Disc brakes have their problems for a motor testing dyno, particularly
in testing induction motors. Speed must be maintained very closely
because the torque-speed is so steep -- 0 to rated horsepower in a
range of about 75 RPM or about 4%. The slightest bit of chatter can
really screw things up, even with a newly turned disc and new pads.
We solved that problem on Jerry's dyno by going to eddy current
braking in an aluminum disc. That was very easy to control and it was
more stable than a disc brake because it's more "viscous": drag
increases with increasing speed. A DCPM motor with external
resistive load would have that characteristic as well.

Even an ordinary induction motor with DC on its windings can work
well as a brake, , though it'll get hot if run very long.

On Sat, 04 Sep 2004 16:09:28 -0400, Gary Coffman
wrote:

On Fri, 03 Sep 2004 18:14:07 -0700, Jon Anderson wrote:
I have a customer that among other things, rewinds obsolete air coil
electric motors. He has asked me about building a dyno so he can test
motors to something other than 'yep, it turns' under power.

Looking through SIC I only find one article on building a dyno. It looks
to be a fair bit of work. As he called me while on the road today and
the connection was bad, we couldn't really get into details. I am unsure
what HP and torque the motors develop, though it's not a lot. I also
don't know what rpm range he wants to test.
Did a bit of Googling and didn't find a whole lot on small model engine
sized dyno building.

Anyone know about web resources for different approaches?

I don't have a URL for you, but I can give a verbal description of a
simple brake dyno. It is simply a brake disc which can be directly
coupled to the motor output shaft. A brake caliper is mounted on
a lever which pivots from the shaft and is restrained at the far end
by a spring scale. If the lever is 1 foot, the scale can be read directly
in foot pounds. If longer or shorter, you have to do a bit of simple
math to convert the reading to foot pounds.

To use, simply apply hydraulic pressure to the caliper until the
motor slows to the desired RPM. Read the spring scale. That's
the motor torque at that RPM. RPM times torque equals power.

By braking to a number of different RPMs and taking readings,
you can construct a graph of torque and power versus RPM
just like the big guys with the expensive engine dynos do. If
you add a current shunt to monitor motor current, you can
also plot motor current at various loads.

For motors up to several hp, you can cobble this together from
junk car parts (rotor, caliper, master cylinder) and a household
scale. Note, when braking heavily, the brake will get hot. Don't
fool around, take your reading then let off on brake pressure
and let it spin to cool.

For fractional hp motors, a motorcycle brake, or even a bicycle
caliper brake, can be used instead of an auto brake. A smaller
spring scale can be used too.

Gary