Re: 7.5 HP Lathe on a 10 HP idler?

On Thu, 14 May 2009 15:03:41 -0500, Ignoramus12712 wrote:
On 2009-05-14, jw <cyberzl1@xxxxxxxxx> wrote:

That's very good to know. Did you try to power reverse it at top
speed, I wonder if instead of reversing the mill, the idler would
reverse instead.

To reverse the idler, you would have to actually stall the idler and
then force it to reverse. Actually you would just need to slow it
enough to throw the phasing out of whack enough to cause the poles to
flip, but .... Anything is possible, but it's unlikely you will cause
that to happen.

More likely cause a surge on the idler mains and trip the breaker
first (assuming those are properly sized).

Think about this. In a running phase converter, that drives a load,
the two motors form a dynamic system. You call one idler and another
load, because the "load" does something useful and the "idler" just
spins. But the motors do not really "know" who is the idler and who is
the load.

So if you reverse one of them via a reverse switch, they will both
generate forces that act against their rotations, until one stops and
reverses. Which would would it be, seems to depend on either their
rotational momentum, or their energy, or something of that sort, but I
can see how it could be the idler motor.

The dynamics of the relation between the two motors is "angels
dancing on the head of a pin..." If you swap the two Utility phases
to the load motor only, the RPC idler should keep turning in the same
direction and producing the phantom third leg to the load motor - but
I still would not suggest plug-reversing any motor in a home shop. For
openers, might be a bit hard on the RPC phase balancing capacitors
getting their voltages slammed around like that.

Unless the power feed to the equipment and the house Main Panel is
ridiculously oversized (and it never is...) and fortuitously
under-loaded because all the major loads are off (and they never
are...) all you are going to do is trip a breaker or blow a fuse.

If you are really unlucky and you beefed up your house panel, the
breaker or fuse that goes open is going to be on the Edison side of
the demarcation line, where you can't fix it yourself. Power
utilities use /very/ generous derating fudge factors on how many
houses they can put on one Pole Pig, and throwing huge mLocked Rotor
Amps shorts across the line is not something they allow much cushion
for - about the biggest motor they plan for is your air conditioner.

All the neighbors that share that transformer or distributuion
circuit with you, and are going to be sitting in the dark for an hour
or three until Edison comes out to reset the line, will not be very
pleased to hear about your hobby...

Industrial 3-phase power feeds *are* sized to handle the extra loads
of plug reversing - which is why commercial power rates are so much
higher, because they have to build excess system capacity that only
gets used for a few seconds a month. Where a house with a 200A Main
might be on a 10 KVA Transformer by itself (there's that over-generous
derating) an industrial building with a 200A Main and nobody sharing
the feed would have anywhere from a 25 KVA to 75 KVA can, depending on
how often you throw hefty loads on.

Not to mention you are running Locked Rotor Amps through that motor
till it spins down to a stop and then spins up in reverse. If the
power feed is hefty enough to feed 20X rated current and hold, that
still isn't good on the motor windings and normally should be avoided.
You get a deep burn in the winding insulation, and the Magic Smoke
will soon escape from that motor.

And if you must plug reverse and the equipment is up to it, you
can't do it more than two or three times in an hour with a bunch of
cool down running time in between.

--<< Bruce >>--
.

Relevant Pages

  • Re: 7.5 HP Lathe on a 10 HP idler?
    ... terminals A, B of the load. ... "1) If you swap two leads L1 and L2 going to the idler, ... No I really meant to say a swap of any 2 leads going to the load would reverse the load. ... motor whose leads are being reversed, ...
    (rec.crafts.metalworking)
  • Re: RPCs: 1700 v 3400, delta wound vs wye
    ... There is no synchronism between the speeds of idler and load motors in a RPC. ... function of the mains frequency, ... This accounts for the well-known phenomena that enables a 3-phase motor to ...
    (rec.crafts.metalworking)
  • Re: Flywheel on a rotary phase convertor
    ... >An induction motor is a consumer, ... >induction generators have to be excited by overdrive from ... A fella by the name of Swinney said elsewhere that idler and load ... Emf is produced in all three windings of the idler. ...
    (rec.crafts.metalworking)
  • Re: 7.5 HP Lathe on a 10 HP idler?
    ... No I really meant to say a swap of any 2 leads going to the load would reverse the load. ... motor whose leads are being reversed, ...
    (rec.crafts.metalworking)
  • Re: 7.5 HP Lathe on a 10 HP idler?
    ... idler / load set up, one side of a single-phase line is connected to A, A and the other side of the ... lightly loaded motor to the more heavily loaded." ... would reverse inuts to both motors and torques, thus direction of rotation, of both motors would ... plug reversing of both idler and load. ...
    (rec.crafts.metalworking)