Help with Myford ML7 Motor

[GrottoParticipant @grotto]

[GrottoParticipant @grotto]

Hi all

I'm a newbie to the forum (and lathes).

I've recently acquired an ML7 which has a 3 phase 1/2 h.p. motor.

the motor is "Y" wired for 415v with no secondary wiring. I can get a VFD inverter to run it, but I've been told there will be a speed penalty (it will run at 800rpm).

Torque would be the same if frequency is set lower.

All the above info has been supplied to me, I not sure on the theory behind it.

My questions are….

Will running at 800 rpm affect anything, if so can I just change the motor pulley to compensate?

Will there be any advantage in running 3 phase using a VFD inverter, or should I just get a single phase motor?

What is the best h.p. motor to use on an ML7?

I've got a unimat3 which I've used over the last few years, and before that my lathe experience was limited to 25 years ago at school. I don't have any specific planned use for the lathe, just playing around and making the odd bit for my bikes, but don't want to go down the wrong route and regret it later.

thanks

[Les Jones 1Participant @lesjones1]

Hi Grotto,
I do not know what you mean when you say "with no secondary wiring" I will assume that you mean that the three windings are not brought out from inside the motor to the terminal box separately so that you cannot change the motor from star to delta connections. I think an ML7 would normally be fitted with a 1/2 or 3/4 HP 1450 rpm motor (A little below the synchronous speed for a 4 pole motor running from a 50 Hz supply.) As you do not give any profile information I do not know which country you are in so I do not know what your mains frequency is. I do not know where the speed of 800 rpm comes from as it does not fit in with a mains frequency of 50 or 60 Hz. Most inverters are designed for 220 volt motors (Which yours would be if you could if you could wire it in delta configuration.) I think there are some inverters with 415 volts output but they are more expensive. If you ran your 415 volt motor from a 220 volt output inverter it would run at about the correct speed on very light load but it would not provide much torque.

Les.

[john fletcher 1Participant @johnfletcher1]

Well I think fit an inverter. Reconnect the motor in Delta, not a long job, some motors have the details of what to do in the terminal box lid. Regarding 800 rpm I don't understand, as one of the many benefits of fitting an inverter is being able to vary the lathe speed. Again its easy to make a remote control box, the details can be obtained on the net and as the voltages are low, cheap components are available.If you decide to make your own speed control the variable resistor needs to be a LINEAR type, not a volume control, they may look the same but are different inside I'm not familiar with setting all the parameter, but some one on this site will already have the same inverter as you intend to buy and I'm sure they would help you out. Best to obtain some 3 core mains screened cable from the motor to inverter (Sy) using the screen and earth one end. I use multi core screened lead ex computer for my control wiring. When using an inverter you can have forward, reverse, speed control,and jog. Ted

[john fletcher 1Participant @johnfletcher1]

Well I think fit an inverter. Reconnect the motor in Delta, not a long job, some motors have the details of what to do in the terminal box lid. Regarding 800 rpm I don't understand, as one of the many benefits of fitting an inverter is being able to vary the lathe speed. Again its easy to make a remote control box, the details can be obtained on the net and as the voltages are low, cheap components are available.If you decide to make your own speed control the variable resistor needs to be a LINEAR type, not a volume control, they may look the same but are different inside I'm not familiar with setting all the parameter, but some one on this site will already have the same inverter as you intend to buy and I'm sure they would help you out. Best to obtain some 3 core mains screened cable from the motor to inverter (Sy) using the screen and earth one end. I use multi core screened lead ex computer for my control wiring. When using an inverter you can have forward, reverse, speed control,and jog. Ted

[LambtonParticipant @lambton]

Grotto,

Connecting the motor into delta is not difficult if it is a dual voltage motor with six terminals in the connector box. Just alter the 3 connector bars to go straight across parallel to each other.

If there are only 3 connections then the motor must be stripped and the "Star point" located and the wires separated and bought out to the terminals. This is not a straightforward job as the star point may be buried in the hardened impregnation material. Unless you are fully competent in doing such work take the motor to a motor repair shop who will do it safely for you.

Eric

[Sub MandrelParticipant @submandrel]

I echo all the above. get a 240V inverter, wire and delta and enjoy variable speed with full torque up to nominal + 20% (tell the inverter the motor's default Hz is 60 not 50 – all modern motors seem to be 'dual frequency). You can also run at higher speeds at less torque (constant power) if your headstock bearings can stand it.

Neil

[GrottoParticipant @grotto]

Thanks for the advice.

my knowledge of electrical stuff is extremely limited and the details I quoted were based on what the supplier of inverters told me.

i'm in New Zealand, so we're 240 volts.

I've added 3 photos to my album which show the motor and it's wiring. I'll talk to a friend who's an electrician and see if he can do the re-wiring for me or point me at someone who can.

[Les Jones 1Participant @lesjones1]

Hi Grotto,
Although your third photo shows how to strap a motor for star or delta your second photo only shows three wires coming from inside the motor to the terminal box. I suspect it will not be possible to strap it for 220 volts (Delta configuration.) If this is the case then you will need an inverter with 415 volts output. It may be just as cheap to bye a new motor which can be used with a 220 volt inverter (Which will be cheaper than a 415 volt inverter.) If you do this then it may also be worth considering getting a 3/4 or 1 HP motor so you have more torque when running the motor at slower speeds. (With the inverter output frequency turned down.)

Les.

[LambtonParticipant @lambton]

Grotto,

Your photo of the motor data plate confirms that this motor is permanently wired in star mode and requires a 415 volt supply. Thus it cannot simply be configured in delta mode regardless of the diagram on the cover plate.

As I said earlier the only way to wire this motor in delta is to dismantle it, dig out the star point and bring new wires to the terminal block. This is a much cheaper option than buying a new motor but should not be undertaken unless you know what you are doing.

I hade exactly the same problem with a Tom Senior mill and a local motor repair company converted it for me at a very reasonable price.

Eric

[Ian S CParticipant @iansc]

Probably best to go to a used machinery place, and get a dual voltage three phase motor, there seems to be plenty around(at least there where), when I wanted a 1/2hp single ph. motor for my wood lathe, there was one .6hp one, at $NZ 90, but truck loads of three ph. motors, $NZ 30, and they would not be too much dearer today, should get a good one for $NZ 50 or so. Ian S C

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