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
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?
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
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?