Obtaining DC motor parameters

You can find the inductance using a resistance box and a storage scope. Are these "specialised"? I found a load of stuff out using Google, including a lot of EE course work.

Thanks for your reply.

Unfortunately that equipment is not available to me. The inductance is not the biggest issue, the torque is more important. I thought maybe someone had a bright idea or an equation or something. I'll look through books and google to find out what to do - I only posted this (relatively) simple question in an attempt to get out of doing that research as I am currently involved in several projects.

I am just a bit unfamiliar with DC motors having only really had experience with ac motors and synchronous machines.

The February issue of "Nuts & Volts" magazine has an article in it that describes how to measure torque of your motors. It is not a trivial task.

for info on how to get a copy. Or you might try your library. Mine has a copy.

Al

On the offchance that this may be useful, I remembered an application note from Linear Technology which described a means of characterising a dc motor for use in a motor speed controller which didn't need a tacho feedback. A little trawling on the Linear Technology website has found:

On page 93 of this PDF tome there is an application note with the title "A tachless motor speed regulator" which has some information, though maybe not exactly what you are looking for (inductance etc) for characterising a motor. Even if not, perhaps this article will still be of interest if you have an interest in driving a dc motor.

On a very different subject, Linear Technology published another application note:

On page 130 they have published (or rather reprinted) a description of Murphy's law as applied to inanimate objects. Since this is an application note from a respectable company it must be true. Well I have often held that it is true when some of my designs haven't quite gone to plan. Sorry for this diversion from your question.

Regards

Bob

------------ The field excitation is not mentioned - if this is a permanent magnet motor then you can say that the back emf =Kw where w is the speed in radians. then torque in Newton meters =KI. You could also drive the motor at a known speed and find the voltage and the constant K. Then all you need is the resistance. You have also not stated whether the speeds given are under load or at no load. The information that you have given is not adequate -partially because you do not specify at what current and voltage you have a particular speed.

Look up Prony brake. You can make one fairly easily with a jar lid, a spring scale and a ribbon. As for resistance. measure the stall current at a known voltage. The stall torque can also be measured at that time with a spring scale.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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Thanks for all the replies, just for interest sake:

The motor is a permanent magent motor. All speeds are quoted under no-load conditions. The higher speed corresponds to the higher voltage. The starting current peaked at around 0.78 A and then settled down to

0.66 A at 9100 rpm.

And thanks for those PDF's, they are going to be an awesome help to me since I was about to post about a cheap alternative to motor speed control.

And at least now I have scientific proof that it is impossible to meet a deadline and likewise impossible to perform a complex calculation flawlessly.