coil ohms

[paul renshaw]

We have a discussion at work about this. A coil on a starter reads 2.0 ohms, it is a 120v coil. How much current will it draw, and why? We have checked two coils in two starters(same size) and each reads about 2 ohms.

 

[Ed MacLaren]

Re: coil ohms

If you are getting a 2 ohm reading with a basic ohmmeter, it is just the DC resistance.
When energised from a 120 volt AC supply, there is also considerable inductive reactance.
Both combined, the impedance, determines the current drawn by the coil.

Ed

 

[dereckbc]

Re: coil ohms

I agree with Ed, then only thing a ohm meter is going to tell you is verify continuity of the coil. The AC impedance will be higher than the DC resistance.

A ohm meter is not a very good test instrument for checking motor windings. A DVM is not very accurate reading below 1 or two ohms. A DLRO is required to get a reading then compare to what the manufacture states what it should be.

[ April 01, 2003, 12:47 PM: Message edited by: dereckbc ]

 

[charlie b]

Re: coil ohms

Taking your information at face value, the current would be 120/2, or 60 amps. That is unreasonably high for a starter coil. The actual current would be much lower, for the reasons given by Ed and Dereck.

 

[bennie]

Re: coil ohms

This 2 ohms show a high inrush, or closing current present.

The holding current is greatly reduced when the magnetic field is closed.

Prevent the armature from closing the air gap and there will be intense smoke. Bad smell

When designing a control circuit, the inrush and holding current must be factored in the calculations.

 

[paul renshaw]

Re: coil ohms

I agree with ED also.(and your smoke theory Bennie) I wanted to show my co-workers(they do not always believe me) that just Ohms law will not always work in all situations. Thanks for the replies.

 

[charlie b]

Re: coil ohms

But "Just Ohm's Law" DOES always work, and in all situations! Voltage will always equal current times resistance.

The tricky part, however, is that voltage, current, and resistance can vary from instant to instant, can be difficult to measure, and can require extraordinarily complicated mathematical expressions to define. Ohm?s Law will work, but you will get a wrong answer if you use wrong input information. ?Just measuring resistance with an ohmmeter? is where you went wrong on this one.

 

[jmc]

Re: coil ohms

Hi Paul, try taking an ampere reading of the coil while energized. Measure the voltage. The "Z" impedance of the coil can be calculated by Ohms law. Z = volts divided by current. To determine the inductive reactance, subtract the square of the current from the impedance squared and take the square root of the result which will equal the impedance in Ohms.
Regards,
John M. Caloggero

 

[physis]

Re: coil ohms

JMC has it right. This is not a discussion about DC resistance, it's "impedance".

The impedance value of your coil is
the sqaure root of (resistance squared + inductive reactance squared).

You can find the inductive reactance of your coil if you have an LCR bridge to measure it's inductance.

Inductive reactance =
2 x 3.1414 x frequency (60) x inductance.

When you divide your supply voltage by the "impedande" you probably wont get 60 amps.