Functional Scope of Solenoids

The basic principle of solenoids is the conversion of electrical energy into force and movement.

The resulting direction of movement is linear or rotary. This movement is determined by the mechanical assembly in which the electromagnetic circuit is encased and can perform the following functions, remotely if required.

The typical applications are:

  • Push > Pull
  • Select > Reject
  • Punch > Index
  • Divert > Kick
  • Hold > Release
  • Eject > Dispense
  • Lock > Latch
  • Position > Pinch
Rotary Solenoid, NSF Controls, Ledex Solenoid

Solenoid Fundamentals

A major objective in the design of a solenoid is to provide an iron path capable of transmitting maximum magnetic flux density with minimum energy input.

The purpose of a solenoid is to get the best relationship between the variable ampere-turns and the working flux density in the air gap.

The working gaps flux density can be increased by increasing the electrical input to the coil. However, as the electrical input increases the coil temperature rises and the work output is reduced. The amount by which the electrical input can be increased depends upon the relative duty cycle and maximum allowable coil temperature.

Many of our solenoids have an auxiliary flux path, not normally found in conventional solenoids, which provides a significant increase in force.

  • The magnetic flux generated is dependent on the number of turns and the current developed when power is applied to the coil. The permissible temperature rise limits the magnitude of the power input.
  • Working flux density is the total magnetic flux divided by the magnetic path (iron path) area.
  • The air gap, the iron path, the pole piece and its contour determine the working flux density.

Duty Cycle

High torques/forces may be obtained from a solenoid if it is used infrequently.

  • The ON and OFF sequence is the time factor that determines the permissible watts input.
  • The formula to calculate the relative duty cycle is: RELATIVE DUTY CYCLE % = (ON TIME / (ON + OFF TIME) x 100

Continuous operation should not be confused with continuous duty cycle.

  • The continuous duty cycle (100% rating) means that the solenoid can be left energised for an indefinite period of time at its rated voltage without overheating.
  • For continuous operation where the solenoid is switched ON and OFF repeatedly, a higher torque/force can be achieved but the permissible ON/OFF time periods for each relative duty cycle % must not be exceeded.

Economy Circuit

An alternative method of increasing the torque/force output is to use a continuous 100% duty cycle rated solenoid and apply an over voltage input for a short period of time and then reduce the voltage across the solenoid coil to the 100% duty rated voltage for the hold-on period.

Influence on over voltage on the solenoid’s characteristic.

  • a = characteristic with normal voltage
  • b = holding force with normal voltage
  • c = effect of over voltage
  • W1 = available work output at normal voltage
  • W2 = additional work output available by over voltage

The method of achieving an increased torque/force and a continuous 100% rating for hold-on is to use an economy circuit which switches a resistor in series with the solenoid coil when the operating stroke is completed and the plunger has reached its hold-on position. The conditions which apply during operation are as follows.

Plunger in start position and during operation:

  • P1 = (l1) 2Rs Where l1 = V / Rs

Plunger in end position (hold-on)

  • P2 = (I2) 2Rs Where I2 = V / Rs + Re
  • P1 = Solenoid input power at start
  • P2 = Solenoid input power at hold-on
  • Rs = Solenoid coil resistance
  • Re = Series economy resistance
  • V = Input supply voltage

The value of Re must be designed to give a solenoid input power equal to the rated 100% ED power value when switched in series.

Operating Speed

The time taken for a solenoid to complete its energised stroke is measured from the moment a voltage is applied to the time the energised position is achieved.

  • This time is dependent upon the load, duty cycle, input power, stroke and temperature range. When a DC voltage is applied the current will rise to point (a) as illustrated below.
  • This time delay occurs prior to any plunger motion until sufficient flux is developed to overcome the load.
  • As the plunger moves through its stroke the change in inductance resulting from the closing air gap causes a dip in the current trace until the solenoid has completed its stroke. From here the current trace begins to rise to a steady-state current value which by Ohms law is I = V / R

At point (a) the solenoid has developed sufficient flux to move the load, as the load increases, more time is required to reach point (c), as shown by the phantom trace.

If the load increased beyond the performance value of the Solenoid the coil will build immediately to a steady-state current, no dip will occur as no plunger movement has taken place (top curve).

Technical Definitions

  • Published torque and forces are specified at an ambient temperature of 20°C.
  • DUTY CYCLE is given at 100% of the rated voltage a reference temperature of +35°C and a cycle time of 5 minutes maximum, with solenoids mounted in an unrestricted flow of air.
  • POWER RATING is the wattage consumed operating at the rated duty cycle and at 20°C coil temperature.
  • AMBIENT TEMPERATURE range -50°C to +35°C.

Insulating materials used with solenoids are classified according to their stability during constant heating. The following table illustrates the insulation classes available for our solenoid ranges.

Insulation Classification Maximum Coil Temperature °C Maximum temperature rise above 35°C ambient Applicable solenoid type
A 105 65 Open frame Magnetic latching Tubular
B 130 95 Tubular*
E 120 80 Open frame* Magnetic latching*
F 155 115 Rotary Stepper Low Profile
H 180 140 Rotary* Stepper* Low Profile*

* High-temperature insulation is available to special order depending on solenoid type and volumes.

Technical Considerations for Ordering Solenoids

  • Dimensions of the space available for the solenoid.
  • Movement required – mm or degrees of rotation.
  • Force or torque required.
  • Voltage available & current limit of supply
  • Duty cycle – If in doubt specify maximum on time and minimum off time for 1 cycle.
  • Temperature of working environment.