Page 181 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/161
Active power 6.23 Number of starts and stops

The power the machine feeds back to the grid is expressed Due to excessive starting and braking heat losses it is
by equation (6.12) discussed earlier. not advisable to switch an induction motor ON and OFF
frequently. The number of starts and stops a motor is
Note A normal motor is designed for a slightly lower voltage than capable of performing will depend upon its working
the system voltage to account for cable drops from the source of conditions such as type of switching, braking and load
supply up to the motor. But as an induction generator, it is designed demand etc. and can be determined from
for a slightly higher voltage than the grid voltage or the primary
voltage of the transformer (when a transformer is installed to raise
the voltage of the wind generator to the grid voltage) to account for (6.14)
the voltage drop from the generator to the grid or transformer.
where
Micro siting ZL= number of starts and equivalent stops per hour
on load. For example, in plugging one start and
To identify the correct location and size of a mill it is one stop will mean four starts and if reversal is
essential to ensure that there is adequate wind at high also involved then five starts.
speeds at the site. The process of identifying the most ZNL = permissible number of starts per hour for a motor
appropriate locations is termed micro siting. Local with a free shaft, using mechanical braking, thus
government and private agencies conduct surveys and placing no strain on the motor. This factor will
compile wind data for potential areas for these mills. A depend upon the electrical and mechanical design
list of these agencies in India is provided at the end of of a motor and will vary from one manufacturer
the chapter. These mills are generally installed away from to another. The cooling capacity, its effectiveness,
areas of habitation for maximum wind and safety for i.e. heating and cooling characteristics and starting
people and animals or in remote areas not connected torque of a motor, are the parameters that would
with power lines. The power so generated may be captive determine this factor. The smaller the motor, the
to supply nearby areas or fedback to a power grid to greater number of starts it will be capable of
augment its capacity. In remote areas where no power performing. For a lower-speed motor, the average
grid exists the power so generated may also be stored in starting torque will be normally less and the inertia
batteries through a voltage source inverter and utilized more. Therefore the permissible number of
when required. Such a system, however, is more convenient switching operations will be comparatively less
for small mills, say, up to 5 kW, otherwise the cost factor for a low-speed motor than for a high-speed motor
may act as a deterrent to such an arrangement. of similar rating. As a rough guide, small motors,
Normally such mills are installed in groups known as say, up to 20 h.p., may have a factor as high as
wind farms to provide a sizeable power source, except in 1000-2000.
remote areas, where power demand may be restricted to
a very limited area and small mills may suffice. When
mills are installed in groups, precautions are necessary
to ensure that there is enough distance between any two 1 .a
mills so that there is no hindrance to routine maintenance,
on the one hand, and obstruction of wind to other mills,
on the other. For more details, refer to the literature 0.8
available on the subject in the Further reading at the end I-
of the chapter.

6.22 Inching or jogging
T o’6
2
This means repeated short-duration application of power 0.4
to the motor to cause small movements of the shaft from
rest to perform certain load requirements. The motor
may normally not reach its full speed, nor at times
complete even one full revolution, and can be rotated in 0.2
either direction. Likely applications may relate to lifting
or hoisting which may call for delicate handling and a
rather slow, smooth and more accurate final movement
for exact positioning, lifting or unloading etc. 0 0.2 0.4 5 0.8 D
This is a severe duty for the switching contactors as Loading Tt I c-
they have to endure repeated arcing of the interrupting
contacts every time they make or break. (Select only Figure 6.66 Average load factor KL when started against
AC-4 duty contactors: see Section 12.10.) load