# how to turn a universal motor into a generator

Discussion in 'Electronic Basics' started by gearhead, Jan 10, 2008.

I took apart an old vacuum cleaner with 12 amp motor, made for U.S. 60
Hz AC household use so I could experiment with turning the universal
motor into a generator. I know it's a universal motor because
applying 12 volts from a battery will get the motor to turn, same
direction regardless of battery polarity. I wanted to experiment with
it as a generator, so I used an electric hand drill to turn the
motor's shaft (quarter inch shaft fit right into the chuck) and
connected a car taillight bulb to the blades on the plug. Spinning
the motor didn't light up the bulb, so I touched a twelve volt battery
to it to get some juice going and set up a field in the motor/
generator. The bulb went out as soon as I took the battery away, so
that was no good. I tried this turning the shaft in both directions,
and with a much smaller bulb that draws less than 100 mA at twelve
volts.
Perhaps the hand drill is just too slow.

First question: Would this experiment likely work if I turn the motor
several thousand rpm?
Second question: Which direction should I turn it?

2. ### defaultGuest

You are going about it all wrong. In order to generate electricity
(and universal motors do make good generators) you need coils of wire
moving through magnetic fields.

The universal motor has a winding in the stator to produce the
magnetic field the rotor-armature needs to turn - and to generate
electricity.

If you disconnect the field on your motor and supply a small amount of
DC to it (a 1.5 volt battery may be enough) and then spin the rotor
(any direction - direction effects polarity not generation) and take
energy off the brushes you should see the light glow.

Beyond that it is a matter of getting the field strength up for more
voltage - the wire size in the armature (rotor) determines the current
output - heavy wire more current - but less wire fits in the space so
less turns and lower voltage - a compromise has to be reached.

There is such a thing as "residual magnetism" that can start a
generator working just by spinning it - a vacuum cleaner is usually a
series wound motor (field in series with the armature) shunt motors
(field put in parallel with the armature) work better for self
starting - the residual magnetism generates a small voltage then it
powers the field and produces more. (better if the load kicks in
after the voltage is up)

Sounds like a fun experiment though.

Universal motors always turn in one direction - in order to reverse
the direction the field must reverse >>> with respect to the armature.

Ditto generation polarity.

Build some holders to put permanent magnets in place of the field
poles and you wouldn't need to "excite" the field to produce
electricity.
--

3. ### defaultGuest

That doesn't make a lot of sense. Sorry.

What I meant was that when you unplug it and just try to use it as a
generator the stator has no power and produces no magnetic field so
generates no voltage. That's where you are.

In truth it does produce tiny amounts of electricity but don't expect
much. Series wound motors can turn tens of thousands of RPM, your
electric drill can turn hundreds of RPM.

Disconnect the field from the armature. Apply DC to the field. Take
power off the brushes (armature) and you have a generator from a
vacuum cleaner motor.

Some (especially very large ones - think old fashioned elevator
motors) will actually self destruct due to over revolutions and
centrifugal force - keep that in mind when playing around with series
motors that have no load and are being used as motors - doesn't apply
to generators.
--

I opened the motor and found out it's not wired like I thought. I
expected it to be wired like a 1950's Chevy generator with the field
tied to ground (instead of going through a voltage regulator):

,---------+----------
| |
| )
) F )
) )
) |
A ) |
) )
) F )
| )
| |
'---------+----------
In which case, once you get a current going, the armature feeds the
field. But that's not how the vac motor is wired. It's wired like
this:
F A F
line----))))------))))))-------))))-------line

So I have to rewire it if I want a self-exciting generator.

The other issue is the core. The old time DC generators used soft
iron pole shoes, which hold enough magnetism to get things going when
the generator spins up. This vacuum motor has laminations, like a
transformer. I suppose it has to, because it's designed to run on AC.

If I can get this thing rewired I'll post back with the results.

5. ### Tim WescottGuest

It's a universal motor, which means it's series-wound. It'll generate a
torque that's proportional to the square of the applied current, so if
you could come up with a load that would apply a regulated _current_,
then turn the motor against the torque, it'll generate power from you.

The load would have to be somewhere between oddball and bizarre, although
you could undoubtedly make a switching converter that would take
universal motor on one side and deliver DC power on the other, at least
after you got it running.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html