When designing a system to utilise variable speed technology there will be some benefits that are relatively easy to quantify, some which are less tangible and some potential problems, which must be avoided.
This Chapter provides extensive practical considerations, which will assist in achieving successful VSD installations .
Tangible benefits to the user
With rotodynamic pump installations , savings of between 30% and 50% have been achieved in many cases by installing VSDs . By adopting all the possible best practices in system improvements identified in this guide , savings as high as 90% have been achieved.
Where positive displacement pumps are used, energy consumption tends to be directly proportional to the volume pumped and savings are readily quantified.
Improved process control
By matching pump output flow or pressure directly to the process require ments, small variations can be corrected more rapidly by variable speed drive than by other control forms , to improve process performance .
There is less likelihood of flow or pressure surges when the control device provides rates of change which are virtually infinitely variable.
Improved system reliability
Any reduction in speed achieved by using a VSD has major benefits in reducing pump wear, particularly in bearings and seals, and by using reliability indices, the additional time between maintenance or breakdowns can be accurately computed.
It is generally considered that wear in bearings and on rubbing surfaces reduces as the seventh power of speed, hence major benefits can be realised by running a pump more slowly.
It therefore follows that the fitting of a VSD has a significant benefit in improving system reliability.
The following additional benefits apply specifically to VFDs:
• Modern power electronics are very reliable: typical MTBF figures achieved by manufacturers now exceed 20 years, subject to routine maintenance.
• Easy to retrofit.
• Good supply side displacement power factor at all loads, power factor compensation unlikely to be required, unlike network connected fixed speed motor.
• May possibly be by-passed in the event of failure.
• Open-loop designs can run without any speed sensor. Few pump applications require the degrees of control accuracy where closed loop control is justified.
• No inrush current from the supply network on starting. Soft starting
and stopping reduces stress on the motor, pump, coupling and supply network.
• Integrated diagnostics and protection helps reduce and identify
faults. VFDs can incorporate several types of dedicated pump and motor protection, (e.g. overload, underload, loss of phase, under/ overvoltage, etc.). With additional sensors the pump can be protected against dry run, cavitation, low flow, seizure etc.
• Many VFDs incorporate control logic: proportional, integral and derivative control loops (three term controller).
• All but the most basic VFDs have serial communications interfaces for networking or for use with over-riding control systems.
Potential drawbacks of PWM VFDs
• PWM drives can be fitted to most existing motors in Europe and other areas, which use a 400 V network. However this is generally not the case in the US, and other areas, where network voltages exceed 440 V and hence motors with reinforced “inverter duty” insulation are often needed.
• Because many standard drives use a simple diode input bridge, there may be some undesirable harmonic distortion. Attenuation methods are discussed in Appendix A3.3.
• The high rate of switching in the PWM waveform can occasionally lead to problems:
• The rate of rise of the wave-front can cause electromagnetic dis turbances. Adequate electrical screening (screened output cables) and rate of rise (dU/dt) or sinusoidal filters in the inverter output can eliminate this problem.
• Older motor insulation systems may deteriorate more rapidly due to the rapid rate of change again a sinusoidal filter will eliminate this problem.
• Long cable runs can cause “transmission line” effects, and cause raised voltages at the motor terminals.
• Voltages can be induced in the shafts of larger motors potentially leading to circulating currents, which can destroy bearings; corrective measures can include:
• Insulated non-drive end bearings are recommended on all motors over 100 kW (150 hp) output rating.
• Common mode filters may additionally be required for higher powers and voltages.
• The converter will have losses, the ventilation requirements for the electronics can be an important issue:
• The life expectancy of the converter is generally directly related to the temperature of the internal components, especially capacitors.
• The converter may require installation in a less onerous environment
than the motor control gear it replaces:
• Electronics are less able to cope with corrosive and damp locations.
• For a motor in a hazardous area the VFD must be installed in a safe environment, which can lead to long cable runs.