Myford Super 7 - Single or Three Phase

I have recently inherited my Fathers Myford Super 7 lathe which I have been using for the last 40 odd years in his workshop. The motor always ran rather hot, and on recently moving the lathe from my fathers workshop to mine, I think I may have found out why. The motor is only rated at 1/3hp. The Myford literature states a single phase motor rating of 3/4hp or a three phase motor rating of 1/2hp. I guess this is why the motor runs hot. Am I likely to be correct? Also why the different ratings for single and 3 phase? Surely if 3/4hp motor is required then the supply is irrelevant to the power required?

Assuming I should upgrade the motor, this leads me to my main question, should I go single or 3 phase? I have read a lot about three phase motors being better than single phase in terms of the motor running smoother, and therefore getting a better finish on the turned work. Is this a theoretical better or is their really a noticeably better finish?

Again, assuming I should go 3 phase, are their any recommendations on what package to get? I have been looking at the transwave inverter kits which include motor, inverter and controller (I only have a single phase supply in my workshop). Are these good kits, or are there better solutions? Would a 3 phase converter be better so that I could use this to also power the larger choice of better machineary available in 3 phase only?

Thanks in advance for any comments and suggestions.

Paul, I doubt that your motor is overheating just because it is rated at 1/3 HP rather than 3/4 HP unless it is constantly running against some load..I suggest you take off the main drive belt and run the motor on its own to see if it still overheats. If it does it could be things like tight bearings, lack of ventilation, damaged wiring or even the starting windings remaining in circuit. Try turning the lathe over by hand with the motor drive belt off. It should turn freely but with some resistance due to the small load imposed by the mandrel bearings and the countershaft. Then slacken the drive from the countershaft to the mandrel and check how freely things work. A motor only produces its rated HP when a matching load is applied to it. When running idle or with a small load the motor will only produce sufficient power to overcome the load. You can easily check this if you put a clamp meter around one of the supply wires and observe the current drawn under various load conditions. I converted my Super 7 to 3 phase inverter drive several years ago and it is a total success. I would never go back to single phase. (Sorry about the lack of paragraphs but for some reason the site will not let me use the return key) .

Paul, Do you stop and start it a lot that will generate heat.

Three phase, if you do this using an Inverter it will give you unparallelled speed control, in the time you have used the machine you must have come across chatter when cutting/facing something with a single phase machine if you wish to do something about it you have to reduce the feed or speed which can mean stopping the machine and in the case of the Super 7 changing pulleys or selecting backgear, imagine instead just reaching for a control pot and simply turning the speed down.

Paul

Paul,

I don't want to pour cold water on the debate, but I too considered switching to inverter drive for lathe and mill in my workshop. I exchanged a lot of informative correspondence with Mr Newton of Newton-Tesla who was most helpful, but unable to resolve my concern over the effects of earth leakage on our over sensitive trip [which is about 15mA]

It is apparently a feature of the construction of these inverter systems that about 10mA is leaked to earth; this is not usually a problem for most domestic earth leakage trips rated at 30mA. It can be adjusted but make it much less and the motors begin to sound 'metallic'

Our computer system at that time was a mains powered tower stack that needed tedious restarts in the event of a power failure, accompanied of course by the loss of data not protected, My wife does a lot of computer designing and saves stuff in slabs as she remembers, laptops have battery back-up so that is not a problem now.

I had no desire at the time to incur domestic wrath every time I powered up machinery, nor the fag of tramping back to the house to reset the trip. so reluctantly I passed the concept by. I manage well enough with a resiliently mounted single phase 3/4 hp Crompton motor for the lathe and the less well silenced single phase 1/2 hp motor on the mill.

Neither of these run excessively warm, you should be able to bear full hand contact on the motor housing after a good run without leaping back in alarm. As Lampton says, check loadings and free running

Brian

Paul, It would make good sense to carry out the checks that Lambton suggests. If after doing that and all is well then the choice is single or 3 phase if you want to get a new motor. 1/3rd hp does seem a bit small.

I have a 3 phase inverter running my milling machine and I have found its not that simple just turning a dial to get the desired speed. Yes I can do that but it quickly runs out of torque as you slow right down. I am going to have to gear the head down to keep the bottom end torque.

Perhaps Transwave are different ?

If you have managed for 40yrs on single phase I wouldn't discount it. What about this

http://www.ebay.co.uk/itm/Electric-Motor-0-75kW-1-0hp-230V-1435-rpm-80-Frame-Foot-Mounted-/181064829871?pt=UK_DIY_Material_Electrical_Fittings_MJ&hash=item2a284e0baf

Quick easy swap and a lot less £££££

Edited By john kennedy 1 on 26/03/2013 16:42:19

I'm with Paul on this one. When my 1/2 HP motor packed up 5 years ago I replaced it with a 1 HP 3-phase motor and VFD package from Newton Tesla, and I wouldn't go back. I set the pulleys to the top speed of the lower range (i.e. the external pair of pulleys on low, the ones inside the headstock on high) and I can't remember when I last needed to move the belts. The motor is a little more powerful than standard, but that means there is plenty of power at low speeds. The NT package comes with proper connectors, and the motor is the correct one to slot straight into the lathe's cradle with minimum fuss. It's quiet and smooth.

I liked it so much that I had no hesitation in fitting a VFD from the same source when I bought an M300 a couple of years later.

David

I note that there are several VFD packages available and wonder how one chooses between the oft mentioned Newton Tesla and the other package options, some of which seem a bit cheaper?

I thinking particularly of fitting to an S7.

Thanks

Phil

Thank you all for your thoughts so far. The motor gets hot even when the drive belt is disconnected, so I think I definately have a motor problem, probably as Lambton suggests, the starter winding is not kicking out as bearings seem fine.

Thanks to _Paul_'s comments, I know understand why 3 phase is a better option than single phase. Chatter can be a problem, and just adjusting the speed to get the tool system out of instability makes a lot of sense. John's comments also make sense as torque will reduce as the speed of motor is adjusted. I beleive that the machines belts/gearing should be used to get to the speed you need, but tweak with speed contoller to get the best from the machine. Is this other peoples belif? I have also read that the better controllers have Vector mapping, which helps with the torque characteristics. Does anyone have experience of these?

Going to 1HP seems to far though. My thoughts are that at this power level serious damage could be done to the lathe if those occational, unplanned and unwanted crashes occur. My question remains as to 1/2 or 3/4 hp? I still do not understand why Myford quote 1/2hp single phase, and 3/4 3 phase motors (admitidly in an old brochure!!). Any comments from anyone would be appreciated?

Thanks

Paul

Paul, I have a S7B which was purchased as a factory recon direct from Myford and it came with new electrics, including a Brook Crompton motor.Similarly, I also purchased a new VMB with VFD already fitted ( Danfoss VFD and Brook Crompton motor) Attracted by the advantages of VFD, I too was impressed by the excellent NT service and package, and subsequently effected the change to the S7B. Overall I am pleased with the conversion, but there are some points of note. In my experience, the NT motor is not as smooth running nor as quiet as either of the BC motors, though it is still acceptable. Adequete ventilation can be a problem with all motors, and this is one reason why NT recommend a minimum motor speed of 800 rpm. Continued use below this speed may result in the heat of the motor exceeding the insulation class of the windings, and thus causing damage. You also need to understand that with VFD there are very real differences at different motor speeds between the power a motor developes and the torque it delivers. Again NT were very helpful in this aspect. In addition, since an electronic VFD can only generally deliver the voltage it is supplied with, NT accomodate this by supplying the motor specialy wound for 220 V. You may find that other companies supply 415V or similar standard motors and this therefore may be one of the ways they are able offer lower prices. In addtion, the NT package came with a 10 year guarantee on the Mitsubishi inverter, but am unsure if this is still the case.

Regarding your question regarding differing power requirements this is actualy related to the different torque characteristics of single and three phase motors.

So, to summarise, in my opinion, whilst VFD is not a panacea, if used properly and in accordance with its properties, which includes the occassional belt change to exploit them, then it is undoubtedly worth the investment.

I hope you find the above of some constructive use. Also, I have no connection with NT in any way other than as a very satisfied customer.

Trevor.

my Fathers Myford Super 7 lathe which I have been using for the last 40 odd years in his workshop. The motor always ran rather hot,

Lots of good advice if you want a new motor, but if it has done everything you and your father needed for the last 40 years and only run hot (without overheating) do you really need to change it?

My own experience for my mini lathe is to change from the ~1/3 HP variable speed DC drive to 1/2 HP fixed speed drive. I find I have LESS usable power as the round belting I am using slips, so I need to upgrade this, (probably to a poly-v belt).

As for speeds, I have twelve combinations available by pulley swapping and back-gear although only 10 significantly different speeds. I think the 145rpm lowest speed is too quick, but even so I find I only appear to have used two combinations between about 300 and 400 rpm, and occassionally the lowest speed - but at the lower speeds the belt slips too easily, and the higher speeds are too fast for all but very small work - I end up doing most work.

That said, I still fancy changing to VFD, but I suspect it's a bit like putting alloy wheels and spoilers on a family car

Neil

Responding to Trevor's point about the torque produced by VFD-powered 3-phase motors, I believe that at low speeds the motor is operating at constant torque. At a certain point, somewhere in mid-range, the motor moves into constant power mode. Certainly I have always found the torque is perfectly adequate even at low speeds; though you do have to be careful not to operate too long at very low speeds, the system has a cut-out to protect against overheating.

And as for motor voltages, the great majority of modern 3-phase motors can be wired in delta or star configuration by a simple shift of links on the terminal block; the former works at 415 V, the latter at 220 V. Older motors may need you to dig out the star point to convert to 220 V, which is a lot more fiddly but you can find instructions on the web. Of course if you buy a kit this is all taken care of for you, and if you are not comfortable with mains electricity I would say this would be the way you should go.

David

My guess as to the reason behind single phase 1/2 or 3 phase 3/4 recommendation is that they are likely to be similar physical size and price, and myford were hinting that 3/4 is better but 1/2 would do if you have single phase supply.

Remember, 'hot' does not necessarily mean 'too hot'! How hot is it?

Some inverters {my 'Powtran' } can happily power a 415v motor from a 240v source. (i presume it uses a voltage doubler or other charge pump process on the input to provide a 1000v DC (from memory) internal bus, which is then switched to provide the required output voltage/frequency. Clever things those chips...)

-regards, russ

Edited By russell on 27/03/2013 01:31:09

I have Transwave ‘Cub’ VFDs on both my S7 and VMC mill and can assure Paul that torque is maintained right down to the lowest speeds. I regularly run mine from up to 100Hz (double speed) down to15Hz (less than one third speed - wonderful for large radius fly-cutting!) and even at the lowest speeds I cannot stop the chuck with my hand.

They will actually run at 10Hz, but not for long. The problem is that as frequency is reduced the reactance of the windings (their resistance to AC current) reduces proportionally and so the current increases until the VFD finally says “enough is enough” and cuts out. Another problem is that at low speeds, the increased current along with the reduced cooling from the motor fan results in motor temperature rising. Luckily, for most model engineering tasks the motor is just ambling along and I have never found it a problem, but it is obviously something to keep an eye on.

Brian

Regarding the use of 100Hz and hence a speed doubling. At these maximum speeds it would be advisable to check the MAX speed rating of any chuck or faceplate in use. They do have safety limitations.

Edited By KWIL on 27/03/2013 09:54:09

You would know if it was the starter windings not switching out, the motor would run at a much reduced speed, the speed would depend on the number of starter poles, your motor is a 4 pole motor, and proberbly has 4 starter poles, so it would run at half speed ie 725rpm. There was an artical in ME back in the 1990s about a bloke who found that he could get extra low speeds for his drill press by switching in the start windings, but they are wound with lighter wire as they only are designed to turn the motor for a second or so.

With modern motors, they can run quite hot, modern insulation stands the heat OK, but old motors you should be able to touch without getting burnt, unless the motor is not rated continuous, and it could have a restriction on the number of starts per hour. Ian S C

Ah yes, plus a large chrome exhaust and lots of noise. And what do you get? A petrol to noise converter, but no extra performance!

To clarify a few (simplified) notes on three phase induction motors. First, a motor will be designed to produce its rated power at a given phase-to-phase voltage and frequency. The actual motor speed will depend upon the number of poles and 'slip', and is known as the base speed.

The current in the windings will depend upon the applied voltage, the impedance of the windings and the back emf, which is proportional to motor speed.

If we double the applied frequency, but keep the same phase-to-phase voltage the motor will run at twice its base speed. But the current in the windings will be reduced due to increased impedance and back emf. It is assumed that we cannot increase the voltage. Since the winding current has decreased the torque will have decreased, but the speed has increased. So, to a first approximation, the power (torque x speed) is constant.

If we now halve the applied frequency the motor will run at half its base speed. If we keep the same phase-to-phase voltage the winding current will increase, due to lower impedance and back emf. However, the losses in the winding resistance will increase as a square law. To counteract the increased losses the phase-to -phase voltage is reduced in order to reduce the current to that which would flow at base speed with rated voltage. The consequence of this is that below base speed the torque stays constant but the power decreases proportionally. So below base speed the motor is essentially constant torque.

Most 'simple' VFDs run a V/F algorithm, ie, as the frequency decreases the applied voltage is decreased to keep the winding currents roughly constant. Sometimes the algorithm is 'tweaked' at low frequencies where the winding resistance causes the basic V/F curve to underdrive the current in the windings. It is also possible to programme the VFD to run the motor below base speed with a percentage 'overload' on the current; fine for short term transients but may lead the motor to overheating if over done.

In summary, above base speed - constant power, below base speed - constant torque.

Russell: I assume that the VFDs that can generate 415V phase-to-phase from a 240V input probably incorporate a power factor corrector, also acting as a boost converter, in the first stage rather than voltage doubling or a charge pump.

Regards,

Andrew

The basic rule-of-thumb for motors running from VFDs is that they give constant torque at speeds less than the standard main frequency, and constant power above.

Since power is a product of torque and RPM, power is proportional to RPM below standard frequency, and torque is inversly proportional to RPM above.

I've purchased VFDs from a well known online auction to convert my lathe, mill, and drill, and seven other machines of friends - in all cases it worked out far cheaper than the commercial offerings mentioned.

Until youv'e used a VFD powered machine you can't really appreciate the advantages.

Paul, I do not profess to be an expert electrician but over the years have found that this type of motor is generally fitted with a centrifugal switch on the starter windings, these switches do gum up over a long period. Mine did on my S7 purchased new in about 1976.

I would suggest dismantling the motor and cleaning the switchgear inside and you may well find your problem is solved.

Regards John

Once again, thanks to all of you for your comments.

I decided to investigate the motor as John suggested, and was not too pleased with what I found. The internal wiring has rubber sleeving and has perished and is beginning to break-up. The motor still runs, but could fail at any time!! Yes, I could replace the wiring but I think soft start and speed control etc. has won the day over single phase. I now have to decide what system to get and who to purchase it from. Life is full of decisions!!

Paul

Drives Direct are very helpful (usual disclaimer) and there is a chap advertises VFD's etc. on Homeworkshop Gavin Oseman.

Paul