myford ML7 - choice of motor

Just got an ML7, which needs a bit of TLC as its not been used for about 20 yrs. It's a tri-lever with a screw-cutting gearbox, so far so good, but: it comes with a 3ph 2 speed motor, and I have a domestic 230v supply. It also comes on a nice industrial Myford stand with all the switch-gear for forward reverse and high/low, which i'd like to re-use, but I'm a bit in the dark. i don't wish to go down the rotary inverter route, as I will only use this occaisioally. I've seen some digital ones on the 'web which sing their own praises- variable frequency allows almost limitless speed variation but: they don't seem to be a real 3ph supply as there's only 230v between phases. I'm actually a bit confused as to what use they are at all. Anyone out there any experience of these?

Also, 2 speed single phase motors apparently exist but are as rare as hens teeth, I'm wondering if my best bet is to buy a 4-pole motor new from somewhere like machine mart- this will work all the low speeds and I could always invest in a bigger pulley if I particularly found I was going to need the higher speeds. Or, go for the 2 pole one which would mean higher speeds only.

Advice please- I'm a spark, so I understand all the motor theory and such, but I have little experience of workimg with motors and the various types available.

Dave

Hi Dave

First thing to check is the motor you already have if the I.D. plate lists the motor as dual voltage i.e. 3 phase 220/440 then things are looking good as you will be able to run the motor of a 1 phase 220v to 3 phase 220 VFD If your motor is 3 phase 440 only then I would look at changing the motor out for a 3 phase 220 motor with a VFD (drives direct do a complete package ready to bolt on to a myford) this will give you the best of both worlds, infinate speed variations and reversing at the flick of a switch. The other advantage with a lot of VFD's is they come with a digital display on them so you can use this as a speed ref.

Regards Jon

Is the motor dual voltage 415v/230v or can it be made dual? for occasional use if you dont want to spend loads of cash the options could be:

• Run using a capacitor to make the third leg.
• Use a Rotary Convertor
• Buy an inverter, some can be had for the cost of a new replacement motor.

Personally I use a homemade 230v RPC (Rotary Phase Convertor) for machines requiring no speed control typically these are my old Norton T&C grinder and some of my Shapers.

My ML7 runs mostly from a Teco 2HP inverter mainly because of the infinitely variable speed control it gives, it's great to just be able to just turn a knob to vary the spindle speed rather than stop the machine and change belts/pulleys.

An RPC will give a good approximation of 3 phase when the device it is driving is actually being powered mine balances out to within a few percent when used with the Grinder as the capacitance was "tuned" to suit this particular machine, when other machines are also running from it it does vary a little more but nothing that will produce any damage.

Paul

The motor plate states 380-420, i reckon this will be the original, cerca 1969 ( I know something of this lathe's history), so definately not designed for dual voltage, although I'll split it this afternoon and see if there's an option to wire it up differently. However, perhaps I asked the wrong question about these inverter/converter units- all the specs say IF the motor can be wired in delta instead of star, it will accept these 3phase 230v digital inverters. My question is, if it's designed to deliver 1/2 HP and I run it on 230 instead of 400 v, I am certain that it will run but surely it will deliver only 60% of the power? Surely that will be insufficient?

Drives Direct and a couple of other places I've looked sing their praises, but surely they should be deiivering 400v and not 230, otherwise everyting will be underpowered? If it did deliver 400v there would be no issues, every piece of three phase equipment would run fine. I thought of giving Drives Direct a call but wondered what others had experienced first, and it would appear to be encouraging, but i find myself still unsure. I am a qualified electrician and the only voltages available for use in our country (excluding specialst stuff) are 230v between phase and neutral or 400v between phases. I am confused.

Thanks again, Dave T

Dave

The motor that you have is a conventional 3 phase motor, so it cannot be run from a 1 phase to 3 phase VFD. 230v 3 phase is the european standard and is very common place in this country in control situations. As you have mentioned you are a sparky so just think about Ohms law for the power output of the motor 3 phase 440v will be less current than 1 phase 220v, but you could still have the same output power (well almost exactly the same) Being a sparky I would look about for a secondhand motor and a secondhand VFD and wire it up yourself, buy a small ABs box for the fwd/rev switch and the speed pot I just picked a new WEG industrial 1HP motor dual voltage still in its box for £50. Looking on ebay VFD's come up for less than £100 so to my way of thinking that would be the way to go.

Regards Jon

I don't see why not? If you hook up a 415V rated star connected 3-phase motor to a 240V phase-to-phase supply it will run. The speed will be the same, but the power available will be reduced, by 1/sqrt(3) in theory, exactly as Dave says.

In the original post Dave says the motor is two speed. Presumably the motor runs in star with 'n' poles, and delta with '2n' poles, so that at the slower speed you get about 86% of the higher speed power rather than 50%.

Technically the 240V and 415V UK supplies are the same, it's just a question of where the voltages are measured.

Regards,

Andrew

Hi Andrew

I should have worded that better yes it could be run but would give out slightly less than half of it's rated power and if you want to control the speed reliably you are still going to need a VFD so you may as well change the motor ?

Regards Jon

As you are a "spark" and understand safety issues you can probably convert the motor from 400 V to 230 V yourself. Many 420 V motors are wired in star configuration. If you can fish out the winding tails you can wire it in Delta configuration and you have a 230 V motor with no loss of power.

Failing that I would get a new 3p motor made specially for use with speed control. They are designed with better cooling so that they can run at full torque at low speed and a 1/2 hp one will cost about £50.

Changing to a 3ph motor and inverter drive was the best mod I made to my lathe. Not only for the convenience but also for the improved finish as a result of the constant torque vs. rotation characteristic of a 3 ph motor.

Russell.

As you are a "spark" and understand safety issues you can probably convert the motor from 420 V to 230 V yourself. Many 420 V motors are wired in star configuration. If you can fish out the winding tails you can wire it in Delta configuration and you have a 230 V motor with no loss of power.

Failing that I would get a new 3p motor made specially for use with speed control. They are designed with better cooling so that they can run at full torque at low speed and a 1/2 hp one will cost about £50.

Changing to a 3ph motor and inverter drive was the best mod I made to my lathe. Not only for the convenience but also for the improved finish as a result of the constant torque vs. rotation characteristic of a 3 ph motor.

Russell.

Well this motor looks in reasonable nick inside- I can' really tell how the windings actually run but there's 6 connections and assuming it is a two -speed then I guess one set runs the lower speed and either the other, (or perhaps both together) work the high speed, which would mean its wired and star ie. all wires come together at a "neutral" point. I get extremly constant, but different resistance readings of 38, 62 and 68 ohms. I get in excess of 200Mohms to earth, so I guess it's likely to be healthy, so: try it with an inverter and if it runs O.K - have a go at switching it to delta: if that fails get another 3 PH motor ( I'm still a bit unsure about this 3phase 230v business, but I'll just ride with it for now). What do you reckon? There seems to be no chance of getting the coils free from the casing, I can't even see how they're fixed in, so access to try to split the joint may not be easy. Any further thoughts?

230 V delta puts the same voltage across each winding as 420 V star.

Sorry, not familiar with 2 speed motors. Personally I'd go for a new motor. Have a look at Marelli motors on this site.

Russell.

Thanks, those motors look very reasonable and it would seem from talking to you all that 3ph is the way to go. Now morre questions! It would seem that if this motor is healthy then it should run with an inverter drive, but underpower by about 40%,.So, as its slightly bigger than the minimum recommended motor for an ML7, is it reasonable to assume if I can get it to work it may do the job? ( I could think about a bigger/better motor later depending on how much I'm using the lathe, I'm not doing it up to sell or impress anybody). If I replace it with a new 3ph single speed one, will I get reasonably constant power when varying the revs using a digital inverter drive?. One more thing, nobody seems to reccomend single phase motors, so, apart from the single speed issue, are there any other drawbacks? I have a mate who has a lathe for occaisional use, woking off a 230v motor, and he's never complained of problems. Regards, Dave T

In a word, no. If you're above base speed of the motor, then yes you will get constant power. But below base speed you will get constant torque, but since the speed is deceasing so will the power.

Let's take some easy numbers. Assume a 1hp motor with a base speed of 1500rpm at a frequency of 50Hz. If you run the motor at 100Hz, you'll get twice the speed, but half the torque, so still 1hp. However, if you run at 25Hz you'll get half the speed, but the same torque, so only 1/2hp. To some extent it is possible to increase the current, and hence torque, below base speed but you then run a serious danger of overheating the motor. Remember that industrial 3-phase motors are dwsgined to run hot at their design speed, often over 100°C.

Regards,

Andrew

If you run a 1/2 hp 230 V motor it will be capable of supplying 1/2 hp at normal speed, not 40% less. However at reduced speed, as Andrew has pointed out, the torque will remain the same so the power is reduced more or less in proportion to the speed reduction. It's not quite that simple but a good enough approximation.

The rated power of a motor is for continuous operation. For the sort of intermittent use it is likely to be subjected to on a home lathe you can squeeze more power out for short periods. The limiting factor is the heating.

Modern motors can run at higher temperatures due to improved insulation materials so can be made smaller fpr a given power. I replaced the old 1/2 hp motor on my lathe (a bit bigger than Myford) with a 3/4 hp 3 ph motor as it had the same frame size. It is more than adequate and has never got too hot. However, you will still need the belt changes for heavy low speed cuts.

Russell

Sorry guys I think i failed to word that clearly: If I run this motor I have (3 phase 400v star wound) I think it will run with a digital 230v inverter but as the voltage will be 1/√3 lower, the power will therefore be the same proportion lower so my 1/2HP/ -1/4HP two speed motor will be about 0.3 HP and 0.15 HP? The recommended size is between 1/4 and a 1/3 HP, so I reckon I should be able to get it to work OK but it may be a bit underpowered, but this could be fine to start with ( no problems lining up or bolting in place, it's all there) Does this sound sensible- get on with the other bits of restoration and worry about the motor at the end? Dave

Go for it. You have nothing to loose. It won't damage your inverter and you can easily change the motor later if you find the lack of power to be a problem. I would buy an inverter capable of driving a 1/2 hp motor though - just in case.

Russell.