- To Pull or to Thrust (Push)
- Stroke (length of travel)
- Force Required
- Supply (AC, DC and variations)
- Ambient Temperature
1, A Solenoid is a linear actuator, which PULLS the plunger toward the stop when the coil is energised. It can only PULL, though a return spring can be fitted if required. To make it Thrust (PUSH) it must have a non-magnetic thrust rod.
2, Force increases as it closes – see graph above, note the difference between AC and DC solenoids. AC solenoids are more powerful than DC on the longer strokes but there are Long Stroke DC types designed for best possible long stroke force. (See data sheets).
3, You decide the stroke – this is set by the way the solenoid is mounted relative to the mechanism it has to operate.
4, Plungers are not captive – a solenoid should be fitted so that the plunger cannot drop out. The maximum practical stroke is shown on its force curve.
A solenoid can be made more powerful by fitting a coil which takes higher watts. It will get hotter, so will need to cool between each operation. Solenoid manufactures use the ‘%’ rating for higher force solenoids.
Watts = heat, and on intermittent ratings there is a limit to the time a coil can safely carry the higher power. This ‘maximum time energised’ depends on the rating, size of solenoid, and any ‘heat sink’ benefit gained from the mounting. Note that % ratings are simply a convenient system which covers most applications, coils can be wound for other duties if required.
‘Cycling’ implies an ON and OFF sequence, and steady cycling should not be called continuous.
A common mistake! Because a solenoid duty cycle is say, 25% it does not have to have a 25% rated coil if a less powerful one will do the job. If for instance a ‘continuous’ (100%) coil gives adequate force it will run cooler, take less current, and reduce the wear on mechanical parts.
A solenoid is more powerful cold, than hot. This is because the coil resistance increases as it warms up, reducing the current and therefore the force. The cold/hot difference is less on an AC than on a DC solenoid. NOTE; our force curves show the coil at maximum working temperature.
Both the ambient temperature of the solenoid environment and the self heating of the solenoid at work must be considered. In a high ambient temperature or a total enclosure a lower-watts, lower-force coil is the conventional way of keeping coil temperature within the safe limit. Coils are normally designed for a temperature rise of about 75°C due to self-heating and we assume an ambient temperature of 20°C.
AC INRUSH CURRENT
An AC solenoid draws a current surge as it closes. The longer the stroke, the greater the surge. On fast cycling, these repeated surges increase the coil heating and upset the ‘% rating’ principle of higher force for intermittent duty. Sometimes a lower than continuous rating is necessary to avoid too high a temperature rise from repeated inrush current. This uses a DE-RATED coil and gives less than continuous force. There is no DC INRUSH CURRENT. All DC intermittent duties can be based on the % rating principle subject to the ‘maximum time energised’.
SUPPLY – AC or DC ?
An AC solenoid gives greater long stroke force than a DC solenoid, gaining this advantage from its ‘inrush current’.
An AC solenoid is fast-closing, but the actual time may vary by a few milliseconds according to the point in the cycle when it is switched.
An AC solenoid tends to hum when closed. Good design keeps this to a minimum, but it can worsen with wear on the mating faces.
A DC solenoid can give high short stroke performance and very high absolutely silent hold. Its pull and hold characteristics can be varied by modifications to the plunger and stop.
A DC solenoid normally closes more slowly than an AC, but can repeat its closing time accurately at a given coil temperature and mechanical load.
An energised DC solenoid can be stopped anywhere along its stroke without overheating the coil or causing hum. An AC solenoid which is prevented from closing will rapidly overheat and will be noisy.
DC solenoids are often supplied from rectifiers. Ripple current can cause hum unless adequately smoothed. Full-wave rectification is better than half-wave. For some intermittent duties however unsmoothed half-wave may be acceptable.
A solenoid gives approximately the same performance with the appropriate coil for any voltage between the limits of the finest and the thickest wire with which the coil can be wound. A low voltage coil tends to have higher efficiency because of the better copper-to-insulation ratio of the thicker wire.
On the smallest solenoids, low voltage is desirable to avoid the combination of high voltage stresses and extremely fine wire.
If you are tempted to use the smallest solenoid that will do your job, remember that it will take more current than a larger one. As an extreme example, a BDC 2 would take 50 watts (cold) to pull 340g through 10mm, where as a BDC 5 would take only about 7 watts. If you need to keep the current down, it makes good sense to step up the solenoid size.
The tapped holes for side mounting are perfectly satisfactory for light to medium duties, but an intermittently rated high-force solenoid should be mounted by the back fixing nut, where available, for mechanical solidarity.