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.