A solenoid coil is crucial for operating a solenoid valve. This essentially converts an electric current into a magnetic field, thereby, enabling operations of the solenoid valve. Today’s feature on #LinkedInsights analyzes the calculation and measurement of this coil through some inputs from industry professionals.

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The life of a solenoid depends on temperature, duty cycle and the temperature rise and class of insulation being used. While at work, an Instrument Engineer from ONGC Petro Additions Limited, India, was stuck checking the solenoid coil. Therefore, he was wondering what the resistance should be for a 110VDC coil?

In answer to this, a Marketing Product Manager from Emerson Automation Solutions, Netherlands, offered his thoughts. “You can calculate the resistance of a DC coil if you know the Nominal Voltage (U), Wattage(P) and/or Current(I) and Voltage(U). Maybe the issue is arising due to the DC switch-off peak suppression diodes integrated into (e.g. our ASCO) DC solenoids.”

He further suggested that calculating and measuring the resistance of a DC coil would be easy, but also pointed out that with these diodes, it would be different. “In this case, the current measurement applying the nominal voltage, e.g. using a multimeter, would be the best solution,” he added.

### Coil Measurement

Getting into the details of the coil measurement, a Senior Electrical Engineer at PneumaticScaleAngelus from Florida, USA has the following to say. “From a go/no-go standpoint, it should read low: Something like 0-100 ohms or so, perhaps a little less or more. If it’s in the k-ohm range, it is most likely not good. The coil is just a big spool of wire. If it reads really low, say 1-2 ohm, it’s probably shorted. Tiny relay coils will read less than larger solenoid coils. You can spend your time calculating it, but if it’s to ascertain whether it’s good or not, then this is the way to go.”

For assessing the coil, having a deeper understanding of the physical performance of the coil is also necessary. Highlighting this, an Electrical & Instrumentation Engineering from Offshore Hook-up Construction Services, India stated that the continuity of the coil needs to be checked primarily. Next comes the coil resistance.

“For example, of the coil has some internal problem, then one should physically check the heating condition, too. This way, you can ensure there is no sound from the coil because of an energy gap.”

### Coil Resistance

Speaking further on the specifications of the coil, an Instrumentation Consultant and Teacher from Brazil explained further. Importantly, if it is fed by DC, one should closely monitor specs for Coil Current by using the use Ohms law (V/I=R) to determine coil resistance.

“In other cases, if the data you have available is Power (watts) then you need to divide Watts per Voltage, that is Current in Amps, Back to Ohms law and find out the resistance in Ohm,” he added.

### Using a Megger

A Supervisor of Instrumentation from Phoenix Calibration, Venezuela spoke about testing and measuring a solenoid coil. In his opinion, the easiest way to test or measure the coil of a solenoid is with a megger. “Placing the megger on the scale of 500 V. Then, you can place the two ends of the megger on two wires of the coil and then hit the test button.”

Also, he stated that the measurement of the megger should be above 1.2 megaohms. “If the measurement is below this indication, this means that the coil has moisture. Another test is with one of the wires from the coil to the megger and the other from the megger to ground. In this case, the reading of the megger must be infinite. Do this to verify that the coil is not shorted and that the insulation of the coil is in good condition. As such, the test must be done on both the solenoid coil cables,” he advised.

### Conclusion

Concluding the discussion was a Consultant at Power Consult, Saudi Arabia. He shared, “Whenever you measure the voltage across any coil, it will be impedance. It is a combination of resistance and inductance. You can consider the power rating of the coil, which will be displayed on the nameplate and divided by voltage that is the current required for energizing the coil.”