Variable Speed Drives - Getting It Right (Part 2)
Part 1 on VSD selection and getting it right has already delved into the importance of understanding the load type (variable torque or constant torque) and temperature ratings when choosing the correct drive for your application. In this article we will look at the importance of understanding whether or not your VSD is fitted with suitable harmonic and EMC mitigation and to help you make an “apples for apples” comparison in these areas.
Harmonic Mitigation
With many Network Authorities now imposing strict installation requirements around VSDs and harmonics it is hard to believe that some suppliers in NZ still offer VSDs with no integrated harmonic filtering - or even offer it as an optional extra.
Harmonic current is real current drawn from the supply at frequencies other than the fundamental , which in NZ is 50HZ. Whilst this current does not deliver real power it does overload transformers, cables, circuit breakers, and causes a distorted voltage waveform. In most standard applications sufficient harmonic filtering is achieved simply through the introduction of inductance in the form of either 3 phase line reactors, or a DC bus choke, with an inductance value of around 3%. Invariably the first time you are aware that your VSD did not have harmonics filtering and you have a problem is when you start tripping protection devices for no apparent reason – the solution – out with the order book and go back to the supplier to buy the harmonic filter, which you then have to retrofit on site. It is much easier to ensure you buy a VSD with these filters included and inside the standard VSD enclosure!
So how do I tell if my VSD has harmonic filtering in it?
A very good indicator is simply to look at the VSD weight. Both 3 phase line reactors and DC Bus chokes are made from copper and iron. This weighs a lot! A heavy VSD will inevitably incorporate these reactors/chokes as standard.
Below is an example of two 75kW pump rated VSDs that have been supplied in the NZ market – The Power Electronics SD750 VSD (Talla 3 size) fitted with 3 phase line reactors and a VSD from a major multi-national supplier. As you can see the SD750 weighs 150% that of the competitor unit.
The SD75014555 (75kW Pump VSD) showing a weight of 67.5kg
Competitor 75kW Pump VSD showing a weight of only 44kg
EMC or RFI Mitigation
This is definitely an area where not all drives are equal!
Every VSD supplier in New Zealand must only sell product that has the RCM mark, ensuring the product complies with IEC 61800:3 – the current EMC regulations. This label must legally be shown on all VSD products sold in NZ. For most applications in New Zealand we are interested in compliance with either the C2 category (consumers who share the secondary of a supply transformer – often referred to as domestic) or the C3 category (consumers who have their own site transformer – often referred to as industrial). VSDs must be fitted with sufficient RFI filtering to meet these requirements and have been tested by the manufacturer to ensure the RFI is below acceptable levels.
So everyone who meets compliance with these standard is the same – right?………Not so!!
A critical factor often overlooked when evaluating the VSD and RFI performance is the length of the output cable between the VSD and motor. When manufacturers conduct their testing they must state the installation conditions and maximum cable length that the VSD can be used whilst still complying with these regulations. The longer the output cable the more RFI is generated.
The VSD switching frequency also contributes to the amount of RFI. The higher the switching frequency – the more RFI is generated. A reputable VSD supplier will conduct their testing using the highest switching frequency and the longest output cable. Their VSD default settings will reflect this. It is not uncommon for VSD manufacturers to use the lowest switching frequency for testing but set their VSD default values to the highest switching frequency. This can have a major impact on RFI performance.
Products such as the Power Electronics SD750 are designed, and tested, to comply with up to 300m of cable connected to the output. This takes into account most New Zealand applications where long output cable runs are common place.
Some NZ suppliers sell VSD products where the tested output cable length for the VSD is only 5m. This means that for every installation that uses an output cable greater than the tested distance RFI compliance can not be assured. If your site develops RFI issues, particularly those that affect other consumers, You as the installer will be responsible for remedying the situation. Once again – get out with that order book!
Competitor VSD showing maximum cable lengths as low as 5m for EMC compliance
Power Electronics is here to assist you in the correct selection of your VSD for your application. We don't expect you have to do this on your own - we can help you.
