Page 67 - Electric Drives and Electromechanical Systems

60 Electric Drives and Electromechanical Systems

insulation will break down. In the selection of a motor, the following points should be
considered:
If the supply voltage is over 440 V (such as the supply of 460e480 V in the USA),
the voltage spikes will be in excess of those experienced in European applications.
If the drive is to be retrofitted to a motor with unknown insulation specifications,
this problem can only be resolved by consultation with the supplier of the original
motor, and it may require replacement of the motor by a motor with enhanced
insulation capabilities.
In the design of the electrical supply system to a drive system, it is important to ensure
that the system is fully and correctly earthed. A good earthing system is required to:
Ensure safety of operators and other personnel by limiting touch voltages to safe
values, by provide a low resistance path for fault current so that the circuit protec-
tive devices operate rapidly to disconnect the supply. The resistance of the earth
path must be low enough so that the potential rise on the earth terminal and any
metalwork connected to it is not hazardous.
Limit the amount of EMI and RFI, as discussed in Section 2.7.2, by providing a
noise-free ground.
Ensuring correct operation of the electricity supply network and ensure good
power quality.
The actual design of a complete earthing system is complex, and reference should be
made to the relevant national standards, within the UK reference should be made to
BS7671:2018 (2018) the requirements for electrical installations, detailing the current
version (currently the 18th) of the IET Wiring Regulations and BS 7430:2011þA1:2015
(2015) that details the code of practice for protective earthing of electrical installations.
In the construction of a drive system, bonding is applied to all accessible metalwork -
whether associated with the electrical installation (known as exposed-metalwork) or not
(extraneous-metalwork) - is connected to the system earth. The bonding must be
installed so that the removal of a bond for maintenance of the equipment does not break
the connection to any other bond.
The provision of a good earth is fundamental to the prevention of EMI and RFI
problems. It is common practice to use a single point or star earthing system to avoid the
problems of common mode impedance coupling. However, care needs to exercised
when shielded or screened cables are used, as loops may inadvertently be formed, which
will provide a path for any noise current.

2.7.4 Supply considerations

While the quality of public-utility supplies in Western Europe is normally controlled
within tight specifications, considerable voltage fluctuations may have to be accom-
modated in a particular application. In cases were the drive system is used on sites with