On 2017-12-04, Martin Eastburn wrote:
You can get this on a common / ground lead and a power lead that
is hooked to the internal switch. The switch is in the start winding
at dead stop. Once turning fast, the switch flips and the run winding
is powered and the start is turned off. It is a Single Pole, double
throw switch that is controlled my speed.
Not any which I have seen when I opened them up. They are
single pole single throw NC (Normally closed), and in series with the
start winding and the start cap.
For a two-voltage motor (120/240VAC), when wired for 120 VAC,
there are two run windings connected in parallel, and the start winding
cap and centrifugal switch are connected from line to neutral.
When wired for 240 VAC, the two run windings are in series,
making the junction point a tap on an autotransformer which gives a
voltage which is 120 VAC different from each end. The switch, cap and
start winding are connected between the center tap formed by the two run
windings and one end. Actually, to reverse the motor (only at start
time), you can select whether the other end of the start winding is
connected to one or the other of the 240 VAC line inputs. (I say "only
at start time", because the relative phase of the run and start windings
only determines the direction it is spun until the centrifugal switch
opens. Afterwards, you can switch quickly between the two positions and
the motor will keep spinning in the same direction. (You can also
switch ends on the start winding, leaving it connected to the same end,
but that requires a bit more complex a switching setup -- especially if
you are using a normal drum switch and want the entirity of the motor to
be disconnected from power in the "stop" position.
Having a phase shifted internal winding gives you on a scope a waveform
and the non-shifted is only the power line. The run doesn't get anything.
The run gets the normal phase from the power line, from 120 VAC
or 240 VAC single phase. The cap and start winding between them
generate a temporary phase in the start winding which differs from that
in the run winding, and which gives the angular field to start the motor
spinning.
The funny motor you are talking about is a milk station that has
A neutral/GND and two Legs of xxx volts that are really 2 legs of 3
phase.
A "milk station"? IIRC, the funny motor which I remember
writing about is the Hysteresis Synchronous motor, with a permanent
magnet rotor and speed locked to the power line frequency. Nothign to
do with milk that I know.
Looking below, I see that I must have written that about the
Hysteresis Synchronous motors in some other article, perhaps in a
different newsgroup or mailing list.
I have 2 phase on my property. Two Highlines and I could have 3 phase
if I wanted with three transformers. I created my own from Single phase.
Two highs and a neutral (if the two highs are fed from two
phases of a three phase power line), yes. If you are calling the two
lines of a 240 VAC power with neutral center tap, you can call those
only either "in phase" or "180 degrees out of phase", depending on how
you look at it, but you cannot generate three phase from that. You need
a 120 degree phase difference to allow this to work.
Of course, you can approximate three phase with a three phase
motor wired to 240 VAC, with a run capacitor shifting the phase to the
third winding. What the actual phase angle is will depend in part on
the value of the capacitor, and the inductance of the motor winding.
This is a rotary converter, and it can be self starting if the
run capacitance is connected only to one of the other windings, not
equally to both.
Or, you can have a separate start capacitor which is
automatically switched by the current in the main two windings.
Or -- you can start it with a pull rope just before switching on
the power. Some people on this newsgroup do this. A bit of a problem
if you walk out of the shop with a machine running, the power drops, and
then returns while you were away. The rotary motor sits there not
starting and getting quite hot.
Martin
On 12/3/2017 10:41 PM, DoN. Nichols wrote:
On 2017-12-03, Martin Eastburn wrote:
I would not call a cap start motor a two phase at all.
Call it a "temporary two phase", as that is present only as the
motor is starting, and is generated by the start cap.
I would call it a dual winding single phase. The starter winding -
large diameter short winding with a cap - dumps the AC power through
the winding as XL is canceled by XC for the tech guys.
Yes, two windings, but one gets standard phase, and the other
gets shifted phase, by the XL and XC causing a shift. This provides a
second magnetic field at an angle to start the motor spinning. You
*can* start the single phase motor with just one widing, if you have a
way to spin it just before you switch on power -- such as a rope wound
on the shaft.
The run winding is used once up to speed and an internal switch throws
at speed - switching run in and start out.
Not quite. The inertial (also internal) switch switches the
start winding out, but the run winding is connected full time -- as long
as there is power to the motor.
Look at the inertial switch when you have a motor open. It
only has the contacts to open the start winding -- not any more contacts
to close the run winding.
Or -- just apply an ohmmeter to the motor from outside (and with
AC power disconnected, too, to keep from frying the ohmmeter). It will
draw current and show a fairly low resistance. If the run winding were
open, you would only see a swing of the needle (if a meter) or a short
display of low resistance moving slowly to infinite resistance, as the
start cap charges. (Well ... some leakage in the cap, so you won't
really get all the way to infinite, as a perfect capacitor would.)
If you apply power *only* to the start winding, you still lack
the shifted phase relative to the run winding, so the motor still won't
start, without something external to spin it up. But, because it is
sitting there drawing lots of current, it *will* blow up the start cap
in short order. :-)
When the power company runs low voltage - starter winding is used
longer, meaning more power used and then the run is enabled.
And then the start winding is switched out, leaving just the run
winding which was already enabled.
Now -- I grant that this is not a precise phase relationship
between the two windings -- dependent on the value of the cap and the
inductance of the winding -- as well as the frequency of the AC applied
to it.
Two phase is 2 of 3 phase in a Delta wiring of a 3 phase motor.
It is called an "Inverted V" or lost leg.
This is when the phases are 120 degrees apart. There are also
two phase power lines (uncommon these days) which have a 90 degree phase
relationship -- and a tricky transformer circuit can convert between
true two phase and true three phase -- either direction. I forget the
full name of it, but 'T' is part of the name.
This is a basis of very high rel 3 phase. Input and output are 3 phase.
If a phase is lost on the input the output has 3 phase at 66% power IIRC.
One could in theory loose two one inside and outside. Dropping more,
but still running just fine. Hospitals and Police/Mil run these like
this in case of attack.
Understood -- but this is still talking about phases shifted 120
degrees relative to each other. So you could call them "partial three
phase". The two phase for motor starting is different, with 90 degrees
being ideal, but usually not hit precisely.
Enjoy,
DoN.
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