 ### Network # 3 pole dc motor with commutator?

B

#### BobG

The standard slot car motor was a 3 pole mabuchi motor with two
magnets on the side of the case. The poles were 120 degrees apart. I
assume that every 60 degrees, the next pole was energized and pulled
toward the closest magnet. Now consider replacing the commutator
segments with slip rings... gnd, and coil a,b,c. Q1) If I energize
coil a, does it rotate to the center of only one magnet? Either
magnet? Or to one edge of either magnet? I assume if I energize the
coils in sequence a,b,c the armature will rotate. I can compute the
ampere turns of each coil. Q2)Is the max torque when the force of the
electromagnet equals the force of the permanent magnet? Q3) Is the
strength of the permanent magnet measured in gauss? webers? teslas?
How many gauss in a 1"" magnet like this? And now the last stupid
question... (yaaay!) Q4) How do I convert magnetic strength of the
permanent magnet to newtons so I can calc torque? I think the force of
the electromagnet is NIB, but the permanent magnet doesnt have any N
or I, so I'm confused as usual.

R

#### Rich Grise

The standard slot car motor was a 3 pole mabuchi motor with two
magnets on the side of the case. The poles were 120 degrees apart. I
assume that every 60 degrees, the next pole was energized and pulled
toward the closest magnet. Now consider replacing the commutator
segments with slip rings... gnd, and coil a,b,c. Q1) If I energize
coil a, does it rotate to the center of only one magnet? Either
magnet? Or to one edge of either magnet? I assume if I energize the
coils in sequence a,b,c the armature will rotate. I can compute the
ampere turns of each coil. Q2)Is the max torque when the force of the
electromagnet equals the force of the permanent magnet? Q3) Is the
strength of the permanent magnet measured in gauss? webers? teslas?
How many gauss in a 1"" magnet like this? And now the last stupid
question... (yaaay!) Q4) How do I convert magnetic strength of the
permanent magnet to newtons so I can calc torque? I think the force of
the electromagnet is NIB, but the permanent magnet doesnt have any N
or I, so I'm confused as usual.

If all you want is speed control, just use PWM. What you're suggesting
could only have value as a learning experience; you can already get
brushless DC motors off the shelf.

Cheers!
Rich

B

#### BobG

Hi Rich. I only converted the commutator to slips rings in my head as
a mental what if. I'm trying to figure out how to compute torque so I
can get a good tight spreadsheet on electric vehicle kwhrs per mile
and argue with the ice heads. Let me rephrase my main question....
there is a big honkin magnet in skycraft that is holding a big steel
bolt straight up in the air on a chain. You can pull it away from the
magnet sideways, but aint no way you're gonna pull it away from that
bad boy on axis. Lets say you have to pull 50lbs just for talking
purposes. I figure if I built a motor out of those things, and it had
a 6in radius, There would be 25 lb-ft of torque just from the magnet
attracting the iron pole piece. Now if I put enough NI (ampere turns)
in the coil to equal the B from the permanent magnet, I assume I'll
have 50 lb-ft of torque. If I double the NI in the coil again, if
thermally possible, I dont get any more torque increase, right? I'm
'swamping out' the permanent magnet?. So I want to calc how much
torque I need to accelerate my EV at .3Gs etc.
Anyone want to agree or disagree with this shade tree mechanic
analysis of dc motor torque?

J

#### John Popelish

BobG said:
The standard slot car motor was a 3 pole mabuchi motor with two
magnets on the side of the case. The poles were 120 degrees apart. I
assume that every 60 degrees, the next pole was energized and pulled
toward the closest magnet.
(snip)

All coils are energized at all times, except when a brush
shorts across the gap between two commutator bars. Then the
coil connected between those two sees no voltage. There are
3 coils, one connected across each of the 3 commutator gaps.
As long as the two brushes are in contact with only two
bars, all three coils are energized in one of two
directions, with either the whole supplied voltage across
them, or half the supply (where there are two gaps between
the brush contact points. When a brush shorts a gap, one
coil gets no voltage, and the other two coils get full
voltage (in opposite directions). A brush crosses a gap
every 60 degrees, so you might make a timing diagram with
waveforms representing the voltage across each coil, showing
how every 60 degrees, either coil passes through the shorted
(pick some number of degrees less than 60, say, 30 that it
takes for the short to pass across the gap) situation and
then has its voltage reversed.

So you have 30 degrees with constant voltage on the coils,
one getting full voltage and two getting half voltage in the
opposite direction.

30 degrees with a short across one of the half voltage coils
(show that as zero volts) while the other coil that had half
voltage now gets full voltage.

30 degrees with the just shorted coil getting half voltage,
reversed from what it last had, and the one that first had
full voltage drops to half, also. The coil that popped up
to full voltage in the last step continues to get full voltage.

etc.

B

#### BobG

Thanks John P. I rewound slot car motors back in '68 in hi school, but
I always thought in my simple mind that one needed 6 poles to get
how they really work, but can you help me with my torque question?
That is, is the max torque possible when the pull from the coil
matches the pull from the magnet?

J

#### John Popelish

BobG said:
Thanks John P. I rewound slot car motors back in '68 in hi school, but
I always thought in my simple mind that one needed 6 poles to get
how they really work, but can you help me with my torque question?
That is, is the max torque possible when the pull from the coil
matches the pull from the magnet?

I might not be much help, since I am not very fluent in the
units of magnetism. I have to look them up every time I
have a problem to solve. But I can tell you that the peak
torque from any armature pole (around which a coil is wound)
occurs when that pole is in the crack between a pair of
permanent magnet poles. In that place, it is simultaneously
repelled by one magnet and attracted to the next. This is
also the place it should be in the middle of its maximum
voltage phase (in the diagram I talked about, earlier),
since this is where the peak rate of change of flux through
the coil occurs (assuming the motor is turning), and so,
generates the peak generated EMF the supply must exceed to
keep forcing current through the coil.

One important limitation to torque any permanent magnets can
produce is the field that it takes to demagnetize them. So
if you have high coercivity magnet material, like neodymium
iron boron, you may be able to push the armature flux above
the magnet flux (actually pushing flux a little backwards or
sideways from the permanent field), but if you have a motor
made with low coercivity material (like some ALNICO that
will be partially demagnetized if you simply pull the
armature out of the field), the armature flux may have to
stay well below the permanent flux to preserve the
magnetization.