I am trying to actuate a solenoid rated 25.4 V and 7,5 A. I have a table top PSU which can supply 12 A at 30 V. If I set the PSU to 25.4 V limited to 12 A the solenoid only draws 4 A and the measured voltage across the terminals on the PSU is about 17 V. I have measured the voltage with respect to time for the full stoke of the solenoid and it starts off at about 24 V dips as the current rises then spikes again at the full stroke length but only returns to 17 V. Why does this happen?
It sounds to me that the power supply voltage and current regulation is poor. How do you know what the setting of the power supply current limit feature actually means?
From what I gather the PSU should be able to supply 12 A when set to 30V. Or am I missing something?
if you disregard the transient behaviour when you switch it on and off, the current through the solenoid during the time that it is on, will be determined by it's resistance.
Activate the solenoid, and measure the voltage at it's terminals and the current going through it. The ratio of these is the resistance of the solenoid coil. Based on your figures, it would be 17/4 , or about 4.2 ohms. This may not actually relate very well to the claimed voltage and current "rating" of the solenoid.
If you then actually put a 24 V source across the terminals of the solenoid, you would expect to get 24/4.2 amps flowing through it. If you actually put a 30 V source across the terminals of the solenoid, you would expect to get 30/4.2 amps flowing through it.
If this current is acceptable, you can probably drive it with 30 V. The issues with doing this would be heat, how long do you plan to leave it "on" for ? And the ability of the solenoid coil's insulation to withstand 30 V.
Then you have to consider the transient behaviour. When you apply voltage to the solenoid, the current will build up slowly to the maximum value determined by the coil's plain resistance. No problem there. When you switch it off, the current will decline slowly. It won't stop right away. This current has to go somewhere. You have to provide a diode and resistance path for this current to dissipate, otherwise dodgy things can happen.
Your description sounds rather strange. I would be wondering what your power supply is actually doing.
pfrost:
From what I gather the PSU should be able to supply 12 A when set to 30V. Or am I missing something?
Even though the power supply can provide 12 Amps, it won't. The voltage and current are not independent. The current that will flow through the solenoid, or anything else you attach to the power supply, depends upon the impedance of the solenoid.
Your power supply would appear to be failing to correctly regulate for the load applied. If it is a variable output supply with preset current regulation then the voltage should remain at the set voltage up to the preset regulation current. If you then try to draw excess current the voltage will be reduced to limit the current to the set value.
With your solenoid connected it would appear the output voltage is drooping from the 25v down to 17v. Clearly the supply is either not working correctly or you have preset too low a current limit.
With respect to the current spike at full stroke :- The solenoid inductance changes from a minimum with the solenoid slug out of the coil to maximum when the slug is fully home. As the slug moves the solenoid inductance has a transient value based upon slug position and speed of movement. When the slug suddenly stops moving as it reaches full stroke the second order transient value (d2L/dt2) is a maximum and is what causes the spike in current reading.
pfrost:
I have a table top PSU which can supply 12 A at 30 V.
Please please post the datasheet (or part number) of the power supply you have, we can't
guess these details!
HI, if the power supply is functioning correctly then it looks like it has a FOLDBACK Current limit.
If the preset current limit is reached the power supply drops it output voltage back to give a preset fault current.
I have this on a 12V 30A linear supply that I built many years ago.
The idea is if there is a component failure in the load, the current being dropped will minimize any damage.
Its con is that it is not tolerant to surge currents that occur especially with loads that have SMPS DC-DC converters or similar high startup currents.
Tom... 