Power, Machines and Renewable Energy
2
Requirements
? Students need to attend one laboratory session, as per the group timetable,
where they will be given all the necessary instructions how to do measurements
relevant for the assignment.
? Prepare a brief report (preferably word edited) to include explicit responses to
the questions below, and hand it in to the FEE Office in Ellison Building, room
B201 by Thursday, 29 April 2016. Please use this Word file as a template and
add the required number of answer sheets as appropriate. Make sure you16
number when you did your lab session, as well as the submission date).
? In case of late submission without a proven good cause, your assignment will
incur fail or zero mark. In the event that you submit the assignment late but with
a genuine reason, you must complete a late submission form available from the
FEE office referenced above, and then have it authorised/signed by the module
tutor.
Content
Induction machines are the main work force in industry today and about 75% of the
installations are based on them one way or another, either in motoring or generating
mode. Although it is possible to achieve variable speed operation using power
electronics, many general-purpose drives or fixed speed wind turbines require a fixed
voltage and frequency supply for the machine. In these systems, 3-phase induction
machines are indispensable due to their unique self-starting ability and possibility of
direct on-line operation unlike any other 3-phase AC machine. This inherent property
represents a significant advantage over its counterparts, and it is mainly for this
reason that they are difficult to beat on the market in a wide range of applications.
The proper understanding of the machine fundamental operating characteristics and
testing methods used in practice is therefore of extreme importance, especially if the
students wish to continue their further professional career in power companies,
electric machines and drives industry or renewable energy sector (e.g. wind turbine
manufacturers). The main objective of this assignment is to allow the students to gain
the necessary practical experience and basic knowledge in this area.
The experimental test rig, procedures and equipment used for testing of a
small-scale cage induction motor have been described in the accompanying lab
sheets hard copies of which you should have already received in the lab sessions.
EN0567 Assignment
3
Tasks
1. From your test results, determine the machine synchronous speed and the
number of poles.
[6 %]
2. The example cage induction machine was started using a variable voltage supply
at 50 Hz.
a. What is the main reason for adopting such a starting procedure as
opposed to dead-on-line start, and especially with larger machines?
[3 %]
b. State alternative electro-mechanical and power electronics starters (at
least one configuration for each) which can be used for the same purpose.
What are the main advantages of the latter over the former?
[4 %]
c. Identify the common problem of fixed frequency operation of the motor with
respect to the starting torque and current?
[3 %]
3. If the same 415 V, Y-connected machine was ?-connected (as you did in your
experiments), what maximum voltage could be applied to its terminals to prevent
the current over-loading? Compare (in relative sense) the line current and torque
values for the two winding connections of the motor for a given speed?
[7 %]
4. Tabulate your measurements (similarly to the table on page 8 of the lab sheets)
showing the following performance parameters: line current, 3-phase electrical
power, power factor, load torque (the measurements may necessarily differ from
the values in the above table), speed, calculated slip in motoring mode, and
speed and electro-magnetic power predictions for the machine operated as a
generator at the same absolute slip and shaft torque values as in motoring mode.
[30 %]
5. Using Excel spreadsheets or similar software available in the computer rooms or
elsewhere, present graphically the following operating characteristics of the motor
in terms of torque: speed, current, power factor, input power and slip (two
plots/figure for each, one for Y and one for ? connection for comparison).
[30 %]
EN0567 Assignment
4
6. Using the approximate equivalent circuit, show that the machine electro-magnetic
torque is proportional to slip in the low slip region, and verify this linear
relationship by the corresponding experimental plot(s) above.
[7 %]
7. In your experiments, 3-phase power has been measured using two single phase
watt-meters connected as illustrated in Fig. 4 of the lab sheets (this is known as
the ‘two-wattmeter method’ in the literature). Using phasor diagrams, an
expression for instantaneous power, or applying any other approach you consider
appropriate, explain how is this possible and why?
[10 %]
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