Siemens A1 User Manual

30 

MICROMASTER  Applications Handbook 

·  The output of all inverters generates high frequency, high voltage switching 

waveforms in the output cables between the motor and inverter. 

·  A lot of EMI occurs at high frequency.  At high frequencies the shape and 

length of the cable has a big effect on its impedance.  Therefore short, thick, 
braided leads will be most effective in grounding, and high quality screened 
cable, grounded at both ends, will be needed to limit effects on signal leads. 

·  If equipment is badly grounded, high levels of EMI may connect from the power 

part of equipment into the control connections.  Similar effects can occur when 
badly grounded equipment is connected together and EMI is conducted via the 
control cables. 

·  Particular care is needed when equipment is used with low signal sensors such 

as load cells and capacitive sensors. 

·  Conducted interference is more likely to cause problems than radiated 

interference. 

·  The signal and control leads in any electronic system are generally low voltage, 

high impedance, and are therefore particularly sensitive to the high level of EMI 
present in industrial systems. 

·  Switching inductive loads, such as electro-mechanical brakes, relay and 

contactor coils generate severe EMI. 

Immunity is very important as damage and nuisance tripping will cause failure and 
service costs irrespective of where the fault lies.  Therefore the tests carried out 
during product development are quite severe, and represent conditions 
encountered in industry. 

Tests include: 

·  Electrostatic Discharge testing of all exposed surfaces, terminal and buttons 

using high voltage discharge gun.  This simulates the effect of discharge from 
a human body electrostatically charged from a carpet for example. 

·  High voltage very high frequency discharges capacitively coupled into the 

control lines.  This simulates the interference that couples from power 
switching (contactors etc.) into control leads. 

·  Similar discharges directly into the supply leads, line to line and line to ground.  

This simulates conducted interference from arcing contacts in other equipment. 

·  High energy, lower frequency interference in the supply leads, line to line and 

line to earth.  This simulates lightning strike interference and similar supply 
disturbance.