Electromagnetic Loads* Application Note

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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 devices 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

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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 abnormal 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 conductive chains through out the device. PolySwitch devices exhibit Outlet port Inlet port low-resistance characteristics under normal operating conditions. but when excessive current flows through the device its temperature increases and the crystalline polymer changes to an amorphous state.

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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 current that can flow through the device to minimal levels.

The PolySwitch device will remain in this state until the circuit 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 problems. Figure 2 displays the normal characteristic for an AC solenoid. 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 approximately 5 amps. When the sole- PolySwitch device trips, preventing noid is energized, the plunger solenoid failure. begins to travel through the solenoid 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 solenoid 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 operation, the higher current will persist, causing the solenoid to generate excessive heat. Under this condition, the solenoid tempera-

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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. 3 where the AC solenoid is prohibit- If the system is installed incor- Most maintenance/repair profes- ed from seating. As shown, a rectly and the load is shorted, sionals can change field outputs, large inrush current in excess of the PolySwitch device will trip, but changing control circuits is 6 A persists. In this scenario, the thus protecting the control usually a problem that only the steady-state condition is never circuits. Once this situation is OEM can address. realized. If this situation continues recognized, the power to the for an extended period of time, circuit is removed, allowing the Problem 2 (DC Applications) the heat generated by the current PolySwitch device to reset. Then, Precautions must also be taken draw would exceed the ratings of the installation can be rewired in DC applications. Electromag- the solenoid and cause prema- correctly and normal operation netic devices are rated in terms ture failure. But in this case, a can ensue without any damage to of duty cycle (continuous duty, PolySwitch device is placed in the control circuits. intermittent duty, and pulse duty). series with the coil. After approxi- The duty cycle is determined by mately 2.5 seconds, the If a stall occurs, the PolySwitch the ratio of the time the device is PolySwitch device has changed device can trip before any of the energized to the time of one from its original low-resistance state, to a high-resistance state, reducing the current draw to neg- Figure 4: PolySwitch Device between the SCR and the Load ligible amounts.

A stalled AC solenoid can easily generate three to ten times the normal steady-state current. This Field output type of stall scenario is not only detrimental to the electromagnetic device, but also can be problem- atic for the entire circuit. For PolySwitch RLOAD example, driver circuits controlling PCB device the electromagnetic device can be damaged. Usually relay contacts, triacs, and SCRs are not rated for much more than the worse case steady-state

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the solenoid is energized, the end Figure 5: Dual-Coil of travel is detected by the sensor. This sensor can then feed back the position of the armature (status of the lock) to the elec- Solenoid tronics, thus turning the power to the solenoid off. If the sensor fails or if the armature fails to pull in, DC Power the intermittent solenoid will supply generate excessive heat and fail. This will result in system down- RL1 RL2 time and maintenance attention. Using a PolySwitch device in the PCB circuit can detect this situation and protect the circuit from damage.

In the automotive market, a designer may use an intermittent- duty solenoid in a trunk-release Figure 6: Oscilliscope Waveform application. In most cases, the lock will only be energized once 3 or twice within a couple of seconds. Tek Stop Single Seq 500 S/s However, if the lock is continuously operated, the solenoid will eventually exceed its thermal rat- Normal ing and fail. Using a PolySwitch operation device in series with the coil can Abnormal operation eliminate this problem.

Problem 3 (DC Applications) 1 Another direct-current situation includes a dual-coil arrangement (Figure 5). This configuration is necessary if a load is unusually heavy or if a large “breaking force” is required. In a dual coil situation, two coils are wound one over the other on a bobbin. Ch 1 2A/DIV M 1.00s Ch 1 5.6V One coil includes a low-resistance path, the other is wound to a higher resistance. These coils are complete cycle [time on/(time on the temperature increase will wired in parallel with each other, + time off)]. In some cases, an quickly exceed the limitations of yielding a low total resistance. electromagnetic device can be the device and cause the device Next, a normally closed switch is driven with a large amount of to fail. placed in the circuit as shown. current for a short period of time, Once the assembly is energized, Solution resulting in higher force during current flows through the parallel this time period. Because the There are a number of scenarios combination, taking advantage of device is energized intermittently, that can lead to the situation the low-resistance (high-current), the heat rises, but stays within discussed above. In the security high-turns path. This generates a an acceptable level. However, if market, for example, an intermit- large amount of force. a device with an intermittent or tent-duty solenoid can be used pulse duty is used continuously, with a sensing mechanism. After

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As the plunger completes its state run current. When the motor such as this one, the PolySwitch travel, it physically hits the switch begins to rotate, a counter elec- device can be placed in series arm, opening up the closed tromotive force (EMF) begins to with the motor. If a stall is contacts. The switch opens up the build. As the motor builds up its encountered, the PolySwitch “pull-in” coil, leaving only speed, the counter EMF builds device will rapidly heat up and the higher resistive coil in series inside the motor. The counter change to its high-resistance with the load. This high-impedance EMF opposes the drive voltage state before the temperature path limits the heat rise and and causes the current to extreme of the wire is exceeded. allows operation with continuous decrease, eventually reaching The PolySwitch device will holding force. its steady state. If a motor is remain in its high-resistance stopped or if a stall occurs during state until the stall is removed, The oscilloscope waveform operation, the motor ceases to thus protecting the motor from (Figure 6) shows this type of rotate, eliminating the counter EMF. premature device failure. Once operation. The first waveform the voltage is removed and the shows the current flow that is If this happens while the motor fault addressed, the PolySwitch expected under normal operation. is energized or operating, little device will return to its low- When the device is first energized, voltage remains to oppose the impedance state. The system the current generated is above 4 drive voltage and dangerously can then resume normal opera- amps. Once the armature seats, high currents will flow. These tion without any intervention. 3 the switch is opened and current currents will flow through the flows only through the series coil. motor, heating up the windings In some motor protection circuits The current at this point is dra- inside the motor until the temper- (Figure 7), a bimetal alternative is matically reduced and, in this ature is exceeded. At this point placed in series with the motor. If case, is approximately 0.25 amps. the coil will open or short, creat- the motor stalls, the bimetallic However, if the armature is ing premature motor failure. contacts heat up and open. When prohibited, it will not switch the contacts open a circuit in this circuit to high resistance. As is true with other electro- fashion, arcing occurs between mechanical devices, motors the contacts, causing the plating Solution will eventually fail. The materials of the contacts to deteriorate. As The second waveform shows used in the construction of a the plating deteriorates, the bare that if the armature is not allowed motor will determine its life. For metal becomes exposed and oxi- to move, the higher current will example, as the motor armature dation begins to occur. This even- persist. At this point, it is only a rotates, the current in the arma- tually causes the contacts to stick matter of time before the solenoid ture windings routinely reverse. together, resulting in a short cir- wire ratings are exceeded. By Due to the inductance of the cuit. This problem can be solved placing a PolySwitch device in windings, the current does not by placing a snubber circuit series with the parallel coil instantaneously reverse, and across the contacts. As the con- combination, you can protect this results in sparking at the tacts open, the capacitor shunts the system. In this example, the commutator brushes. This the arc away from the contacts PolySwitch device trips to its eventually leads to device failure. providing a level of protection for high-resistance state after about Side loading and other forms of the bimetal breaker. 4 seconds and the system is improper use also contribute to salvaged without incident. device failure. While the failure The PolySwitch device has of an electromechanical device advantages over the bimetal Problem 4 (Motors) is accepted, one can certainly alternative. Because the Due to its inherent properties, a limit the damage that can be PolySwitch device is a solid-state motor will require higher currents done to the entire circuit. solution, arcing is not an issue. during start-up. When the motor Therefore, one does not need the is first energized, the armature Solution extra RC network defined above. resistance is usually quite low. If stalls are created during normal The PolySwitch device replaces Therefore on start-up, the inrush operation, higher currents will the bimetal breaker, the resistor, currents can be very high, several persist. In many applications, and the capacitor (Figure 8). times greater than its steady-

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Reliability is enhanced, space is saved, and the cost to the end Figure 7: Bimetal Alternative customer is reduced. The PolySwitch device also is not as susceptible to vibration as is the bimetal breaker. Bi-metel R Motors are usually controlled by breaker Snubber relays or switches. Typically, DC Power circuit C these components are part of the supply control circuitry found in a panel or on a printed circuit board. Wiring is then used to connect the control circuitry out to the Motor motors in the field. Implementing a PolySwitch device on the control board is an easy way of isolating the control circuitry from the motor or from the field outputs. If an electromagnetic device should 3 fail due to its designed end of life, the PolySwitch device will protect the contacts, SCRs, TRIACs, etc., Figure 8: PolySwitch Device Solution that drive these devices. Many times, it is easy to replace a motor in the field, but difficult to replace a relay on a PCB. Replacing a relay may require the user to remove the entire control PolySwitch R device Snubber board and send it back DC Power circuit to the OEM, a costly solution. supply C

Conclusion Electromagnetic devices are found in numerous markets, such Motor as automotive, medical, security, and consumer. The PolySwitch device is an efficient product that can eliminate many of the problems that one encounters when using an electromagnetic device. *Special thanks to Brian Cahill, Deltrol The resettability of the PolySwitch Controls, Inc., for his assistance on this device allows the user to enhance application note. the reliability of the system and to provide the OEM with a more robust solution. Including the PolySwitch product solution can lead to such benefits as lower field returns, better warranties, and greater customer satisfaction.

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