Re: [Electronics_101] Re: Hand mixer speed control

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John Popelish

On 10/08/2011 07:35 PM, laser100 wrote: (snip) ... You are welcome. Universal motors are not hard to understand, in a sort of hand waving way, but the details

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, Oct 9, 2011

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On 10/08/2011 07:35 PM, laser100 wrote:
(snip)
> Thank you so much for your help. I'm not
> sure I understand the different taps on the coil. Does it
> adjust voltage to the motor?

You are welcome.

Universal motors are not hard to understand, in a sort of
hand waving way, but the details get a little tricky.

The most important fact of their operation is that the field
coils and armature are in series, so both sets of poles (the
stationary ones and the rotating ones are energized by the
same current. The interaction between these two sets of
poles is proportional to the strength of each, multiplied
together, which makes it proportional to that single current
to the second power.

The second important fact about universal (or series wound)
motors is that their speed is very poorly regulated and
thus, highly dependent on their load torque. Here is a
little hand waving to explain why.

The speed settles at whatever is required for the armature
to generate almost the full line voltage (since the field
voltage drop is usually a small part of the total line
voltage, except near stall conditions, when the high current
produces large resistive and inductive drops). But that
speed depends inversely on the field magnetic strength. The
load determines the torque required at any given speed (and
most loads require higher torque for higher speed) so the
speed is the intersection of that load speed torque curve
and speed torque of the motor with a given number of field
windings. Lower load torque runs the speed up the motor
curve, where the armature generates lots of voltage, driving
the current down, which weakens the generated voltage which
runs the speed up higher, etc., until load and motor torque
match.

Switching in extra field winding turns increases the field
strength at all currents, which causes the armature to
generate more voltage at every speed, current combination,
so that slower speeds buck the current down than with fewer
field turns. This shifts the motor speed torque to more
torque at all speeds so that it will intersect the given
load curve at a lower speed.

That is a bunch of words trying to substitute for
simultaneous second order equations.

--
Regards,

John Popelish

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