Brooks motor 415V to 240V conversion result and problems

You should be using only 2 of the 3 windings.

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.

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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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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It might be worth going back and verifying that each red is associated with the winding you think it is. Getting two reds transposed would leave one winding floating but shorted and only two windings active.

Alan

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Checked that the other day and all seems OK. I think that would show up when the phase to phase resistance was checked and they're equal.

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

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

Thanks for your thoughts so far.

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.

ioamateur.co.uk

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

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.

Jim

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?.

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

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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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

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:

I thought that was the idea of the diesel generator in the garage :-)

Mark Rand RTFM

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

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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

So, did you ever finish the restoration of that Hardinge lathe ?...

Regards,

Chris

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