I just got roped into helping a friend of my son balance a phase
converter for him. Asking me to help with electric is kinda like
asking a blind man to point out the best looking woman in the room.
There may be more wrong as the mill won't even start. I'll be taking
another motor along for testing.
Right now, he's getting 260 volts between one of the legs and the
Anyway, Fitch or somebody else, wrote up a balance procedure at one
time. Can someone point a link to it or send it to me?
Excellent article, Jim covers the whole subject well. But only a
couple of paragraphs on balancing. I'm almost sure Fitch had an
excellent guide just for balancing. Google search gets thousands of
hits with fitch and any subject I could think of.
Here are some posts and reposts that I have saved from Fitch on rotary
converters. The simplest stuff is at the end and the sections are
seperated with strings of ****
Fitch R. Williams wrote:
Back in 1997 I posted the following two posts on measurements taken on
converter. for those who are interested, they are re posted here in
------- First post ----------
I took some more data this afternoon on the two configurations
and the bare motor. Motivated by the discussion in another thread and
follow up (Help Electrical Mavens) or something like that
I was trying to measure several things, the response time
to turn on the lathe (shorter is better), the minimum voltage turning
lathe (higher is better), the minimum voltage reversing the lathe
better), the current into the third phase of the idling mill (more is
the stall current into the locked mill motor (more is better), and the
impedance of the converter (less is better) as a function of the
configuration. I made up a simple brake for the mill (didn't want to
built in one) and used the "MAX" current function of the FLUKE 36
ammeter and the record voltage function of the FLUKE 97scope meter.
mill brake wore out just as I finished so I made the final measurement
built in brake.
The output impedance seems to be adjustable in the low current range
7A on a 5hp idler) by using capacitors to boost the generated phase.
point, the inherent impedance characteristic of the size of rotary
comes into play. One speculates that larger idlers will have lower
characteristic output impedances for lots of reasons. On my little
sketched graph, the impedance goes flat at ~5 ohms at 2A for the bare
caps), at about 4.5A for the balanced configuration, and at about 7A
The start times are the time from the last peak before the voltage dip
first peak at the new loaded line voltage measured by running the
.5sec/div in single shot mode, and then using the cursor function to
My reading of the data is that the configuration optimized to balance
converter is not as good as the unbalanced one that was optimized
Zuppan's approach plus a power factor compensation cap) for balancing
the idling lathe.
For those that have not been following this series of posts, the
are quickly summarized as follows:
Bare idler motor (same in all cases) Gould 5hp 3ph 1725 rpm, 220/440V
wired in the low voltage configuration. Cp = run cap in parallel with
cap. Cs = run cap opposite, Cp (line to third phase), Cpf = power
correction capacitor. There were no capacitors on the bare motor.
Balanced Configuration: Cp = 60uF, Cs = 50uF, Cpf = 50uF
Unbalanced: Cp = 110uF, Cs = 100uF, Cpf = 12.5uF
The results follow:
If you change your display font to courier this table will be easier
Parameter Bare Idler Balanced
Third Phase Voc 223.7V 239.9V 252.6V
Current/Voltage Into .7A/223.2V 2.4A/234.5V
Third Phase of Idling
Mill Stall I/V 18.1A/140.8V 19.5A/141.3V
Lathe Start Time 1.3sec 0.9sec 0.68sec
Lathe Start Vac Min 142V 144V 148V
Lathe Rev Vac Min 125V 130V 133V
Zout @ 2A (Ohms) 5 2 1.7
Zout @ 5A (Ohms) 6 5 3
Zout @ 10A 6 5.5 5.2
Zout @ 15A 6 5.5 5+
Zout @ Stall 4.6 5.1 4.9
I've about run out of things to measure except power factor and
will tackel that when I get the time. The unbalanced configuration is
lot like it is the "final" design. Since I bought the enclosure when
the balanced one was "it" I need to get a bit clever about packaging.
--------------------- end of first post --------
Power factor and power consumption are covered in the next post. I
measured efficiency directly yet - need a load motor dyno to do that.
efficiency of interest is single phase input to load motor shaft
There were several posts in between these - but these two capture most
------- Second post --------
I created an Excel spread sheet and printed it to provide a format in
record the data. After the numbers were entered I had it do the
and printed to a comma seperated ascii file. This views like a sober
you have your browser font set to courier or some other monospace
font. If not,
it views like a drunk table.
A bit of explanation for any first time viewers or those unfamiliar
series of posts on the 5hp rotary converter project.
This is a table of measurements related to the single phase 220V input
to a 5hp
rotary converter evaluated with a number of capacitor combinations and
The converter idler motor is a Gould 5hp 1725 rpm "Y" wound
induction motor. A number of posts have been made detailing other
associated with these converter combinations. The purpose of this
in a series of measurement sets is to explore the effect of capacitor
configuration on power factor, and idler power consumption.
A brief tour of the table and definition of variables. CP is the run
in parallel with the start capacitor (connected between one single
and the third phase). Note that CP values of 125uF and larger make
particular converter self starting if there are no other run
capacitors. CS is
the other run capacitor which is between the other single phase line
third phase. Cpf is the power factor correction capacitor when one is
All capacitor values are expressed in microfarads.
V(Sh) is the rms voltage in volts developed across the current shunt
It is multiplied by the Shunt constant of 159.59 to convert it to
is the converter input line to line voltage in volts.
Ph is the phase angle between voltage and current. The positive value
voltage is leading current which indicates that the input is in all
least slightly inductive. Pf is the cosine of angle Ph.
W is the actual power input to the system in watts which is calculated
W = 159.59 * V(Sh) * V(LL) * Pf
The instrument used was a FLUKE 97 scope with 10:1 probes except for
converter measurement on the last two cases where a 1:1 probe was used
channel B (current measurement) to increase the size of the wave form
improve the accuracy of the phase angle measurements. The current
.006266 ohms as measured on a very accurate Valahalla micro-ohmmeter.
calculated rms values of the voltages were used. The scope determined
angle measurements were used. Because of some variability in the
the phase angle variations were studied and the data from the best
placement (closest to zero reference crossing) was used.
Power and Power Factor Measurements
Shunt constant = 159.591
Description V(Sh) V(LL) Ph W Pf
CP = 0
Idler 0.078 232 70 988 0.34
Idler + L(N) 0.116 232 70 1,469 0.34
Idler + L(1500) 0.116 232 69 1,539 0.36
Idler + L(1500) + M(223 0.148 232 69 1,964 0.36
Idler + M(stall) 0.420 224 35 12,299 0.82
CP = 50uF
Idler 0.060 232 77 500 0.22
Idler + L(N) 0.096 232 77 800 0.22
Idler + L(1500) 0.096 232 69 1,274 0.36
Idler + L(1500) + M(223 0.132 232 69 1,751 0.36
Idler + M(stall) 0.408 224 51 9,179 0.63
CP = 100uF
Idler 0.044 232 69 584 0.36
Idler + L(N) 0.076 232 69 1,008 0.36
Idler + L(1500) 0.080 232 60 1,481 0.50
Idler + L(1500) + M(223 0.116 232 60 2,147 0.50
Idler + M(stall) 0.380 224 43 9,935 0.73
CP = 125uF
Idler 0.038 232 61 682 0.48
Idler + L(N) 0.068 232 69 902 0.36
Idler + L(1500) 0.072 232 51 1,678 0.63
Idler + L(1500) + M(223 0.104 232 60 1,925 0.50
Idler + M(stall) 0.380 224 43 9,935 0.73
CP = 160uF
Idler 0.030 232 51 699 0.63
Idler + L(N) 0.056 232 60 1,037 0.50
Idler + L(1500) 0.062 232 44 1,651 0.72
Idler + L(1500) + M(223 0.096 232 51 2,237 0.63
Idler + M(stall) 0.360 224 43 9,412 0.73
CP = 60uF CS=50 Cpf=50 Balanced as an Idler AKA "Balanced Config."
Idler 0.012 232 9 439 0.99
Idler + L(N) 0.040 232 60 741 0.50
Idler + L(1500) 0.048 232 44 1,278 0.72
Idler + L(1500) + M(223 0.080 232 51 1,864 0.63
Idler + M(stall) 0.360 224 43 9,412 0.73
CP = 110uF CS=50 Cpf=12.5 Balanced lathe currents AKA "Prefered"
Idler 0.016 232 26 532 0.90
Idler + L(N) 0.046 232 60 852 0.50
Idler + L(1500) 0.060 232 35 1,820 0.82
Idler + L(1500) + M(223 0.088 232 53 1,961 0.60
Idler + M(stall) 0.384 224 43 10,040 0.73
Of interest to me is the 17% lower power consumption of the converter
balanced as an idler when compared to the "prefered configuration"
optimized to balance the lathe currents. The total rms current taken
scope from the waveform was 1.91A for the idling balanced converter.
cooler and maybe quieter (although I have no objective way to evaluate
I don't have the ability to print waveforms from this scope (optical
cable is missing). That could change shortly after the first of the
year when I
am supposed to get the chance to use a TEK THS720 with PC connect
cable and SW
for a short time. However I made a note that the current wave form
noticably non sinusoidal and "pinched" at the peaks in the case where
160uF, I might recreate it and take a picture of it with my digital
Based on data taken during previous testing the increasing power
CP is increased is due to the increase in current flowing in the third
the idler motor. While it is reactive current, it still dissipates
power as an
IR drop in the third phase windings. The output voltage of the third
increases markedly with increases in CP. 125uF or 130uF is about as
far as I am
comfortable with (153+volts).
The differences in power consumption between the last two
easily explained by the significnatly lower circulating currents in
the idler in
the balanced configuration.
I will be adding a few more cases to this table shortly after the
first of the
year after I pick up a buck/boost converter to try in place of CS. I
currently expecting to try it with CP values of 25, 50, 75, and 100
Note that the power factor of unloaded induction motors is worse
when they are loaded and working hard. This is because the
is such a larg part of the total current when the motor is idling and
part grows as the motor is loaded and does "real" work.
There is more, but that is probably more than enough for now. I keep
will put this in the Metal Working News page - roundtuit needed!
In So. Cal. **************************************************************************************************
Subject: Re: rotary phase converter - parts ?
From: Fitch R. Williams
Date: Tue, 02 Jan 2001 19:31:13 -0800
If you add capacitors so that when the converter is idling
the voltage of the two generated phases is 108% to 110% of
the incoming line voltage, and then add capacitance across
int incoming line to minimize the measured line current you
may find that it works even better. That will give you
nearly balanced voltages when the load is connected, but
will not cause an idling over voltage that will damage the
run caps (assuming they are 370V caps).
I would be interested in a copy of your data if you care to
In So. Cal. *******************************************************************************************
Subject: Re: Power factor: the real deal
From: Fitch R. Williams
Date: Sun, 28 Apr 2002 13:43:40 -0700
Rich Kasik wrote:
There are three opportunities to add capacitors to a rotary converter.
L1 and L2 are the incoming 240V single phase leads.
Cp is the run cap that is in parallel with the start cap. Assuming
the start cap
is between Line 1 (L1) and the third phase lead, then Cp is also
L1 and the third phase lead.
Cs is the other run cap and is connected between L2 and the third
Cpf is the power factor correction capacitor and is connected between
L1 and L2.
You select Cp and Cs so that the idling converter has voltages from
L1 and L2 to
the third phase lead that are equal and at or up to 108% more than the
line voltage between L1 and L2. To do this, you only need a volt
meter and a
selection of run caps.
Once the two run caps are selected, you need a current measuring
device to select
Cpf. Assuming a clamp on ammeter is available, clamp that around one
incoming lines, L21 or L2, "before" the junction where the run caps
Start adding Cpf in increments. After each addition (capacitors are
parallel), read the line current. As capacitance is added, the line
decrease to a minimum, and then start back up. After the first
indicating an increase in current, remove the most recently added
call it quits.
The rule of thumb is to add Cpf until the idling converter line
Hope this adds more clarification than confusion.
In So. Cal. *********************************************************************************************
Many thanks - just what I was looking for.
Also, Thanks Bob Swinney; for the information you sent by email.
FWIW, the motor being used as the phase converter was WAY to small.
Switched to a five horse and it took right off. I left the guy a copy
of Fitch and Bob's information so he could learn the fine art of