# question and need help on 9-18V DC motor

Discussion in 'The Projects Forum' started by KevinisSaint, Apr 18, 2012.

1. ### KevinisSaint Thread Starter New Member

Apr 18, 2012
1
0
Hi

I bought 9-18V range DC motor ---> this one http://www.amazon.co.uk/Super-Speed-9-18VDC-Hobby-Motor/dp/B0014BT0C8

and I'm trying to find out the moment of inertial of the rotor (J), damping ratio of the mechanical system (b),
electromotive force constant (K=Ke=Kt), electric resistance (R), and electric inductance (L) of this dc motor.

All I know is that this dc motor has max of 1.98A, 9-18V range with maximum RPM of 18000. The max rpm of this when 12V is applied
is around 15047 rpm. The speed of this dc motor has 24000 rpm (which is 400 rev/s) when there is no load.

from this information, how do I find obtain values of J, b, K, R, and L for this 9-12V dc motor?

I looked online for something similar but it didn't help me. I'm trying to obtain these values. Are all DC motors
have different values?

I would really appreciate the help.

2. ### kkazem Active Member

Jul 23, 2009
160
31
You forgot the back-emf constant. Well, if you have the motor, you can measure the DC resistance. Using a capacitor and a sine-wave signal generator, you can make a series LC ckt and find it's resonant frequency and from there, easily compute the inductance. Here's a formula, but you need to rearrange it to solve for L: Fo=1/2*pi*SQRT(LC). For the torque constant, you need to install a torque driver or torque wrench and keep the handle fixed. Apply enough DC current to get a mid-range to upper-range reading on your torque wrench and divide the torque by the current. You may need to convert the American inch-pounds or foot-pounds to Newton-Meters or something else to get it in the units you need. For the damping ratio, that requires measuring the friction when moving, not when starting (that's called stiction friction) and make a calculation I'd have to look up, but I seem to recall that a 2nd order equation is involved. There are ways to measure the inertia (I assume the mass moment of inertia), I recommend consulting a good university physics book and you should be on your way.
But, the best thing to do is to contact the manufacturer and ask for a datasheet on that motor. Assuming the mfg's name is on the motor.
Also, for the damping ratio of the system, it depends not just on the motor, but also on the load you connect to the motor and you'll have to model that. The cheapest way is to use one of the free electronic circuit simulators called SPICE, like the free Linear Technology LTSPICE or similar. The electrical equations are exactly the same as the mechanical equations, all you have to do is know what to xchange for what. For example, mass would be a capacitor, speed would be equivalent to voltage, and so on. Or, you could buy a mechanical modelling and analysis software program, many have free trials for 30 or more days.

Good luck,
Kamran Kazem, V.P., CTO
Magnetic Design Labs, Inc.