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COLLECTED QUESTIONS FROM CEE II
1
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The cost-effectiveness of the PLC
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The speed PLC’s processor or processors performance
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The ability to simulate the program without having to connect to the PLC
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The communication protocols used by the PLC
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Outline FOUR mechanical checks carried out on mounted and wired electrical machines
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Describe mechanical checks performed on electrical machines
Temperature of machine
Vibration if any
Balance and alignment of the rotor
Oil or grease in bearings
Leakages if any
4
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Non-uniform
foundation stiffness leads to excessive vibrations in the motor frame
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g
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Alignment is achieved via dial indicators and
feeler gauges
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c) Coupling: coupling is a connection component used to transmit torque between a drive
shaft and a driven part e
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between a motor shaft and a ball screw
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5
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g electric
excavators, elevators etc
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is AC motor or DC motor with constant speed
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whose
speed is to be controlled
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Simiyu D
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Hence, very smooth speed control of the dc motor can be
obtained by this method
...
)
It is a very smooth speed control system over a very wide range (from zero to normal speed of
the motor)
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) The speed can be controlled in both the direction of rotation of the motor easily
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) The motor can run with a uniform acceleration
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) Speed regulation of DC motor in this Ward Leonard system is very good
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) It has inherent regenerative braking property
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)
Higher initial cost due to use of two additional machines of the same rating as the main dc
motor
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) Larger size and weight
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) Requires more floor area and costly foundation
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) Frequent maintenance is needed
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) Lower efficiency due to higher losses
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) The drive produces more noise
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List TWO tests carried out on industrial control panels
Power and cable size check
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Megger and high voltage test for the bus bar panel
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The start-stop sequencing
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Simiyu D
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Apply a voltage of 500 volts DC between phase R to earth for one minute with phase Y, B & N
shorted and earthed and record the values
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Also connect the enclosure to earth
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Discharge pole and also the test kit
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Commissioning procedure for motor control center
Make sure that all the pre-commissioning verifications are executed effectively
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List components found in industrial control panels
General industrial control panels have: breakers, suppressors, and fuses
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) Circuit breaker to control power supply
ii
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) Terminal blocks housing different voltage level lines
iv
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) Relays for switched controls
8
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)
Change the level or value of the signal e
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voltage level
ii
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g
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Simiyu D
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) Converting analogue and digital signals from one to the other
iv
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Explain each of the following job distribution with respect to preventive maintenance
program
a) Planning techniques: an organized approach to maintaining the condition of a
machine, or equipment
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b) Staffing: Ensuring expertise/personnel in preventive maintenance to never miss a
scheduled task
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Explain each of the following fault in asynchronous motor
a) 3-phase fault: Faults in where all the phases (Yellow, Blue and Red) are shorted together
and they are grounded
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b) Single line to ground fault: occurs when one of the power lines comes into direct contact
with the ground or any other low-impedance path
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c) Line-to-line fault: i
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unsymmetrical fault that occur when lines (conductor) come into
physical contact or short, e
...
due to a broken insulator
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Simiyu D
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Fault current flowing from the line to the ground
within the involved phases, say, Yellow Phase and Blue Phase
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11
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)
Clean motor, blowing out dirt from windings, and wipe commutator and brushes
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) Visually inspect commutator clamping ring
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) Check and replace brushes that are more than half worn
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) Examine brush holders, and clean them if dirty
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) Check brush pressure and position
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Explain the functions of each of the following components in a 3-point starter for a dc
motor
a) No-volts release coil (NVC): the no volt coil retains the starter knob in the last step, as far
as the supply voltage is available
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NVC coil loses its magnetism and leaves the handle, the handle the retracts back to
OFF position because of spring action
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OLC is a small electromagnet
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c) Hinged spring on starter handle: has spring force, the handle comes back to OFF
position, opening the circuit and thus switching off the motor
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Mr
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E
Page 5 of 7
Three-point starter of DC shunt motor
1) Starting resistance 2) Starting arm 3) NVC - No-Volt Release coil (Hold-ON Coil) 4) Soft Iron
Piece
5) Overload relay 6) Arm/strip 7) Spring L-Line F-field winding
13
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Simiyu D
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With aid of a labelled circuit diagram explain the operation of a capacitor-start single
phase induction motor
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A centrifugal switch S C is
connected to the circuit
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Thus, a single-phase supply current is split into two phases
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Simiyu D