I can have a motor rewound for about 500 US $. I'm led to wonder the value of a transformer it the switchgear is sufficient for the newly planned voltage.
The original 3 speed motors have an 'interesting' design. If you want to go down this route I would make quite sure that the rewinders understand how it works before entrusting yours to them. The original motor + brake system was expensive when new and are now reaching unobtanium level. I desided to keep mine intact in store so a new owner could return everything to original condition if they wished.
Monarchist expressed interest so I will put the write up here.
Be warned, it was written for a different target audience in the
UK - entirely different electrical standards and codes- and it is a long read.
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1024 Electrical Rebuild
About two years ago I started looking for a good Smart & Brown 1024 lathe for my workshop. I believed that this was the ideal lathe for my size and type of projects, but accepted that there would be one or two challenges in getting such a lathe set up and working. Foremost amongst these challenges was running the lathe from a single phase supply. I hoped to find a VSL model which has a Reeves type variable ratio drive, but despite looking at a couple of these I was unable to find one in good enough condition. In the end I bought a Mk1 1024 (no Reeves drive, just a gearbox) which had been all its life in a university workshop and was in generally good mechanical condition. This meant that I had to find a way to run a lathe with a 3 speed single voltage 415V main motor from a single phase supply.
There were several options:
1. Use a rotary converter, either bought or home built to produce a 415V three phase supply. This would mean that the lathe could be run as-is without any further modification. My experience with these units is that the noise from them although not especially loud is annoying, since it is always present. In addition it can be difficult to get a good balanced three phase supply at varying loads. A transformer would be needed to obtain the 415V supply.
2. Buy a ‘Phase Perfect’ type of unit that would provide 415V three phase supply for the whole of my workshop. This is an attractive option from the viewpoint of simplicity and silence, but a unit that could deliver sufficient power is extremely expensive, and my other machine tools have already been converted to run off single phase, so they would not benefit from this.
3. Convert to single phase motors for the main motor and suds pump. This is feasible, but a bit ‘agricultural’. The 1024 is a really high class lathe and it seemed a pity to inflict a rough running slam bang start single phase motor on it. Even worse, a single phase motor would have a single speed and this would drastically limit the range of speeds available.
4. Replace the main motor with a new dual voltage single speed inverter duty motor and build an inverter based power supply that would provide all the facilities of the original design. This is probably the most expensive option and the most complex by a large margin. It would provide a few major advantages if it could be made to work. The most significant to my mind were sophisticated speed control with good motor protection, soft starting enabling the initial current draw to be kept low, and a smooth quiet drive.
In the end I chose the last option and started a project that took many months to bring to a successful conclusion.
Design Issues
The Mk1 Smart & Brown 1024 has an unusual spindle drive. The main motor is a large, high quality three speed unit with (in my case) a brake. The motor changes speed via some complex wiring and a rotary switch mounted on the front control panel of the lathe. The drive is taken from the motor to an intermediate gearbox by a flat belt drive and crowned pulleys. The gearbox (made by Matrix) has two ratios, mechanically selected via a linkage to a control lever again on the control panel of the lathe. The gearbox can change speeds ‘on the fly’ via a pair of internal Matrix multi plate clutches. The drive is taken from the gearbox to the lathe spindle via a second flat belt drive. This drives a pulley mounted on its own set of bearings to avoid transmitting vibration and belt stresses to the spindle. Although this design of drive might seem archaic in some respects, it does provide a silky smooth and nearly silent drive when in good condition and contributes to the excellent finish for which this lathe is renowned. The design means that six spindle speeds are immediately available from the control panel without stopping the spindle to change speeds. An additional back gear is available selected by a handle on the headstock. This provides a total of twelve spindle speeds from 30 – 2500 rpm.
The original motor speeds are 2800, 1400 and 940 rpm, and the corresponding outputs are 2.5, 2 and 1.5 hp. That means that there is a ratio of nearly 3:1 between the lowest and highest motor speeds. A motor driven by a VFD can provide variable speeds of course, but a range as large as this is rather problematic. A normal 3 phase induction motor has a characteristic output with varying speeds. From zero up to its base speed it delivers roughly constant torque. From its base speed to its rated maximum speed it delivers roughly constant power. The corollary to this is that from zero to base speed the power of the motor rises with the speed, from very low power at low speeds to reach the rated output at the base speed. Over the base speed, the torque drops off as the speed rises, so that at high speeds the torque available is significantly diminished.
The original motor is single voltage – 415V. This has significant implications for a rebuild intended for domestic 240V use. Many modern 3 phase motors of around 3hp are dual voltage. This means that they can be connected to either 415V or 230V supplies. In the case of 415 V supply, the motors are connected in star configuration and for 230V in delta. A delta connected motor is very convenient since it can be driven by a low cost single phase 230V in and 230V three phase out VFD. Unfortunately a single voltage 415V motor cannot be driven in this way – transformers can be used but life becomes a bit more complex. Single phase 230V in, 3 phase 415V out VFDs are available, but they are expensive or the result of local modifications that may not have the manufacturers full approval.
Connecting a suitable VFD to a three speed motor might seem relatively easy – simply mimic the original wiring and feed the VFD provided 415V three phase supply to the rotary switch to allow the three speeds to be switched on the fly. If you read the manuals provided by the VFD suppliers you will find that they specifically warn against this type of arrangement. Any switches between the VFD and the motor that are capable of interrupting the supply under load might cause VFD failures. In reality modern VFDs may be capable of living with this, but the risk is still real, so best avoided.
I could have retained the original motor and used just one winding – say the middle speed. I didn’t do this because the motor isn’t designed for inverter use in this way and I felt that I would be stretching its design far too much to get the speed ratio I needed from it. The difficulty of supplying it with 415V further put me off pursuing this option. I decided to bite the bullet and buy a new, good quality, inverter rated, dual voltage motor.
The motor size I selected was 2.2 kW or 3 hp, so slightly larger than the original motor’s maximum power output. I chose a 4 pole motor, meaning that the motors synchronous speed at 50Hz would be 1500 rpm. The speed under its rated load would be slightly slower, say 1425 rpm. A 2 pole motor would have been twice this speed and an 8 pole half the speed. The choice was an engineering compromise as is often the case. A 4 pole motor favours the lower speed end of the range. I wanted to ensure sufficient torque in the lower spindle speeds, at the expense of torque in the highest speed of 2500, since I did not expect to use this high speed very often, and certainly not with a big chuck.
Modern VFDs have a ‘sensorless vector’ mode. Without getting into the details, this allows the VFD to deliver better performance at lower speeds. I selected a VFD that had sensorless vector control and was confident that with the four pole motor, I could successfully run at 940 rpm (or even less) with no problems. This just leaves the problem of reaching the higher motor speeds needed to deliver a spindle speed of 2500 rpm. That would require a VFD output of about 98Hz. This is well beyond sensible frequency limits for a normal motor. I decided that the maximum frequency I could use (based on some reading and study of motor performance charts) was about 80Hz. I parked this issue for a while as I considered how the new motor could be fitted mechanically.
Mechanical Matters
The original three speed motor is large, very heavy and was suspended by its feet from the underside of the gearbox mounting plate. The modern single speed motor is smaller and lighter, and had completely different foot/shaft dimensions. This meant that some new parts were needed to allow the new motor to line up with the gearbox input pulley. The flat belts used on this lathe are endless so ideally the new motor should be located in such a way that the original belt could be re-used. To get the motor into the required position, I designed a welded framework that dropped the motor by the required amount and allowed the existing tapped holes in the gearbox support plate to be used. The framework also moved the motor sideways so that the shaft would line up with the gearbox pulley.
To provide a bit of added interest, the position and mounting of the motor in this lathe is not user friendly. Getting the motor out safely required a timber framework to be made. I reused this when getting the new motor in position, but the whole exercise of measuring up for the weldment and moving motors in and out (several times) was extremely physical.
The new motor was metric and of course the shaft was a different size – 28 mm compared to 1”. As you would expect, the keyway was metric as well. I decided that it would be simpler to make a new pulley rather than trying to modify the existing pulley. Given this decision, I had the opportunity to change the ratio between the motor and gearbox a little to resolve the problem that I had with the higher speeds. I made the new pulley a little larger than the old one, sufficiently so as to bring the maximum spindle speed within reach of the motor running at 80Hz. This was a balanced change that did not take the lower limit of speed out of reach for the motor.
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