Siemens manufactures an industry leading variable frequency drive, the BT-300, designed specifically for the HVAC market. With sizes ranging from 1 HP to 250 HP and numerous customization options available, Siemens has introduced the next generation VFD for HVAC applications.

Standard Features


HIGHMARK recommends the use of standalone variable frequency drives for applications where disconnects already exist or the disconnects are to be field installed.


Siemens can supply a standard variable frequency drive with an integral disconnect. This reduces field work and streamlines installation. HIGHMARK recommends the use of the integral disconnect in instances where redundancy already exists on the device side of system.  For instance, pumping systems are typically designed with redundant pumps, so in the event of a drive failure, the building operators can switch to the redundant pump. Disconnect packages may also be suitable for ancillary systems that, in the event of failure, will cause little to no impact to day-to-day operations. It is also important to note that variable frequency drives are considerably more robust and readily available than in the past, which was why the US industry had previously standardized around bypasses.


Siemens' BT-300 drives are available with either conventional and electronic bypasses.  Common practice in the past has been to configure all variable frequency drive packages with integrated bypasses in order to provide redundancy for a given device should the VFD fail. Essentially a bypass acts as an across the line starter in the event of a VFD failure, which allows the motor run at full speed (60 HZ). Now that more advanced options are available, HIGHMARK only recommends bypasses be used in applications where VFDs are used for balancing or are on relatively small devices that will not critically impact the infrastructure or operation of the building (such as a small exhaust fan).  In the event a bypass is required, HIGHMARK also recommends the use of electronic (E-Bypass) over conventional bypasses due to the smaller footprint, lower cost and integration of the bypass into the communication with the drive. Also, unlike many competitive products, the Siemens E-bypass can still operate even if the controller fails, just like a conventional bypass.

Motor Switch ride through

During maintenance, other drive manufacturers will advise not to disconnect power between the drive and the motor without first disconnecting the power to the drive.  This is because disconnecting and reconnecting power between the motor and VFD while the VFD is powered can cause damage to the drive. Siemens has solved this problems with motor switch ride through. The VFD can detect when power has been disconnected between the drive and motor and protects itself until normal operation can be restored.


Due to the principle of operation of a variable frequency drive, unwanted harmonics are typically generated on both the line and load (building and motor side) of the VFD. These harmonics are typically measured and represented in total harmonic distortion per IEEE 519 or THD. The larger the motor, the more noticeable the harmonics will be on the line side of the drive, which can disrupt other devices in the building or even at the utility.  A typical 6 pulse drive without any harmonic mitigation can create up to 70% THD.


Every BT-300 drive is provided with an integral DC link reactor that provides industry leading harmonic reduction without the addition of additional filters or reactors.  The BT-300 uses thin film capacitors that do not dry out like electrolytic types, meaning that there is no need to condition the drive prior to install. Resulting in a drive that can be stocked and stored on the shelf for years without issue. 

Integral Controls

Thin Film Capacitor.png

The BT-300 drive has integral PID functions that are easily programmed via the startup wizard to control any type of HVAC fan or pump application.  Every drive is also embedded with standard communication protocols: BACNet MS/TP, BACNet IP, Modbus RTU, Modbus TCP, and N2.  Lonworks is also available as an option.

standard enclosures

Standard enclosures for VFDs in the HVAC market include NEMA 1, NEMA 12 and NEMA 3R.  NEMA 1 is a regular electrical box that is designed to protect users from electric shock.  HIGHMARK recommends a minimum of NEMA 12 on all drives as these enclosures are designed to be dust proof.  Being that a drive requires air movement via fans to cool the drive circuitry, it is advisable to utilize an enclosure that will protect those components from dust and debris.  NEMA 3R is the advisable enclosure for outdoor installations.


The BT-300 has easy to use, step-by-step startup wizards. The LCD interface is easy to read, and the main screens are customizable to show information that is most pertinent to the user and application. It is also possible to save parameters to the screen, remove the screen and transfer those parameters to the another drive; or transfer parameters from a computer directly to the drive, dramatically reducing startup time. 

Specialty and Custom options

Ultra low harmonic drives

An untreated drive can generate 6 pulses which can create Total Harmonic Distortion up to 70%. With standard harmonic mitigation means, such as DC link reactors or line reactors, THD can be reduced to between 20% and 35%, which is suitable for most HVAC applications. However, in the event that further harmonic mitigation is required, Siemens offers numerous options.  12 and 18 pulse drives are available which would reduce THD to approximately 12% and 7% respectively. This drives carry a significantly higher price tag and are much larger than conventional drives. Alternatively it is possible to fit BT-300 drives with either passive or active harmonic filters which result in similar reductions. HIGHMARK recommends exploring all solutions to figure out which is best for every opportunity.


In lieu of bypasses, HIGHMARK recommends the use of redundant drive packages for most applications where redundant devices are not already present. Siemens' redundant drive package offers a complete backup variable frequency drive in the event of a drive failure. Additionally, controls are integrated into the packages so that the system will automatically switch from the non-functional drive to the redundant one AND maintain the exact same setpoint, catching the motor on the fly. This prevents the overpressurization of systems and excess energy usage that could be experienced through the use of a bypass.  Ideal applications are variable speed air handlers, fans and any other large pressure dependent devices.

SPECIALTY Enclosures

Aside from standard enclosures such as NEMA 1, 3R and 12, custom enclosures, such as NEMA 4, 4X and 8 are available to deal with extreme ambient or corrosive conditions.


The cooling tower controller is an application specific panel adaptable to any tower type. Single source control reduces components and increases the reliability. Requiring one single point of power per cell, all of a cooling tower’s power and control options can be self-contained. Available in both outdoor (NEMA 3R) or indoor (NEMA 1) enclosure, the controller provides flexibility for any installation. 

Additional factory installed options include:
•    Panel space heater and enclosure fan (Outdoor Applications)
•    Basin heater, spray pump and discharge damper control including automatic staging
•    Vibration switch cut-out
•    Electronic water level controls
•    Communication Options (BACnet, Modbus, LonWorks, N2)
•    Output Load Reactors or other specialty harmonic options
•    Water temperature controller (standalone operation)