Page 65 - Electric Drives and Electromechanical Systems

58 Electric Drives and Electromechanical Systems

FIG. 2.12 The use of a counterbalance to reduce the continuous motor load in a vertical drive application using a
conventional mass counterbalance, (A) or a pneumatic system, (B). In the pneumatic version, (B), the two valves V 1
fed from a compressed air supply and V 2 which vents to atmosphere are adjusted a maintain a cylinder pressure
that will balance the load. If the air supply is lost a pressure switch activates a brake and disables the drive.

actual coefficients of friction can be many times the nominal value for the given material-
to-material contact. In practice, the actual friction should be carefully determined by
practical measurement or an adequate safety margin should be used in the sizing of the
motor-drive in order to reflect the degree of uncertainty which is present.

2.7.2 Electromagnetic compatibility

Electromagnetic interference (EMI) can affect all types of electrical and electronic
equipment to varying degrees; such interference has increased in importance because of
recent European legislation, which recognises the importance of removing these potential
problem areas at the design stage. It should be recognised that a system can both emit and
be susceptible to EMI, hence care must be taken in the design and use of a system. In an
assessment of equipment for compatibility, the emissions and the susceptibility of the
equipment must be considered over a very broad frequency range, normally d.c. (0 Hz) to
110 MHz. In consequence, electromagnetic compatibility has a considerable influence
over the design and application of a system. The most obvious sources of electromagnetic
radiation are the power converters which are used in motor controllers and any associated
switch gear. With the increasing use of microprocessor-based controllers, any interference
can have serious safety implications. In general, the main route of interference into or
out of a piece of equipment is via the cabling or direct radiation. A cable which is longer
than one metre should be considered to be a problem area. The use of the grounding
and shielding of cables must be a high priority, as must careful design of the panel
layout. While these measures cost little to implement, failure to do so will be costly.