Motor Controller EMC Testing - Loaded Vs Unloaded

We manufacture EMC compliant solid state relays, (up to 35db quieter than standard SSRs) which can be used with a wide variety of loads and during EMC tests always operate them at maximum loading - several motors in series or 20kW resistive for example. We find that the amount of 'switching noise' produced is roughly proportional to the current being switched, so better to go for the worst-case load scenario and avoid any problems.

The motors are usually pretty quiet as far as conducted noise is concerned and we have found that the motor controller/switching device is often the noisiest part of the whole apparatus. The conducted noise is low frequency and very difficult to get rid of using EMC filters which is another good reason to design the switch/controller to be as 'quiet' as possible at maximum loading rather than adding expensive and awkward filters afterwards.