Transients-1

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  1. Lighting strikes are passed on to the distribution lines in two ways and are defined how? What are they? Keith H. Sueker, Power Electronics Design, 2005 PP 57 (NC set: transients)
    • differential modes: Voltage between the power line conductors
    • common modes: voltage of the several conductors to ground
  2. Differential-mode voltages are passed directly through transformers and appear where & how? Keith H. Sueker, Power Electronics Design, 2005 PP 57 (NC set: transients)
    on the secondaries as transformed by the turns ratio.
  3. Explain the construction of an MOV or metal oxide varistor with a non-linear V/I relationship and thus its ability to suppress voltage surges.
    A MOV contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). Source: http://www.electroschematics.com/5224/metal-oxide-varistor/
  4. MOV's, metal oxide varistors are components that have what characteristic? Keith H. Sueker, Power Electronics Design, 2005 PP 35 (NC set: transients)Image Upload
    • A nonlinear V/I characteristic 
    • Explanation: When exposed to high transient voltage, the MOV clamps voltage to a safe level. A metal oxide varistor absorbs potentially destructive energy and dissipates it as heat, thus protecting vulnerable circuit components and preventing system damage. Varistors can absorb part of a surge. Source: http://www.electroschematics.com/5224/metal-oxide-varistor/
  5. Metal oxide Varistors of the types used for power electronics are made by pressing and sintering wafers of __________. Keith H. Sueker, Power Electronics Design, 2005 PP 37 (NC set: transients)
    zinc oxide ceramic.
  6. MOV's have a little power dissipation capability, and can experience what as a result of repetitive transients such as produced by SCR commutation? Keith H. Sueker, Power Electronics Design, 2005 PP 36 (NC set: transients)
    Destruction
  7. MOV are generally applied at their nominal RMS voltage rating and are expected to clamp transients to a peak voltage of _____ times their RMS rating. Keith H. Sueker, Power Electronics Design, 2005 PP 36 (NC set: transients)
    (formula) 2.5 for 2.5 times their RMS or 2.5(.707x peak voltage). RMS is Root Means Squared and also the same heating effect AC as it’s DC counterpart.
  8. The best protection from line-induced transients of all types on secondaries is? Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)
    A set of MOVs
  9. Common-mode voltages (voltage to ground) can be effectively stopped at a transformer by what? Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)

    Also called what? PP 58 (NC set: transients)
    An electrostatic shield between the windings.

    A Faraday screen
  10. Dry type transformers should be equipped with at least what on medium voltage circuits to avoid transient over voltage issues? Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)
    Distribution-class lightning arrestors.Image Upload
  11. Most gate drives for SCRs (Silicone Controlled Rectifiers) or insulated gate bipolar transistors (IGBT’s) are supplied in one way or another through the transformers with primaries at what potential and secondaries at what voltage? Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)
    With primaries at control circuit potential and secondaries at the cathode or emitter voltage.
  12. Transformers have ________ that will couple the power voltage transients on the semiconductors to the low-voltage control circuits. Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)
    interwinding capacitances.
  13. The best protection to transient over voltages through printed circuit board traces is what? Keith H. Sueker, Power Electronics Design, 2005 PP 58 (NC set: transients)
    To equip the gate drive transformers with electrostatic shields and to be sure their primaries are tied directly to ground whenever possible.
  14. Ground, common, and powers leads entering and exiting a PC board should be located where? Keith H. Sueker, Power Electronics Design, 2005 PP 59 (NC set: transients)
    on the same end if possible, due to inductance ground planes.
  15. The operating coils on contactors and power relays will generate what when they are de-energized? Keith H. Sueker, Power Electronics Design, 2005 PP 59 (NC set: transients)
    transient voltages, or inductive kick.
  16. Regarding transients and power relays, what on the coils will reduce the effect on other circuit elements? Define this. Keith H. Sueker, Power Electronics Design, 2005 PP 59 (NC set: transients)
    R/C snubbers. : A simple RC snubber uses a small resistor (R) in series with a small capacitor (C). An appropriately-designed RC snubber can be used with either DC or AC loads. This sort of snubber is commonly used with inductive loads such as electric motors.
  17. Signal circuit should be further protected by using shielding wire, the shields should be continuously connected but they should be grounded only at what point? Keith H. Sueker, Power Electronics Design, 2005 PP 59 (NC set: transients)
    Only at a single point that serves as a earth ground for all signal commons.

    Image Upload
  18. If the signal lead is routed along the ground paths what happens to ground-loop? Keith H. Sueker, Power Electronics Design, 2005 PP 59 (NC set: transients)
    The loop is eliminated.
  19. What is the relationship between shielding electrostatic effects and electromagnetic coupling? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)

    What is the answer to electromagnetic coupling? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    Shielding Can Eliminate electro-static effects but it does little to prevent electromagnetic couplings.

    Twisted pairs are the answer.
  20. What can switching currents in semi conductors cause? Keith H. Sueker, Power Electronics Design, 2005 PP60 (NC set: transients)
    Switching currents and semi conductors can cause very high DI/DT (Developed current/developed time) levels particularly in IGBT's (insulated gate bipolar transistors) and these transients can couple to other circuitry gate drives, and low-level circuitry should be kept away from power switching circuits.
  21. What will drop the source voltage to zero in a microsecond or so and hold it there regarding SER's? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    Commutation period.
  22. The commutation period: dropping the source voltage to zero in a microsecond or so and holding it there until the commutation is completed is in effect called what? Keith H. Sueker, Power Electronics Design, 2005 PP60 (NC set: transients)
    Notching
  23. Notching is a disturbance that can propagate to other equipment in the area through what? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)

    One way to reduce notching is to install what on the power lines? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    Common power lines

    R/C snubbers

    Image Upload
  24. Most but not all transients are associated with what? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    Rising voltages, but falling voltages can also interrupt operation.
  25. If the control transformer is supplying power to large contactor coils or fan motors the inrush current may cause what over relays or operations? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    The voltage loss sufficient to drop relays or disrupt operations.
  26. The cure to inrush current causing the voltage loss sufficient to drop relays or disrupt operations is what? Keith H. Sueker, Power Electronics Design, 2005 PP 60 (NC set: transients)
    To oversize the control transformer or use low-leakage reactance design so that the control voltage can be held above the required minimum.
  27. Name three signal processing electronic components. Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)

    Signal processing electronic components have a limited ability to reject what on differential inputs Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    Opamps, digital processors, and multipliers.

    Common mode noise on differential inputs.
  28. A circuit is presumably immune to high-frequency noise because of _______ of the multiplier. Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    Because of the common mode rejection of the multiplier
  29. If a noise level is not really known it is a good idea to install what ahead of each multiplier input? Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    Pole of rolloff: A low pass filter
  30. EMI or electromagnetic interference is defined levels for both _____________levels. Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    Radiated and Conducted levels
  31. Regarding an unknown noise level and installing a poll of rolloff Ahead of each multiplier input, the pole should be located about how far above the highest frequency that the multipliers expected to pass?Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    A decade.
  32. Radiated interference into and out of equipment can usually be thwarted by good grounding techniques and shielding of critical circuits. Interference conducted by supply control or load wiring may require the installation of what to reduce conducting noise in both directions into it out of the equipment? Keith H. Sueker, Power Electronics Design, 2005 PP 61 (NC set: transients)
    A T-filter

    Image Upload
  33. Re: EMI and instrumentation what is often used to measure DC current levels and occasionally for AC? Keith H. Sueker, Power Electronics Design, 2005 PP 62 (NC set: transients)
    Shunts: In electronics, a shunt is a device which allows electric current to pass around another point in the circuit by creating a low resistance path.
  34. Regarding EMI problems with instrumentation what is used to amplify the shunt output voltage that is usually in the 50 to 100 mV range? Keith H. Sueker, Power Electronics Design, 2005 PP 62 (NC set: transients)
    • Isolation amplifiers. 
    • informational: Isolation amplifiers are a form of differential amplifier that allow measurement of small signals in the presence of a high common mode voltage by providing electrical isolation and an electrical safety barrier. Isolation amplifiers are used in medical instruments to ensure isolation of a patient from power supply leakage current.[1]
  35. What is also known as an isolation amplifier?
    source: http://www.learningaboutelectronics.com/Articles/Isolation-amplifier
    Also called a unity-gain amplifier, this is an op-amp circuit which provides isolation of one part of a circuit from another, so that power is not used, drawn, or wasted in a part of the circuit.
  36. The purpose of an isolation amplifier isn’t to amplify the signal.  So what happens to the voltage?
    Image Upload
    • The same signal that is input into the op amp gets passed out exactly the same. This means that output voltage is the same exact as the input voltage, meaning if 10V AC is input into a circuit, 10V AC is output. An op amp is a device with a very high input impedance. This high input impedance is how it causes isolation.
    • http://www.learningaboutelectronics.com/Articles/Isolation-amplifier
  37. Difficulties may arise if care is not taken with what in regards to the shunt especially if the power circuit has a high ripple level? see figure below. Keith H. Sueker, Power Electronics Design, 2005 PP 62 (NC set: transients)
    Lead dress.

    Image Upload
  38. The shunt should be made such that the terminals are symmetric about what of the shunt? Keith H. Sueker, Power Electronics Design, 2005 PP 62 (NC set: transients)
    Symmetric about the geometric center of the shunt.

    Image Upload
  39. Contaminating signals can be neutralized with what of a few turns connected in series with what to shunt output? Keith H. Sueker, Power Electronics Design, 2005 PP 62 (NC set: transients)
    • Air core "pick up” coil.
    • Image Upload

    • Informational: 
    • Its inductance is unaffected by the current it carries. This contrasts with the situation with coils using ferromagnetic cores whose inductance tends to reach a peak at moderate field strengths before dropping towards zero as saturation approaches. Sometimes non-linearity in the magnetization curve can be tolerated; for example in switching converters. In circuits such as audio cross over networks in hi-fi speaker systems you must avoid distortion; then you need an air coil. Most radio transmitters rely on air coils to prevent the production of harmonics. Source: http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html
  40. The precautions on shunt metering apply also to oscilloscope measurements of low-level signals in the presence of interference the ground lead of the scope should be looped back and forth along ________ to minimize loop area and the resultant induced voltages. Keith H. Sueker, Power Electronics Design, 2005 PP 63 (NC set: transients)
    Back and forth along the probe.
Author:
johnbowens
ID:
124951
Card Set:
Transients-1
Updated:
2018-01-26 22:45:20
Tags:
transients JATC IBEW NJATC electrical over voltages
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Description:
transients in electrical power systems
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