In the small machine shop there often comes the time when a new machine
arrives for adoption. If this machine spent its previous life
in an
industrial
setting, odds are pretty good that it runs from 3-phase power.... But
you only
have single phase residential power in your shop... What should you do?
There are a number of commercial solutions available, you could buy a
solid state motor speed controller (expensive), or buy one of those
static converters you see in catalogs (expensive, but less so), or buy
a rotary converter (very expensive).
1) Solid State Motor Speed Controller The solid
state motor
controllers are a great solution, but they cost $500 to $700, and are
usually limited to small sub 1 HP motors. They whine
and
sing, and usually want to run off of 3-phase power.
Since
by design, motor speed controllers have no interest in the frequency of
the original power source, they first convert the 3-phase AC into very
smooth DC, using a 3-phase full wave rectifier. You can
usually
fool these units into running off of single phase power by
just
connecting up 2 of the 3 wires to normal 240V AC, and promising
yourself that you will not load the converter beyond about 66%
of
its name plate rating. The converter's added benefit of
motor
speed control, is a feature you should not use, unless your motor has a
separately powered cooling fan. It can be fatal for many
older
motors that cannot handle the high temperatures that result when the
motor is run significantly below or above its name plate speed... You
have been warned!
2) Static 3-Phase Converter. Static 3 phase converters take
advantage of a feature of all 3-phase motors, that is the ability to
self generate a missing phase leg. If you start a 3-phase
motor,
and then remove one phase of the power source, the motor will continue
to run. Because the motor is now running with only 2-legs of
the
3 phase power, there is going to be a reduction of its name plate
power. Your motor will burn out if you attempt to load it to
more than about 66% of its name plate power. In practice, it
is
best to stay below 50%. When you run a 3-phase motor this
way, it
will also become more noisy, and its shaft torque will no longer be
silky smooth. It will now have as much torque variation as a
single phase motor.
3) Rotary 3-Phase Converter. A rotary 3 phase converter is
basically a 3 phase idler motor that is operated as a rotary
transformer. It is excited on only 2 out of the 3 phases, so
it
regenerates the missing phase. Unlike a motor running under a
static 3-phase converter, the idler motor is not mechanically loaded.
This allows it to put all of its capabilities into
regenerating
the missing third phase. Motors that are powered by Rotary
3-Phase converters can generally be operated at 80 to 90% of their
nameplate HP rating.
A simple, inexpensive Static Converter
So, how do you start a 3-phase motor with only 2 phases connected?
You can take a cue from all of the single phase motors that
are
in use, and add a capacitor from one side of the power line to the
missing phase leg. When you connect the capacitor, the motor
will
start to spin up to speed, and when it gets most of the way there,
disconnect the capacitor and watch it lock in to speed.
Though a
manual solution would be to just add a button, and start your motor by
pressing the button, a more automated approach is easily made:
How it works: When a 3-phase motor, whose shaft is stopped,
has
240V applied to only two of its phases, say B and C, it acts
like
a voltage divider. You will find 120V between A and B, and 120V between
A and C. When a 3-phase motor, whose shaft is spinning, has
240V
applied to only two of its phases, say B and C, it acts like a rotary
transformer, and regenerates its missing A phase. If you
measure
the voltage on the motor, you will find about 240V between A and B, and
about 240V between A and C. That is the trick! At
some
point between stalled rotor, and full speed rotor, the voltage across
points A and B will go from 120V to 240V. So, what
we need
is a relay connected to those points with a set of contacts that are
closed when the voltage across A and B is 120V to about 200V,
and when the voltage across A and B is about 200V to 240V.
This SPST relay can be almost most any 120VAC SPST relay you can
find. I have used a cheap one found at RadioShack.
The series
resistor is a compromise. It should be picked so that the
relay armature just
starts to close when there is about 200V between A and B. It
is important that the relay
armature not close when there is 120V between L1 and A. Some
experimentation
will be necessary. Second, the Motor Contactor solenoid needs
to be
sized to handle your motor load. It can be any old surplus
1-phase,
2-phase, or 3-phase contactor you can find that has a 120V
control coil.
Your start capacitor will have to be picked by trial and error.
Some 3-phase motors just love to start, and others, really
need
to be kicked hard. 200uf should start a 1hp motor without any
problem. I use 500uf to start a 7-1/2 HP motor.
3-Phase Rotary Converter
In the above note, I mentioned that a 3-phase motor that is missing one
phase, will act as a rotary transformer, and regenerate the missing
phase. This is a vital fact that is used extensively in
industry.
In factories where there is an imbalance between the 3
phases,
usually due to single phase motor loads that aren't evenly distributed,
they will often install a huge 3 phase idler motor across the line, and
use its natural tendencies to balance the phases. A factory
using such an idler motor will continue to run even if one phase
disappears. Well, we can do that too! In fact, the
static
motor converter shown above looks remarkably a lot like a rotary
converter. That is because it is one. If you build
such a
converter, you will notice that the voltages between A, B, and C, are
not all quite equal. The voltage from B to C is fixed at 240V
by
the power line, but A to B, and A to C will differ. You can
run 3
phase motors off of this regenerated 3 phase just as it is, but it will
work better if you use a run capacitor to help with the imbalance.
A run capacitor is an oil type capacitor that is designed to
be
continuously connected to the AC line.
There is no easy calculation that I know of that
will help
you find the best run capacitor, so, you will have to use trial and
error. It helps if you have some clip leads, and a bunch of
run
capacitors to play with. The basic trick is to hook up two AC
volt meters, one A to C, and one A to B. Try a capacitor
across A
to B, and see what happens. If the balance gets better, add
more
capacitance, until it gets worse. On my rotary converter, I
ended
up with 60uf between A and B. Your mileage may vary.
Adding
a capacitor from L1 to L2 can sometimes improve the power factor of
your new rotary converter. This will reduce the current draw
when
the motor is unloaded. Most of this current draw is out of
phase
with the voltage, so it won't spin the usual power meter.
Improving your powerfactor will reduce your fusing
requirement,
and will make your power company happy.
It is best to use as large an idler motor as you can. I would
go
no smaller than equal to the largest motor you intend to start.
I
use a 7-1/2 HP, 3-Phase, pump motor. I have started, and hot
plugged 3HP motors with no problem, infact the phase balance improves
with the addition of more motor loads.
Oh, and it is a good idea to put a resistor in series with the start
capacitor to bleed of its charge. This will keep the
capacitor
from becoming overvoltaged if the converter's input power is started,
stopped, and restarted by some power company flaw.
