Re: [CAD_CAM_EDM_DRO] Re: Converter or Inverter for 2hp Bridgeport mill - OT?

Hi gang;

Basically, motors will generate, and generators will motor. The thing is, that
to do either _well_, you have to optimize the commutator timing for one or the
other. At least you did on the motors we were playing with in heavy equipment
mechanics school, some 30 odd years ago.....:-)

Bill

On Sun Jan 4 19:05:16 2004 washcomp wrote:

>I don't pretend to be an expert on generators either, but I remember
>seeing steam engine plants from the late 1800's whch had a small
>generator hooked up to supply EMF to the coils of a much larger
>generator (or maybe dynamo, foret whether it was for AC or DC).
>
>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "vavaroutsos" <pete@v...>
>wrote:
>> Bob, I'm no expert on generators, but I don't think many
>generators
>> are built like an induction motor. My guess is generator rotors
>> either have permanent magnets or coils with slip rings.
>>
>> While it's true that over speeding an induction motor will
>generate
>> power, it only works because there is a field in the stator to
>induce
>> current in the rotor. I don't know how you would start a generator
>> like this unless there was enough residual magnetism in the rotor.
>>
>> ~petev
>>
>> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bob McKnight"
>> <bobmcknight@c...> wrote:
>> > To: CAD_CAM_EDM_DRO@yahoogroups.com
>> > From: "vavaroutsos" <pete@v...>
>> > Date sent: Mon, 05 Jan 2004 00:42:44 -0000
>> > Subject: [CAD_CAM_EDM_DRO] Re: Converter or
>> > Inverter for 2hp Bridgeport mill
>> > Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com
>> >
>> > petev
>> >
>> > OK
>> > Let me explain where I am coming from and I sure never got this
>> > out of a text book. Generators are Motors and Motors are
>> Generators.
>> >
>> > I question this with some of the static motors, but it is from
>lack
>> of
>> > knowledge, not misinformation.
>> >
>> > I have been told and again never checked it out that if you over
>> > speed an induction motor it will become a generator. I kinda
>> bought
>> > this based on residual magnetism.
>> >
>> > So your explaination makes sense because I would be operating in
>> > the middle of the area between motor and generator which would
>> > no current flowing or no energy applied. Except I figured I
>> would
>> > be supplying a little energy, but I figured I was supplying
>energy
>> to
>> > overcome mainly friction forces, but I can see that is not the
>> case.
>> >
>> > I am at the stage I was many years ago with pumps. I thought
>that
>> > shutting off the flow of water would make the pump work harder
>(true
>> > for piston pumps). But in centrifugal pumps.either air or
>water,
>> > throttling the input reduces current flow. I learned this the
>hard
>> way.
>> > I took a blower unit out of a furnace and ran it just as a vent
>fan
>> in
>> > the shop. It burned up. Fortunately I had more and shut off
>> almost
>> > all the air on the side opposite the motor to get the current
>down
>> > where it belonged.
>> > Which brings up another question. Most convertors are used to
>> > run motors. I am using this to run large a large welding
>> transformer.
>> > See any problems with the weak leg. The local power company
>> > can supply 3 phase for a price and it will have a weak leg.
>> >
>> > Bob McKnight
>> >
>> > [ Double-click this line for list subscription options ]
>> >
>> > Bob, I assume you are using an induction motor? If so, the
>magnetic
>> > field in the rotor is caused by the slippage. If the rotor is
>> turning
>> > at the same speed as the stator magnetic field, it does not see a
>> > moving field and no currents will be induced. Without a magnetic
>> field
>> > in the rotor, you won't get any voltage on your generated leg.
>If
>> you
>> > don't beleive me try just spinning the motor with no windings
>> > connected. No voltage will be generated because there is no
>field in
>> > the rotor.
>> >
>> > When an induction motor is loaded, more slippage occurs. This
>> creates
>> > more current in the rotor and a stronger magnetic field. This in
>> turn
>> > balances the force of the load on the rotor.
>> >
>> > ~petev
>> >
>> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bob McKnight"
>> > <bobmcknight@c...> wrote:
>> > > To: CAD_CAM_EDM_DRO@yahoogroups.com
>> > > From: "vavaroutsos" <pete@v...>
>> > > Date sent: Sun, 04 Jan 2004 18:33:33 -0000
>> > > Subject: [CAD_CAM_EDM_DRO] Re: Converter or
>> > > Inverter for 2hp Bridgeport mill
>> > > Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com
>> > >
>> > > Petev
>> > >
>> > > It started out that way. The motor I am using is a 15 HP unit.
>> > > After looking at the voltage differences and the phasing, I
>got
>> the
>> > > idea that some of that may be caused by the slippage from
>> > > synchronous. So, since I was going to use a motor to start the
>> > > convertor, I decided to try to eliminate the slippage. I have
>> asked
>> > > around and even my EE friends don't know. I thought somebody
>here
>> > > might know, if not, I will just plug along.
>> > >
>> > > Thanks
>> > > Bob McKnight
>> > > [ Double-click this line for list subscription options ]
>> > >
>> > > Bob, what are you using the small motor for? Is this just to
>> start
>> > the
>> > > rotary converter? Why don't you just add a start capacitor and
>> > > potential relay. If you're building a small unit, you could
>use a
>> > > Supco SPP-8E hard start available from www.grainger.com
>(4MG78).
>> > >
>> > > If you use run caps to both single phase legs and a good motor
>you
>> > > should get a nice generated leg. My generated leg varies about
>10V
>> > > from no load to full load and the phase angle is pretty good.
>> > >
>> > > ~petev
>> > >
>> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bob McKnight"
>> > > <bobmcknight@c...> wrote:
>> > > > To: CAD_CAM_EDM_DRO@yahoogroups.com
>> > > > From: "ibewgypsie" <ibewgypsie@h...>
>> > > > Date sent: Sun, 04 Jan 2004 06:50:28 -0000
>> > > > Subject: [CAD_CAM_EDM_DRO] Re: Converter or
>> > > > Inverter for 2hp Bridgeport mill
>> > > > Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com
>> > > >
>> > > > Hi
>> > > >
>> > > > am new to the group, so hope this is appropriate.
>> > > > The Home Shop Machinist just completed a series on
>rebuilding
>> the
>> > > > Bridgeport. I am building a rotary converter and wonder if
>> anybody
>> > > > knows if turning the rotary transformer at synchronous speed
>> would
>> > > > help even out the current available from each leg. I have a
>> small
>> > > > motor with a mechanical variable speed drive that I am
>> attaching
>> > to
>> > > > the old motor that is the heart of the machine. Also have
>> > > > capacitors to help 'tune' the system.
>> > > >
>> > > > Bob McKnight
>> > > > [ Double-click this line for list subscription options ]
>> > > >
>> > > > I used a toshiba single phase in, 3 phase out inverter. I
>used
>> the
>> > > > bridgeport relay to turn on with dry contacts, left the
>> overload
>> > in
>> > > > place (overkill since inverter monitors too). I gutted the
>cnc
>> > > drives
>> > > > and upgraded at that time too thou. Inverter works great.
>> > > >
>> > > > My phase convertor made the head rattle, inverter is much
>> > smoother.
>> > > > Money well spent. I have my machine on a 30 amp 220 single
>phase
>> > > > circuit now. Like I said I upgraded the cnc drives too. The
>3
>> > phase
>> > > > cnc drive transformer had to go.
>> > > >
>> > > > David Cofer
>> > > >
>> > > >
>> > > >
>> > > >
>> > > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "john_glynn57"
>> > > > <john_glynn57@y...> wrote:
>> > > > > I have just taken delivery of my own Bridgeport mill and
>want
>> to
>> > > > > know which is the best option for powering the machine :
>an
>> > > inverter
>> > > > > or a converter. My research to date suggests that a
>Hitachi
>> L100
>> > > > > inverter is the way to go however I want to question this
>as
>> > > > > follows: 1. I can run a dedicated 20A or 30A supply to the
>> > machine
>> > > > > which should be able to handle the startup current demand
>of a
>> > > > > converter. 2. If I use a converter I can use all the
>original
>> > > > > Bridgeport systems without mosification ie Bridgeport speed
>> > > > > control,forward and reverse,3phase coolant pump and
>shaping
>> unit
>> > > > > which is attached to the machine.
>> > > > >
>> > > > > My plan is to circumvent the original Bridgeport wiring
>and
>> > drive
>> > > > > the Main motor(s) directly from the hitachi unit and
>setting
>> the
>> > > > > motor speed at the low speed of 1400 and not using the
>> valiable
>> > > > > speed control of the machine at all. The hitachi will give
>me
>> > > speed
>> > > > > control by varying the frequency however I do intend to
>use
>> the
>> > > > > original forward reverse switch. In addition I plan to
>> replace
>> > the
>> > > > > coolant motor with a single phase model.
>> > > > >
>> > > > > Am I doing the right thing.


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