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Variable Speed Drives
September 17, 2009

Vacon Drives, Coles Myer Mulgrave

Variable Speed Drives (VSDs) are now being widely used across the HVAC industry in many different applications. The benefits of a VSD, including their ability to regulate fan speed, save energy consumption (up to 70% in some instances) and provide motor protection, are now well recognised. However, in certain applications a number of possible issues must also be considered.

This article looks at these issues and provides practical solutions that may help ensure reliable, trouble-free operation.

AC electrical power emitted from the VSD device will differ from a normal AC supply in the shape of the voltage waveforms released. The waveforms from the output side of the VSD are achieved with what is known as ‘Pulse Width Modulation’ (PWM), which essentially turns the power on and off for a given period of time several thousand times a second.

The PWM method of operation enables the VSD to have a limitless number of operating speeds for an induction motor. However this can also cause certain issues during operation which do not appear in fan installations where normal mains AC is used.

Some of these are:

  • VSD-induced motor noise
    The ‘jagged’ nature of the waveforms sent to the motor by the VSD device can cause some motors to produce vibration in tune with the switching frequency of the VSD output. These vibrations are actually harmonics in the motor windings which then excite the fan casing or the ductwork attached to it. This as a result, creates noise.

    The current generation of VSDs are available with filters which will resolve this issue and reduce the noise output.

    Alternatively, the switching frequency of the VSD output can be changed. A drawback to this approach is a reduction in the power handling of the VSD that ultimately limits how high this frequency can be set. Setting the switching frequency of the VSD higher may produce noise that is less audible, but this depends on what is practically achievable with the motor and VSD combination.

  • Voltage spikes
    Momentary high voltage spikes can be experienced at the motor terminals as a result of the voltage output of the VSD. These spikes can be 4 to 5 times greater than the peak AC voltage and can greatly reduce the lifespan of the motor, particularly if the motor has very compact electrical windings. The reason for this is that the high voltages can try to arc between spots in the motor windings where the insulation is thin, and compact windings with thinner wire and thinner insulation are more susceptible to this. Motors in an External Rotor (ER) configuration are a good example of this, as the windings have to be extremely compact to fit in this application.

    This can be resolved using special filters such as dV/dT and all-pole sinusoidal filters, which effectively ‘smooth out’ the voltage waveforms from the VSD.

    Another way of avoiding the problem is to aim for very short wiring lengths between the VSD unit and the motor.

  • EM interference from cables
    The nature of voltage waveforms from a VSD can induce a lot of Electro Magnetic (EM) interference. This may be reflected in malfunctioning computer equipment, inconsistent sensor readings in control systems or poor television and radio reception. Using shielded cables between the VSD and the motor will help to reduce this problem.

  • Natural Frequency / Resonance Coupling
    Another issue that can sometimes occur in variable speed fan applications relates to a ‘natural frequency’ or ‘resonance phenomenon’. All structures have a certain stiffness and will deflect under applied load from either its own weight or the weight of other objects placed upon it.

    This is simply known as a spring and mass system. This system also has a certain ‘resonance’ frequency that is a characteristic of that system. If it is then excited or vibrated at the same frequency from another source, the entire structure can start to deflect and move enormously. For a spectacular example of this look up the Tacoma Narrows bridge. The catastrophic vibrations that destroyed the bridge was the result of a complicated oscillation between the bridge and the winds passing through it.

    An example can be the observation of a change in the vibration or movement of a fan at certain speeds.

    In the case of a variable speed fan installation, care must be made so that the operating speeds of the fan do not correlate with the natural frequencies of the mounts, building structure or the fan itself.

    Examples of how this can be achieved include: -
    - Upgrading or resizing anti-vibration mounts
    - Replacing or adjusting fan specifications including speeds
    - Adding weight/mass to the structure
    - Programming the VSD to avoid the resonant speeds

    As you can see there are a number of issues that may arise through the use of a VSD, but in many cases, potential problems can be easily avoided or simple solutions put in place, particularly if they are considered when selecting equipment. VSDs are still an efficient and effective method for programming fan speed, reducing energy consumption and providing motor protection.

    For assistance on VSD selection or technical advice, please call your Fantech branch or agent.

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