Handbook of Electrical Installation Practice - BeKnowledge

418 Handbook of Electrical Installation Practice
To achieve higher machine efficiency the designer will incorporate more copper
in the windings to reduce the I 2 R copper loss. This may mean some increase in
machine size to accommodate the additional copper.
Iron losses also will be reduced by utilising more expensive lower loss and possibly
thinner electrical sheet steel for the laminations and by specially treating the
laminations to improve the stacking factor.
Fewer losses mean less heat generated and therefore less cooling, with consequent
reduction in windage losses.
Some companies already offer, at a premium, motors having an efficiency 2–3%
higher than a normal commercially available machine. That is, efficiencies of 93%
compared with 90% for 100kW motors are readily available. One UK company is
consistently achieving an efficiency of 97% on specialised 100kW cage motors. The
lower cooling demands of a high efficiency motor will also reduce the associated
noise level.
STORAGE
Efforts should be made to minimise the deterioration of the machine during storage
on site or in a warehouse. Dry conditions, protected from damp and condensation
and from extremes of temperature, are obviously desirable. Ingress of dust to a
machine on site should be avoided, especially of concrete dust which may be present
if building and civil work are still going on. Machines should not be stored in an
area which is subject to vibration because this could cause damage to the bearings.
If some degree of vibration cannot be avoided, the shaft should be turned by hand
every few weeks to load different parts of the bearings.
Where doubt exists about the dryness of the storage area, and the machine is
fitted with anti-condensation heaters, it may well be worth considering using
the heaters during storage. If the heaters are used for this purpose the safety
aspects should be borne in mind and terminal box covers be replaced and warning
notices attached. It is important to inspect the machine in storage every six months
or so and to maintain protective coatings on bright parts which may be liable to
rust.
INSTALLATION
Careless installation can affect the life and operation of a machine. Contractors and
users are urged to follow the machine manufacture’s instructions. General considerations
are indicated below.
Checking
Machine manufacturers are not always made aware of any need to pack machines
specially for prolonged storage. Storage, for however long, may have been under
unsuitable conditions and it is essential to check before installation that the motors
have not deteriorated. While signs of rusting are obvious, the insulation resistance