Search in the document preview
Cork Institute of Technology Higher Certificate in Engineering in Electrical Engineering – Award
(NFQ Level 6)
Autumn 2006 Electrical Power Systems & Equipment
(Time: 3 Hours) Instructions Answer FIVE questions. All questions carry equal marks
Examiners: Mr. J. C. Duggan Mr. M. Hennessy Prof. E. McQuade
Q1. The start-up sequence for an industrial plant is specified below. Control is by single stop
and start push buttons. Motors M1, M2 and M3 are to be started in sequence with 10s
intervals between each start.
After M3 has started a solenoid value, SV1, is to operate for 1 minute and then turn off.
An O/L on any motor stops all. An emergency stop is required. Develop a suitable
control circuit. (20 marks)
Q2. (a) A 400V. 50Hz industrial supply has an impedance of 0.0013 + j 0.0061Ω. Cables from
transformer to L.T bus-bars have a resistance of 0.15mΩ. A balanced three phase short-
circuit occurs on the bus-bars.
Using the information supplied in appendix A and diagrams 3 or 4 determine
(i) The steady state RMS symmetrical current.
(ii) The initial peak asymmetrical current.
Take the value of E as 410 and F as 1.1 (10 marks)
(b) A H.T supply has a capacity of 20 MVA and a Zpu of 0.25. Two transformers T1 and T2
are supplied from this source. T1 and T2 are identical and have values of 6 MVA and
Zpu of 0.05.
Determine the maximum fault level when the transformers are connected in parallel.
Q3. (a) Equipment for use in potentially explosive gas atmospheres is classified in terms of
(i) Gas Groups.
(ii) Hazardous Areas.
(iii) Temperature Class.
Explain the significance of each. (9 marks)
(b) A squirrel cage induction motor intended for use in a potentially explosive gas
atmosphere has part of its marking labelled as EEx ‘d,e’.
Explain the principle of the protection method(s) provided by this motor. (6 marks)
(c) Write a brief note on the ATEX directives. (5 marks)
Q4. (a) Describe a moulded case circuit breaker (MCCB). (5 marks)
(b) State the advantages of using solid state (electronic) trip units in MCCBs. (5 marks)
(c) Outline the arrangements of switch contacts in an MCCB to facilitate
(i) Conduction of current.
(ii) Making and breaking of current. (5 marks)
(d) A reverse loop magnetic blow-apart contact system is often used in MCCBs. Briefly
describe how this system works. (5 marks)
Q5. (a) Describe the construction of
(i) A standard rotor of a squirrel cage induction motor (SCIM)
(ii) The rotor of a double cage or high torque SCIM. (8 marks)
(b) Sketch and explain the speed / torque characteristic of the motors in (a) above. (6 marks)
(c) Describe the arrangements for starting a wound rotor induction motor (WRIM). Explain
how the rotor of such a motor produces an increased torque during starting conditions.
Q6. (a) Write a concise note to explain why reduced voltage starting methods may sometimes be
required for SCIM. (6 marks)
(b) List four such starting methods. (4 marks)
(c) Sketch the power (mains) circuit for a star-delta starter. (6 marks)
(d) Explain a delta-loop star-delta starter and state its advantage. (4 marks)
Q7. (a) Describe how an inductor motor may be protected against
(i) Short Circuit.
(ii) Overload. (6 marks)
(b) A SCIM requires a long run-up time when starting against a load known to have a high
inertia. Describe a method of overload protection which may be applied to such a
starting condition. (8 marks)
(c) What is meant by the term “single phasing” as applied to a three phase motor? What
arrangements are made against this condition causing stator burn-out? (6 marks)
Q8. (a) For an ACB or MCCB what is meant by the following
(i) Rated current, IN
(ii) Frame size rating.
(iii) Rated ultimate breaking capacity, ICU
(iv) Rated short-time withstand current, ICW
(v) Rated insulation voltage, Ui (10 marks)
(b) The supply arrangements to an industry requires that a number of circuit breakers may
only be closed in a defined way.
Breakers C1 and C2 may be closed together.
Breaker C3 may be closed in combination with either C1 or C2, but not both.
Describe a key interlocking system to facilitate this. (6 marks)
(c) Distinguish between a “short key” and a key exchange box system of Cartell interlocks.