They must have centrifugal switches .
You can start with a series cap on the start winding and
if your rich and can afford the parallel cap on the run winding
( it improves power factor only , it is not abs needed ).
an 8 amp CSR 117vac motor maybe needs a ..... shit i forgot ,
why not compare another similar . Maybe its around
10 Mikes ? starting . The bigger it is , the more torque .
Does HF still use PSC motors on the $40 drill press ?
I returned mine years ago , it had no power at all !
All tool motors must have CSR motors , PSC wont work .
Jim> I just came across 2 1.5 HP induction motors- they seem in fine shape
According to JimInsolo :
Are you sure that they even *need* capacitors? If they are
three phase motors, they don't need them -- just three phase power.
For single phase motors, some (most) will only need a starting
capacitor, but there are some which need both a start and a run cap.
Check out the motor's nameplate. They often specify the size of
capacitor needed as part of the data. (It will be in uFd or MFd -- the
same units, but different age of motor.)
And without that -- the motor's voltage will also be important,
as the needed capacitor changes with voltage -- and some dual-voltage
motors use the same cap for either voltage, by a wiring trick which
applies the same voltage to the start winding and cap either way.
Single phase (including switched start/run winding, split phase,
capacitor start or non-capacitor) induction motors don't need start
capacitors to put the motors into operation.
This is to mean that in some applications, a motor wouldn't need the
start capacitor to run the motor.
A call or visit to a motor shop might provide you the values of the
The start capacitor will increase the motor's starting torque, where a
motor has a load connected to it at the time of startup.
Many single phase, split phase induction motors operate for years
without start capacitors, when they are used for application where no
load is applied at startup.
A typical switched start/run motor without a start capacitor, can be
modified to increase the starting torque, by adding a start capacitor
to the proper internal terminals/connectors.
When I asked about sizing motor start capacitor values at my local
motor shop, and they said that the rule of thumb is 500 microfarad per
Since then, I've noticed that motor manufacturers seem to follow that
rule, as the start capacitors in motors that I've serviced have always
had capacitor ratings that follow the 500uF/HP rule.
Motor starting capacitors are AC dry types, and rated for a higher AC
voltage than the motors windings will apply to the capacitor. Starting
capacitors are only capable of having the voltage applied to them for
brief durations (the short interval that the motor gets up to run
winding switching speed).
Start and run capacitors have different characteristics. Start caps are
compact for their value (typically over 100uF), compared to run caps,
which are large but have lower values (typically less than 50uF).
Single phase motors that utilize both start and run capacitors (such as
air compressor-duty rated motors) will also have run capacitors. The
run capacitors are a small value (about 10 microfarad), physically
larger AC oil types, and are capable of sustained voltage application.
Books on motor theory explain that run capacitors will increase the
motor's run torque.
Jim> I just came across 2 1.5 HP induction motors- they seem in fine shape
For a single phase 220V 60 Hz 1.5 HP capacitor start
and run motor, 45 to 50uF is typical for the run capacitor. Start
capacitor is not critical - it depends how much starting torque
you need. 3 times the run capacitance is usually sufficient, up
to twice as much if you need more torque.
Because the market for largish single phase machines
is fairly small, some of these "single phase" capacitor start and
run machines actually use standard symmetrical 3 phase windings
with added capacitors and starting switch for single phase
If the resistance of the separate (disconnected) windings between each line
and the capacitor and between the two lines are the same, then it will almost
certainly be a three phase motor.
Depends on how many wires you can get at. A 3 phase machine
will have three equal windings that can be connected in star or
delta - any pair of wires will measure equal resistance.
A true capacitor start/run machine will normally have only
two windings and although one end may be commoned to give three
leadouts it will not measure equal resistance between any pair.
Like i said , if they have a run cap , they are expensive motors .
If you remove a run cap , you only pay more for electricity
and get less torque at max load and it will "dropout " sooner .
we all have house wired 3 phase so we use
3 PH motors on all our machine tools ..
D> According to JimInsolo :
I suspect that the manufacturer may have used two start capacitors (in
parallel to attain the high value required for a 1.5 HP single phase
If the two cover cans are the same size, this would lead me to believe
this is true. As compressor-duty rated motors are usually marked as
such, leaving information out of the original post isn't very likely to
result in conclusive replies.
Compressor-duty rated motors generally have start and run caps, and the
run cap is typically much larger in physical size (explained earlier).
The best source of info about motors would be the manufacturer.
A newsgroup may be the second best source, when the poster includes all
of the info present on the motor's data plate (and many RCMers have
seen/used lots of motors).
It seems that most posts (motors or other equipment) don't include all
of the pertinent info, and a lot of speculation follows, until several
days later, when the OP gets around to posting the "rest of the story",
in a Paul Harvey sort-of way. I think of these posts as being PHPs..
If the OP can determine if the motors have centrifugal switches, then
there is little point in exploring the possibility of whether they're
three phase motors (which I've already discounted).
There are several posts with good information . . . but lets start with
the basics. Let's assume Cap-start/Car-run. First off, you need to know which
cap went in which can. Mixing up two wires will let the smoke out.
The basic circuit consists of two "sets" of windings, the "run winds"
(which carry the bulk of the current), and the "start (or auxiliary) winding.
" The leads at the terminal board or box usually will have a connection
diagram for the end-user, but the internal wiring has to be figured out.
Checking with an ohm meter can be confusing, as the motor is stationary, ie.
the start switch is closed until the motor comes up to ~75% running speed.
Continuity through the start switch can make the cap leads difficult to sort
out. It is often simpler to remove the "opposite drive-end" endbell (bearing
housing) in order to trace the wiring.
Single phase motors are often provided in a dual volage configuration. The
run windings are divided in half, and how they are connected to each other
determines the voltage the motor will run on. If the two halves are connected
in series (current has to pass through one half and then the other), the
motor will run on the higher voltage (usually 230V). If, on the other hand,
the two halves are connected in parallel (current can pass equally through
either winding), the motor will run on the lower voltage (typically 115V).
Series: ---------1------------2-3-----------4-------- 230V
Run winds only
/ 115 \
Parallel -------L N--------- 115V
\ 115 /
Dual voltage motors usually have the start (or aux) winding rated for the
lower of the two voltages. The start wind is connected in parallel with one
half of the run winding, therefore it always sees the lower voltage. If the
motor were connected for high voltage, one end of the start wind with it's
caps and start switch would be connected to the mid-point of the run windings.
Run wind Run wind
115 \ 115
The purpose of the capacitors is to shift the phasing of the A/C power
feeding the start winds. The windings are laid into the stator of the motor
with the centers of the coils half-way inbetween the coils of the run
windings. On a split-phase motor (start switch, but no caps), the start winds
have MANY more turns of much smaller wire than the run winds. Because the
current has so much further to go through a smaller conductor, it takes
longer for the magnetic field to build to full strength. There is a real
limit to how much torque these types can have on start, as increasing the
current in the start winding by reducing the length of the wire and/or it's
resistance (larger wire) allows the magnetic field to build almost as quickly
as that in the run windings. If they peak at the same time, there is NO start
torque, the motor just hums or growls instead of accelerating.
Capacitors "slow down" the current, without limiting it to the same extent.
They have to be "filled" with electrons before they will allow any to pass
through. They act like a balloon or air compressor tank, absorbing and
releasing pressure. This allows the engineer to design his start wind with
fewer turns of larger wire to carry more current, thereby increasing the
torque available from the start winds. The proper value of start cap will
maximize the current flow in the start windings ~90 degrees behind the run
winds. This amount of phase shifting releases tremendous energy into the
auxiliary windings, far exceeding the continuous current rating of the
winding. Start caps have high mfd ratings, but are intended for intermittant
duty only, or they will blow up. The start switch is wired in series with the
start winds and start caps. It opens at ~75% of full load speed, cutting
current through the start caps.
s = switch
---l(--- = start capacitor
This is a cap start motor. Once it is up to speed, the start winds don't do
diddly-squat until it has to start the next time. It didn't take designers to
long to figure out that if they put another, smaller, continuous duty
capacitor across the windings, then those windings could be used while the
motor was running as well. The phase shift was less, but the current was
manageable as well.
s = switch
---l(--- = start capacitor
---ll--- = run capacitor
These symbols both refer to capacitors, using different ones just makes the
diagram easier to figure out. The caps are both non-polarized (A/C), but the
run cap has a higher voltage rating.
Not only is the entire winding working all the time, but the run winds and
aux winds are working in conjunction with each other, (You just doubled the
"number of cylinders" of your "engine", so to speak).
That analogy also serves to demonstrate the importance of the correct value
of capacitor to match the windings. If the cap is wrong, the "timing" will be
off. The motor will be unable to develop it's full torque, as the aux winding
wil "fire" at the wrong time. If you have make and model # (or Catalog #),
any motor shop should be able to get the proper values from the manufacturer,
assuming they are still in business. The rough figure of 500 mfd/hp is
perhaps on the high side for cap-start-cap run, especially if the motor is
dual voltage. I'd suggest that you post the particulars of the motors taken
off the nameplates, and see if any one can get you definate answers.
Wow- that was a really informative explanation- I learned something new, as
I always thought the start winding simply shut off once the motor came up to
speed. Some of the other guys got a bit off the original question and
started talking about 3 phase etc., but your explanation was right to the
point- I once had a start/run induction motor that every now and then would
buzz loudly and break the breaker- no one could tell me the problem- from
your post, I am thinking it had a weak or failing start capacitor which was
getting the windings out of phase-
> duty only, or they will blow up. The start switch is wired in series with the
> start winds and start caps. It opens at ~75% of full load speed, cutting
> current through the start caps.
The main part of this post is OK but it's gone slighly adrift in
the simplified description of the start winding operation.
In split phase start motors the start winding is
wound with slighly fewer turns of much finer wire than the main
winding. This gives it a little lower inductance than the main
winding but much higher resistance. The phase angle of the
currents in the windings is controlled by this inductance to
resistance ratio and it is this difference in ratio that causes
the start winding current phase angle to lead that of the main
winding. Unfortunately the phase difference is much less than the
ideal 90 deg. This reduces the available starting torque so this
type of motor has high starting current and poor starting torque.
In a capacitor start motor the effect of the
capacitance is to partially cancel the effect of the winding
inductance so that the change in effective inductance to
resistance ratio (which gives greater phase difference) can be
achieved without the power loss and inefficiency of a high
resistance start winding. Because of this the capacitor start
winding is typically wound with only slightly finer wire with and
with about the same or greater number of turns than the main
This arrangement gets closer to the ideal 90 deg
shift without the heavy losses of the split phase system so it
achieves both higher starting torque AND lower starting current.
20 to 30 degrees.
This reduces the available starting torque so this
Ok, you made me dig out my books. You are absolutely right, I explained it
exactly backwards. I also made a mistaken assumption that a split-phase motor
would have even more turns in the auxiliary than a cap-start. I guess I
should read a bit more. . . more often.The book I go to for single phase is
Veinott's FHP Electric Motors, and perhaps this is just a case of terminology
changing over time, but the term used in my book is "reactance". Inductance
was reserved for the current in the rotor, reactance being the counter EMF in
the winding to which the voltage is applied DUE to the current being induced
in the secondary (or rotor, in this instance)
Because of this the capacitor start
I'll second THATas a statement by itself, I was wrong on the split-phase. I
don't rewind furnace fan or above-ground pool pump motors, so the winding
data wasn't something I gave much thought to.
Question for you. . . what type of cap-run motor has two sets of identical
windings displaced 90 mechanical degrees apart, and what are they used for.
Hint? Very limited duty cycle.
An intermittant "hummer" is much more likely to be due to a "sticky" or
"wobbley" centrifugal mechanism, and/or a mis-adjusted start switch. If the
switch dues not close when the motor stops, then there will be no current in
the auxiliary winding at the next start-up ( or not enough phase shift in the
case of a cap-start/car-run motor). If the switch closes the next times, the
motor will start then.
Jim I once had a start/run induction motor that every now and then would
Two identical sets of windings (i.e 2 pairs of
leadout wires) 90 deg apart is a true 2 phase wind and can be
driven as such from a 2 phase supply. It also performs pretty
well as a capacitor run motor but needs a largish capacitor.
It's commonplace in aircraft and military small servo
systems. Most industrial/domestic machines use more turns of a
bit finer wire in the capacitor phase because this permits the
use of a smaller value capacitor.
Not sure if I've correctly understood your question. If
you mean 2 separate sets of 0deg&90deg windings (i.e.4 pairs of
leadout wires) it's either a dual voltage motor or a very strange
beast indeed. One possibility is a 2 speed pole changing machine.
The starting torque of the low speed mode would be much better
than the high speed mode which might be a useful attribute.
Sorry, my verbal description was not perfectly clear. Conventional four-
pole series connection in both starts and runs, four leads out possible, but
more usually found with three leads (internally connected common lead). The
only unusual part is that the starts and runs have the exact same turns and
wire size. The capacitor is connected across the leads which show the highest
resistance ( the common lead being the "center-tap"). Voltage is applied to
the common lead and either one of the other leads, depending on desired
direction. Typical duty cycle is 20%. (2min. in 10min. intermittant) The only
application I have seen them put in is actuators. Handicap chair lifts,
remote valve actuators, and the strangest one was a motor for vertical blinds,
of all things.
Fairly good torque for motors of their size, instant reversing with low in-
rush, but low in effieciency (sp).
I guess we are getting a little off-topic for this group?
One type of AC motor that utilizes a run capacitor is the PSC motor.
These types have 2 identical windings (three leads), connected in
series to form a center-tap. The AC line is connected to center and one
end lead, and the capacitor is connected to the center and the other
The PSC motor is easily reversed, and responds quickly to having the
connections of the AC power and the capacitor swapped.
PSC motors are used in a wide variety of applications from small fans,
cheap bench grinders, and gearhead motors.