Why coil ohm don't match measurement?

[sagaboy]

I have a few coil with ohm specified imprinted on the outside but when I measure the resistance with an ohm meter--they don't match. Is that normal?
A 3 Ohm coil when measured with ohm meter turn out to be 4.5 ohm
another 3 ohm coil measured to be 5 ohm
an another 3 ohm coil measured 4 ohm instead

[Dale M.]

I have a few coil with ohm specified imprinted on the outside but when I measure the resistance with an ohm meter--they don't match. Is that normal?
A 3 Ohm coil when measured with ohm meter turn out to be 4.5 ohm
another 3 ohm coil measured to be 5 ohm
an another 3 ohm coil measured 4 ohm instead

Meter accuracy plays a big role in all this.... Cheap meter will only get you close....

Dale

[sagaboy]

I used a digital Ohm meter and the coil resistance measured matched current consumption when one end of the coil is hooked-up to a battery terminal and the other end in series with an amp meter. Two of the coil are Brazil made Bosch Blue coil, I guess we can't depend on what's specified on the box!

[E_bug]

Check your multimeter,

Connect the two leads and read out the resistance, should be close to zero.



If you can get it calibrated do so,.. If its a fluke you can send it for calibration to your local fluke authorized repairer.

[Marc]

The total impedance of a coil is the sum of the resistive component (the resistance the primary wiinding wire would have if laid out straight, plus that of any internal ballast resistance present) and the inductive reactance component, which is dependent upon the applied frequency (RPM in this instance) and is 90° out of phase - it's not basic math.
Without knowing if the coil manufacturer rates their product by resistance or impedance (and the assumed frequency if the latter) one cannot draw meaningful conclusions from static ohmmeter readings unless there's a gross difference. I'd say anything within ±⅓ of the marked number is probably fine. Significantly lower would indicate that some adjacent windings have shorted together. High resistance is not likely to be encountered, other than ∞ from an open circuit or degradation of the ballast resistor (if present).

[sagaboy]

The total impedance of a coil is the sum of the resistive component (the resistance the primary wiinding wire would have if laid out straight, plus that of any internal ballast resistance present) and the inductive reactance component, which is dependent upon the applied frequency (RPM in this instance) and is 90° out of phase - it's not basic math.
Without knowing if the coil manufacturer rates their product by resistance or impedance (and the assumed frequency if the latter) one cannot draw meaningful conclusions from static ohmmeter readings unless there's a gross difference. I'd say anything within ±⅓ of the marked number is probably fine. Significantly lower would indicate that some adjacent windings have shorted together. High resistance is not likely to be encountered, other than ∞ from an open circuit or degradation of the ballast resistor (if present).

Marc,
Thank you. I get understand now! -- I recalled what I have learned 40 years ago

[sagaboy]

I have a few coil with ohm specified imprinted on the outside but when I measure the resistance with an ohm meter--they don't match. Is that normal?
A 3 Ohm coil when measured with ohm meter turn out to be 4.5 ohm
another 3 ohm coil measured to be 5 ohm
an another 3 ohm coil measured 4 ohm instead

Upon checking the packing box, it state the internal ballast resistor is 3 ohm and does not includes the coil impedance (the resistance of the coil wire) which is 1.5 ohm making it a total of 4.5 ohm. So, it is normal
Of course, the impedance varies from one brand to another--depending on the length of the coil/copper being wound inside the coil.