I recently bought a baldor 10 inch 3ph grinder with dust collector.
I use VFDs for most of the machines in my shop, but it seems to
make more sense to use a rotary phase converter (RPC) for this application.
I searched the web and found various different articles on building
a RPC. However they don't always agree on things like the run caps.
Can someone recommend a good set of plans.
Also where is a good source for capacitors and used 3PH motors?
I'm in a chicago suburb (naperville).
I think I have plenty of relays
thanks
chuck
I found a 10HP motor but I think there are some disadvantages to using
a big idler motor.
- hard to start
- big starting currents
- require large start caps
- inefficient
Should I keep looking for a smaller motor?
thanks
chuck
You will need a rotary phase converter of at least 1-1/2 times the HP of the
load motor. There was a good article in the Nov/Dec, 2001 issue of "Home
Shop Machinist" describing Rotary Phase converters. That article has good
recommendations for start and run capcitance. Capacitors are available on
the surplus market, and new from AC supply houses.
Bob Swinney
application.
The size of your largest load motor will determine the size of the idler motor
you need. The size of the idler motor will determine the amount of run
capacitance. The rules of thumb developed on this NG over time are:
Idler motor: 1.5x the largest load motor's horsepower (e.g. 3hp load->5hp idler)
Run caps : 12 uf (microfarads) per horsepower of idler motor
Start caps : 70 uf per horsepower of idler motor
These numbers have been used many times and are reliable. When you're looking
for 3 phase idler motors, take a good quality multimeter with you and check
the ohm reading from L1->L2->L3->L1. All should be about the same, and very
low readings, like 0.2 ohms. No open circuit. Then check each leg to the
motor's case. All should look like an open circuit. Then spin the shaft and
listen to the bearings. If you feel any crunch be ready to spend $40-50 on
new bearings. Inspect the motor case closely. Many times motors are scrapped
because of mechanical damage usually from being dropped. If the case is clean
or at least undamaged, and the bearings are good, and the meter readings are
OK, then consider the motor as a good candidate for an idler motor. The
slower it spins the better in my opinion (less noise). Also, if it has
sleeve bearings it will be quieter but you will have to oil them periodically.
There is a good source for used run caps in Seattle. I go there a lot. Email
me if you want me to look for some.
Grant Erwin
Kirkland, Washington
To email me, see http://www.t> I recently bought a baldor 10 inch 3ph grinder with dust collector.
For a bench grinder I wouldn't overlook a static converter,
either store bought or home brew. I have VFDs on one lathe
and the mill, but despite also having a rotary converter,
prefer to run the 3/4HP bandsaw, 3/4HP belt sander, and
1/2HP grinder on a static converter.
Ned Simmons
Is all the capacitance in one place or is it divided across legs?
One set of plans used 5uf across L1:L3 and 12uf across L2:L3
Input is L1 & L2. Generated leg is L3.
All across L1:L3 right?
I found a good 10HP motor. Seller claims it runs on real 3ph power, so
I think that means its good. Using your formula I would need a 120uf
of run caps and 700uf start caps. Will these caps cost a lot?
I think a 10HP idler is way overkill for a 1.5 HP grinder and 1/2HP
dust collector. I am concerned that I will need massive contactors
and wiring. I am also concerned that the caps will cost me a
small forturne.
chuck
I'm about to assemble one for myself (3 hp idler). Grainger has starting
caps at a decent price ($10 for the one I need), but
formatting link
has run caps at about 1/4 the Grainger price.
I also bought the HSM issue that Bob Swinney mentioned, and it has some good
information in it. Also check out some of the old threads at
formatting link
(see VFD and converter topic area). Then, if you
really want more information, there are a number of good r.c.m articles from
3-6 years ago that you can find by a bit of google searching. Look for Bob
Swinney and Fitch Williams as authors. There were many other contributors,
but google will get you to the threads and you can move back and forth from
there.
Pete
Buy new start capacitor(s) because those are electrolytic and dry out.
You can scrounge run caps safely. Split the run caps about half between
L1-L3 and other half between L2-L3. As you've seen, some guys get way into
tweaking these things and add a little more on one leg or the other. The
big reason to use run caps is they make the converter run much more quietly.
As long as the load runs smoothly and the converter isn't buzzing you're
fine. Mine has half/half. All the start capacitance goes L1-L3, yes. And
you do have to do something to take the start cap out of the circuit. I
use a potential relay, but you can also use a regular relay wired for
momentary contact, plus a pushbutton. The relay has to be rated to break
the full starting current, which can be much higher than the full load
amperage.
A 10 hp idler is way overkill. Loud, big, wasteful. OTOH a buddy of mine
dragged over a real nice Toshiba 15 hp motor the other day which we started
up in my shop and it ran real quiet. I'd look for a 3hp idler. - GWE
Charles A. Sherwood wrote:
If you already have some VFD's in your shop, why not use some of those
relays to switch the output of a vfd to the grinder?
The reason that the various plans don't agree on the run caps is that
different motors require different values. So the values are a
starting point for something that is not very critical.
Try looking at places that do air conditioning work. Most single
phase airconditioners use run caps. And a three phase compressor
might make a reasonable motor.
Dan
I guess I could buy a static phase converter to run
the grinder and then connect the dust collector to the
running grinder. This might work fairly well.
I should be able to test this out with a few caps.
If I'm not happy with it, I'm half way to building
a rotary phase converter.
chuck
My machines are not physically all together. The mill and lathe are in
one room and the grinders are in a different room. The grinder needs a
big VFD which are on the machines in the other room.
The bench grinder is right next to the surface grinder because I want
to use the dust collector for both machines. The surface grinder has
a 208V motor so the VFD needs to be configured for a lower voltage
output. Too many variables for changing things around.
Starting surge on a 10 hp idler is substantial. I built a 20 hp rotary,
and starting surge was horrific. To avoid that, I eliminated the self-
start feature (and capacitors) and simply spin it up with a modest
1 ph motor before applying power. Starting surge is negligible that
way, and I only need permanently wired run caps for phase balance,
no starting caps or cap start circuits required.
The control circuit uses a spring loaded DPDT center OFF switch.
In one position it applies power to the little 1 ph motor to spin the
assembly up. In the other position it closes the contactor to throw
240 to the idler. Note that this removes power from the little pony
motor once the rotary is started. It then just freewheels along for
the ride.
Start procedure is to hold the switch to the pony side until the
motors come up to speed, then flip it the other way momentarily
to close the contactor and throw 240 to the rotary.
You could use two NO push button switches if you don't have
a spring loaded DPDT center off switch, just make sure you don't
hold them both in at the same time. If you do, and the two motors'
synchronous speeds are different (they will be slightly), the little
pony may overheat and fail. The spring loaded DPDT center off
switch avoids this possibility.
A safety relay's coil is wired to be held in by L2 (the manufactured
leg). Its NO contacts are in parallel with the start switch. So once
the rotary starts making L2 (this occurs in a fraction of a second
after you hit the rotary with 240), the relay pulls in, holds the contactor
closed, and you can release the start switch.
If the power is interrupted and the idler stops, the safety relay drops
out, the contactor opens, and will remain open until you work the start
switch again. This prevents the idler from attempting to restart on its
own (which it can't do), and burning up its windings after restoration
from a power failure.
The stop circuit is just a NC push button wired in series with the
contactor coil. Push it, the contactor drops out. Let the rotary wind
down, then release. The converter won't restart until you operate
the start switch again. Safe, simple.
On the subject of sizing breakers and wiring for this project, they
only have to be rated slightly heavier than the *load* you're going
to put on the converter. If the converter is properly balanced and
power factor corrected with capacitors, it won't have high circulating
reactive currents through the primary wiring and breaker. The only
parasitic power draw is the small amount needed to overcome
windage and bearing drag. All other real power will be consumed
by the load(s), and so the primary wiring and breaker only need to
be sized to accommodate load consumption.
For my converter, I actually sized everything to handle a 20 hp
load, because I might run several large motors from my converter
at the same time. But that's not necessary if you're only going to
have a max load of 2 hp on the converter.
Gary
Up here in the great white north, the average house wife not only can
change the engine
on the family Buick, but can also come up with ways on how to save cash
with the bat
of an eye lash.
First, your values are a good ball park figure on your caps, of course
you'll have to do
some tweaking for balancing the the phases. The actual values to balance
the phases
actually vary between different makes, models and winding, so line
balancing is a must.
I presently run a 5 hp rotary phase, home built to power the shop
equipment . I run a
Machine shop, and it is a legitimate business, up here where Poly Phase
is no where to be
found. To run a branch from the nearest city was quoted to me at $
27,000.00 + $ 4000
per pole.
Cost of my rotary converter: $ 25.00
I have a couple of schematics for a rotary PC that might be of some help.
P.S. I've been running mine steadily for 4 years now, and not even a
whimper!
contact me
Charles A. Sherwood wrote:
I agree. The idler in a phase converter is basically a 3-phase motor
running on twophase power with runcaps, or singlephase power if run
caps aren't used. The main advantage to having an idler is the
ability to reverse -- but you don't reverse a grinder often, right?
A popular "rule of thumb" is that 3phase motors operated on
singlephase power must be derated. This is true for sustained
operation at full load because of heating, but does NOT mean that the
motor can't deliver full rated torque intermittently, albeit with
perhaps a very slight reduction in run speed -- a percent or two.
This may seem contrary to intuition or "conventional wisdom", but
Jerry Martes has actual dynomometer data that clearly supports this
assertion.
This makes sense to me, Don. Isn't the issue unbalanced current in the
windings (hence too much current in one winding) as opposed to not being
able to produce the torque?
Steve Smith
That's interesting Don, I used to run my bridgeport with a static
phase converter. After switching to a rotary it was obvious that the
power was increased. The machine would bog down on heavy cuts with the
static converter and not with the rotary. I had repeat jobs so was
able to really tell. Was this difference in power more because the
caps in the static unit were not matched well enough to the motor?
ERS
It's true that static converters (start and run capacitor systems
with no idler) can deliver the full rated power of the motor for
surprisingly long periods but that is not the whole story.
A converter of this type is basically a capacitor/inductor phase
shift system which produces an open vee 3 phase system. This phase
shifter is a series resonant circuit and when it is set up to give the
60 deg phase shift it is working a long way below its natural resonant
frequency. 60 deg is of course the correct phase angle between the two
legs of an open vee system.
The motor(s) is the inductor in the system and unfortunately the
apparent inductance of the motor changes with rotor speed. For any
particular rotor speed greater than about 90% of synchronous speed
(the lower limit varies a bit with motor type) it is possible to
choose a capacitor combination which produces a pretty close
approximation to balanced 3 phase at the motor terminals.
For near the full load rated speed of the motor, large run
capacitance is needed with most or all of it as a single capacitor
feeding the phantom phase from supply live. At light load the speed of
the rotor rises and if the capacitor value is chosen to achieve the
right phase angle the phantom phase voltage will be excessive. This
could be corrected by feeding the capacitor from a lower voltage
single phase source but this would mean feeding it from an auto
transformer across the supply.
It is much simpler (and of course everybody does this) to use
two capacitors arranged as a voltage divider to simultaneously achieve
the correct phase angle and phase voltage. The effective capacitance
of the two capacitors connected in series across the supply is the sum
of the capacitances because the source impedance of the supply is zero
and this effectively parallels the two capacitors. Because the they
also act as a voltage divider, this sum capacitance is effectively fed
from a voltage of supply voltage times C1/(C1+C2) where C1 is the top
capacitor and C2 is connected phantom phase to neutral.
Because it looks nicely symmetrical there seems to be a
tendency to believe that C1 and C2 should be equal and any inequality
in their optimum value must result from some strange second order
effect. This is NOT true. There is nothing magic about equal C1 and
C2. It simply results in a capacitor of value C1+C2 fed from half the
supply voltage. At this low effective supply voltage it is only
possible to get close to balanced operation at no load or light loads
which enable the rotor to operate close to synchronous speed.
As the load increases with consequent slowing of the rotor speed the
total capacitance needs to increase with both more in C1 and less in
C2. By the time full load is reached the optimum value for C2 is
usually zero.
These effects are very noticeable if you're using a single
motor on a variable load up to near rated full load power and some
compromise necessary. The saving grace is that industrial motors are
surprisingly tolerant of reasonable overvoltage when operating at
light loads so the trick is to size the capacitors for at or near full
loads and to accept some overvoltaqe at light loads. This increases
the motor losses at light load but the total motor losses still remain
below the losses at rated full load so temperature rise is acceptable.
Summing up - if you need to cope with heavy loads on a static
converter throw away the bottom capacitor and be sure to choose C1 for
operation near full load.
None of this helps with starting torque - this is inherently
poor with the static converter arrangement however large the starting
capacitor. This is because correct low speed phasing requires the
capacitor to be fed fed from a voltage many times the supply voltage.
Jim
I consider that unlikely, because the caps in a "static" phase
converter are only connected long enough to spin the motor up to
speed, and then disconnect. (Since they are usually motor-starting
caps, they aren't rated for continuous duty, anyway, and would let the
"magic smoke" out rather quickly.
Enjoy,
DoN.
What is your definition of C1 and C2? Won't things be wrong if you run
the load motor the other way? I seem to run my mill backwards a fair
amount. I have to admit I have C1 and C2 equal, but that isn't because
I can't do it any other way, it's because I don't really know exactly
how to tune my converter to be "right" for the entire range of load
motors, directions and speeds it sees. I have one rotary converter and
about 8 machines it drives, one at a time of course. Smallest motor is
a 3/4hp 1140 rpm unit on the vertical bandsaw, then a 1hp 1760 on the
surface grinder with another 1hp 3450 (I think) on the tool & cutter
grinder. My Bridgeport is 2hp 1760 and my Cincinnati lathe is 3hp 1760.
My press has a 3hp 1760 rpm motor. The power hammer has a 1hp 1140 rpm
motor. I simply don't know how to optimize across all of these machines.
Plus, everything works well and fairly quietly. Thus while there may
be merit in fine-tuning in an academic or theoretical sense, my experience
(limited though it is) is that it isn't really necessary in practice.
I will freely admit that I don't think I've ever run any of my motors
at anything like their rated capacity. I go slower than that, never
having worked in a pro shop, even when I'm working for money.
Grant Erwin
Kirkland, Washington
Jim
It isnt likely that anyone in this news group would have need for this
information, but, I disconnected one of the windings in a 3 phase motor to
evaluate the start torque from a 3 phase motor on single phase. I
reconnected that winding so it was like the start winding in a regular
single phase motor with a capacitor in series with it across the single
phase input to the motor.
If the start capacitor is selected to be "just right", the start torque of
a three phase motor (reconnected for single phase use) is almost double
(200%) the motor's rated max running torque.
I dont know where this "reconnecting" of a 3 phase motor for use on single
phase would be usefull. But it was interesting to get an idea of how
similar 3 phase and single phase motors are.
Jerry
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