Page 66 - Electric Drives and Electromechanical Systems

Chapter 2 Analysing a drive system 59

FIG. 2.13 A possible wiring layout for a drive system in the normal industrial environment. The motor cables are
twisted to reduce interference and the cabinet is provided with a bonding point. While the controller is
separately earthed, care needs to be taken to prevent earth loops. To prevent problems, any sensitive electronics
should be placed a distance away from various elements as shown by the shaded areas. The separation distance
on the drive's voltage and current rating and are specified in the relevant RFI and EMI standards.
Fig. 2.13 shows the precautions which are normally taken in a typical motor-drive
system.
Radio-frequency interference (RFI) is electromagnetic emission in the range
150 kHze110 MHz which are of sufficient strength to be capable of interfering with any
form of telecommunications; the main sources of RFI are power electronics, relays, and
motor commutators. Recent legislation has placed strict limits on RFI emissions, and
these limits should be met before a piece of equipment can be placed in service.
Due to problems of interference between systems, a range of standards (for example
BS EN 61000-6-3:2007þA1:2011, 2011 and BS EN 55035:2017, 2017) detail the amount of
RFI which is permitted to be emitted and, perhaps more importantly a systems
susceptibility. The suppression of RFI requires considerable thought and can only be
achieved by the use of filters and good design practice. The screening of cables will not
affect the RFI being transmitted around a system. It should be noted that these directive
state that screened cables are not an acceptable form of RFI emission prevention. The
only acceptable form of suppression is by the use of in-line filters in all the supply lines,
including the earth return. The design of suitable filtering should be carried out in
partnership with the system user, its supplier, and the electricity supply authority.

2.7.3 Wiring considerations
While the installation of motor power cables needs to be considered for any EMI and RFI
effects, other factors must also be considered if the separation of the motor and drive is
excessive. The pulses from a pulse width modulated drive will act as an impulse on the
motor cables, and reflection will occur due to the mismatch in impedance between the
motor and the cable. If the length of the cable is excessive, the reflected pulse will
combine with other pulses to increase the motor voltage to over twice the nominal
supply voltage. This reflected-wave theory is well understood in terms of transmission-
line theory. The net effect of this high-voltage spike is the possibility that the motor’s