine some light on the situation. I wish I knew more on how excitation works - or worked 30 years ago! Would it be true to say that most alternators use the same sort of excitation, and only the details are different between manufacturers? If the voltage output of an alternator is only dependant on the loading and the DC current being fed into the excitation winding of the exciter, it may be that I can build a circuit that does this in a very simple way. My load is not going to be anywhere near 65 KVA in any case.

[BigWallop]

The best thing to do, is fire it up. Get it spinning and see sort of thing.

The Auto' Volt' Reg' will be regulating both internal and output supplies, I would have thought. Also, If you can't see any sort of external bridging between the units, then it would seem that the AVR would already be connected and ready to go.

The other Regulators sound like drop downs for external low voltage supply, maybe from the AVR to sensors on the exciter, to check the main output and automatically adjust engine speed Etc.

Can you post pictures for us?

Have you read anything like this

to get a better idea of how the workings go together with each other? It might be worth a read through, to see if it gives any pointers on the connection layout and internal workings.

[Andy Dingley]

Try uk.rec.engines.stationary Lots of generator people thereabouts

[Fred]

Can you post some pictures somewhere. I know a lot about the theory but nothing of the alternator you're describing.

[Andy Wade]

Not even some documentation, connection diagrams, etc.?

Well it might or might not be something like the set I used to look after a few years ago. This was of similar rating to yours (100 kVA) and your description sounds somewhat familiar. AFAICR, 'mine' worked as follows:

The electronic AVR, of which more later, provides a variable DC output current to the stator (field) of the exciter machine - this will be the "Exc. volts 16.5, amps 1.8" in your description.

The exciter's rotor produces a polyphase AC output which is immediately rectified by diodes built into the combined rotor assemblies of the two machines. The rectified output provides the DC excitation for the rotor of the main alternator - this is is the "Excitation volts 71. amps 18" in your description. It's an elegant arrangement since there are no slip rings or brushes anywhere.

The main 3-phase AC output comes from the stator windings of the main machine, which will be star-connected so as to provide a neutral point.

The AVR is essential to the operation of the whole set. It's a fairly simple bit of circuitry, powered from one phase of the AC output via a small mains transformer, bridge rectifier and reservoir capacitor. The AC output voltage is sampled, rectified and compared to a Zener reference, the difference being used to drive the exciter stator winding via a switching power transistor (PWM control). A 'freewheel' diode and the self inductance of the winding turn the hard-switched voltage drive to the winding into a fairly steady DC excitation current.

A negative feedback loop is now in place - if the main AC output voltage falls the AVR increases the exciter's stator current which increases the output of the exciter which increases the rotor current in the main alternator, thus tending to increase the AC output voltage. A pre-set pot on the AVR module allows fine adjustment of the AC output voltage.

I hope this helps a little. For test purposes you could feed the exciter stator from a car battery or an appropriately rated DC PSU. Connect a shunt (reversed biased) diode firmly across the winding terminals to prevent arcs, sparks and the risk of serious shocks and/or insulation damage occurring when you disconnected the excitation. Beware that, if you over-excite, the main output phase voltage may rise to 300 V or more - enough to damage some loads (DAMHIKT).

It's also essential to observe correct polarity of the excitation current and direction of rotation. The whole thing relies on residual magnetism to boot itself up and you don't want to reverse this.

[Fred]

If the unit is as you say a brushless unit it's unlikely to be self exciting and therefore unlikely that any residual magnetism will be sufficient to for the rotor voltage to overcome the forward diode drops. If it's brushed then it is more likely since brushes have very little forward drop.

I would have thought the AVR would have terminals for a battery for the initial excitation? Given that this is a 1980's alternator I doubt the electronics at the time would have stretched to supplying 71V and 18A. I may well be wrong but I would have thought a mechanical vibrating regulator would be more likely.

[meow2222]

That sounds more like a description of 1950s kit. 1980s electronic power supplies went up to 100s of 1000s of volts and hundreds of amps.

I dont know the specific gen set, but the following setup is a classic arrangement, and probably what you've got:

The small machine is a permanent magnet generator. This produces a low power output which is used to excite the main machine. The latter produces high power, and its V_out is regulated by adjusting the exictation voltage. Since this is a modern gen it'll all be done automatically and electronically.

Such a 2 machine set doesn't rely on residual magnetism at any time.

You can test the main gen's output safely by using a very low excitation voltage, such as a 1.5v D cell, which will give you somewhere vaguely in the region of 5v on the mains output terminals.

5v 90A from an inductive source should not be disrespected too much, its still capable of turning metal red, fire or possibly fatal shock (due to inductive kickback).

The electronic regulator will likely not tolerate any nonsense, so it must be wired up correctly first time to avoid releasing any smoke. The inital run can be done at much reduced speed to check the basic function is ok, though expect the voltage regulation to malfunction at low speed. Since you've got electronics in there, and probably unprotected electronics, do _not_ just spin it up and connect wires any which way to see what works.

NT

[Andy Champ]

Back in the 1980s we had these things called "computers" which were sometimes the size of a small car, and ate 10s of kW at 5V. And we had these other things called "Radars" which sometimes ate megawatts - at a somewhat higher voltage.

So I wouldn't be at all surprised to find an electronic one in equipment of that vintage.

Andy

[Fred]

I said I might be wrong. I think you'll find the cost of a radar or even a sensible computer in the early 80's probably cost more than this generator.

Until recently the regulator in your car dashboard was a vibrating reed affair. It worked, was cheaper and more rugged to abuse than semiconductor equivalents. Similarly flasher relays. The access of new technology doesn't mean it's immediately a cost effective solution to every engineering problem.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Fred" saying something like:

I was building those into gensets in the early 80s, and yes, they were self-exciting. Mechanical vibrators are so 50s, daddy-oh.

[Andy Champ]

I'll give you that!

Can't say for sure. I only know about the bits that went wrong(and there were enough of those!). But in this case, I'll defer to the GC.

Andy

[Fred]

Thanks, I keep an open mind what this generator uses, it a shame the OP hasn't come back, not has he posted on uk.rec.engine.stationary. It's always possible my ISP hasn't picked it up, it won't be the first time!

[Andy Wade]

The one I looked after was definitely self-excited. The only battery involved was for the engine starter motor.

Did you read my description properly? That level of voltage and current are for the main alternator and the only 'electronics' involved is the rectifier diodes built into the rotor. The PWM o/p from the AVR was in the same 16 V, 1.8 A range mentioned by the OP, and was handled by a single TO3 power transistor - possibly an OC35(!) although I can't remember for sure.

[George]

Hi there,

In fact I was not expecting so much information, having searched all over for several months. I posted first in alt.electrical engineering. I have not been back there either as I was disappointed by the lack of interest.

Having read all these posts I am very motivated now. However this is a big machine and I was planning to build it all on two heavy bits of channel and make up the drive coupling to one of two Perkins P3 engines I have - or even sacrifice the 1.9 litre engine out of an old Mondeo I am still using, but which has a limited expected life. This is pretty efficient and most of all, - quiet, at this sort of speed. However it is not something I can just Hook Up. I wanted to be fairly sure that it could be made to work and was very disappointed when Mawdsley told me that they had ditched all their historical data during past reorganisations.

The technology is actually 1970,s but it is way ahead of the old car regulator. I shall remove the covers to the exciter and take some pictures. but it will take a few days. Watch this space. The alternator was driven in line by a similar sized 3 phase motor continuously, to provide a smooth supply to a medium sized ICL computer installation. A Quick Start diesel Generator came up in a few seconds in the event of a complete mains failure, but this set could spin on with little drop in speed for this time. There was a lot of rotating mass. It certainly worked, but the next generation was a big UPS. Although I was the resident ICL Rep. I was the only one with any technical training and so everything of this nature fell in my lap by default. Even the old equipment that was taken out!

My sincere thanks to those who have helped, it has given me a whole lot to investigate and the will to get started.

Many thanks, George Bell.

[John Weston]

Ah the nostalgia... Storing loads of energy in flywheels to cover the start-up time of diesel generators - and trying to get it runnning in the first place. Loads of switchery...

Reminds me of when, as a DEC product support engineer, I was asked if our computer would run on the same motor generator set as the ICL it was replacing. The reply was that it would and, what's more, would fit in the corner of the same room :-) The customer desided to accept our recommendation for a new SS UPS which lived in the corner of the large, almost empty, computer room

[John Weston]

desided - who wrote that?

[George]

Little gremlins do that, their favourite toys are typos.

I got my trusty tractor out today and moved 7 layers of "useful" things and finally got to the alternator, which of course was at the back.

Why can't I copy and paste a picture??? I shall make a page on my website when I get a few more. This is no problem now I have it out in the open. I took the cover off the exciter and there were a bunch of rotating rectifiers and when I took off the cover where all the cables come out there were 3 fat phases and a neutral and two obvious excitation wires labelled x and xx. I don't think there will be a problem sorting out which is which. 6 wires in all, - what could be simpler?

It's quite clean inside but I shall high pressure water it and then wire brush and paint the outside. I looked at my scrap pile, but I only have one length of big channel. I'll get another. I think this is going to be easier than I first thought. I do have a long railway sleeper which I could cut in half and mount the alt. and an engine on that. I think I prefer a good solid chunk of steel.

More later. Regards George.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember George saying something like:

What's the rated speed of the alternator?

3000rpm - you're laughing for direct drive.

If it's 1500rpm, you'll find a car engine with a road camshaft to be right on the edge of its torque curve if you pull any real power out of that set at low speeds if it's direct coupled. I'd look at an HD pulley drive to bring the engine speed up to ~2000rpm if possible.

If the speed is 750 rpm, forget direct coupling entirely.

[George]

Thanks for that. It is 1500. rpm. But I am only going to try and use this 65 Kva alternator because I have it. The only other one I have is about 5Kva and not strong enough to drive the 3 main motors I have in my workshop, currently a Herbert Lathe 5 or 7 1/2 HP . A Denbigh Mill and a low HP pillar drill. I am also thinking of a big welding,

- plasma cutter set. Second hand 3 phase is cheaper as not so many people have it.

My worst load is the Herbert and I am still able to get it up to speed if I wait a bit, on a cold morning, using a 3HP. 1 to 3 phase converter. The sheer weight of the alternator rotating parts will give a nice start-up capability. I am not all that concerned about exact frequency stability. A pretty simple AVR will also suffice. In the future I may use it to charge up inverter batteries for a combined heat and power set up, running on bio-diesel. It will be neater to directly couple it so I would prefer to run at 1500. It will go in an insulated space and so not annoy neighbours, the water being plumbed in to a hot water tank and central heating. Waste not, want not. I shall only start it up when I need it.

What do you think?

Regards George.

[tony sayer]

Set up a project website so we can see the Gennie's progress:-))...