Reversing single phase induction motors


In my last year’s video
about induction motors, I really didn’t go
into any details about starting circuits and reversing
of induction motors. So, I’ll cover
that now. The rotor of an induction
motor is essentially a permeable
iron core with a short circuit winding
that’s cased in place. And that causes it to resist
changes in the magnetic field, which causes it to
follow any sort of spinning magnetic field.
For 3-phase motors, the sense of rotation is
inherent in the 3 phases coming into the motor. But
for a single-phase motor, like this split-phase
induction motor, a sense of rotation has
to be created with a bit of trickery,
using a capacitor. This is the schematic
for the induction motor where we have 2 windings
that are at a right angle. And, one of these windings
is connected directly to AC whereas the other one is in
series with the capacitor. On this induction motor,one
of the windings is these 2 coils and
the other winding is these 2 coils. So, this
winding goes this way, forming this field, the
other one this way. And the interaction of the
inductance of the coils here, and the capacitor causes
1 of the windings to be shifted
90 degrees so, as this one
goes up and down, and this one goes
side to side, they’re out of phase 90
degrees, so we get a sense of rotation. Reversing
this type of motor is trivially easy. All I
have to do is move the AC from directly
to this coil to directly to this coil and
then through the capacitor the other winding
is delayed. So, essentially all I have to
do is move the power connection from here to here. So, here’s the capacitor
and all I have to do is move this clip to the other
side of the capacitor to reverse it. And this becomes much
easier to see if I slow down the motor a lot
by reducing the voltage. So, you can see now it
goes counter-clockwise and now I move this clip,
and now its clockwise. Now, if we look inside
this 3/4 horsepower motor, you can see the windings are
not as easy to discern. Essentially the winding
is split over many slots in the stater, here. And
that makes the transition from pole to pole,
a lot smoother, which makes for a quieter,
more efficient motor. Looking closely at the
windings, you can see there is a winding
that goes here. And then there’s another
one that goes here, and that’s made out
of thicker wire, and that’s the
main winding. And then, if we have another
winding that’s kind of to the inside of that, it’s
made out of thinner wire that goes here
and here. So, now if I put a compass
in here and I put DC through the
main winding, you can we have a
horizontal magnetic field. And if I move that clip to
activate the starter winding, you can see we have a
vertical magnetic field so those windings are 90 degrees
out of phase with each other. One end of the starter
winding is connected to this red wire and that goes to
this capacitor and then the other side of the
capacitor goes through this black wire onto here and
that’s the starter switch, and that gets disconnected
once the motor’s up to speed. So, the schematic for this
motor is very similar to our split-phase motor with
1 winding on the AC and the other one in
series with the capacitor on AC. Except
that this winding is much thinner and we
turn off the winding once the motor’s
up to speed. The starter winding is
automatically turned off with a centrifugal switch. and
the way that works is these 2 metal clips get
pulled out by centrifugal force once it spins at full
speed, and that pulls back this plate here. When the motor’s
not at full speed this plate rubs against
this plastic tab, here, and as long as that
plastic tab is pushed, this starter contact
is closed and then once it’s up to speed,
the plate pulls away and it no longer rubs against
here so we don’t get friction and wear. So, if the starter winding
helps to create a rotating magnetic field
to spin up the motor, why disconnect it once the
motor’s actually up to speed? The whole phase shifting,
the starter winding with the capacitor is not as
elegant as it would sound. The size of the capacitor
very much depends on the expected load
of the motor. And, if we just connect
it briefly, temporarily at the start, it doesn’t
matter if it’s a little bit inefficient or the wrong
size, because we won’t leave it engaged. Also,
this capacitor is an electrolytic capacitor.
And it’s not designed to take a
continuous AC load. If we got that kind of
load, it would burn out. And to have a condenser
for this size motor, it would be quite
expensive. And, because we turn off
the winding once the motor’s up to speed, you
can also use a much thinner winding for
the starter winding and save a bit of money
that way because copper is
expensive. So, just to turn up
the starter winding, makes an awful
lot of sense. But, once we turn off
the starter winding, the magnetic field just
goes back and forth. So, why does the motor
keep on spinning? You can compare that to
moving a screwdriver back and forth with
this pendulum. Just moving it side to side
doesn’t get the pendulum rotating. But, if I rotate it,
now it goes around and round. And, now even if I
just go side to side, I can keep that thing spinning
for as long as I want to. Having said that,
there are some motors called capacitor start
capacitor run motors, that have a beefier starter
winding and then they have a second capacitor that is in parallel
with the starter switch and the starter
capacitor that stays engaged, and
that helps the motor run a little bit better.
The motor in my table saw has that kind of
motor, and it’s got 2 bulges on there. 1
for a start capacitor the other 1 for a
run capacitor. So, out of personal
curiosity, I brought a wire out
from this motor, it goes to the
starter winding and I hooked that up to 1
channel of the oscilloscope and the power in is
on the other side, and here’s my scope and
I’ve got the motor hooked up to
a variac. so I can run it at
a reduced voltage So, let’s slowly
apply some voltage, and on the screen here you
can see 2 sine waves, 1 of which is shifted to
the side a little bit. Each period of the
sine wave represents 1 full circle turn.
And the shift on here is about a quarter sine
wave, so that’s about 90 degrees,
which is good. Now, so far this motor
is not spinning, so let’s give it a
bit more voltage to actually get
it running. And, there it goes. And
you hear the click. That’s the starter
switch disengaging. So, now we still have a
voltage on the main winding, and the starter winding,
even though we’ve actually disconnected the
starter winding, And, interestingly enough,
the taller of these 2 sine waves is actually
the starter winding. Because if I
disconnect this, you can see the taller
wave form disappears. So, it appears that the
starter winding has more turns on it than the
main winding, and the induced voltage is
actually greater than the voltage going
into the motor. To explain this a little bit
better, I made a little diagram of the motor. The
main winding is in green and the starter
winding is in red. And, once that
motor is running, we have this passively
magnetized rotor and it’s in sync with the
back and forth magnetic field from the main
winding. But, rather than demagnetize
itself, when it’s at 90 degrees, it just simply magnetizes the
stater from what’s in here. So, basically that
means we have, aside from a magnetic
field this way, we have also 1 this
way, together making a rotating
magnetic field. And, that means we have
a magnetic field going through the windings
of the starter. And, that in
turn induces a 90 degrees
out of phase voltage in the starter
winding, even though there’s nothing
hooked up to it. So, how to reverse
this motor? I can’t just move the AC
power connection from here to here. That would
reverse the motor but that would make the starter
winding the main winding and they’re very
different and that’s not gonna work
very well. What we need to do instead
is just to reverse 1 of these windings,
so that essentially we make a mirror image
of the magnetic field, and anything that
was clockwise becomes
counter-clockwise. So, if we can, for instance,
just connect this to here, and connect this to here, and
break these connections, we’ve swapped the
polarity of this and it will start in the
opposite direction. Except, if I look on the
label for this motor, it says here, “Motor
is non-reversible”. It should be reversible
but why is it not? The problem is really
one of cost-cutting. There’s only 3 wires coming
out from the windings of this motor. Basically
what was done is inside the windings,
the starter winding and the main winding, share
1 wire that comes out and so we only have 1 end
of the starter winding and 1 end of the
main winding. And, unless I go in
there and dig out this connection to bring this
wire out separately, I can’t swap the terminals
on the starter winding, which means I can’t
reverse the motor. Of course, where there’s
a will, there’s a way. So, I could potentially dig
out this connection in there and break it and bring
out another wire. But, the problem is
there’s no way to remove the stater from the case,
so I better not have to cut a hole in the
case to get at it. And, I think that would just be
more trouble than it’s worth. So, for all
practical purposes, this motor can’t be reversed,
because they saved 1 wire. But these motors
here can be reversed and reversing is always
reversing the starter winding. So, see here, “For reversing,
interchange red and green leads in terminal
box in end casing.” Basically reversing the polarity
of the starter winding. On this one
it says, “For counter-clockwise rotation,
interchange red and black.” Again, reversing the
starter winding. And on this
one it says, “To reverse rotation,
interchange red and black
wires.” So, reversing a single-phase
induction motor is always a function of
reversing the polarity of the starting
winding. The challenge is,
on some motors, you just don’t have access
to the starter winding, and on other motors, you
may have access to it but first you have to
figure out which one the starter winding actually is.

100 thoughts on “Reversing single phase induction motors

  1. I use this type of motor to run a pump. I can reverse it by swapping the black and red wires. How would I wire a switch to do this? I have an On-On DPDT switch on hand. Thanks!

  2. hello mr Wandel, i have a single phase motor with 4 wires coming out of the motor. The windings are identical. one winding is connected by a capacitor(permanent). Can i just use the other winding and get it going by hand?

  3. I have a motor like this on my bandsaw. I want to replace the switch with a magnetic relay switch but the old switch only had one black and one white while I need the two black and two white to hold the magnet together during operation… do you have any experience with this? Where to find the other white and black wire? Thanks a lot. Love your videos.

  4. Considering myself highly interested in all kinds of technical stuff, I had to live 67 years until getting such clear explanations. You are a genius AND very gifted teacher! Thanks!

  5. hello Mr. Wandel,
    could you help me solving this compressor motor (230V-2HP) problem
    – when I use a wall socket with 230V AC on right side (* see note), the compressor runs
    – when I use another socket where connections are reversed (* see note), I get a short circuit:
    a spark and my mains fuse or differential circuit breaker trips
    The start capacitor (35 uF, 450V) checked with an analogue ohmmeter seems OK: not shorted, not open (there is a charge current, the needle moves accordingly)
    (note *) I checked the different wall sockets by measuring between 1 side of the outlet and the earthing pin

  6. Well if you can't reverse it isn't that the time to become a little more primitive? Weld the right gear on that motor and now you can make it move any way you like. It would by nice though if the maker whould give you the option to reverse it :p

  7. Thanks for valued info.
    I have removed a motor from toilet ventilation fan, it is induction type and it has only 2 wires and without capacitor. how it can be that??

  8. I've been Googling, but can't find an answer. Hopefully someone can answer. Can I swap the rotor in an induction motor? Providing that the size of the rotor core is the same. My goal is to get a different length shaft into the motor. I know the bearings may need to changed to accommodate the shaft. Thanks.

  9. Matthias,
    You left out nothing is this video, excellent. Now to go cut a hole in my new jet lathe's motor…bummer.

  10. Oh beautiful. very good explanation.
    May you live a long life Full of gladness and health . May all God's blessings descend upon you. May peace be within you may your heart be strong. May you find what you're seeking wherever you roam.?

  11. Would a clothes dryer that reverses during cycle have a have a single phase induction motor or an inverter attached to an inverter duty motor?

  12. My dad's belt sander has had this problem with starting up eversince I can remember it. When you push the start button it sounds like there's too much load on the electric motor so you have to give the belt a push by hand, then it starts up and works just fine.
    Now, I'm not really good with electronics but is it possible that it is caused by the fault of the start capacitor you mention in the "capacitor start capacitor run engine" segment of the video?

  13. Hi Mathias, I have a doubt about this kind of motor. Some of this motors works with high speed and low speed. They used it in some models of old washing machines. How it works? May be it has two main windings like dahlander three phase motors, or not?

  14. You answered a question I had about my old GE reversible boat davit motors. One of them wont start so i have to give its large pulley a shove to get it going. I plan to first replace the start capacitor. If that doesnt fix it I'll assume its the connections to the start winding or the winding itself. Thanks!

  15. Great explanation of the why! I have known the how for 40 years but have never known the why. Thanks for that. It never ceases to amaze me how giant corporations will step over a dollar to save a dime. It would cost next to nothing to add 1 wire pigtail to that motor and make it exactly twice as useful having a dual rotation. I really enjoy all of your videos.

  16. Thank you so much for making this very informative and easy to understand (if you're versed in electronics) video.

    I was looking to see if I could reverse the rotation on my drill press and found this video and it had every piece of information I was looking for. Thanks for taking the time to put it together.

  17. this worked for my rc helicopter. it's very appreciated that people are so willing to share such pertinent information to fix some things. Thank you for making this video

  18. Very Informative and explained in very clear terms. You present the information at just the right speed keeping my complete attention. Cant stand people who speak way too slow wasting time.

  19. Thank you Matthias, very useful. I like your videos because of the decent quality of sound and vision as well as your clear explanations and diction. Keep them coming!

  20. I have been doing research on making motors spin different directions and your video is the best! I have Car lift from Atlas that was purchased in US with single phase 230v motor and the motor is spinning in wrong direction. I had changed polarity according to the wiring diagram and it does the same. There are 2 metal strips that has to be changed from vertical to horizontal position and it should make it spin opposite, but it does not. Also swapping 2 positive leads did not do any different. I had another new replacement motor installed and it does the same. Atlas customer service representative suggested the motor winding being defective/installed wrong from the factory. Got new motor installed and it does the same. Any tips what could be wrong with it?

  21. Why does a squirrel cage air conditioner motor generate more power spinning it clockwise, but less power counter clockwise?

  22. I saw you changed the motor speed by changing the voltage. Does that motor have capacitor in it? In that way, does the motor get overheated?

  23. Thumbs up with the pendulum example. Note: you left out the other side of the coils in your first 2 diagrams, nice video.

  24. do you have the make and model # of the 4 pole split phase motor (small one) so i can replicate? what did it come out of? thanks.

  25. hi, got a Grizzly g1022 motor it says in the manual I need to switch wire number 5 and 8 but can't figure out which are those wires. thanks in advance

  26. This is cool. IF I can reverse the motor in my old swamp cooler, I can have a whole house fan and suck out the hot air when it cools down.

  27. Matthias, I was able to reverse my non-reversible motor (an A.O. Smith, 1/8 horse power) without altering the coils. I simply pulled off the casing ends and flipped the rotor end-for-end. After this, the motor ran in the opposite direction. It may not be possible to do this with all non-reversing motors, but on this one it worked perfectly! The motor powered a heavy-duty orbital buffer that shook so violently that it was nearly impossible to use. I disassembled it, removing the orbital counterweight unit and bolted the buffer disc holder directly to the motor shaft. I thought I had to reverse the motor, which it turned out that I did not have to do, however, curiosity got the better of me and I had to test my theory: that I could just turn the rotor end-for-end to get reverse rotation. It works and was easy to do on this motor. Thanks for your educational videos! Will

  28. OMG!!!… You've just explained so much in such a short amount of time in three videos, why can't schools do this? I always got the DC bit, forward/reverse/speed, 3 phase I'll never use domestically, so not a big deal, but at least I understand the theory now, but it was always single phase AC motors I didn't quite get because of the way AC alternates, I 'assumed' the AC capacitor was there for a 'kick' but had never bothered to go any further into it because I didn't think I'd ever understand it, but Woohoo!!!… Single phase AC, swapping the capacitor round, changes the rotation, so simple!!!… Why did they never bother to tell me that at school!… A video is worth a entire book!!!… Thank you!!!

  29. This is a really good video have had training on motors in college but it's been a while since I touched one and your video was a great refresher

  30. Matthias, on the the motor you said was not easy to reverse, would it work if you tied a pice of rope on shaft pulled it like starting a lawn mower, then engaged power ? All the best, great video by the way !

  31. That's the first time I actually understood the explanation on the start/run windings and circuit of an induction AC motor.   Thank you !

  32. This is super informative and well presented. Thank you! Now I just need to get my hands on some induction motor…but I don't see any furnace motor on craigslist =(.

  33. Hi Matthias, I have 3 phase motor which I got working using single phase power using capacitor, however when the motor is under load, it stops. When I look at this video, you have two capacitors, one that starts and one that stays engaged while the motor is running. What kind of second capacitor to use as I'm afraid it'll burn out when the motor is under load continuously?

  34. I have a Delta Rockwell 12 14 table saw. Is there an easy way to determine if the motor is 3 phase or not? I believe these saws originally were set up for 3 phase but I'm not positive the existing motor is original.

  35. Hello,I've come across your video, and it's just amazing the ability to communicate simply … Thank you! … I have a slight problem that I have no idea how to solve it …I have an asynchronous one-phase asynchronous motor with four wires on one side – red and orange and red and orange on the other. my question is,How to place them on four pins? …So that I would connect the source of the voltage and the phase.best regardsitamar

  36. As you said, the start winding uses a thinner copper wire than the run winding. So if you throw an ohm meter across the windings the Start winding will be somewhere about twice the resistance of the run winding.

  37. Matthias – Thanks for a great video and excellent explanations too. I have been reading some of the comments. Interestingly ALL the comments are sensible and appropriate. Very often when someone does a good video, people spoil it with their irrelevant and irreverent comments which is annoying. Kudos to you, and keep up your good work.

  38. Nice explanation….. kept it up. thanks

    I have an question regarding 1ph motor with 4 wire connection which can be run reverse and fwd direction. However, this motor voltage through SCR control card. The motor has 3 wires connected to motor. During running of motor i have measured the voltage common to one terminal has 125VAC, and common to another terminal 100VAC in CW direction. For CCW change over the voltage of terminal 100VAC instead of 125VAC and 125VAC instead of 100VAC. and running well with existing motor.
    But the problem is after install new motor (spare) the system control voltage fluctuating 15V, when running in both direction.
    Could you pls explain what cause u can suspect.
    Thank you..

  39. I have 110v motor and I would like to change rotation , it has 2 speeds but I cannot seem to find the answer.

  40. I have a 1/3 attic old fan motor that only that only spin when you spin the blade .The it will run really good.But when you turn it off with then on again the switch it will just buzzes and doesn't spin at all until you make it spin.I tried oiling the motor but it is still doing the same thing.So is my 1/3 attic motor gone bad ?

  41. What would happen If I connect the phase to both sides of the capacitor at the same time ?
    Let's say I connect with relays, and one of the relays gets stuck.

    I see that the capacitor is basically shorted, so it would do nothing, but what about the windings ?
    Thank you

  42. I'm having an issue with a compressor motor tripping a breaker. Would you be willing to give me some tips based on my current findings?

  43. What if I drill hole through the outer casing bring the three wires out extend them and bring them back in on the what is essentially the front of the motor then switch the front and back relocating the wiring of the whining?

  44. people don't want that they want to know which ones don't for stator and rotor pedal systems so you don't crash! make an ebike with 120 rotor and stator and show which one to use and you will get millions of views from poor countries like africa and thailand.

  45. Hello, I have an old meat grinder (H.P. 1/4, Current: AC, Volts: 110/200, Cycles: 60, Phase: 1, AMP: 8/4) that is spinning right, but we need for it to spin left. WITHOUT a centrifugal switch, starter contact with plastic that it sits on and a capacitor how can we make the grinder spin left?

  46. What a pity part of the video was full of de-interlace/field order errors. Apart from that thanks for the excellent follow along style of presentation as opposed the usual terrifying maths kind.

  47. Old Matthias is a pretty smart guy, I have an old Logan Lathe w/reversible AC 110v 1 phase motor 1/2 HP which just burned out.  Not sure how to fix it except taking it to a motor shop or replacement on Ebay hopefully.  Lots of wires, qty 4 going into the motor

  48. Good. I knew all that. I'm heading out to look at a table saw which I'm told two other guys tried and failed to reverse the direction. No history but I am guessing someone bought the wrong motor. It's got two new capacitors. Lol. Your awesome and clear is a great brush up and will save time and make me look like i know what im doing. Thanks. Rich

  49. After all that crap and not actually see the motor change directions!?!? Why waste both our times?!?!

  50. So if your start winding is inducing a voltage yet is disconnected, could you draw that current for some purpose? Given that a circuit were built to allow switching.

  51. I rewatch your videos on motors every once in a while and I take in a little bit more each time.
    Good stuff to know, I sometimes find myself repairing swamp coolers and air conditioners, and these videos have helped me out with it twice now.

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