"DoN. Nichols" wrote in message
...
On 2008-02-16, Jerry wrote:
"DoN. Nichols" wrote in message
...
On 2008-02-14, Jerry wrote:
[ ... ]
Why do you write that a static converter with more capacity will have
difficulty starting a smaller motor?
I refer to your statement -
This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly.
Because the phase shift from the capacitor is a function of the
motor horsepower (and related inductance) the capacitance, and the
frequency. Since the frequency is fixed at either 60 Hz (here) or 50 Hz
(UK) we have the motor inductance/horsepower and the capacitance. Too
small a capacitor will not produce enough phase shift to get the motor
started quickly enough to avoid blowing the capacitor, and too large a
capacitor will generate too much phase shift, again resulting in very
slow motor starting and likely failure.
Note that the static converter was spec'd for a range of
horsepower (in the part of the article now trimmed), not "up to
such-and-so horsepower".
[ ... ]
Obviously -- if it is a homebuilt static converter, or if the
owner is competent to open it up and replace the capacitor with the
appropriate size for the other motor, then this can be overcome.
Also -- if he is not careful to wire it so the real 240 VAC
comes in to the terminals which feed the control circuitry, he may not
be able to get the spindle motor to start with the static converter
alone.
[ ... ]
Hi Don
I have fallen short on my study in this 3 phase stuff. When I was
doing
the testing with my litle dyno on 3 phase motors, I recognized that the
bigger the capacitor I used to spin up even the fractional HP 3 phase
motors, the quicker the motors spun up. I suspect I never approached
the
"too much" capacity.
You may not have even reached the level of "right value". :-)
You were perhaps using oil-filled AC capacitors, instead of the
electrolytic motor starting capacitors which are more common in motor
applications -- because they are smaller and they cost a lot less.
Oil-filled AC capacitors are a lot more tolerant of long start
times.
At one time I thought the static converter was a partially useful
method
of running 3 phase machines at home, on single phase. But, the more I
learn about them, the more I recognize that they are an excellant device
for
most home users for running 3 phase machines.
That depends. If you need to "plug reverse" a motor -- say do
it to a lathe spindle motor when you are threading up to a shoulder to
avoid a crash -- a static converter is useless.
The static converter (one version) is something like this (view
with a fixed pitch font such as Courier to avoid distortion of the
drawing):
(1)--------+ +----------------------------------(A)
| | +-- |
+----------+ | |
| I R | | |
+----------+ | |
o o
| |
(2)-------------------+-----------------------------(B)
|
| C1
+---|(------------------(C)
The block marked "I R" is a current sensing relay. When the motor is
drawing the high current when it is switched onto power with no ability
to start, it is sufficient current to close the contacts. One contact
connects to either (1) or (2) of the 240 VAC input lines (neutral is
ignored, so I did not draw it here). The capacitor (C1) applies
phase-shifted current to the motor terminal (C).
Once the motor spins up to perhaps half of its normal speed, the
relay is no longer seeing enough current, and thus the contacts open,
leaving terminal (C) on the motor totally disconnected.
That's OK for most things, because the single-phase power
applied between (A) and (B) is sufficient to keep the motor spinning at
normal speed -- though if it is loaded to full horsepower rating, the
current through the single winding will be higher than it was designed
for, and the winding will overheat and eventually burn out if you are
running it near its load rating for a long time.
However -- if you want to reverse the motor quickly, you are out
of luck. Normally, a three-phase motor is reversed by interchanging any
two of the three power leads. At that point, the three-phase power will
be trying to turn it the other way and it will reverse *quickly* (and
with a lot of current for a very short period). But, remember that the
(C) feed to the motor goes to nothing as long as the motor is not
drawing a lot of power. And once the motor is spinning, it doesn't care
which way it is going when fed by single phase.
So -- it won't reverse until the relay contacts close, and that
will require a lot of current, such as you get from a stalled motor.
But since it is spinning at normal speed, you won't get that current and
the relay will keep the connection to the (C) terminal open, so the
motor will keep running in the original direction, and your lathe tool
will crash into the shoulder.
A VFD does not want switches between the VFD and the motor, so
you have to ask it to reverse the motor, and that will require it to
slow the motor down and then speed it up in the other direction. Not
quite a fast as just reversing two leads on a real three phase, but
usually quick enough.
A rotary converter (which can be thought of as a static
converter starting a motor (called the idler motor) which then acts to
generate the third phase) can handle plug reversing -- as long as the
idler motor is enough larger than the load motor. I have heard of
attempts to plug reverse with too small an idler motor resulting in
reversing the idler motor instead of the load motor. Plan on at least
1.5 time the load motor rating for the idler motor -- or two times is
even better.
The only literature I have read related to static converters ghas been
sales brochures. I am a little sceptical of what is written in those
sales
papers.
Good!
I accumulated some data while dyno testing the effects of 3 phase
motors
and rotary convertyers. I saw that a very much smaller idler could spin
up
a larger tool motor when the tool motor is spun up while very lightly
loaded. But, as you would expect, the little idler helps little or
nothing
to the tool motor's ability to deliver power.
Where can a guy find some good tech info on static converter design?
Not from the makers -- they don't want you to see how simple
what they are selling you is, and how little it would cost for you to
make your own.
BTW That current relay is another reason for the minimum horsepower.
If the motor is two small, even with it stalled it won't draw
enough current to draw in the relay and engage the start
capacitor.
Some may be built using voltage sensing relays, waiting for
enough voltage to come back from terminal C instead of current sensing
relays -- or perhaps even a timer which just assumes that the motor must
have started by now and thus disengages the capacitor. All of this is
doing just what the centrifugal switch in a motor built for single phase
does.
If you want to study circuits which do what a static converter
does -- look at the home-brew plans for rotary converters -- they often
use the "static converter" design to start the idler.
And good rotary converter designs add oil-filled AC capacitors
to tune the output of the rotary converter for better balanced three
phase. You need to adjust the total capacitance according to the idler
motor, and ideally with the load motor connected too, so you are tuning
for best balance when under load.
Now -- I have seen mention of static converters which use a
tapped inductor to accomplish the phase shift instead of a capacitor.
That might avoid the need to switch out the third phase as soon as the
motor is spun up -- but it certainly increases the cost of
construction. Basically -- if you have difficulty lifing a static
converter, it is probably an inductor-based one, and may be better.
But personally, I like the VFD for most home shop conversion,
since it lets you tune the speed. The only place where I would consider
the rotary converter better is when you have a lot of three phase motors
on a single machine (which I believe is what started this). And of
course real three phase from the power company is even better -- if you
can get it.
Thanks for the "heads up" on excess capacity for spinning the 3 phase
motor.
You're welcome.
Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
Hi Don
You have alot of information on Converters. I began a study a few years
ago by building a dyno and using Excel to make charts of HP while running 3
phase motors from a single phase line. Don Foreman taught me lots and lots
about all the stuff I built and tested. he is world class instructor. I
have disassembled the dyno but keep the set of four 200 mic electrolytics in
parallel that I can apply to the unfed motor lead thru a solid state switch.
I use the capacitors to spin up 3 phase motors here at home. I wont be
surprised if i sometime fail to spin up a small 3 phase motor due to the
excess capacity.
I have several VFDs in systems I work on. So, I do like them and have
used alot for stuff I've built. I even repaired a 120 KW solid state
converter last week. It is fixed frequency at 60 Hz in and 400 Hz out.
The problem is 'I dont know what I did to make it work'. Thats not too
satisfying, but the unit did get shipped after I disassembled part of it and
then put it back together.
But, the more I learn about "Static Converters", the more I respect their
value to home use where space and budget is a factor.
Jerry