Solenoid assembly: force, stroke and heat

Three curves, one actuator

A solenoid is specified by a force at a stroke, but the force depends on the current, and the current depends on the coil temperature. The three curves belong together. A force stroke curve at one current, a current rise curve at one voltage and a temperature rise curve at one duty cycle describe the same actuator from three sides.

A bench check at 20 °C with a coil that has run for ten seconds is the easiest test to pass and the least useful. In service the coil sits at its equilibrium temperature, the copper resistance is higher, the current at a fixed voltage is lower and the force at the end of the stroke is lower again. The verification has to reproduce that state, or the acceptance number describes a different product.

Reading a force stroke curve

A force stroke curve plots pull against armature position at a held current. The shape comes from the magnetic circuit: the force is usually highest near the seated position and falls as the air gap opens. The curve only means something when the current, the coil temperature, the stroke datum and the return spring are stated with it.

The worst point is not always at the start. In a short stroke actuator the minimum may sit mid travel where the magnetic force and the spring load cross. Plot the spring load line on the same axes, because the actuator can only hold a position where the magnetic force exceeds the spring. If the crossing point sits close to the required stroke, tolerance in the spring or the coil will move it out of the working range.

State where stroke zero is. Some drawings measure from the fully seated armature, others from the rest stop with the return spring fitted. A force quoted at 2 mm means nothing until the datum is fixed, and the two conventions can differ by the residual gap and the spring preload.

Where the heat comes from

Copper loss dominates. The coil dissipates current squared times resistance, and almost all of that heat leaves through the bobbin, the housing and the mounting. Hysteresis and eddy current loss are small in a DC coil, but they matter when the coil is driven with pulse width modulation or AC.

As the winding heats, resistance rises. At a fixed voltage the current falls, so the force falls with it. The coil is self limiting to a degree, but the actuator loses the margin the designer counted on. A coil rated for a 55 K rise at 20 °C ambient is not at 20 °C when the panel around it sits at 50 °C.

Duty cycle and thermal equilibrium

Duty cycle converts a continuous rating into an intermittent one. A coil that can run continuously at one watt may run at two or three times that loss for a short on time if the off time lets the winding cool. The energy average matters, not the instantaneous current.

The mounting is part of the thermal circuit. A coil clamped to a metal bracket conducts heat away; the same coil in still air inside a sealed housing runs hotter. Orientation, nearby heat sources and altitude all move the equilibrium. The duty cycle should be stated with the mounting and the ambient, or the number cannot be checked.

The cold and hot verification set

The same actuator has to be measured twice, because the cold check proves the mechanism and the hot check proves the application. The table shows what to record at each state.

How to run the verification

The sequence matters, because a force measured before the coil has stabilised will be optimistic. Run the actuator at its specified duty and ambient until the resistance change over a fixed interval is small enough to call equilibrium, then measure.

1. 01

   Stabilise the coil

   Run at the specified duty, voltage and ambient until the winding temperature stops climbing.

2. 02

   Measure resistance

   Use the resistance change from the reference reading to calculate the mean winding temperature.

3. 03

   Sweep the stroke

   Hold current constant and record force at each position with a load cell aligned to the axis.

4. 04

   Add the spring

   Overlay the return spring load line and find the crossing point with the force curve.

5. 05

   Repeat at the limits

   Run again at the minimum and maximum supply voltage and at the ambient extremes.

What moves the number

Five things account for most of the difference between a good bench result and a field result. Check each one before the design is released.

• Supply voltage tolerance, including the drop during inrush from a shared rail.
• Coil temperature at equilibrium, which sets the resistance and the current.
• Air gap and residual gap, which change the magnetic circuit and the force at the seat.
• Friction in the bore and the plunger guide, which consumes force before the load sees it.
• Return spring tolerance and preload, which set the position where the force crosses the load.
• Mounting and heat sinking, which decide how hot the coil runs in the application.

Specifying acceptance

Specify the force at the rated stroke at the minimum supply voltage and the maximum ambient, after thermal equilibrium. Add the duty cycle, the mounting condition and the temperature rise limit. A single force figure at room temperature cannot be verified in a way that protects the application.

State the measurement method as well: the stroke datum, whether current or voltage is held, the load cell direction and the soak time. Then ask for the trace, not only the pass mark, so the shape of the curve is visible. A part that passes at one point but has a flat or falling curve can still fail in the mechanism.