ELECTRON.ICL 1/15/99 3:06 PM Page 49 Electromagnetic Loads* Application Note The ability to open a valve, lock a door, or extend an actuator relies on the use of electroma- chanical forces. These types of applications are executed by electromagnetic devices that turn electrical signals into mechanical movement. Solenoids, valves and motors are some of the de- vices that are used to accomplish these tasks. 3 Since these devices are inherently electromechanical, application reliability and product life are criti- cal design considerations. The environments where these magnetic device, this movement mechanically secured together. devices are used can be harsh can be linear or rotational. The armature is then inserted and unforgiving. Installation into the completed solenoid problems, stalls in the field and When an electromagnetic device assembly. When the coil is excit- short circuits can permanently is required to act, current passing ed with current, a magnetomotive damage these systems if not through a coil generates a mag- force is created, causing the protected properly. netic field the strength of which is plunger to be pulled into the coil measured in ampere-turns (NI). and to seat on the backstop. The quality of these electro- This magnetomotive force causes mechanical devices and the relia- the core piece to move as a Solenoid-Operated Valves bility of their interface circuits can result of the magnetic attraction A solenoid-operated valve con- be enhanced in several ways, between it and its magnetic coun- sists of a solenoid operator along one of which is with the use of a terpart. Different types of move- with the components of the valve. PolySwitch device. ments, magnitudes of forces, etc., While there are many ways to are controlled by the device con- construct a solenoid-operated Construction of Electromagnetic struction and the magnetomotive valve, a typical construction is Devices force generated by the coil. one in which the solenoid part Solenoids, valves, relays, and simply opens and closes the motors are electromechanical Solenoids valve. When the solenoid is in devices that generate mechanical A solenoid is an electromagnetic the de-energized state, a spring force by converting electrical device with four basic parts: a coil pushes the armature with an energy into mechanical energy. assembly, frame, armature, and attached seal against the valve Typically, a magnetic core piece backstop. The coil assembly is seat. This creates an interface (commonly referred to as a plunger constructed by winding magnet that prevents the flow of the or armature) moves as a result of wire around a bobbin. The coil medium that the valve controls. being part of a magnetic circuit. assembly along with the backstop When the solenoid is energized, Depending on the type of electro- are placed into a frame and the armature of the solenoid Raychem Circuit Protection Devices Electromagnetic Loads 49 ELECTRON.ICL 1/15/99 3:06 PM Page 50 moves away from the valve seat tion. These components are then careful that the current that flows toward the backstop and allows placed in a protective magnetic through the device, keeps the I2R the medium to pass through the housing. heating below the temperature valve (Figure 1). rating of the device. In other Design Issues words, the heat rise generated by Motors Design engineers are continually the current flow in addition to the A motor is another device that trying to optimize the ampere ambient temperature must stay converts electrical energy into turns equation by maximizing below the temperature rating of mechanical motion. The current the amount of current and the the device. The materials that capability of the motor is deter- number of turns on a given coil. designers use can vary greatly mined by two conductors. First, Theoretically, as the amount of with respect to temperature limi- a field winding generates flux current that passes through a tations. Temperature ratings of through the device. Also, the given number of turns in a coil standard materials range from armature incorporates conductor increases, the force generated 105°C to over 200°C. Typically, loops that carry current through on the armature also increases. designers will attempt to approach the device. These current-carrying However, the electromagnetic these limitations, but not exceed conductors add to the total current device can be only a finite size, them. Generally, the closer one used by the motor. In DC circuits, thus, the number of turns and can get to the maximum tempera- a commutator ring is used to the wire diameter become con- tures, the more efficient the elec- 3 produce torque in one direction. strained. Also, materials used tromagnetic device becomes. If Brushes are also used to make to manufacture these devices the temperature of the coil contact with the rotating commu- have temperature limitations. exceeds the device rating, the tator and to ensure proper opera- Designers must, therefore, be wire insulation can burn away, causing the coil to short to adja- Figure 1: Pressure Valve cent windings or even burn through the magnet wire itself, creating an open circuit. Back stop and tube assembly Hex nut In most cases of electromagnetic C-frame device failure, something abnor- mal happened in the application or the product was simply mis- Coil assembly used. A PolySwitch device can eliminate these problems. Return spring Armature PolySwitch Resettable Fuses The PolySwitch resettable fuse is made from a conductive polymer blend of a crystalline polymer and Seal Valve body carbon black that provides con- ductive chains through out the device. PolySwitch devices exhibit Outlet port Inlet port low-resistance characteristics under normal operating condi- tions. but when excessive current flows through the device its tem- perature increases and the crys- talline polymer changes to an amorphous state. 50 Electromagnetic Loads Raychem Circuit Protection Devices ELECTRON.ICL 1/15/99 3:06 PM Page 51 This transition causes the device to expand, breaking the conduc- Figure 2: Normal Characteristic for an AC Solenoid tive paths inside the conductive polymer. The change causes a dramatic increase in the device’s Tek Stop Single Seq 1.00 kS/s resistance. This increase in resis- Ch 1 Zoom 1.0X Vert 1.0X Horiz tance reduces the amount of cur- rent that can flow through the device to minimal levels. The PolySwitch device will remain in this state until the cir- cuit is opened. Once this occurs 1 the device cools, the carbon chains reconnect and the device returns to a low-resistance state. Problem 1 (AC Applications) Solenoid and valve products are susceptible to problems in the Ch 1 2A/DIV M50.0 ms Ch 1 5.6 V 3 field. Devices designed for AC applications have inherent prob- lems. Figure 2 displays the nor- mal characteristic for an AC sole- noid. Upon energizing an AC Figure 3: Protected Oscilloscope Trace solenoid, a high inrush current is generated due to low inductive reactance since the plunger (armature) is in the extended Tek Stop Single Seq 50.0 S/s position. This oscilloscope trace High inrush due shows that the inrush current Ch 1 Zoom 1.0X Vert 2.0Xto stuckHoriz plunger through the solenoid is approxi- mately 5 amps. When the sole- PolySwitch device trips, preventing noid is energized, the plunger solenoid failure. begins to travel through the sole- noid body, causing the inductive reactance to increase, thus low- 1 ering the current until a steady state is reached. This occurs when the plunger inside the sole- noid is fully retracted or seated. As indicated in Figure 2, it takes approximately 250 ms for the plunger to fully seat. At this time, Ch 1 2A/DIV M.50 s Ch 1 1.6V the steady-state current is less than 0.4 A. If the plunger is obstructed or bound during oper- ation, the higher current will per- sist, causing the solenoid to gen- erate excessive heat. Under this condition, the solenoid tempera- Raychem Circuit Protection Devices Electromagnetic Loads 51 ELECTRON.ICL 1/15/99 3:06 PM Page 52 ture will begin to rise until it condition. When higher than components on the PCB fail or exceeds the thermal rating of the expected currents exist for before the electromagnetic device materials used in the solenoid extended periods of time, the itself fails. Once the stall construction. Typically, the mag- switching circuit may fail along with is eliminated, the PolySwitch net wire will fail or the other mate- the electromagnetic device. Also, device can reset and normal rials will break down. In addition, traces on the printed circuit board operation can resume. If the sys- the bobbin, tapes, and other insu- can open if they are not designed tem fails be-cause of the electro- lating materials are all thermally to handle this type of situation. magnetic device’s life limitation, constrained so excessive temper- This also can be true for the wiring. (usually determined under ideal atures could lead to shorted coils, conditions and measured in open coils, bobbins collapsing, The PolySwitch device provides cycles or number of operations) and other undesirable situations. several advantages in this case. the excessive current will then All of these conditions are cata- The circuit in Figure 4 places the disable the control circuits. If the strophic and lead to permanent PolySwitch device in series be- PolySwitch device is used as por- device failure. tween the SCR and the load. trayed in Figure 4, it can isolate This load can be a damper, a these control circuits from the Solution valve, a solenoid, a motor, or electromagnetic device. In this The oscilloscope trace in Figure 3 any other electromagnetic device. case, the electromagnetic device depicts a hypothetical situation is thought of as a field output.