Variable Speed Drives and HT Fans

For the first time, it allows the use of Frequency Converters during a fire event, enabling dual mode fans (those used for both everyday ventilation and high temperature smoke extraction) to be controlled with Variable Speed Drives (VSDs), even during fire events.  Prior to the change, the VSD had to be bypassed during a fire event, with the fan running at 100%, which is often not the optimum speed for a given situation.

This change in the regulations offers designers far more flexibility when it comes to designing solutions.  Essentially, the new regulations allow for three possible solutions – one of which has enabled the best possible outcome.

Evolution of the standard

One option is a continuation of what was allowed prior to the change, which is that the VSD is bypassed during a fire event.  In this case, the fan motor must be connected directly to the mains electrical supply during a fire event, such that the VSD is not within the electrical circuit which supplies power to the fan drive motor (see Figure 1).  This means that VSD control can only be used when the fan is performing a normal ventilation function.

On the face of it, this might seem like a more robust solution, forcing the fan to run at full speed.  But if used in a pressurisation system, doing so could cause component or duct work damage, and not using a VSD could make it more difficult for the designer to achieve the desired smoke control and extraction strategies.  Most seriously, over-pressurising stairwell pressurisation systems may make it impossible for occupants to open escape route doors.

The second option is to use either a sinusoidal or du/dt Voltage Waveform Filter (VWF) between the VSD and the fan motor.  This necessitates the motor being de-rated by 20% (i.e. a larger capacity motor being required), as shown in Figure 2.  Even with the new regulations, this is the only solution which is permitted where the whole package (fan, drive motor and VSD) have not been third party tested and certified.

There are several negative implications to this solution.  VWFs are very expensive (often costing more than the VSD), and there is also extra cost in using a larger fan motor, along with associated uprated components such as wiring and control equipment.  The fan size might also need to be increased.  In addition, the panel enclosure will need to be bigger, to house the VWF alongside the VSD.

The game changer

The real game-changing part of the new regulation is the ability to use a dual mode VSD-controlled system which not only does not require a VWF, but also only requires a marginal de-rating of the fan motor (over-sized by just 5%) – the same as if it were operating at normal ambient air temperature (see Figure 3).

This solution is only allowed where the complete package (VSD and fan) have been third part tested and certified by a body such as BSRIA or BSI.

There are clear advantages to this approach.  Using a VSD during emergency smoke extraction adds control flexibility to help optimise smoke control and extraction performance, and offers a practical way of achieving strategies such as zonal control and reversing smoke extraction flows without the need for additional equipment such as dampers and contractors.

VSD ‘Fire Mode’ 

A Variable Speed Drive ‘Fire Mode’, or smoke clearance function, is a vital tool in keeping buildings safe in the event of fire, and in particular mitigating the effects of smoke in such situations.

Smoke obscures vision, preventing the building’s occupants from finding safe exit routes in the event of fire.  It also hinders the fire brigade in search and rescue operations.  Smoke can kill people by asphyxiation or poisoning (if toxic) well before the temperature of the fire causes any physical injuries.

Fire protection should always be classed as a ‘system’, whose elements must work together to achieve a consolidated and co-ordinated approach to fire safety.

The reliability of a smoke control system decreases as the number of components increased, unless the system includes component redundancy.  Such systems can be categorised as ‘dedicated’ or ‘non-dedicated’.  VSD Fire mode creates a priority function which over-rides all other functions, to ensure that the drive performs its primary task for the duration of the fire event.

In Fire Mode, the VSD ignores the ambient conditions and over-rides its safety devices which might in normal operation lead to the VSD tripping and resetting.  Although this mode is only designed for short-term use, it will operate for long enough to ensure the system’s reliability.  The VSD can be enabled either automatically or via digital input.

It can be very difficult to integrate smoke direction control logic into systems which use a VSD bypass solution, and as an overall system approach, this option may not be more reliable.

Reliability is, of course, a vital consideration for fire safety fans.  Dedicated fire mode fans will typically only be tested at best monthly, and quite possibly only once or twice a year.  If a fan and its motor bearings are not turned regularly this can cause degradation, either because the grease solidifies, or because the bearings get flat spots (‘brinelling’).

A non-dedicated, dual mode fan controlled by a VSD which is used for both day-to-day ventilation and smoke extraction, will effectively be being run-tested every day – which means the reliability is likely to be higher.

A packaged solution

The Woods/Danfoss HT Fan and VSD packaged solution allows a dual mode axial fan to be speed controlled during both normal ventilation mode and emergency HT mode.  Using tried and tested design principles, the advanced VSD design allows multiple duty set points to be defined, whilst the Building Management System compatibility adds additional control flexibility and high levels of design strategy confidence.

Most importantly, the VSD ‘Fire Mode’ ensures that the fan and VSD package performs a smoke extract function for a minimum of two hours.

Our solution was the first fully EN12101-3:2015 (F300 and F400) certified fire mode packaged solution in the UK, fully tested and certified by BSRIA and BSI.

The features of the package all incorporate best practice when it comes to HT fan design.  This includes the integrity of the impeller design, with choice of materials, component fit and stress limits, which we optimise; the correct impeller tip to casing gap, which is critical for the correct operation and performance of the fan; and high level motor specifications, including bearing design, grease and cooling systems.

Finite Element Analysis design tools have been used in the design of critical components, and the impeller, motor, casing and motor mounting have all been HT tested in-house, before third-party testing at BSRIA to deliver a robust solution.

If you'd like to know more on this topic or need advice on your next project please get in touch.