When it comes to installing electric and electronic equipment – like frequency converters – proper grounding of the equipment and using shielded cables plays an important role in creating a safe and noise free working environment.
Grounding means to connect electrical equipment to a common reference ground or earth. Shielding is used both for immunity (protecting against external interference) and emission (preventing interference to be radiated).
There are two reasons why you should take really good care of your grounding:
- Electrical safety: Safety grounding ensures that in the case of the degradation of electrical isolation no live voltage is present on conductive parts that can be touched by humans – thus avoiding the risk of electric shock.
- Reduce interference: Signal grounding reduces voltage differences that might cause noise emission or susceptibility problems.
It is very important to note that electrical safety always has the highest priority – higher than EMC.
There are different types of grounding; single point grounding whereby multiple pieces of equipment are connected to one ground connection – in series or parallel – and multi-point grounding whereby each piece of equipment has its own ground connection.
The different types of grounding have advantages and disadvantages but, what matters at the end of the day is that the impedance of the grounding connection for each piece of equipment is as low as possible in order to provide potential equalisation of the connected equipment.
In frequency converter applications, shielded cables are used both for power (motor cable and brake resistor cable) and for signals (analog reference signals, bus communication).
The shielding performance of a cable is indicated by its transfer impedance Zt. The transfer impedance relates a current on the surface of the shield to the voltage drop generated by this current on the opposite surface of the shield:
Zt = U2 / (I1 x L) where L is the cable length
The lower the transfer impedance value the better the shielding performance. The table below shows typical values of transfer impedance for different kinds of motor cable. The most common type of motor cable is the single layer braided copper wire as it off ers a good shielding performance at a reasonable price.
Transfer impedance can be drastically increased by incorrect shield termination. The shield of a cable needs to be connected to the chassis of the equipment through a 360 degree connection. Using “pigtails” to connect the shield increases transfer impedance and ruins the shielding effect of the cable.
One or Two Sides Termination
The question that often comes up is whether both ends or only one end of a shielded cable should be terminated. It is important to realise that the effect of a shielded cable is reduced when only one end is terminated. It is very important to terminate correctly both ends of the motor cable, otherwise interference problems may occur.
The reason why in some situations only one end is terminated has to do with ground loops in signal cables. This means that there is a voltage potential difference between the chassis of the two pieces of equipment that are connected (for example frequency converter and PLC) and if the shield connects the two chassis a ground current will occur (with the frequency of 50 Hz/60 Hz). This current then couples into the useful signal disturbing it – in audio applications this is commonly known as “hum”. The best solution is to use an equalising connection in parallel with the shielded cable. If this is not possible then one end of the shielded cable can be terminated via a 100 nF capacitor. This breaks the ground loop at low frequency (50 Hz) while maintaining the shield connection in the high frequency range. In some equipment this capacitor is already built in.
This information comes from Danfoss’ Facts Worth Knowing About Frequency Converters – free download here