SAMPLE. MOVs are sometimes referred

Transcription

1 attenuate (weaken or reduce) a spike or transient pulse. Recall that in a capacitive circuit the voltage lags the current, and in an inductive circuit the current lags the voltage. Inductance added METAL-OXIDE VARISTORS ac input To power supply rectifier A varistor is a semiconductor device whose resistance drops as the applied voltage is increased. A metal-oxide varistor, or MOV, is a solid-state device used for Inductance transient voltage suppression. added MOVs are sometimes referred FIGURE 9-3. Pre-filter circuit with inductance added to as breakover devices. The breakover voltage is a threshold or limit. When it is exceeded, the MOV is designed to short circuit, opening a path to divert the current. The schematic symbol for a MOV is shown below: The MOV is a bidirectional metal-oxide device whose resistance is a nonlinear inverse function of applied voltage. This means that it offers a very high resistance at the rated voltage. But at higher voltages (voltages above the breakover voltage), its resistance decreases sharply. The transient voltage is then reduced (or clamped ) to a safe level and the energy in the transient is dissipated in the varistor. Because they are bidirectional, MOVs will suppress a transient pulse on either the positive or negative side of an ac waveform. MOVs are rated by the applied voltage and the maximum current and voltage that they will suppress. Normally, they start at 115 V ac RMS, and go as high as 250 V ac RMS. Their current rating can range from 1 A to several hundred amperes. Figure 9-4 shows a typical MOV used as the input to a transformer that provides power for some electronic device. Note that it is placed directly across the ac 104

2 LESSON 9 line, so its rating must be equal to or higher than the line voltage. Because the resistance of the MOV is very low when high-voltage transients are present, the transformer primary is bypassed. The type of protection shown in Figure 9-4 is designed for single-phase power. Today more equipment is being designed to operate on three-phase power, especially larger systems. The controls are taken from the three-phase line and then a control transformer drops the voltage down to the controls circuits. Normally six MOVs are used in a three-phase power system. Three of them will direct the transient to ground, as shown in Figure 9-5. Note that none of the circuits that have been discussed up to this point have shown fuse protection. This is because fuses and similar circuit protection devices are always installed prior to any of the components that have been examined in this Lesson. TRANSIENT ABSORBERS Sometimes a computer or microprocessor is located in an area where there are inductive devices motors, compressors, heavy-duty contactors, motor starters, etc. All of these at one time or another can and will impress a counter EMF pulse or spike on the ac line. As noted previously, these pulses can destroy sensitive electronic components. Both pi filters and MOVs suppress very fast transients. But there are some very slow pulses capable of getting past an MOV or pi filter that can do a lot of damage to electronic circuits. There are circuits that will literally take a slow pulse and change its shape and level. They are known by several names, depending on the manufacturer. Transorbs are made by General Electric Company. Quencharcs are made by the ITE Company. In general, such devices are called snubbers. A snubber is basically an RC (resistor-capacitor) circuit. It consists of a capacitor and a resistor, connected in series and built into one two-lead package. A snubber L1 L2 ac input MOV Primary FIGURE 9-4. Typical MOV circuit L3 Secondary FIGURE 9-5. MOVs in three-phase power system To power supply rectifier 105

3 network, shown in Figure 9-6, can be used to protect a circuit or a device from sharp increases in voltage. In essence, a snubber creates a time delay for any pulse higher than what the circuit is designed for. The charging capacitor momentarily places the voltage across the resistor, FIGURE 9-6. Snubber network where the energy contained in the sharply rising portion of the voltage waveform is dissipated. Figure 9-7 illustrates how a snubber network tends to short out a transient voltage spike across a device in the off-state condition. Look at the circuit shown in Figure 9-7A. At time zero, the switch is closed and the capacitor starts to charge. Since the voltage across the capacitor (E c ) is zero at this time, the capacitor acts as a direct short. As shown in Figure 9-7B, E c rises to approximately 63% of the supply voltage after one time constant. After five time constants, E c is approximately equal to the supply voltage, and the charging current (I ) drops to nearly zero. This circuit will reduce voltage transients during the off-state condition. Snubber networks are also used for suppressing arcs across relay contacts. Figure 9-8 illustrates how a snubber can be placed across the contacts of a control relay. Not only does the snubber clamp or quench the pulse, but it also helps to keep the contacts from arcing too much. The design of snubber networks must take into account peak line voltages and load characteristics or conditions. Supply voltage A. Circuit with snubber Current R Supply voltage, % B. Graphing current and voltage E c C 20 S I Switch closes Time constants FIGURE 9-7. Snubber network operation 106

4 LESSON 9 SPARK ARRESTORS Spark arrestors can be likened to small lightning arrestors. A spark arrestor is usually made with a small ceramic capacitor. The capacitor is used in a line-filtering circuit like the one shown in Figure 9-2. The leads of the capacitor protrude from the top of the device, forming a small gap, as shown in Figure 9-9. A spark of high voltage will jump across this gap and then go to ground without damaging any component, or getting into any other circuit. CROWBAR CIRCUITS An incoming voltage that exceeds any of the power supply s regulators can cause electronic components to fail. In a regulated power supply, the only component standing between an elevated dc source voltage and the components in the rest of the system is one transistor, or perhaps a group of transistors wired in parallel. If the transistor, or one of the transistors in the group, happens to short internally, the whole system could be damaged or destroyed. Placing a momentary short across the output terminals of the power supply is called throwing a crowbar across the circuit. This crowbar action usually FIGURE 9-8. Snubber used for arc suppression µf FIGURE 9-9. Spark arrestor To controlled circuit Spark gap R To electronic device Zener diode SCR Crowbar circuit FIGURE Crowbar circuit 107

5 results in high current in the power supply, and will blow the power supply s fuses. To prevent this, a form of protection known as a crowbar circuit is used to safeguard many complex systems and other equipment against possible overvoltage. This circuit usually consists of a silicon-controlled rectifier (SCR) connected directly across the output of the power supply, with a voltage-sensing trigger circuit tied to its gate. The trigger circuit shown in Figure 9-10 on the previous page consists of the voltage-dropping resistor (R) and a Zener diode. If the output voltage exceeds the trigger setpoint (a preset voltage limit), the SCR fires, and the output of the power supply is short-circuited to ground. SUMMARY Today s sophisticated electronic equipment and its associated controls require reliable protection. This Lesson has concerned itself only with a few of the most popular protective devices. There are many others, but they are all based on these few basic circuits. 108