Motor Controls

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  1. A motor thermal device is either of the__________ or the ____________.
    Bi-metal type or the melting alloy (solder pot) design. pp. 7Image Upload
  2. The bi-metal type, as the name implies, has two (BI) dissimilar metals laminated together. As the two metals are of different types, the expansion rates are different when heat is applied. This difference in expansion rates causes the laminated piece to bend or warp. This warping action_____________.
    Opens a set of normally closed contacts. PP7
  3. The overload section of the starter, as the name implies, protects the motor from overloads and/or?
    Phase failure. PP6
  4. The contactor opens and closes contacts using the electromagnet principle, is restricted to?
    Fixed lighting loads, electric furnaces and other resistive loads that have set current values, or portions of a motor control circuit that do not require OVERLOAD protection. PP6
  5. Overload devices are of three basic types:
    Thermal, magnetic, and solid state. PP6
  6. Heaters have letter and/or number designations which are used for proper sizing. Each manufacturer uses?
    Different designations, so there is no industry standard. The manufacturer’s data must be referred to for the being used. PP8
  7. Bi-metal overload device is adjustable, much like a thermostat. The standard adjustment range is from 85% (closer) to 115% (farther away? The adjustment is often used to increase the overload setting to eliminate?
    Nuisance tripping. PP8
  8. The second type of thermal overload device is the melting alloy type, commonly called the solder pot type. This device basically consists of?
    • A ratchet wheel with a shaft that sets in a tube; : tube is filled with a metal
    • (a metal with a low melting point) After the metal has cooled, the device can be manually reset. pushing the reset button is like cocking a gun. PP9
  9. In regards to the solder pot overload. Installed in the Low position the heater coil is lower on the tube and…? A.The eutectic metal will melt with _____ current, and higher up in position it will melt with _____ current. PP9
    less …. more.
  10. To eliminate the need for gigantic sized heater elements and associated overload relays, current transformers are used (pis. 129) current transformers are standard on what size starters?
    • NEMA size 5 and larger starters. Motor Controls- Richard Cox, 2nd edition. PP11
    • 60HP for 240 volt and 125HP for 480 volt motors start at NEMA size 5.
  11. To compensate for starters that are located in high operating temperature locations, _________must be installed conversely, in low temperature areas, ________ elements are installed. PP10
    Oversized heaters …undersized heaters.
  12. Some bi-metal units are designed to compensate for changes in ambient temperature, This type is labeled as?
    Compensated overload relay. PP10Image Upload
  13. On the Window or Donut-type CT’s, the conductor, bus bar, or bushing which passes
    through the center of the transformer constitutes one primary turn. On Window-type units with low primary current ratings, where the primary conductor size is small, the ratio of the transformer can be changed by taking multiple wraps of the primary conductor through the window. If, for example,
    a window CT has a ratio of 100:5, placing two primary conductor wraps (two primary turns)through the window will
    change the ratio to ______. Some types of equipment employ this method to calibrate the equipment or to permit a single ratio CT to be utilized for several different ampacities of
    • 50:5 ratio, or halfing the primary ratio is accomplished with each additional wrap of conductor through these.
    • source:
  14. The output of standard CT’s are usually, but not always, _____ max.
    5 amp maximum output for these. Electricians Guide to AC Motor Controls- Richard Cox 1981 edition. PP11

    • The most common CT secondary full-load current is 5 amps which matches the standard 5 amp full-scale current rating of switchboard indicating devices, power metering equipment, and protective relays. CT’s with a 1 amp full-load value and matching instruments with a 1 amp full-range value are also available. Many new protective relays are programmable for either value.
  15. Caution: The secondary of a Current Transformer must always have a burden (load) connected; an open-circuited secondary can result in the development of a dangerously high secondary voltage. Energized but
    unused CT’s must be kept________.
    • The secondary of these must always be kept short circuited to avoid a buildup of high voltage.
    • source:
  16. All current transformers are ________ polarity. Polarity refers to the instantaneous direction of the primary cur-
    rent with respect to the secondary current and is determined by the way the transformer leads are brought out of
    the case. On subtractive polarity transformers the H1 primary lead and the X1 secondary lead will be on the same
    side of the transformer (the left side when facing the low-side bushings).
    • All of these have subtractive polarity, meaning they are polarity sensitive.
  17. The secondary of a CT has one terminal marked for polarity, as indicated by? And this is what polarity?
    The black dot represents the negative terminal on the secondary of this device. PP11
  18. When CT’s are used in conjunction with thermal overload relays they should be grounded on the?
    • secondary represented by a black dot on a CT with thermal overload relays. PP11
    • Image Upload
  19. Regarding the bushing CT, care must be taken with the installation to insure that the mounting clamp bolts do not contact what and why?
    Also because the case is metal and is installed
    externally it can decrease the_______. The circuit breaker or transformer manufacturer should be consulted to verify acceptability of the installation.
    Image Upload
    • Care for these types of CT’s must be taken for contact to the metal case with the mounting clamp bolts resulting in a one-turn primary short circuit.
    • The bushing strike distance can be reduced due to the metal outside case of these type of CT’s.
    • source:
  20. Magnetic types of overload relays are of two basic types?
    • time limiting (Inverse time) and instantaneous trip. PP12
    • Inverse Time: done with a purposely delayed action that decreases as the operating force increases —used especially of electrical relays: Mariam Webster Dictionary (OR)
    • Inverse time tripping is a characteristic of circuit breakers in which the breaker trips in more time with lower overcurrent, and less time with higher overcurrent. source:
  21. Time limiting or delay type (Dash pot) of magnetic overload, as shown, has a heavy gauge wire-wound wired directly in series with the motor leads or by use of what for larger motors, usually NEMA size 5 or larger? PP12
    Image Upload
    CT’s to monitor motor current are used for these two types of overloads.
  22. In a time limiting or delay type magnetic overload, the trip current level can be made by screwing the core of the relay? These are also called what type of relay or overload? Describe its operation.PP12
    Up or down, and is also called a Dashpot Relay. As the current increases, the pressure in the oil around the core will pass through small orifices and open the contacts.
  23. instantaneous trip magnetic relay is not designed for normal overload use, as the primary use of this device is to? PP13
    • Trip instantaneously at a preset current level.
    • Image Upload
  24. Solid state overload relays are sealed electronic devices with three current sensing loops through which the ________ pass? PP13
    • motor circuit conductors…
    • Image Upload
  25. Solid state overload relays are adjustable from? You can do what to provide for lower ranges? PP13
    80 to 120% of the units rated value. By using multiple turns through the sensing loop (one for ea. phase), for instance, two turns in a 10 amp relay can provide protection for a 20 amp motor.
  26. Solid state overload relays are virtually unaffected by? PP 13
    ambient temperatures
  27. Due to the large size and weight involved in closing the contacts of size 6 and larger starters, DC coils are used by most manufacturers to provide faster, more positive closure. ________ is used to produce the DC voltage necessary for the coil. DC eliminates the _____ that is associated with most larger sized starters and contactors. PP13
    A bridge rectifier and eliminates hum.
  28. Arc quenching is also called?
    Arc rupturing. PP13
  29. Two methods of arc quenching are? PP13 & 14
    The arc to rises in the arc box or chute where it contacts de-ionization grids or plates and a series of small arcs, cooled and is rapidly extinguished. Another method used to quickly extinguish the arc caused by opening contacts under load is the blowout coil. Blowout coils are coils of wire connected in series with the contacts and the motor leads. Current flow through the coil sets up a magnetic field across the pole pieces as shown.
  30. No matter what method is used for arc-quenching, the arc chutes or boxes also prevent the arcs from crossing phase to phase or phase to ground. For this reason, the starter or contactor must never be……? PP17
    …operated if the arc chutes are not in place If the chutes moved for cleaning or replacement, tag or lockout the starter to prevent it from being energized.
  31. Normally in AC magnetic equipment, black wires are used in _______ and red wires are used for _______. bold lines denote the power circuit, and thin lines used to show the control circuit. PP18
    power circuits & control circuits.
  32. To energize the coil, it is necessary that what two potentials be available at the coil? PP 19.
    Both L1 & L2.
  33. Three wire control, as the name implies, will require three wires to control the motor circuit.
    The third wire will allow use of momentary pushbuttons, float switches, limit switches, pressure switches, etc., the motor control
  34. (Sp) Single pole, indicates?  ST (Single Throw) indicates?
    (Single Pole) indicates one circuit conductor can be opened or closed. Single throw indicates possible positions of the switch other than OFF. PP 30
  35. silver or silver alloy reduces contact maintenance because?
    the film caused by the arc when the contacts are opened and closed is silver oxide, which is itself a good conductor. PP31
  36. There are basically two types of contact ratings?
    and CONTROL- The power rated contacts are to carry and to interrupt large amounts of electrical current, whereas control contacts are smaller in physical size and are designed to carry only small amounts of electrical current (normally 10 amp continuous rating). PP31
  37. A 115 volt single phase motor can be controlled by opening and closing the hot (Ll) conductor. The exception would be?
    A 115V single phase motor used for a gasoline pump National Electric Code requires that all conductors be disconnected from the source of supply. A double pole switch then would be needed, one pole for each conductor LI and L2. PP32
  38. SPDT is sometimes called a _________, because it can direct or transfer power to either of two loads. It will not, however, operate both loads at the same time (Fig- 3.7). PP33.
    transfer switch
  39. To reverse the direction of rotation of a single phase (split phase) motor requires that the leads of the start winding be reversed • reversing the direction of rotation of a standard 3 phase induction motor can be accomplished changing or reversing any two line or motor leads. This can also be accomplished by a DPDT switch, or a four way, by switching what two leads?
    L1 and L2.Image UploadImage Upload

Card Set Information

Motor Controls
2017-04-22 19:56:37
motors controls overloads Richard Cox

Source- Electricians Guide to AC Motor Controls; 1981, Richard Cox.
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