Test a coil of a hydraulic directional valve
The electric coils for controlling the hydraulic directional valves can be all or nothing or proportional control.
The power supply can be done:
- In direct current (DC).
- Alternating current (AC)
- PWM (Pulse Width Modulation) current - RCO (Cyclic Opening Ratio).
The coils are generally supplied with very low voltage:
- Voltage less than or equal to 50 Volts AC.
- Voltage less than or equal to 120 volts DC.
The intensity that passes through the coils depends on the impedance (resistance) of the winding as well as the supply voltage.
The law of Ohm highlights the relationship between resistance, voltage and intensity. The formula allows you to do the calculations.
U = R x I
U in Volt
R in Ohm
I in Ampere
1- Visual inspection: corrosion at the core, state of the connection terminals.
2- Test the continuity of the winding using a multimeter positioned on Ohmmeter.
The value read varies depending on the coil tested. The bigger the coil, the lower the resistance.
3-Test the insulation between the coil connections and the ground using a voltmeter positioned on Ohmmeter.
The value must be infinite (O.L).
4- Test the supply voltage using a multimeter set to AC Volt or DC Volt to control the power connections. It is imperative to make this test with coil connected.
For PWM-fed coils this test is impossible. Only the intensity test should be performed.
For this test a control box allows to deport the measurement and avoids to disassemble the DIN plug. It is no longer necessary to perform hazardous manipulations with the multimeter's touch points. Thus the risks of courses circuits will be discarded.
The voltage reading must not be less than 3% of the circuit voltage. For example, 11.64 V for a 12V power supply.
If the loss is too important check the connections.
5- Test the power supply using a multimeter set to alternating amperes (AC) or continuous amps (DC).
For this test a control box allows to deport the measurement and to easily carry out the control because the ammeter must be connected in series. Insert the multimeter instead of the shunt.
For the proportional coils the variation of the intensity makes it possible to control the good functioning of the electronic card, computer or automaton.
To note : AC powered windings tend to heat up when energized and disassembled. Put a screwdriver in the coil to avoid this phenomenon and prevent damage.