Handbook of Electrical Installation Practice - BeKnowledge

Motor Control Gear 455
Another choice the designer must make is that of insulating materials. These not
only act as a mounting base but as the mechanical slides and guides and also the
wall of the arc chamber within the contactor. A rule of thumb is that most moulded
components in contact with current-carrying parts will be produced from thermoset
materials, usually a polyester glass material stable up to at least 160°C, but the use
of high temperature thermoplastic materials is increasing. The use of asbestos has
now been phased out by all manufacturers with replacement materials being
adopted for the continued production of existing designs.
The contactor is magnetically held closed by maintaining the current flow through
the coil. If the voltage to the coil fails or falls below a defined level, the contactor
opens, thus disconnecting the motor from the supply. The coil must be constantly
energised in order to keep the contactor closed. Alternatively, contactors can be
fitted with a mechanical latch which does not require continuous energisation.
In order to provide quiet operation, a.c. magnets are fitted with shading rings
which normally consist of a single short-circuited loop of copper alloy. They fulfil
the basic function of providing a secondary magnetic flux to prevent the magnetic
pole faces parting and reclosing when the flux produced by the operating coil passes
through zero. If a shading ring should be omitted when reassembling some types of
contactor after maintenance, the omission will be obvious as the magnet will ‘hum’
loudly when energised. This same situation may also arise if a shading ring should
break in service, although this is unlikely as modern contactors have captive shading
rings of proven design.
A further advantage of well-designed shading rings is the reduction or elimination
of magnet bounce which contributes to contact bounce during closing of the
contactor; if a contactor has severe magnet bounce, short contact life in service will
result.
The remaining design parameters such as the ability of terminals to accept cables,
service temperature rise limits, the ability to switch currents under defined conditions
and the performance under short circuit conditions, are now determined by
British and International standards.
The standards applicable to low voltage motor controlgear are mandated
CENELEC publications. Compliance with these satisfies the requirements of the
Low Voltage Directive and the Electromagnetic Compatibility Directive and
permits the use of the CE mark after drawing up the appropriate Technical
Construction File and Declaration of Conformity.
These British Standards publications are essentially the English text of a comprehensive
document, IEC 947, which is published in the UK as BS EN 60947 under
CENELEC rules. IEC 947 and BS EN 60947 are published in several parts which
are to be read in conjunction with Part 1, the ‘General Rules’ document. In the case
of the devices under consideration in this chapter, the applicable parts of the
document are:
IEC 947–1 BS EN 60947–1 Low Voltage Switchgear and Controlgear:
Part 1. General Rules.
IEC 947–4 1–BS EN 60947–4–1 Low Voltage Switchgear and Controlgear:
Part 4. Section 1. Electromechanical Con-
tactors and Motor Starters