Description : The material for commutator brushes is generally (a) mica (b) copper (c) cast iron (d) carbon
Last Answer : Ans: d
Description : In a commutator (a) copper is harder than mica (b) mica and copper are equally hard (c) mica is harder than copper (d) none of the above
Last Answer : Ans: c
Description : In D.C. generators, current to the external circuit from armature is given through (a) commutator (b) solid connection (c) slip rings (d) none of above
Last Answer : Ans: a
Description : In addition to undercutting the mica, proper maintenance of a D.C. motor's commutator includes _______________. A. side-cutting the copper segments and insulating the slots B. coating the copper surface with light machine ... in an oven at a maximum of 95°C for not over 8 hours D. all of the above
Last Answer : Answer: A
Last Answer : The insulating material used between the commutator segments is normally mica.
Description : Which of the following statements best describes the material known as varnished cambric? A. Felted asbestos sealed with varnish. B. Cotton cloth coated with insulating varnish. C. Rubber ... Paper impregnated with mineral oil, specially wrapped with nonmetallic tape, and coated with varnish.
Last Answer : Answer: B
Description : Etched or burned bands on the contact faces of the brushes in a direct current generator can be caused by ____________. A. high mica segments B. copper drag on the commutator C. brushes improperly positioned D. copper embedded in the brushes
Last Answer : Answer: C
Description : Brushes of D.C. machines are made of (a) carbon (b) soft copper (c) hard copper (d) all of above
Description : Iron losses in a D.C. machine are independent of variations in (a) speed (b) load (c) voltage (d) speed and voltage
Last Answer : Ans: b
Description : In case of D.C. machines, mechanical losses are primary function of (a) current (b) voltage (c) speed (d) none of above
Description : A separately excited generator as compared to a self-excited generator (a) is amenable to better voltage con-trol (b) is more stable (c) has exciting current independent of load current (d) has all above features
Description : Copper brushes in D.C. machine are used (a) where low voltage and high currents are involved (b) where high voltage and small cur-rents are involved (c) in both of the above cases (d) in none of the above cases
Description : In a four-pole D.C. machine (a) all the four poles are north poles (b) alternate poles are north and south (c) all the four poles are south poles (d) two north poles follow two south poles
Description : In lap winding, the number of brushes is always (a) double the number of poles (b) same as the number of poles (c) half the number of poles (d) two
Description : 77 The transformer laminations are insulated from each other by A. Mica strip B. Thin coat of varnish C. Paper D. Any of the above
Last Answer : B. Thin coat of varnish
Description : Mica is a (A) Dielectric material. (B) Insulating material. (C) Magnetic material. (D) Both insulating and dielectric material.
Last Answer : (D) Both insulating and dielectric material.
Description : The material used in the manufacture of lead pencil is – (1) Graphite (2) Lead (3) Carbon (4) Mica
Last Answer : (1) Graphite Explanation: Pencils create marks via physical abrasion, leaving behind a trail of solid core material that adheres to a sheet of paper or other surface. They are noticeably distinct from ... of graphite mixed with a clay binder, leaving grey or black marks that can be easily erased.
Description : The material used in the manufacture of lead pencil is (1) Graphite (2) Lead (3) Carbon (4) Mica
Last Answer : Graphite
Description : Electrical leads and insulation on a motor should be painted with ____________. A. heat-resisting acrylic B. heat-resisting aluminum C. insulating varnish D. insulating white lead
Description : Temporary repairs to an open DC propulsion armature coil can be made by _____________. A. connecting the coil ends directly to a pair of negative brushes B. disconnecting coil ends, insulating ... C. grounding the coil ends and short circuiting the commutator bar D. removing the sparking brushes
Description : Which of the listed conditions might contribute to very rapid wearing of a DC machine's commutator bars? A. A grounded commutator bar B. Using improper carbon brushes C. Aligning the front and rear mica V-rings improperly D. An open circuit in the armature
Description : Which of the listed faults cannot be eliminated, EXCEPT by turning or grinding a commutator with a rigidly supported tool? A. Sparking brushes B. Eccentricity C. High mica D. Blackened commutator
Description : Which of the listed conditions will occur if dirt and grease are allowed to accumulate between the commutator segments of a motor? A. A partial short circuit. B. A dead short circuit. C. Misalignment of the motor shaft. D. Overspeeding of the motor.
Description : How many commutator segments are required in a two-coil generator?
Last Answer : Four
Description : The current from the stator of an alternator is taken out to the external load circuit through (A) slip rings. (B) commutator segments. (C) solid connections. (D) carbon brushes.
Last Answer : (C) solid connections.
Last Answer : In case of D.C. machine winding, number of commutator segments is equal to number of armature coils.
Last Answer : The commutator segments are connected to the armature conductors by means of copper lugs.
Description : _____ brushes are fragile and cause excessive wear of commutator, (a) Natural graphite (b) Hard carbon (c) Electro graphitic (d) Metal graphite
Last Answer : (a) Natural graphite
Description : Voltage failure of an AC generator may be caused by ____________. A. failure of the exciter generator B. a tripped bus circuit breaker C. high mica segments on the stator bus bar D. excessive prime mover speed
Description : The disadvantage with paper as insulating material is (a) it is hygroscopic (6) it has high capacitance (c) it is an organic material (d) none of the above
Last Answer : (a) it is hygroscopic
Description : State two applications of : (i) PVC paper (ii) Porcelain with type of class based on withstand temperature is insulating material.
Last Answer : (i) Application of PVC paper : 1. It is used in slot lining or motor winding insulation purpose 2. For Insulation of Transformer winding (ii) Application of Porcelain : 1. Mainly used for manufacturing ... on withstand temperature : * H' & C' Class of insulation i e. 180° and above 180°C
Description : The 'dielectric constant' is a numerical value indicating the effectiveness of a dielectric material in comparison to that of a standard, which is _______________. A. paper or cloth B. glass or mica C. plastic or Teflon D. dry air or a vacuum
Last Answer : Answer: D
Description : Which of the following is not a dielectric material? (A) Cotton, silk and paper (B) Asbestos, glass, porcelain and mica (C) Rubber and polymeric resins (D) All refractory materials
Last Answer : Option C
Description : Which of the listed groups of electrical insulation materials is best suited for the highest operating temperatures? A. impregnated cotton and silk B. unimpregnated paper and cotton C. pure glass and quartz D. mica and porcelain with bonding agents
Description : Cables for 220 kV lines are invariably (a) mica insulated (b) paper insulated (c) compressed oil or compressed gas insulated (d) rubber insulated (e) none of the above
Last Answer : (c) compressed oil or compressed gas insulated
Description : Good conductor of electricity is – (1) dry air (2) paper (3) kerosene (4) graphite
Last Answer : (4) graphite Explanation: Graphite has a tendency to behave very much like a metal because the carbon molecules arrange themselves into a lattice structure. The crystal lattice is the same orientation that ... better than the dry air paper and kerosene and that's what makes it a good conductor.
Description : Good conductor of electricity is (1) dry air (2) paper (3) kerosene (4) graphite
Last Answer : graphite
Description : D.C. motor brush adjustment includes attention to _________. A. angle with the commutator and clearance within the holder B. brush length and pigtail tightness C. grade of material and surface dirt conditions D. all of the above
Description : State the function of commutator in DC generator and name the material used for commutator.
Last Answer : Function of Commutator: Commutator converts AC induced in armature winding into DC for external load. Material: i) Commutator segments are made up of Copper. ii) Commutator segments are insulated from each other by Mica.
Last Answer : The material for commutator brushes is generally carbon.
Description : What is the effect of heat on insulating material ?
Last Answer : With increasing temperature, the resistance of almost all solid insulators except Avro decreases. When the temperature drops again, the resistance increases.
Description : Biological functions of lipids include (A) Source of energy (B) Insulating material (C) Maintenance of cellular integrity (D) All of these
Last Answer : Answer : D
Description : A circuit breaker differs from a fuse in that a circuit breaker _____________. A. melts and must be replaced B. is enclosed in a tube of insulating material with metal ferrules at each end C. gives no visual indication of having opened the circuit D. trips to break the circuit and may be reset
Description : Which of the following is the most widely used insulating material for steam carrying pipelines? (A) Ceramic fibre blanket (B) Glass wool and 85% magnesia (C) Vermiculite (D) Slag wool
Last Answer : (B) Glass wool and 85% magnesia
Description : The compound of silicon used as insulating material for electric motors/appliances is
Last Answer : silicone.
Description : The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface) (A) K/h0 (B) 2K/h0 (C) h0 /K (D) h0 /2K
Last Answer : (B) 2K/h0
Description : What do you understand by tan delta for a insulating material?
Last Answer : Tan delta measurement is done to find the qualities of insulating material. Tan delta is angle between current due to surface leakage or current due to capacitance and the capacitive current. That is Tan δ = Ir / Ic.
Description : Which of the following is the most widely used heat insulating material for pipelines carrying steam? (A) Tar dolomite bricks followed by asbestos (B) Fireclay refractory followed by aluminium sheet (C) Cotton followed by aluminium foil (D) 85% magnesia cement and glass wool
Last Answer : (D) 85% magnesia cement and glass wool
Description : What is the major factor that makes coaxial cable less susceptible to noisethan twisted-pair cable? A) inner conductor B) diameterof cable C) outer conductor D) insulating material
Last Answer : outer conductor
Description : A steam pipe is intended to be insulated with two layers of insulating materials of different thermal conductivities. For minimum heat transfer to take place (A) The better insulation material should be put just ... to which insulating material is to be put inside (i.e., just next to the pipe)
Last Answer : (A) The better insulation material should be put just next to pipe (i.e., inside)