[1] On an energy point of view, discuss why is it advantageous to run induction motors in star
mode when the load less than 50%?
Discuss how stator and rotor losses are reduced in an energy efficient motor as compared
to normal motors.
[2] A 15 kW rated motor burns out. The financial manager of the firm wants to rewind the
motor for R3000 to save money. The Energy Manager wants to buy a new premium motor
for R20000; after selling motor for R5000. He claims that he can save much more money
in the next five years than the cost difference of the above two options. Other data is as
under:
Operating hours/year
= 8000
Rewound motor efficiency
= 89%
New premium motor efficiency
= 93%
Motor loading
= 75%
Energy cost
= R4/kWh
[3] How much money does the energy manager actually save over 5 years and what is the
simple payback period?
[4] A multi-storied shopping mall has installed 5 x 110 TR reciprocating compressors of which
four compressors are in use and fully loaded for 14 hours per day. The specific power
consumption of reciprocating compressor is 0.8 kW/TR. Due to higher energy cost the
shopping mall chief engineer has decided to replace reciprocating compressors with screw
compressors having specific power consumption of 0.65 kW/TR. The chief engineer needs
following input from energy consultant:
• Comparison of power and electricity consumption of both reciprocating and screw
compressors?
• Annual energy bill savings (for 320 days operation). Present unit cost is R 6.00 per
kWh.
[5] 100 numbers of fused 60-Watt incandescent light bulbs (ILB) are replaced by same
numbers of 12 Watt CFL instead of new ILB. Calculate the following for 4000 hours of
operation per year.
• The annual “kWh saved”
• The annual “kVAh saved” if the power factor of the CFL is 0.6.
• The annual reduction in electricity costs if R 4 per kWh is the energy charge and Rs.
250 per kVA per month is the demand charge.
• The simple payback period if the ILB costs R 10 and the CFL costs R 100 (assume life
of ILB and CFL as 1000 hours and 4000 hours respectively).