APPLICATION NOTES Littelfuse Varistor Design Examples

This note is meant to be a guide for the so some suppression must be added. It is 3000 UL1449 CORD CONNECTED AND DIRECT PLUG-IN user in selecting a varistor by describing desirable to use the built-in impedance of 2000 CATEGORY common application examples, and illus- the coil to drop the remaining voltage, so 1500 V130LA2 trating the solution process to determine the suppressor would best be applied as 1000 V130LA5 800 V130LA10A the appropriate varisor. The note also shown. A selection process for a Littelfuse 600 V130LA20A describes series/parallel connection rules. varistor is as follows: 500 400 300 MAXIMUM PEAK (V) IMPULSE GENERATOR LOAD LINES 200 (IMPLIED) UL1449 PERMANENTLY APPLICATIONS: Solution CONNECTED CATEGORY, AND ANSI Stead-State Voltage IEEE C61.41 (IEEE587) CATEGORY B 100 POWER SUPPLY 100 101 102 103 104 The 117VAC, 110% high line condition is 27 31 PROTECTION AGAINST 129V. The closest voltage rating available is PEAK AMPERES 8/20µs WAVESHAPE LINE TRANSIENT 130V. Figure 2a.V130LA Varistor V-I Characteristics DAMAGE Energy and Current 1000 Problem The 100µH inductor will appear to be 800 Ω 600 It is desired to prevent failure of the power about 30 is derived from the inductive 500 supply shown in Figure 1b to be used on reactance at the transient generator source 400 5π residential 117VAC lines. A representative frequency of 10 rad.Taking a first estimate 300 transient generator is to be used for testing of peak varistor current, 2500V/80Ω=31A. as shown in Figure 1a. (This first estimate is high, since it assumes MAXIMUM PEAK (V) 200 varistor clamping voltage is zero.) With a 50 5kV tentative selection of a 130V Harris Varistor, 100 100 101 102 27 31 VT = we find that a current of 31A yields a volt- 5 age of from 325V to 380V, depending on – 5kV sin 10 „ t X PEAK AMPERES 8/20µs WAVESHAPE e-10-5t the model size, as shown in Figure 2a and Figure 2b.V130LA Varistor V-I Characteristics Figure 2b. Figure 1a.Transient Generator Revising the estimate, I=(2500V- IV If the transient is applied to the existing 325V)/80Ω=27.2A. For model V130LA20A, 27 circuit, the rectifier will receive high negative 27.2A coincides closely with a 320V clamp- voltages, transmitted through the filter ing level.There is no need to further refine capacitor.The LC network is there to the estimate of peak current if model 20A CURRENT VARISTOR t prevent RFI from being transmitted into the remains the final selection. 10µs 20µs power line (as in a TV set), but also serves to reduce the transient voltage. An analysis To arrive at an energy figure, assume a Figure 3. Energy Approximation shows that the transient will be reduced sawtooth current waveform of 27A peak, approximately by half, resulting in about dropping to zero in two time constants, or the number of transient current pulses 2.5kV instead of 5kV at the rectifier. 20µs. expected in the life of the equipment. A 70J Rated varistor will clamp at 315V and be 6 100 H D Energy is then roughly equal to capable of handling over 10 such pulses. An (27Ax320Vx20µs)/2, the area under the 11J unit will clamp to approximately 385V + and be capable of handling over 105 such 0.1 F 150 F power waveform.The result is 0.086J, well - within the capability of the varistor (70J). pulses. Furthermore, the clamping voltage Peak current is also within the 6500A rating. determines the cost of the rectifier by deter- mining the voltage rating required. A smaller, lower cost varistor may result in a Figure 1b.Typical Power Supply Circuit Model Selection The actual varistor selection is a trade-off more expensive higher voltage rectifier This is still to high for any practical rectifier, between the clamping voltage desired and diode. SCR MOTOR CONTROL formed in the secondary circuit due to the mutual inductance, LM . At no load, the 14 Problem magnetizing current, (INL ), is essentially reac- 12 The circuit shown in Figure 4. experiences tive and is equal to IM .This assumes that the failures of the rectifiers and SCR when the primary copper resistance, leakage reactance 10 transformer primary is switched off.The and equivalent core resistive loss compo- 8 6 manufacturer has tried 600V components nents are small compared to L .This is a f = 50...60Hz M 4 with little improvement. valid assumption for all but the smallest 2

control transformers. Since I is assumed CURRENT PERCENT MAGNETIZING NL 2 4 6 8 10 12 purely reactive, then: TRANSFORMER RATING (kVA) V Figure 6. Magnetizing current of transformers with low sili- ARMATURE pri R1 X = ------LM I con steel core 330k SPEED NL CONTROL and

iM = INL 480V R3 6

AC 60Hz FIELD 250k SCR I can be determined from nameplate 5 SUS NL 4:1 R2 data.Where nameplate is not available, 4 15k CI 0.2µF Figure 6 and Figure 7 can guide the designer. 3 f = 50...60Hz Assuming a 3.5% value of magnetizing FIGURE 4 SCR MOTOR CONTROL 2 Figure 4. SCR Motor Control current from Figure 7 for a 20kVA trans- former with 480V AC primary, and 120V AC 1 secondary: CURRENT PERCENT MAGNETIZING 110 102 103 104 Solution 20kVA L =/X w i = ()0.035 ------M L TRANSFORMER RATING (kVA) Add a varistor to the transformer secondary M 480V M = 0.872H Figure 7. Magnetizing current of transformers with high sili- = 1.46A to clamp the transformer inductive transient 2 con steel core or square loop core 0.872() 2.06 iM =i2 M E = ------voltage spike. Select the lowest voltage LM 2 X = 480V/1.46A Should the transient be repetitive, then the Littelfuse Varistor that is equal to or greater LM = 1.85J = 329‰ average power is calculated from the prod- than the maximum high line secondary AC uct of the repetition rate times the energy voltage.The V130LA types fulfills this With this information one can select the of the transient. If this value exceeds the requirement. needed semiconductor voltage ratings and V130LA20B capability of 1.0W, power varis- required varistor energy rating. tors of the HA, DA, or DB Series may be Determine the peak suppressed transient required. Should the ambient temperature voltage produced by the transient energy Peak varistor current is equal to transformed exceed 85˚C or the surface temperature source.This is based on the peak transient secondary magnetizing current, i.e., îM(N), or exceed 85˚C, the single pulse energy ratings current to the suppressor, assuming the 8.24A. From Figure 2, the peak suppressed and the average power ratings must be worst-case condition of zero load current. transient voltage is 310V with the derated by the appropriate derating factors Zero load current is normally a valid V130LA10A selection, 295V with the supplied on the data sheet. assumption. V130LA20B.This allows the use of 300V rated semiconductors. Safety margins exist in CONTACT ARCING DUE Since the dynamic transient impedance of the above approach as a result of the the Littelfuse Varistor is generally quite low, following assumptions: TO INDUCTIVE LOAD the parallel higher impedance load path can be neglected. Problem 1. All of the energy available in the mutual To extend the life of the relay contacts inductance is transferred to the varistor. shown in Figure 8 and reduce radiated Since transient current is the result of stored Because of core hysteresis and secondary energy in the core of the transformer, the noise, it is desired to eliminate the contact winding capacitance, only a fraction less than arcing. transformer equivalent circuit shown in two-thirds is available. Figure 5 will be helpful for analysis.The 2.The exciting current is not purely reactive. stored inductive energy is: There is a 10% to 20% safety margin in the 1 2 E =--- LI RELAY L 2 M M peak current assumption. M L

+ CC The designer needs to know the total R After determining voltage and peak current, 28V C C = STRAY CAPACITANCE DC C energy stored and the peak current trans- energy and power dissipation requirements L = RELAY COIL INDUCTANCE R = RELAY COIL RESISTANCE must be checked. For the given example, the C single pulse energy is well below the V130LA20B varistor rating of 70J at 85˚C FIGURE 8. RELAY CIRCUIT ZP ZS N maximum ambient temperature. Average IM MUTUAL Figure 8. Relay Circuit INDUCTANCE power dissipation requirements over idling VPRIMARY REPRESENTED LM VSECONDARY BY IRON CORE power are not needed because of the non- When relays or mechanical switches are repetitive nature of the expected transient. used to control inductive loads, it is neces- IDEAL TRANSFORMER Figure 5. Simplified Equivalent Circuit of A Transformer sary to use the contacts at only about 50% The capacitor technique requires the capaci- flow when the contacts are open.The of their resistive load current rating to tance to be sufficiently large to conduct the combination technique of a small R-C reduce the wear caused by arcing of the inductor current with a voltage rate-of-rise network in conjunction with a varistor is of contacts.The energy in the arcing is propor- tracking the breakdown voltage rate-of-rise advantage here, too. tional to the inductance and to the square of the contacts as they mechanically move of the current. apart.This is shown in Figure 10a. In this example a 0.22µF capacitor and 10Ω resistor will suppress arcing completely, but Each time the current in the inductive load by reducing the capacitance is interrupted by the mechanical contacts, CONTACT BREAKDOWN LEVEL to 0.047µF, arcing will start at 70V. 100 the voltage across the contacts builds up as - L di/dt.When the contacts arc, the voltage Thus, to use a varistor as a clamp in LARGE C WITH R (NO ARCING) across the arc decreases and the current in 50 conjunction with the R-C network, it must

the coil can increase somewhat.The extin- (V) ARC VOLTAGE suppress the voltage to below 70V at 1A SMALL C WITH R guishing of the arc causes an additional (ARCING) and be capable of operating at a steady- voltage transient which can again cause the 0 state maximum DC voltage of 28V + 10%, 025 50 75 100 contacts to arc. It is not unusual for the BREAK TIME ( s) or 30.8V (assumes a ±10% regulated restriking to occur several times with the Figure 10a. R-C arc suppression 28V DC supply). total energy in the arc several times that which was originally stored in the inductive The limitations in using the capacitor The three candidates that come closest to load. It is this repetitive arcing that is so approach are size and cost.This is particu- meeting the above requirement are the MA destructive to the contacts. larly true for those cases involving large series V39MA2B model and the ZA series amounts of inductive stored energy. V39ZA1 and V39ZA05 models, all of which In the example, RC is 30 and the relay Furthermore, the use of a large capacitor have maximum steady-state DC voltage contacts are conducting nearly 1A.The alone creates large discharge currents upon ratings of 31V.The V39MA2B and V39ZA05 contacts will draw an arc upon opening with contact reclosure during contact bouncing. V-I characteristics at 1A shows a maximum more than approximately 0.4A or 12V.The As a result, the contact material may melt at voltage of 73V, while the V39ZA1 character- arc continues until current falls below 0.4A. the point of contact with subsequent weld- istic at 1A shows a maximum voltage of 67V. ing.To avoid this inrush current, it is Thus, the latter varistor is selected. Use of a Solution customary to add a series resistor to limit 0.068µF capacitor in place of the 0.047µF the capacitive discharge current. However, previously chosen would allow use of the To prevent initiation of the arc, it is neces- this additional component reduces the V39MA2B or V39ZA05. sary to reduce the current and voltage of network effectiveness and adds additional the contacts below the arc threshold levels cost to the solution. Placing only a Littelfuse Varistor rated for at the time of opening, and then keep them 31V DC across the contacts results in arcing below breakdown threshold of the contacts A third technique, while not as obvious as up to the 66V level. By combining the two, as they open.Two obvious techniques come the previous two, is to use a combination the capacitor size and voltage rating are to mind to accomplish this: approach.This technique shown in Figure reduced and suppression complete. 1) use of a large capacitor across the 10b parallels a voltage clamp component contacts with an R-C network.This allows the R-C Besides checking the varistor voltage and 2) a voltage clamp (such as a varistor).The network to prevent the low voltage initial arcing elimination, the designer should clamp technique can be effective only arcing and the clamp to prevent the arcing review energy and peak current require- when the minimum arc voltage exceeds that would occur later in time as the capaci- ments.Varistor energy is determined from a the supply voltage. tor voltage builds up.This approach is often measurement of the coil inductance and the more cost effective and reliable then using a calculation E = 1/2 Li2 . Peak current, of In this example a clamping device operating large capacitor. course, is under 1A. Power dissipation is above the supply voltage will not prevent negligible unless the coil is switched often arcing.This is shown in Figure 9. (several times per minute).

100 CONTACT BREAKDOWN LEVEL BREAKDOWN LEVEL In those cases where multiple arcs occur, the ARCING 100 varistor energy will be a multiple of the 2 50 above 1/2 Li value.The peak current is SMALL C WITH R AND

ARC VOLTAGE (V) ARC VOLTAGE VOLTAGE CLAMP COMBINATION well within the rating of either the MA or VOLTAGE CLAMP ABOVE ARC VOLTAGE ZA series of varistors, but the number of 50 0 ARC VOLTAGE (V) ARC VOLTAGE 0 25 50 75 100 contact operations allowable for either VOLTAGE CLAMP BELOW BREAK TIME ( s) ARC VOLTAGE varistor is a function of the impulse dura- Figure 10b. R-C and clamp arc suppression tion.This can be estimated by assuming a 0 0 25 50 75 100 L/RC time constant at the 1A or peak BREAK TIME ( s) Also, with AC power relays the impedance current value. Since the voltage across the Figure 9.Voltage clamp used as arc suppressor of a single large R-C suppressor might be so varistor is 67V at 1A, the varistor static low that it would allow too much current to resistance is 67.The coil RC value is 28V/1A, or 28.The coil inductance was found to be full range of current that could flow through transistor. 20mH. the switch in the normal AC operation. A The inductor drives the collector voltage up Thus, the approximate time constant is: diode detector was used to observe the RF voltage developed across a 2” length of wire VC From the pulse rating curves of the V39ZA1 (50nH of inductance). 26V V+ 50 τ 20mH = L/RC=------= 210 s 200mH V+ 95 26V The supply is set at 25mA to represent the VC t model, the number of allowable pulses peak motor current in normal 120V AC IC exceeds 100 million. operation. As switch S1 was opened, the 470 waveform in Figure 12 was recorded. Note t the “showering arc” effect.The highest break- NOISE SUPPRESSION PERIOD OF HIGH SOA down voltage recorded here is 1020V, and REQUIREMENT Problem the highest RF detector output (shown in Figure 14a. Basic solenoid circuit Switching of a small timer motor at 120V, the lower trace) is 32V. 60Hz, was causing serious malfunctions of an VC = COLLECTOR electronic device operating from the same A 26V V+ VC EMITTER VOLTAGE 0 power line. Attempts were made to observe V+ 200V/cm 26V t the transient noise on the line with an oscil- I C I loscope as the first step in curing the IV C problem. Observed waveforms were B t “hash,” i.e., not readily identifiable. IV 0 20V/cm 200 s/cm Noise in an electromechanical system is a t UPPER V1:200V/cm LOWER VRF: 20V/cm commonly experienced result of interrupting t: 0.2ms/cm Figure 14b. Solenoid circuit with varistor protection current by mechanical contacts.When the Figure 12. Unprotected Contacts switch contacts open, a hot cathode arc when the transistor base is grounded (turn- may occur if the current is high enough. On Obviously, some corrective action should be ing “off”).The inductor forces current to the other hand, low current will permit taken and the most effective one is that flow until the energy stored in its field is switch opening without an arc, but with ring- which prevents the repeated breakdown of dissipated.This energy is dissipated in the ing of circuit resonances. As a consequence, the gap. Figure 13 shows the waveform of reverse bias condition of the transistor and voltages can exceed the contact gap break- V1 (upper trace) and VRF (lower trace) for is sufficient to cause breakdown (indicated down resulting in a replica of the old spark the same test conditions with a Littelfuse by a sudden collapse of collector voltage gap transmitter. It is the low current Varistor, type V130LA10A, connected during the pulse). case that produces the most serious noise directly across the switch terminals.The disturbances which can result in malfunctions varistor completely eliminates the relaxation oscillations by holding the voltage below the Solution or damage to electrical equipment.These This condition can be eliminated either by pulses cause noise problems on adjacent gap breakdown voltage (about 300V) while dissipating the stored energy in the system. shunting the transistor with a suppressor or lines, trigger SCRs and triacs, and damage by turning it on with a varistor connected semiconductors. In addition, they can disrupt collector-to-base.The first method will microprocessor operation causing memory considerably reduce the demands upon the to be lost and vital instructions to be missed. safe operating area (SOA) of the transistor. If 200V/cm 0 the voltage is kept below its breakdown Solution level, all energy will be dissipated in the A test circuit (Figure 11) was set up with suppressor.The latter method will cause the 0 transistor to once again dissipate the stored lumped elements replacing the measured 10V/cm circuit values.The motor impedance was 200 s/cm energy, but in the forward-bias state in which UPPER V1:200V/cm LOWER VRF: 20V/cm the transistor can safely dissipate limited simulated by R1 ,L , and C , and the AC t: 0.2ms/cm 1 1 amounts of energy.The choice is determined line impedance by L2 and C2 . A DC source Figure 13.Varistor protected contacts allowed repeatable observations over the by economics and reliability. A suppressor connected collector-emitter (C-E) will be PROTECTION OF TRAN- more expensive than one connected C-B, since it is required to absorb more energy, L 5H S1 SISTORS SWITCHING 2 INDUCTIVE LOADS but will allow the use of a transistor with L1 6.8H reduced SOA. C2 V + 1 C1 V V Problem CC 14 F 2 80 F R 1448 P P 1 The transistor in Figure 14 is to operate a If a collector-emitter varistor is used in the solenoid. It may operate as frequently as above example, it is required to withstand

V 28.6V DC worst-case (26 + 10% regula- 2" AWG #22 WIRE RF once per second.The circuit (without any suppression) consistently damages the tion).The stored energy is 1/2 Li2 or 1/2

Figure 11.Test Circuit (0.20) (0.572)2 =0.0327J.The energy exceeded. Figure 15B shows the curves for provide guidance when specific motor data contributed by the power supply is roughly the larger, 14mm V39ZA6 device and, illus- is lacking.The data is conservative as it equal to this (coil voltage ≈ supply voltage, trates the resultant higher capability in assumes maximum motor torque, a condi- since varistor clipping voltage ≈ 2 x supply terms of number of transients for a given tion that is not the typical running voltage). Ignoring coil resistance losses for a peak pulse current and duration. condition. Stored energy decreases consid- conservative estimate, varistor energy dissi- erably as the motor loading is reduced. pation is 0.065J per pulse.The peak current Also, it may be necessary to extrapolate Experience with the suppression of will be 0.572A, the same as the coil current the pulse rating curves.This has been done magnetic energy stored in transformers when the transistor is switched off. in Figure 16 where the data from Figure indicates that Littelfuse Varistors may be 15B is transposed. At low currents the used at their maximum energy ratings, If the transistor operates once per second, extrapolation is a straight line. even when multiple operations are the average power dissipation in the varis- required.This is because of the conser- tor will be 0.065W.This is less than the Finally, the V-I characteristics curves must be vatism in the application requirements, as 0.20W rating of a small 31VDC varistor consulted to determine the varistor maxi- indicated above, and in the varistor ratings. (V39ZA1). From the data sheet it can be mum clamping voltage in order to select Thus, no attempt is made to derate the seen that if the device temperature the minimum transistor breakdown voltage. varistor for multiple operation because of exceeds 85˚C, derating is required.The In this example, at 0.572A the V39ZA6 (if the random nature of the transient energy non-recurrent joule rating is 1.5J, well in chosen) provides a maximum of 61V experienced. excess of the recurrent value.To determine requiring that the transistor have about a 600 460VRMS LINE - LINE the repetitive joule capability, the current 65V or 70V capability. 400 4 POLE 2 POLE 230VRMS LINE pulse rating curves for the ZA series must 200 V150PA20 be consulted.Two are shown in Figure 15. 100 32 80 V151HA 60 2 POLE 4 POLE To use Figure 27, the impulse duration (to 15 40 40 V321HA32 the 50% point) is estimated from the circuit 10 V320PA 20 8 ZA SERIES V18ZA3 TO V68ZA10 Y CONNECTED

time constants and is found to be 1240µs. 6 PER PHASE (J) ENERGY STORED 5 10 From Figure 27A, for this example, the 4 8 6 3 10 20 40 60 80 100 200 400 600800 1000 MOTOR (hp) 500 2 103 MODEL SIZE 7mm 1 4 V18ZA1 - V68ZA2 Figure 17. Stored energy curves for typical wye-connected 200 2 10 5 induction motor 100 10 10 1

6 PEAK PULSE CURRENT (A) NOTE: 50 102 10 PULSE RATING CURVE FOR 1,240µs PULSE WIDTH NOTES: 20 1.Y connected 60Hz. 103 104 105 106 107 108 109 1010 10 2. Energy at Max torque slip speed. NUMBER OF PULSES 5 3. See Figure 20 for varistor circuit placement. Figure 16. Extrapolated pulse rating curves 2 INDEFINITE 1 600 DELTA CONNECTED 400 RATED PEAK PULSE CURRENT (A) RATED 0.5 Motor Protection 460VRMS LINE - LINE 4 POLE 0.2 200 20 100 1,000 10,000 Frequently, the cause of motor failures can 230VRMS LINE - LINE 2 POLE IMPULSE DURATION (µs) 100 be traced to insulation breakdown of the 80 32 60 4 POLE Figure 15a. ZA Series V18ZA1-V68ZA2 (7mm) V511HA motor windings.The source of the tran- 40 2 POLE

510PA80 1,000 sients causing the breakdown may be from V 1 20 2 3 MODEL SIZE 14mm 10 V18ZA3 - V68ZA10 500 4 either internal magnetic stored energy or PER PHASE (J) ENERGY STORED V271HA32/V275PA40 10 10 105 6 10 200 102 10 from external sources.This section deals 8 6 100 10 20 40 60 80 100 200 400 600 800 1000 with the self-generated motor transients MOTOR (hp) 50 due to motor starting and circuit breaker Figure 18. Stored energy curves for typical delta-connection 20 operation. induction motor 10 INDEFINITE NOTES: 5 In the case of DC motors the equivalent 4. Delta connected at 60Hz.

RATED PEAK PULSE CURRENT (A) RATED 2 circuit consists of a single branch.The 5. Energy at maximum torque slip speed. 1 6. See Figure 20 for varistor circuit placement. 20 100 1,000 10,000 magnetic stored energy can be easily IMPULSE DURATION (µs) calculated in the armature or field circuits Figure 15b. ZA Series V18ZA3 to V68ZA10 (14mm) using the nameplate motor constants.With As an aid in selecting the proper operating 7mm V39ZA1 would not be limited to a AC induction motors the equivalent voltage for Littelfuse Varistors,Table 1 gives cumulative number of pulses. magnetic motor circuit is more complex guidelines for wye-connected and delta- and the circuit constants are not always connected motor circuits at different In cases where the peak current is greater given on the motor nameplate.To provide a line-to-line applied voltages. Figure 20 and intersects with the recommended guide for motor protection, Figures 17, 18, provides guidance in proper placement of pulse life curves, the designer must deter- 19 were drawn from typical induction the varistor. mine the maximum number of operations motor data.While the actual stored energy expected over the life of the circuit and will vary according to motor frame size and confirm that the pulse life curves are not construction techniques, these curves S O O G GS O CO C O O S

RMS Line Voltage 230 380 460 550 600 (Line-Line) 1A C.B. Delta Connected Applied V. 230 380 460 550 600 Varistor Ratings 250/275 420/480 510/575 575/660 660 Y Connected Applied V. 133 220 266 318 346 Varistor Ratings 150 250/275 320 420 420 V + C106D Table 1.. Littelfuse varistor selection guideline for 117VAC applications V NORMAL VOLTAGE < 240V PEAK - t ABNORMALVOLTAGE 600 has 52J of stored magnetic energy per FULL WAVE > 400V PEAK DELTA CONNECTED (RECTIFIED) 400 phase. Either a V320PA40 series or a 460VRMS LINE - LINE 4 POLE V321HA32 series varistor will meet this 200 230VRMS LINE - LINE 2 POLE requirement.The HA series Littelfuse Figure 21. Crowbar circuit 100 80 32 Varistor provides a greater margin of safety, 60 4 POLE V511HA although the PA series Littelfuse Varistor The supply shown can provide 2A RMS of 40 2 POLE fully meets the application requirements. V510PA80 short-circuit current and has a 1A circuit 20 Three varistors are required, connected breaker. A C106D SCR having a 4A RMS STORED ENERGY PER PHASE (J) ENERGY STORED V271HA32/V275PA40 directly across the motor terminals as 10 capability is chosen.Triggering will require at 8 shown in Figure 20. 6 least 0.4V gate-to-cathode, and no more 10 20 40 60 80 100 200 400 600 800 1000 MOTOR (hp) than 0.8V at 200A at 25˚C ambient. Figure 19. Stored energy curves for a typical motor with stalled rotor NOTES: Solution 7. 60Hz, see Figure 20 for varistor circuit placement. VL-L Check the MA series Littelfuse Varistor 8. Energy at start, i.e., SLIP = 1. specifications for a device capable of 9. Induction motor. 10. 2, and 4 pole motors. supporting 240V peak.The V270MA4B can handle √2 (171V RMS ) = 242V. According Interruption of motor starting currents to its specification of 270V ±10%, the 1 V = ------V V270MA4B will conduct 1mA DC at no presents special problems to the user as VARISTOR 3 LL— shown in Figure 19. Since the stored Figure 20a.Wye connected less than 243V.The gate-cathode resistor magnetic energy values are approximately FIGURE 20A. WYE CONNECTED can be chosen to provide 0.4V (the mini- 10 times the running values, protection is mum trigger voltage) at 1mA, and the SCR difficult at the higher horsepower levels. will not trigger below 243V.Therefore, RGK Often the motor is started by use of a should be less than 400.The highest value 5% tolerance resistor falling below reduced voltage which will substantially VL-L reduce the stored energy. A reduction in M 400 is a 360 resistor, which is selected.Thus, starting current of a factor of two results in RGK is 378 maximum and 342 minimum. a fourfold reduction in stored energy. If a Minimum SCR trigger voltage of 0.4V reduced voltage starter is not used, then a requires a varistor of 0.4V/378, or 1.06mA ≈ decision must be made between protection VVARISTOR = VL-L for a minimum varistor voltage of 245V. The maximum voltage to trigger the circuit for the run condition only, and the condi- Figure 20b. Delta connected tion of locked rotor motor current. For is dependent upon the maximum current most applications, the starting condition can the varistor is required to pass to trigger be ignored in favor of selecting the varistor the SCR. For the C106 at 25˚C, this is for the worst-case run condition. Power Supply Crowbar determined by calculating the maximum Occasionally it is possible for a power current required to provide 0.8V across a supply to generate excessively high voltage. parallel resistor comprised of the 360 RGK Problem An accidental removal of load can cause selected and the equivalent gate-cathode To protect a two-pole, 75hp, 3φ , 460V damage to the rest of the circuit. A simple SCR resistor of 0.8V/200A, since the C106 RMS line-to-line wye-connected motor safeguard is to crowbar or short circuit the requires a maximum of 200A trigger from interruption of running transients. supply with an SCR.To provide the trigger- current.The SCR gate input resistance is 4k ing to the SCR, a high-voltage detector is and the minimum equivalent gate-cathode Specific Motor Data Is Not Available needed. High voltage avalanche diodes are resistance is the parallel combination of 4k effective but expensive. An axial leaded Ω and RGK(MIN) , or 360 -5%, 342.The Littelfuse Varistor provides an effective, Ω Solution parallel combination is 315 .Thus, I VARIS- inexpensive substitute. Consult Figure 17 along with Table 1. TOR for maximum voltage-to-trigger the Standard varistors having the required volt- C106 is 0.8V/315, or 2.54mA. According to the specification sheet for the V270MA4B, age ratings are the 320VRMS rated models. Problem This allows a 20% high-line voltage condi- In the circuit of Figure 21, the voltage, with- the varistor will not exceed 330V with this tion on the nominal 460V line-to-line out protection, can exceed twice the current.The circuit will, therefore, trigger at voltage, or 266V line-neutral voltage. Figure normal 240V peaks, damaging components between 245 and 330V peak, and a 400V 17 shows a two-pole 75hp, wye-connected downstream. A simple arrangement to rated C106 can be used.The reader is induction motor, at the running condition, crowbar the supply is shown. cautioned that SCR gate characteristics are sensitive to junction temperatures, and The second category is that of surges This latter mode of stress may result in the a value of 25˚C for the SCR temperature produced by nearby lightning strokes.The eventual open-circuiting of the device due was merely chosen as a convenient value severity of a lightning stroke is characterized to melting of the lead solder joint. for demonstrating design procedures. in terms of its peak current.The probability of a direct stroke of a given severity can be When the device fails in the shorted mode The maximum energy per pulse with this determined. However, since the lightning the current through the varistor becomes waveform is determined as approximately current divides in many paths, the peak limited mainly by the source impedance. 1/2 x K x IPK x VPK x t (duration of 1/2 current available at an AC outlet within a Consequently, a large amount of energy can wave pulse), or 0.52mJ for this example. building is much less than the total current be introduced, causing mechanical rupture Since the voltage does not drop to zero in of the stroke.The standard impulse used to of the package accompanied by expulsion this case, the SCR remains on, and the represent lightning and to test surge of package material in both solid and varistor sees only one pulse; thus, no protective devices is an 8/20µs current gaseous forms. Steps may be taken to mini- steady-state power consideration exists. waveshape as defined by ANSI Standard mize this potential hazard by the following C68.2, and also described in ANSI/IEEE techniques: 1) fusing the varistor to limit General Protection of Standard C62.41-1991 and IEC 60664-1. high fault currents, and, 2) protecting the Solid State Circuitry surrounding circuitry by physical shielding, Against Transients On A third category of surges are those or by locating the varistor away from other 117VAC Lines produced by the discharge of energy stored components. in inductive elements such as motors and Problem transformers. A test current of 10/1000s waveshape is an accepted industry test Series and Parallel Modern electronic equipment and home impulse and can be considered representa- Operation of Varistors appliances contain solid state circuitry that tive of these surges. In most cases the designer can select a is susceptible to malfunction or damage varistor that meets the desired voltage caused by transient voltage spikes.The Although no hard-and-fast rules can be ratings from standard catalog models. equipment is used in residential, commer- drawn as to the category and severity of Occasionally the standard catalog models cial, and industrial buildings. Some test surges which will occur, a helpful do not fit the requirements either due to standards have been adopted by various guideline can be given to suggest varistors voltage ratings or energy/current ratings. agencies and further definition of the envi- suitable in typical applications. When this happens, two options are avail- ronment is underway by the IEEE and other able: varistors can be arranged in series or organizations. The guideline of Table 2 recognizes consid- parallel to make up the desired ratings, or erations such as equipment cost, equipment the factory can be asked to produce a The transients which may occur on residen- duty cycle, effect equipment downtime, and “special” to meet the unique application tial and commercial AC lines are of many balances the economics of equipment requirement. waveshapes and of varying severity in terms damage risk against surge protection cost. of peak voltage, current, or energy. For Series Operation of suppressor application purposes, these may Varistors be reduced to three categories. Failure Modes and Varistor Protection Varistors are applied in series for one of two reasons: to provide voltage ratings in First, the most frequent transient might be Varistors are inherently rugged and are excess of those available, or to provide a the one represented by a 30kHz or conservatively rated and exhibit a low fail- voltage rating between the standard model 100kHz ring wave.This test surge is defined ure rate.The designer may wish to plan voltages. As a side benefit, higher energy by an oscillatory exponentially decaying for potential failure modes and the resultant ratings can be achieved with series voltage wave with a peak open circuit volt- effects should the varistor be subjected to connected varistors over an equivalent age of 6kV.This wave is considered surge currents or energy levels above its single device. For instance, assume the representative of transients observed and rating. application calls for a lead mounted varistor reported by studies in Europe and North with an V rating of 375VAC and America.These transients can be caused RMS Failure Modes having a I peak current capability of by distant lightning strikes or distribution TM Varistors initially fail in a short-circuit mode 6000A.The I requirement fixes the varis- line switching. Due to the relatively high TM when subjected to surges beyond their tor size. Examining the LA series impedance and short duration of these peak current/energy ratings.They also voltage ratings near 375VAC , only 320V transients, peak current and surge energy short-circuit when operated at steady-state and 420V units are available.The 320V is are lower than the second and third cate- voltages well beyond their voltage ratings. too low and the 420V unit (V420LA40B) gories. results in too high a clamp voltage (VC of APPLICATION TYPE DUTY CYCLE LOCATION EXAMPLE SUGGESTED MODEL 1060V at 100A). For a V130LA20B and a Light Consumer Very Low A Mixer/Blender V07E130 or V10E130 V250LA40B in series, the maximum rated Consumer Low A Portable TV/Electronics V14E130 Consumer Medium A Home Theater, PC V14E130, V20E130 voltage is now the sum of the voltages, or Light Industrial/Office Medium B Copier, Server V20E130, V20E140 380V.The clamping voltage,VC , is now the Industrial Medium B Motors, Solenoid, Relay V20E140, V131HA32 sum of the individual varistor clamping volt- Industrial High B Large Computer Motor Control V131DA40 or DB40 Industrial High B Elevator Control Heavy Motors V151DA40 or DB40 ages, or 945V at 100A.The peak current capability is still 6500A but the energy Table 2. Littelfuse varistor selection guideline for 117VAC applications rating is now the sum of the individual SERIES PARALLEL energy ratings, or 200J. Objective Higher voltage capability. Higher Current Capability Higher energy capability. Higher Energy Capability Non-Standard voltage capability. In summary, varistors can be connected in Selection Required No Yes series providing they have identical peak Models Applicable All, must have same ITM rating. All models current ratings (ITM), i.e., same disc Application Range All voltages and currents. All voltages - only high currents, i.e., >100A. diameter.The composite V-I characteristic, Precautions ITM ratings must be equal. Must be identical voltage rated models. energy rating, and maximum clamp voltages Must test and select units for similar V-I characteristics. Effect on Ratings Clamp voltages additive. Current ratings function of current sharing as determined graphically. are all determined by summing the respec- Voltage ratings additive. Energy ratings as above in proportion to current sharing. tive characteristics and/or ratings of the Current ratings that of single device. Clamp voltages determined by composite V-I characteristic of matched units. Energy W , ratings additive. Voltage ratings that of single unit. individual varistors. TM Table 3. Checklist for series and parallel operation of varistors Parallel Operation of ing improves markedly. For instance, at a Reference Varistors clamp voltage of 900V, the respective varis- For more information concerning Littelfuse Application requirements may necessitate tor currents (Figure 20) are 2500A and Industrial application solutions visit the higher peak currents and energy dissipation 6000A, respectively.While far from ideal Littelfuse web site-http://www.littelfuse.com than the high energy series of varistors can sharing, this illustration shows the feasibility [1] Kaufman, R.,“The Magic of I2t,” IEEE supply individually.When this occurs, the of paralleling to achieve higher currents and Trans. IGA-2, No. logical alternative is to examine the possibil- energy than achievable with a single model 5, Sept.-Oct. 1966. ity of paralleling varistors. Fortunately, all varistor. Littelfuse Varistors have a property at high current levels that makes paralleling Practically, varistors must be matched by feasible.This property is the varistor's means of high current pulse tests to make series-resistance that is prominent during parallel operation feasible. Pulse the “up-turn region” of the V-I characteristic. testing should be in the range of over 1kA, This up-turn is due to the inherent linear using an 8/20µs, or similar pulse. Peak volt- resistance component of the varistor char- ages must he read and recorded. High acteristic. It acts as a series balancing, current characteristics could then be orballasting, impedance to force a degree of extrapolated in the range of 100A - sharing that is not possible at lower current 10,000A.This is done by using the meas- levels.This is depicted in Figure 20. At a ured data points to plot curves parallel to clamp voltage of 600V, the difference in the data sheet curves.With this technique current between a maximum specified current sharing can be considerable sample unit and a hypothetical 20% lower improved from the near worst-case condi- bound sample would be more than 20 to tions of the hypothetical example given in 1.Thus, there is almost no current sharing figure 22. and only a single varistor carries the current. Of course, at low current levels in In summary, varistors can be paralleled, but the range of 10A -100A, this may well be good current sharing is only possible if the acceptable. devices are matched over the total range of the voltage-current characteristic. In At high current levels exceeding 1000A, the applications requiring paralleling, Littelfuse up-turn region is reached and current shar- should be consulted.

LIMIT SAMPLE Some guidelines for series and parallel 1000 800 operation of varistors are given in Table 3. 600 500 400 300 LOWER BOUND (20%) 200 SAMPLE UNIT PEAK VOLTAGE (V) PEAK VOLTAGE MODEL V251BA60 o o TA = -40 C TO 85 C 100 0.1 0.5 1 5 10 50 100 500 1000 5000 10000 Littelfuse, Inc. PEAK CURRENT (A) 800 E. Northwest Highway Figure 22. Parallel operation of varistors by graphical tech- Des Plaines, IL 60016 nique www.littelfuse.com

Specifications, descriptions and illustrative material in this literature are as accurate as known at time of publication, but are subject to change without notice. Littelfuse is a registered trademark of Littelfuse Incorporated.

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