Measuring armature resistance

CircuitZord

Joined Oct 8, 2012
59
I am aware of a number of methods of measuring the armature resistance of a DC motor including locking the rotor, giving a fraction of the rated voltage, measuring the current and calculating the resistance that way.

I have access to two reasonably accurate multimeters at the moment at least 0.1 ohm accuracy which I believe should be OK for my purposes.

The DC motor has some vent holes through which I can clearly see four areas of windings, I believe it's a 4 pole motor, and when I use two probes and put it between two windings I get a reading of approximately 0.1 to 0.2 ohms. Each measurement I do across two adjacent windings gives me the same result. Does this mean the combined series resistance of the winding is just four times the amount? 0.2 x 4 = 0.8 ohms?

The hand-held Agilent multimeter I'm using is only accurate to 0.1 ohm and sometimes it flicks between 0.1 and 0.2, so in reality it's probably more like 0.15 x 4 = 0.6 ohms.

One thing that confuses me though is that a Tektronix digital precision meter which gives a reading up to several more decimal places gives a reading of 1.8xxx ohms. I do not understand why one meter would give a reading of 0.2 ohms and the other one give 1.8 ohms. They are both operating correctly.

Any insight is greatly appreciated.

Here are the links to the datasheets of the two multimeters I've used, I suspect this sub-1ohm range of measurement is beyond the capability of the Tektronix, right?

http://www.tek.com/datasheet/dmm4050-4040-digital-multimeter

KEY PERFORMANCE SPECIFICATIONS
• 6.5 Digit Resolution
• Basic VDC Accuracy of up to 0.0024% (1 yr.)
• 100 mV to 1000 V Voltage Range, with up to 100 nV Resolution
• 100 μA to 10 A Current Range, with up to 100 pA Resolution
• 10 Ω to 1 GΩ Range, with up to 10 μΩ Resolution
So this means even though it has a very fine resolution, under 10 ohms readings could be inaccurate?

AGILENT U1240
http://www.farnell.com/datasheets/1788148.pdf

Resistance: 0.1 ohm to 100Mohm

So this is more suitable? Which one should I trust?

Joined Jan 15, 2015
6,061
If you want to measure armature or stator windings the type meter generally used is a DLRO (Digital Low Resistance Ohmmeter). A Google of "DLRO Meter" will bring up some very good information. Low resistance measurements are generally done using a 4 wire measurement process using Kelvin Leads (4 wire leads). The idea is we pass a known very accurate current through a resistance and measure the voltage drop across the resistance. For example if I pass a well regulated stable 10mA through a .5 Ohm resistor I actually have .010A * 0.5 Ohm = .005 Volts from there it is just a matter of scaling the meter. The meter is actually reading volts but scaled to display Ohms.

For redundant task, like testing armatures on a motor assembly line you can buy a turn key solution DLRO or fabricate your own depending on the resistances you plan to measure. Obviously there are limitations here. Using for example 10 Amps of current to measure a 0.5 Ohm armature rated for 5 Amps will lead to problems as the wire begins to cook and the resistance increases due to heating effect, there is that acrid odor of burning resin in the armature and things get ugly. So current ranges are selected to afford good resolution and an accurate stable measurement. General handheld and bench type DMMs while they will measure low resistance, are not really designed for low resistance measurements.

If you look at the data sheet for the Agilent U1240 page 2 they cover the test currents used for the 0.1 to 1,000 Ohm range and it's 0.5 mA. A good DLRO will have a 1.9999 Ohm range and use a test current of around 10 to 100 mA.

Ron

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CircuitZord

Joined Oct 8, 2012
59

Now that you have mentioned this "4 wire measurement" I remember seeing that function on the Tektronix. I have looked through their datasheet and the technique is mentioned in section 4-9:

http://www.afc-ingenieros.com/uploads/Afc/Productos/Tektronix/Multimetro/pdf/DMM4040-DMM4050 Manuales/DMM4040-DMM4050 Manual del usuario 077036100.pdf

Unfortunately, I do not think I can do this properly as I just wouldn't be able to get 4 probes into the motor ventilation holes and dismantling is not an option, and the range appears to be 10 ohms anyway so maybe it won't be too accurate. My question earlier still remains however, given that I have 4 poles in this motor and 4 windings, can I simply add the resistances of each winding to get the combined resistance?

Or is 0.2 ohms I measured simply the resistance of the entire winding (which is what I suspect it is).

Thanks!

Kermit2

Joined Feb 5, 2010
4,162
if it is not an induction motor you must consider the variation of resistance due to the brushes. The four windings are most likely paired and could be series or parallel. Starting current is almost 100% due to DC resistance and you can decipher most of your question by knowing what the current measures at startup

alfacliff

Joined Dec 13, 2013
2,458
the armature windings are almost always paralelled. you would have to disconnect them for a good measurement. also, you didnt mention if its a series or shunt wound motor, a shunt wound motor has the field in paralell with the armature, also throwing off measurements till disconnected.

Joined Jan 15, 2015
6,061
This would go much better with a drawing or schematic of the actual armature windings. Much, much better.

Ron

Joined Jul 18, 2013
23,619
I am aware of a number of methods of measuring the armature resistance of a DC motor including locking the rotor, giving a fraction of the rated voltage, measuring the current and calculating the resistance that way.

Which one should I trust?
This is considered the only practical way to get a true reading, using high impedance ohm meters is often not reliable.
Max.