Ohm Meter and Measuring

I am trying to measure the resistance of the coils in a motor with an ohm meter. I was told that some of the new meters measure resistance by means of a time constant calculation while older meters send a current and measure the voltage drop using the equation V=IR. I was told by my professor to make sure that I use an older meter that does not use the time constant calculation as the inductance of the coils will throw off the reading.

How can I tell which way the multi-meter measures resistance?

Thank You!

I was told that some of the new meters measure resistance by means of a time constant calculation

That’s news to me! IMO, any good meter should be able to measure the DC resistance of a coil or the infinite resistance (or leakage resistance) of a capacitor.

How can I tell which way the multi-meter measure resistance?

Maybe connect the meter to an oscilloscope while measuring resistance?

You could connect a small battery, measure voltage & current, and do the calculation yourself. But, maybe stick a resistor in series because even a small battery can put-out a lot of current and you wouldn’t want to blow the fuse in your meter.

DVDdoug: That's news to me! IMO, any good meter should be able to measure the DC resistance of a coil or the infinite resistance (or leakage resistance) of a capacitor.

Maybe connect the meter to an oscilloscope while measuring resistance?

You could connect a small battery, measure voltage & current, and do the calculation yourself. But, maybe stick a resistor in series because even a small battery can put-out a lot of current and you wouldn't want to blow the fuse in your meter.

Thanks for the reply. I will see what I can do for both of those suggestions.

Helicopter12: How can I tell which way the multi-meter measures resistance?

Thank You!

The answer to that is to learn about methods of measuring resistance. What kind of subjects or topics are you studying right now? Electronics?

General multimeters don't cater for transient response measurements like time constants. And to get resistance from time constants would likely require inductance L to be extracted.

If you have 1 coil... just have the coil de-energised... ie. disconnected from power. And use a multimeter to measure the DC coil resistance.

Helicopter12: I am trying to measure the resistance of the coils in a motor with an ohm meter. I was told that some of the new meters measure resistance by means of a time constant calculation while older meters send a current and measure the voltage drop using the equation V=IR. I was told by my professor to make sure that I use an older meter that does not use the time constant calculation as the inductance of the coils will throw off the reading.

How can I tell which way the multi-meter measures resistance?

Thank You!

Never heard of that, sounds completely bogus to me - you can't distinguish resistor from capacitor unless you use strictly DC. Anyway you can measure a multimeter with another multimeter - set one to ohms and the other to volts. Standard ohms measurements will (after auto-ranging settles down) show a constant voltage (no AC component).

Hi,
Some times I have had problems measuring DC resistance of an inductor with a DMM if it is in AUTO range.

When you connect the leads and the DMM changes down to lower ranges, the change in current through the inductor, as ranges change, can cause back EMF that fools the DMM and it keeps changing ranges.

Even FLUKE meters.

So if that happens, just lock the DMM in low ohms.

Tom… :slight_smile:

Helicopter12:
I am trying to measure the resistance of the coils in a motor with an ohm meter. I was told that some of the new meters measure resistance by means of a time constant calculation while older meters send a current and measure the voltage drop using the equation V=IR. I was told by my professor to make sure that I use an older meter that does not use the time constant calculation as the inductance of the coils will throw off the reading.

How can I tell which way the multi-meter measures resistance?

Thank You!

This sounds like a question from your prof to investigate how meters work. :slight_smile:

Modern DMM are very convenient but used for some measurments may give erroneous results if used without knowlege of basic principles.

DVDdoug:
Maybe connect the meter to an oscilloscope while measuring resistance?

Most DMM have an input impedence of 10 Mohm same as most scopes.
Depending what you are measuring that will give a rather erroneous result.

He may be referring to an impedance measurement as against a resistance measurement. Any inductance or capacitance in a circuit will affect the measured impedance depending upon the frequency of test/operation.

Helicopter12: I use an older meter that does not use the time constant calculation as the inductance of the coils will throw off the reading.

Thank You!

Modern DMM use digital sampling which introduces a time constant .older types such as Coil meters do not, perhaps that is what he is referring to.

Thanks for all of the suggestions guys I will play around with them in the lab this week and get back to you guys with my findings.

I am studying electrical engineering right now and more specifically I am in a control theory class where we are looking to find the motor parameters: armature resistance, armature inductance, torque constant, back emf constant, motor intertie (with and without the inertial disk), motor viscous friction (with and without the inertial disk)

Boardburner2: Modern DMM use digital sampling which introduces a time constant .older types such as Coil meters do not, perhaps that is what he is referring to.

Yes this is what i was after.. I wanted to know how you could tell which meter uses what method

analog vs digital. If it has a lcd, it's digital.

Boardburner2: Most DMM have an input impedence of 10 Mohm same as most scopes. Depending what you are measuring that will give a rather erroneous result.

Scopes have a 1M input impedance as standard. Its the 10x probe that has 10M impedance.

MarkT: Scopes have a 1M input impedance as standard. Its the 10x probe that has 10M impedance.

True but i cannot remember the last time i used a scope without a x10 probe on it.

From memory the reason given was to protect the scope input from damage although most scopes are good for 400V i think..

The reason is to reduce the capacitance/impedance seen by the circuit being probed normally - or to allow sensing of higher voltages. Most 10x probes can easily be switched to 1x without noticing so I would not trust them as high voltage protection devices really.

Modern DMM use digital sampling which introduces a time constant .older types such as Coil meters do not, perhaps that is what he is referring to.

I suppose thats it, the use of dual-slope integration ADC. Many newer DMM's have faster bargraph displays to allow better recognition of rapidly fluctuating signals.