On Mon, 02 Jul 2012 10:01:02 -0400, Scott Dorsey wrote:

Antonio I0JX wrote:
I am not familiar with that particular dynamotor, so I am unable to tell
you which lead is positive and which negative (although it should not be
too difficult to determine it).

Some of them will run properly in reverse, and of course you get reverse
voltage on the output.

In all cases, please be informed that dynamotors typically have a very
high inrush current. If you use a modern stabilized power supply, the
overcurrent protection will surely trip.

Figure if you have a 25A running current that your starting current is
50-75 amps. Not for very long, only for a very short time. Adding a
big capacitance can help if you're stuck using a current-limited supply.

HOWEVER, if you see anything higher than 25A running current under load
and it's rated for that, something is wrong. Before I would fire up an
old dynamotor, I would pull the thing apart, remove all old grease on
bearings, and relube everything with modern lubricants, and I would
check the brushes at the same time.

Old and crusty lube will increase your starting and running current and
eventually damage the bearings and overheat the motor windings.

After problem will be solved, I fear you will anyway want to build an
ad-hoc P.S. replacing the dynamotor. Most often dynamotors are so noisy
one can't stand them!

That's why you put it in the trunk of the car where it belongs. --scott

I'm speaking as a guy who controls motors for a living, not someone who's
ever even touched a Dynamotor. So size your grain of salt appropriately.

First, everything that Scott says about making sure that the Dynamotor is
up to snuff before you start is good advice. I would add to Scott's
advice that while you're checking brushes, you make sure the commutators
are in good shape -- if they're all corroded, then you should be thinking
of rebuilding the thing. Check both the generator side and the motor
side.

I'm not sure about the start-up current only being 3x the run current,
unless that's run current while loaded, and an unloaded start. DC motors
_really_ like to run at a speed determined by their terminal voltage, and
will draw _lots_ of current starting from a stop.

If you have a power supply that gracefully limits the current to 10 or
20% higher than the unloaded run speed or just enough to overcome the
starting bearing friction, whichever is higher, then it'll almost
certainly bring the motor up to speed, albeit more slowly than just
strapping the thing onto +12V (or whatever its run voltage is).

If your power supply does something unsociable in the face of too-high
current, like shutting off, you can try this dodge:

1: measure the terminal resistance of the motor. Do this while rotating
the motor in small increments, stopping, then measuring -- the resistance
will change depending on the brush position on the armature. Choose the
lowest resistance measured for the next step.

2: find the resistance that you can hang off of your supply such that at
rated voltage you don't exceed the maximum supply current. Subtract the
motor armature resistance from step 1. Remember this number.

2a: if you're lazy, just assume 0 ohms of armature resistance in the
motor, and calculate your resistance from the supply's rated voltage and
current.

3: calculate the resistor dissipation (P = I^2 * R). Go shopping for a
resistor. You can probably go about 1/3 as much power as you calculate,
because if the motor starts at all it won't draw that much current for
very long (and if it doesn't start, you know enough to slap the power
switch off, don't you).

4: wire your resistor in series with your dynamotor, with a switch to
short out the resistor at need.

5: switch the switch off -- this is your start position. Turn on your
power supply. If the motor starts spinning -- horay! If it doesn't --
slap that there switch off before you burn up your resistor.

6: when the motor seems to be going as fast as it'll go (or if you have
an ammeter in there, when it seems to settle out to a minimum), switch
the switch on. There should be a current spike (hopefully not enough to
screw up your power supply), then the motor should settle out at a
slightly higher speed.

7a: tell your loved ones not to sue me if you die

7: turn the HV side on

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com