Hello, I need to measure the inductance of a DC motor of about 12V. Which instrument do you recommend? Ideally low cost (below $90) but capable of measuring the inductance accurately. I read that there are LCR meters such as:
M4070 AutoRanging LCR Meter Up to 100H 100mF 20MR
MASTECH MS8260E Digital Multimeter DMM LCR Meter Inductance Capacitance Tester w/hFE Test & LCD Backlight
Peak Atlas LCR45 – LCR and Impedance Meter
DE-5000 Handheld LCR Meter
How are they compared with each other?
Since you haven't stated why you need know the inductance of a D.C. Motor, we're left questing what tool would be suitable. I'm sure all the devices you listed function in one way or another to measure inductance, we just have no idea which device is most suitable...
Can you help us understand what you're trying to accomplish since the inductance of the motor is usually not a concern?
Thanks. I am trying to model the motor in computer simulation to study its dynamics behavior.
You won't be able to trust the simulation, unless you have actually measured the motor's "dynamic behavior" and have something to compare with theory.
If you are doing this as an exercise, use a motor for which you know all the values needed for the simulation.
jremington:
You won't be able to trust the simulation, unless you have actually measured the motor's "dynamic behavior" and have something to compare with theory.If you are doing this as an exercise, use a motor for which you know all the values needed for the simulation.
Unfortunately I am stuck with this motor and I have to measure its inductance.
Then buy or borrow an inductance meter.
jremington:
Then buy or borrow an inductance meter.
Do you have any good recommendation?
They all should work well. Why does it matter what you get for a one-shot experiment?
If you are doing this for a university class, then there should be some available to borrow in various laboratories.
I think what you really want is a map of voltage and current vs load.
A single inductance number won't cut it.
This whole posting is confusing to me. Inductance is a resistance to a varying voltage, AC of some type. Are you going to apply AC to your DC motor? IF not, what frequency do you intend to measure the inductance?
What kind of DC motor? Does it have armature as well as field windings? Does it have an armature?
Paul
@Paul: The OP says he wants to construct a mathematical model for the motor, which is likely part of an assignment in an engineering school Control Theory class.
Part of that model would normally require knowing the winding inductance, which is a constant (and is often specified in motor data sheets). Other required constants would include the rotor moment of inertia, etc.
With an oscilloscope (preferably storage) you can find it by experimentation. Place a capacitor in parallel with the armature (I'm assuming pole pieces are magnets) Lock motor shaft to prevent motor running, place scope probes across the armature. Via a switch, apply say 10% of rated voltage to motor for about a second. Open switch and observe resultant ring oscillation on oscilloscope (it will decay very fast). The frequency of oscillation is a function of Xc, Xl and Xr. You will have to experiment with values of C to get a suitable response. From the observed frequency you can determine Xl and hence L QED.
You couldn't measure it through the leads anyway, the brushes will bridge 2 or more winds, you would have to measure directly on the commutator bars.
The brushes do not 'shorten' out any windings, they connect adjacent windings so windings in parallel would also give an inductance value. The rotor may have to be turned slightly to accommodate both cases but that's what experimentation and thinking is all about.
Edit : I correct myself.
As previous author correctly states the brushes do indeed 'bridge' adjacent commutator segments and hence adjacent windings.
However, they do not short out the remaining windings which form the major portion of any reasonably sized armature, so inductance can be measured at "non-bridged" and "bridged" conditions.
The effect of the bridged turns will be to dampen the oscillation at a much faster rate then the non-bridged state. In fact that effect was used by myself many years ago as a means of detecting for shortened turns when winding transformers.
Thanks. In the case of using the LCR meter, am I right that all I need to do is to connect the two probes from the meter to the two wires/terminals coming out from the DC motor? Do I need to choose a meter that supports a particular frequency range?
Is this a parallel or series wound motor? or does it have a permanent magnet field?
It makes a difference.
Allan
Could probably just inject a small AC voltage with a 1 Ohm resistor (of suitable power handling) in series the motor, and measure the voltage across the resistor and the voltage of the supply on an oscilloscope. Convert these voltages to complex-valued form (to account for phase difference between those two voltages). Then estimate the impedance = Vsource divided by Vresistor. This can be done if the resistor is 1 Ohm. And use a multimeter with cable resistance (calibrated away) to estimate the resistance. What is left over could be the reactance. And knowing the frequency of the AC voltage could allow an inductance to estimated.
Or knowing an equivalent resistance from multimeter measurements, and using a DC supply with switch (and storage oscilloscope) do some step response tests on the DC motor, and observe the step response as a function of time. Estimate the inductance from the time constant under the assumption of a first order series resistor inductor response. I guess you could just try a few of those kinds of things and see how it goes.
bbqq:
Hello, I need to measure the inductance of a DC motor of about 12V. Which instrument do you recommend? Ideally low cost (below $90) but capable of measuring the inductance accurately.
Accuracy is not appropriate, the inductance of a motor only needs to be a rough measurement as is will
vary depending on rotor position and applied frequency and amplitude, and the Q will likely be very low.
Also the DC bias currents in a motor are large and will reduce the dynamic inductance under load.