Brooks motor 415V to 240V conversion result and problems

A few months ago I asked about converting an old Brooks 415V 3 phase 1hp 1420rpm motor to 240V and got some good advice here. An electrician I
know checked the insulation and pronounced it spot on although the 3 lead out wires were a bit perished, we replaced them with suitable black wire as per the original. We then went searching for the star point and found that straight off so separated those 3 wires and verified we had 3 distinct windings of equal resistance, we did. We added new leads to the start point ends using suitable red wire this time and connected them up to the main terminal block as per a normal star to delta connection re-orientation so ended up with a red and black wire connecting at each of the 3 motor terminal block connections. From the book mentioned on electric motors by Jim Cox we were looking for about 40uF run capacitance so my mate acquired some capacitors from flourescent fittings and we rigged it up with 36uF (3 x 12uF) and another 36uF (3 x 12uF) for start capacitance which we switched manually. This seemed to work OK no load but when fitted to the machine it wasn't happy. The mill, a Hayes DieMaster, has a gearbox with 9 speeds ranging from 50rpm to 2400rpm and it would start most times at 400rpm but wasn't having any of it above that. My mate acquired a larger start capacitor, about 300uF, as the book mentioned that up to 8 or 10 times the run capacitance might be required for high starting torque applications but when fitted it made little improvement. One of the thing noticeable is that in some positions the motor seems to jitter backwards and forwards slightly when starting under load and at others it starts much more strongly but not with enough torque to run up to speed in the higher gears.
The new larger capacitor is supposed to be new but none have been checked. I haven't got a capacitance meter and don't really won't to have to test it using a scope and comparing the charging characteristics against a known reference. Got the scope but haven't done that sort of thing since physics class over 25 years ago and my mate is not local. Might be an excuse to buy a cheap DVM with capacitance checking though.
Does anyone have any thoughts on what might be wrong, if not the motor will be retired and a single phase cap start/cap run one purchased for the machine as currently the motor is the only thing preventing it being used. I suppose now the motor is wired for 240V an inverter is an option but my mate is tight and I think the 1ph motor is the cheaper option unless he is lucky and comes across a bargain inverter. With 9 speeds I think the inverter may not give any real major benefits and the mill is not likely to be used that often.
Any thoughts appreciated.
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You should be using only 2 of the 3 windings.
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Magnum wrote:

The information I was provided by one responder and in the book by Jim Cox shows all 3 windings being used, the motor being connected for 240V delta operation with L+N directly across one winding and the other 2, the capacitor then being connected between the L and the junction between the 2 windings . Do you have any experience with the operating difference between using 2 windings and 3. I presume using 2 windings it would be more like a single phase motor with the start winding being disconnected after the motor is up to speed, IIRC that won't give the full motor power continuously from what I've read.
L --------.------- / \ | / \ _ / \ _ / \ | N ---.---------.--
That may come out if displayed with a fixed font. I'm not going to give up my day job to take up ASCII art.
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Hello Dave, I think I was one who responded before. I struggle with ASCII art too, but you are correct with you motor connection; you should indeed be using all three windings. Using caps. from flories is a bit borderline and you possibly could do with a little more for the run cap. (Cr) total value. I suspect that the start cap. (Cs ), however is too big. Starting a milling machine does represent 'some' load, but is not really 'starting under load' as eg driving a compressor.
When the motor is running, you should check the phase voltages with a volt meter (set to ac obviously). You want the voltage from the non-line connected phase (Pc) to the live and to the neutral to be about 5~10% higher than the L-N voltage and ideally within not more than about 5% of each other. Likely values might be L-N 245, L-Pc 260, N-Pc 255
I have had experience of too big a Cs. and too small a Cr. where the motor starts, but because Cs is too big, the motor does not reach full speed. Then when Cs is switched out, there is not enough Cr to enable the motor to pick up. I'm not sure if this is what you are experiencing, but the values you are quoting for a 1 HP motor lead me to suspect that it may be.
Richard
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Richard Shute wrote:

Richard,
You were one of the ones that responded before IIRC, also Pentagrid. From the information in the book my mate and I got the impression that more start capacitance was better for the starting performance but from what you say maybe not. Your description of too much start capacitance may be a factor as we have seen that it doesn't run upto full speed with the large start cap in place but does pickup speed when the contactor is released, at least starting no load on the bench. When I see him next week I'll take my DVM down and do a check on the voltages and get him to see if he can scrounge some more caps in the mean time, his company was throwing loads of light fittings out so that's where he got them, the large start cap he had to buy and too many of those and a cheap single phase motor starts looking more attractive.
My current concern is the starting performance, while it's not a compressor which would normally have a step down drive, the inertia of the drive train does present a significant factor when the motor speed is being stepped up, the max being 7:12. Currently it won't start above 400rpm and that isn't great, that's a 3.5:1 reduction. We need to improve the starting quite a bit or look for some other option.
BTW the drive train in this machine is the motor driving the gearbox via a belt drive at about 1:1, the output pulley is about 6" diameter and 3" wide flat belt and not far off being solid, the flat belt drives a smaller pulley on the quill shaft in the upper ram, which then drives the main quill spindle via a spiral bevel reduction gear set. I suspect the gearbox output pulley runs at about the quill speed. A picture of the mill here http://www.lathes.co.uk/hayes/ . The motor sits in the bottom of the base at the back and the mill will sit against the wall when finished so we don't want to have to play with the motor much after that.
Thanks for your thoughts so far.
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On Tue, 25 May 2010 11:17:34 +0100, David Billington

This method of motor conversion is well proven and normally works like a charm. You have accurately described the correct reconnection of the windings and unless there is some unnoticed connection error all should be OK.
I think you and Richard are on the right track.The fact that your motor speeds up! when the starting capacitor is disconnected points to a faulty component or a wildly inappropriate value. With the correct value there should be little or no speed change with starting capacitor disconnect but a marked reduction in motor noise as the phantom phase voltage moves to its proper working level.
36uF extra capacity for starting is not enough for even the fairly light starting load of your machine but the nominal 300uF capacitor which may be at the top end of it's - 50% +100% tolerance is excessive. Motors vary a bit in their starting requirements but I would expect your machine to give it's best starting performance with about 100uF to 150uF extra capacitance. Ex Fluorescent lighting capacitors are ideal for this purpose - their capacitance tolerance is much closer - typically +/-10% and their losses lower than the equivalent electrolytic motor start capacitor.
Jim
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snipped-for-privacy@yahoo.com wrote:

One of the things I was wondering was whether it's possible to connect the windings up in the wrong manner given that we have the 3 original star connection wires in black and the 3 star point wires in red, and have connected a red to the black of another winding. Can this connection scheme be done wrong or is it like swapping the phases to reverse the direction of a 3 phase motor. Do you have any idea as to whether 2 pole or 4 pole will effect the outcome?. Where is Nikola Tesla when you need him?.
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On Tue, 25 May 2010 21:12:33 +0100, David Billington

If you had one of the three phases connected back to front it would be similar, in effect, to having a short circuit. (one phase winding in opposition to the other pair).
No motion and a strong possibility of the magic smoke needing to be reinstalled in the motor if the fuse was too big.
Luckily, in the OP's case, since they had to find the star point and make new leads, it would be fairly hard to get one back to front...
Mark Rand RTFM
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David, Providing you really have got the reds from the (originally) star points and the blacks from the other ends of each winding and you now have red-black from a pair of windings at each corner of the delta then it doesn't matter. Swapping the 'position' of a winding in the delta will simply reverse the motor rotation.
To comment on another response, all the (motor run) capacitors I have are either +/-5% or +/-10% not -50%/+100%, I suspect that must be either a typo or a mis-read of the component.
Because I have done a few of these, I have built up a fair collection of motor run capacitors of various values so I can swap them in and out to get the best result. I have probably a dozen, but usually obviously only use two or maybe three. They are not cheap I agree and if you can come by some scrap ones for free so much the better. A little patience on fleabay and you can usually pick some up for around 6~8 each
Richard
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On Tue, 25 May 2010 23:06:30 +0100, Richard Shute

A bit more on the connection sequence might be helpful. With the winding ends coded red and black, the torque component it generates can be regarded as an arrow located between each red &.black end. With the arrows shown on a delta connected motor diagram it is quite clear the that the correct terminal connections are red to black in ANY order. Winding sequences ABC or ACB etc. are all equally valid and produce torque in the same direction
No single red/black pair is allowed to be reversed as this destroys the addition of torque.
However, if ALL pairs are reversed, torque addition is restored in the opposite direction and the motor rotation is reversed.
This is not a sensible method to reverse because it is so much easier to reverse any pair of the input wires to the motor. This reverses the direction of supply rotation and this reverses all three torque directions.
This is not affected by the number of poles in the motor - 2,4,6 pole motors will all behave in an essentially similar manner.'.
I agree with Richards comments on tolerance, -50% +100% belongs to a long outdated commercial world. Dedicated motor start capacitors are much tighter than this and even general purpose electrolytics are now no worse than -20% +30%.
Jim
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Hi,
The problem with 3 phase motors is just that, they are 3 phase :-). You never get full power and torque trying to run the thing in single phase mode and you could save a lot of time by rewiring as a 3 phase 240 volt motor (red to black, red to black, red to black, to effectively series connect all the windings), then buy a cheap single to 3 phase 240 in and out inverter from Ebay.
My solution, after finding a collection of 3 phase i/p inverters in a junkyard, was to build a 500 volt dc @ 10a power supply, then feed this to 2 of the 3 phase inputs of the inverter. Most inverters seem to have a 3 phase bridge on the input, followed by reservoir caps, then straight into a 3 phase bridge pwm inverter, so straight dc input is ok, though you may have to derate slightly if the input bridge is near it's rating. The 500v psu was just a 240-110v 10a isolation tx, with the 110v secondary in series with the mains, feeding a 40a bridge rectifier and into a couple of very large ex ups caps, with a 2 sec time delay relay to charge the caps up slowly on switchon. Output is about 550 volts dc, with a very nonstandard socket on the output for safety reasons !. Works like a charm and has enough welly to drive 7.5HP motor and probably more...
Regards,
Chris
snipped-for-privacy@yahoo.com wrote:

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I thought that was the idea of the diesel generator in the garage :-)
Mark Rand RTFM
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Mark Rand wrote:

Hi Mark,
That was actually at the end of the garden and went some time ago :-). The Rover apu turbine was sold a couple of years ago as well to a preservation group and am told is now in the hands of the Gatwick Air Museum. At least it's found a good home. Probably spent 100's of hours building and restoring that unit.
Anyway, If you consider that 3 phase motors and power tools are far cheaper than single phase units, it makes sense to set up for default 3 phase operation. Hence, the big psu and inverter with a variety of output 3ph sockets. By the time you've finished faffing around with caps, you could have the job done with all the advantages of speed control, power limiting, slow ramp up and down. Quite usefull for compressors and other large loads that would normally exceed the capacity of the supply on startup.
The 3 phase inverters really can be quite a bargain as well. eg: 3 phase 17.5 Kva Lenze unit went for less than 5.00 on Ebay a week or so ago and they used to appear regularly at the local scrapyard near Banbury. Of course, you have to take a chance, but the manuals are all downloadable and most are not too difficult to set up.
So, did you ever finish the restoration of that Hardinge lathe ?...
Regards,
Chris
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Still working on it, especially last night :-(
Apron gearbox completely re-manufactured. Saddle, cross and top-slides re-scraped. Bed reground and some lapping done with a cast iron lapping plate to remove the divots and troughs formed by grinding it in two halves and by it hogging up between it's mounting bolts. Everything repainted, subject to some minor tidying on one cabinet door.
Cabinet is assembled with motor, speed control and electrical cabinet.
Currently working on re-assembling the thread cutting gearbox. Two more bearings on the way to replace for ones I'd mis-ordered. I have parts from several lathes, assembly is by selecting the best parts out of the heap. Hardinge changed between ball and needle roller between the two heaps...
Everything takes longer than it should, but the end is in sight.
Mark Rand RTFM
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We have a pillar drill which came from Holland with a 3-phase motor on it, that has run since the 1970's on single-phase with a capacitor only as a means of using it on single-phase. Power is more than sufficient for up to its maximum drill size, and we have used it beyond those sizes with no problems.
What IS affected is slow-speed motor torque, but in many ways that is good as the drill will stall before yoiu do any damage if the drill jams.
This implies a motor with windings that can be reconnected to 240V Delta connection from 440V star.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk
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Peter A Forbes wrote:

Hi Peter,
Agreed, it can be done, but the result is always a compromise. The phase voltages vary with load, so if you set up just right at no load, it's usually miles out with load. Have tried several methods, starting with an alternator belt driven by a single phase motor, caps, transformers and inductors (eg: scott connected transformer), but the results were always unsatisfactory in terms of flexibility and power output.
I needed a universal solution to drive *all* present and future 3 phase stuff and an inverter solution just gets the job done affordably, quietly, with little hassle and footprint :-)...
Regards,
Chris
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snipped-for-privacy@yahoo.com wrote:

Jim,
Indeed around 100uF to 150uF. Last night we had the opportunity to try again and started by connecting a 80uF start cap from another motor, that was a significant improvement on the previous 40uF or 320uF, but not totally OK. We continued adding some more fluorescent caps up to about 130uF, which is all we had, and that seemed to be a major improvement and worthy of trial back in the mill. I operated the start cap contacter manually while by mate operated the mains switch and loaded the flat belt pulley with some timber and 130uF looks very promising. Next step is to put the motor back in the mill and connect the new belt drive and purchase a suitably sized start cap and try again. He has enough fluorescent caps that we can do some more tuning when fitted in the mill to try the real starting performance when driving the intended load. From some initial checks the 40uF run capacitance seems OK from the voltages mentioned by Richard, we'll check again under working conditions. Will report back again with hopeful success and a working Hayes Diemaster mill and original, if slightly altered, Brooks 3 phase motor. Maybe my mate will respond when he figures out newsgroups, I think this is the first he has looked at.
Many thanks to all for your information.
Dave
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I meant to add that I would expect a Cs for this application to be about 60~100Uf. 300uF is BIG! in this context.
Unfortunately, this task is a 'select on test' type of job. I have recently done two apparently identical lathes with apparently identical motors, yet the capacitor values required for best performance and balance were not identical. I'm still confused by that, but I can't argue with what the DVM tells me.
Richard
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Richard Shute presented the following explanation :

Typically, capacitors of this type have quite a wide tolerance - could be -50% to +100%. That might explain the variations between the same setups.
--
Regards,
Harry (M1BYT) (L)
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On May 25, 1:14�pm, Harry Bloomfield

Just read your posting and it sounds like there may be a wiring fault perhaps. Got some motors like this running with just capacitors on them. In my experience I start off with smallish values a bit below per Jim Coxes book with the motor running free and measure the speed. I have loaded the motor sometimes with a wooden block to judge the power and trim/add capacitance. I seem to get around 2/3 power at least on average
My Myford S7 has a 1 hp motor and it will start the lathe without the clutch, it does not have a start cap but only a 30uf run 440vwg and has been doing this for 13yrs. This is a fairly small frame motor but I only have to use start caps on a fan with a similar motor.
Think you are on the right lines, I like 3phase motors, no arcing start contacts!
Peter
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