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Posted by Christopher Tidy on August 21, 2006, 7:29 pm
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Peter Harrison wrote:
> Greg wrote:
>
>>
>>> Also, it turns out there are 2-pole and 4-pole motors. What on earth is
>>> all that about? I was only just coming to terms with the idea of star
>>> and delta winding. Nobody mentions what their motor is so I assume that
>>> can be adjusted internally to either configuration.
>>
>>
>> As no one has mentioned it, no you can't change the number of poles
>> internally, a motor is wound one way or the other, the more poles the
>> slower
>> it turns for a given mains frequency.
>>
>> Greg
>>
>>
> I see.
>
> I was thinking of the star vs delta winding pattern which some sources
> suggest is a common thing to have configurable by the user.
>
> Pete Harrison
The option to connect a three phase motor as either star or delta offers
a choice of two operating voltages. The operating voltage required for
the star connection is approximately 1.7 times the operating voltage
required for the delta connection. Most small three phase motors operate
at 240 V in the delta configuration or 415 V in the star configuration.
This doesn't affect the shaft speed.
Chris
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Posted by Christopher Tidy on August 21, 2006, 7:23 pm
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Greg wrote:
>
>
>>Also, it turns out there are 2-pole and 4-pole motors. What on earth is
>>all that about? I was only just coming to terms with the idea of star
>>and delta winding. Nobody mentions what their motor is so I assume that
>>can be adjusted internally to either configuration.
>
>
> As no one has mentioned it, no you can't change the number of poles
> internally, a motor is wound one way or the other, the more poles the slower
> it turns for a given mains frequency.
There are a few rare three phase motors which have a tapped winding
connected to a multi-pole switch which allows the number of poles to be
changed by a factor of 2. Somewhere I have a diagram explaining how the
arrangement works. I can dig it out if anyone is interested. There are
also a few three phase motors which have two main windings, and I've
even read about one which has two tapped main windings, giving a total
of four speeds. But in general you can't change the number of poles.
Chris
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Posted by Peter Harrison on August 22, 2006, 10:49 am
Please log in for more thread options Well, it looks like a trip to Newton Tesla. They have a ready-to-go
motor which they claim is a drop-in replacement. Their motor also has
the appropriately sized output shaft so that is one less task to do. It
would appear that their motor is not a totally enclosed type which might
have been better. There is, of course a premium for this at £100 inc Vat
collected from the door but I can't seem to find much else that I can
rely on as an alternative - unless anyone knows better.
Thanks for the help
Pete Harrison
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Posted by Roy on August 23, 2006, 1:36 pm
Please log in for more thread options Hi Peter,
I have just been through the same for my Myford ML7. I bought the
Mitsubishi S520 Inverter and a motor from John at Newton Tesla and I
can say i am very happy with the end result. John is very helpfull with
setting up the motor with the inverter even though he didnt supply it.
The difference in motor poles is what sets the rotational speed.
For a 50 hertz system, the formula would be:
50 x 60 x 2 = 6000 no-load RPM divided by the number of poles.
YOU CAN'T CHANGE THE NUMBER OF POLES
Most 3 Phase motors are wound for both Star and Delta and on smaller
motors this can be changed to suite the electrical supply you want to
run on. But mostly high torque motors are started in Star (Initial high
torque for starting under load) and then when up to speed switched to
Delta to run. But ignore this because if you stick with Newtons motor
it comes correctly configured.
Roy
Peter Harrison wrote:
> Well, in a fit of madness, I bought a variable frequency inverter. These
> are easy to find and I rather thought that finding a motor would be as
> simple.
>
> Daft really but what can you do when the buying mist descends?
>
> I have spent (wasted) the last couple of hours trying to find out
> exactly what motor spec I will need. My lathe is a Myford Super7B.
>
> All I seem to have discovered is that the output shaft is 5/8ths. No
> doubt they can't be had anymore and I will not be able to fit the
> current pulley to the new motor.
>
> Can anyone tell me what the mounting dimensions are. I could go and
> measure mine but it is a bugger to move until I actually come to swap
> out the motor. I would rather not have to do it twice. In any case, I
> seem to need the actual frame size and, probably, some more modern NEMA
> equivalent.
>
> Also, it turns out there are 2-pole and 4-pole motors. What on earth is
> all that about? I was only just coming to terms with the idea of star
> and delta winding. Nobody mentions what their motor is so I assume that
> can be adjusted internally to either configuration.
>
> Can anyone help me with the appropriate motor specs? Oh, and a supplier
> or two would be nice.
>
> Pete Harrison
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Posted by Peter Harrison on August 23, 2006, 4:36 pm
Please log in for more thread options Roy wrote:
> Hi Peter,
>
> I have just been through the same for my Myford ML7. I bought the
> Mitsubishi S520 Inverter and a motor from John at Newton Tesla and I
> can say i am very happy with the end result. John is very helpfull with
> setting up the motor with the inverter even though he didnt supply it.
>
> The difference in motor poles is what sets the rotational speed.
> For a 50 hertz system, the formula would be:
> 50 x 60 x 2 = 6000 no-load RPM divided by the number of poles.
> YOU CAN'T CHANGE THE NUMBER OF POLES
>
> Most 3 Phase motors are wound for both Star and Delta and on smaller
> motors this can be changed to suite the electrical supply you want to
> run on. But mostly high torque motors are started in Star (Initial high
> torque for starting under load) and then when up to speed switched to
> Delta to run. But ignore this because if you stick with Newtons motor
> it comes correctly configured.
>
> Roy
>
That is all good to know. I shall be calling in to pick up a motor
tomorrow (Thursday).
Hope they have some 4 core flex. That seems unnaturally hard to find in
small amounts.
Then all I shall have to do is persuade My wife to hold up one end of
the lathe while I change the motor. Shouldn't take long, better have her
eat extra weetabix though - just in case.
Pete Harrison
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