It is now over 40 years since electronic VSDs were introduced and during that time the scope of what a drive can now do is beyond any resemblance of the early days. In the past 15 years alone, the size of drives, measured in volume, has decreased by up to 80 percent, while the number of components has come down by some 60 to 70 percent. Thus, drives’ mean time between failure (MTBF) has increased five-fold. Today the MTBF is better than 1 in 50 years.
The fundamental reasons for using VSDs in the first place remain the same:
– Improved energy efficiency, leading to lower energy bills
– Faster installation and commissioning times
– Reduced maintenance routines
– Superior motor control
– Greater adaptability when using VSDs in buildings
– Greater connectivity through automation networks
The problem is that HVAC technical specifications drawn up several years ago may not be making the most of the technological evolutions that have been going on inside VSDs.
The challenges are compounded by the developments in products associated with VSDs, such as the electric motor. Some specifications still assume that motors require a large de-rating when being controlled by a VSD. Also building consultants are confronted with electronically commutated motors, permanent magnet (PM) motors and synchronous reluctance motors (SynRM) – all competing with the traditional induction motor. Yet, each motor type has its pros and cons.
Yet despite the technology innovations, not all VSDs are the same. ABB has identified some 30 technical challenges which need to be considered when specifying VSDs, such as:
Motor control platforms
- Ensure that your VSD can control a synchronous reluctance motor (SynRM) properly. Specifically watch out for poor control at low speeds and the inability to catch spinning loads – not ideal for data centres. Ensure that your VSD contains enough DC capacitance so that it can effectively ride through power dips and recover through brown outs without issues. The drive should be able to recover energy from the spinning load to maintain maximum brownout time. Ensure that your VSD can control all motor types - including induction, permanent magnet and synchronous reluctance (SynRM) and across the speed and torque range in all applications.
- Ensure that your VSD fulfils the requirements of the EMC Directive and the EMC Product Standard. To ensure the EMC compatibility of the entire installation, there are some basic principles that need to be followed. Conducted disturbances can propagate to other equipment via the cabling, earthing and the metal frame of an enclosure. Conductive emissions can be reduced by the VSD which should include a suitable RFI filter for high frequency disturbances and by following good earthing practices. It is also important that the RFI filter complies with the drive standard EN61800-3 (which requires a motor to be connected to qualify), rather than the generic standard EN55011 (which does not). To avoid disturbance through the air (radiated), all parts of the power drive system should form a Faraday cage against these radiated emissions. This includes drive modules, cabinets, auxiliary isolation boxes, cabling, motors, etc. The manufacturer needs to provide adequate installation instructions.
- If the drive detects issues with the load, the faults and warnings that appear should be clear and contain a full text explanation about what to do to clear the problem and get the application running again. Make sure that your interface offers an easy connection to PC tools with no need for special cables and has the ability to connect to a modern mobile device over Bluetooth and use Apps to operate and register the drive. Ensure the drive can show the parameters that have been changed during the commissioning process, so that they can be easily recorded, and more importantly, it is then easy to determine if unauthorised changes have been made after the installation is signed off.
Counting energy use, CO2 and money
- Adding a VSD to a direct-on-line (DOL) controlled motor saves a massive amount of energy. For example, slowing a 30 kW motor down by just 10 percent (5Hz) saves 27 percent energy. That can equate to £3,500 per annum. Controlling a single 400 kW DOL motor slowed to 70 percent flow with a VSD can save over £25,000 per annum, the cost of the drive being paid back in less than half a year - a saving of over £200,000 after 10 years. Advanced motor control features, such as energy optimisation, helps lower energy use. With energy optimisation, the magnitude of the motor’s magnetic field is controlled according to the actual load, giving live energy optimisation. This results in reduced energy consumption and lower audible noise. And of course the drive should calculate the savings and present them for viewing or transmission to the BMS.
- Swinging chokes alter their value to ensure the best possible harmonic mitigation at full or partial loads, typically giving 25 percent less harmonics compared to a traditional choke. Invest in active rectifier technology or active harmonic mitigation using active filters to get the lowest possible harmonic footprint.
- VSDs are typically designed with a keypad that can accommodate programming and controls. When the smoke control system is activated either automatically or manually at the firefighter’s control panel, this keypad needs to be overridden and all control/programming functions disabled. The fireman’s override should set the drive into a “run to destruction” mode so that smoke venting occurs without interruption, and PID loops should be employed to allow stairwell pressurisation, keeping escape routes clear.
Part of the reason consultants’ specifications may fall behind is that they do not purchase drives and so are not always targeted with the latest information by manufacturers. This is despite consultants being key influencers in the choice of drives with a need to understand the latest developments more than most.