There are a number of things to consider before deciding if a variable speed drive (VSD) is right for your application. These considerations can be roughly assigned to one of three groups: energy usage, process/application, and hardware.
With the rising cost of electricity and increasing demands on aging power grids, there are large incentives to reduce power consumption wherever possible. According to US Department of Energy statistics, electric motors consume approximately 68% of industrial energy usage, and a significant majority of these could see energy reductions when controlled by VSDs. Couple this with the fact that nearly 96% of the total lifecycle cost of motor ownership is electricity consumed, and you can see that there can be real cost benefits to VSD use.
If the process or application you are considering uses a motor to drive a variable torque load (also referred to as a quadratic torque load), such as a centrifugal fan or centrifugal pump, and the motor is not required to run at full speed to meet the demands of the application, then the choice to use a VSD is a simple one. Because in this case motor power consumption is proportional to the cube of the speed, per the Affinity Laws a 20% drop in speed provides a nearly 50% reduction in power used:
For other load profiles the decision is not as clear-cut. Often the process does not allow operation at less than full speed, which means there is no significant advantage in using a VSD in terms of energy cost. That said, there are many processes that can benefit from a VSD’s ability to control torque applied during starting and stopping, better controlling process output and/or reducing mechanical wear and tear on driven components. VSDs not only have this soft-starting/stopping capability, but they can reduce current draw during starting (i.e. inrush) to approximately the motor’s FLA. This is better than a standard soft-starter, which only reduces inrush to about 300 – 350% of FLA (or higher for smaller frame motors/high torque applications).
So two pertinent things to determine regarding application are:
- What is the torque profile of my application – variable, constant (e.g. compressor, PD pump, conveyor), or constant horsepower (e.g. drill press, lathe)? Constant torque and constant HP profiles require heavy duty drives, which may increase capital cost and somewhat reduce energy savings opportunities.
- Can my application be served by a motor running at reduced speed (i.e. at less than the motor’s full (base) speed)? If not, then VSDs and/or soft-starters should be considered only to reduce mechanical wear and tear on connected equipment; energy savings would typically not be significant.
It is also important to know whether the equipment you want to control will benefit from, or can tolerate, being controlled by a VSD. As noted above, VSDs can provide reduced voltage starting, which translates to lower current inrush to the motor, and lower torque to the connected equipment. As long as the heavy inrush-related torque isn’t needed, this means less stress on bearings, couplings, pulleys, and belts. This can lead to longer equipment life and lower maintenance costs.
On the other hand, most VSDs generate high-frequency pulses to synthesize an AC output to the motor. These pulses are generated at frequencies of 12kHz – 120kHz, with very rapid rise times. As a result, pulses with very steep wave fronts are sent through the motor cables to the motor. Depending on factors such as cable length and cable and motor impedances, this can result in resonances being induced in the cable, which can create high voltage values at the motor terminals. Most motors manufactured over the last 10 – 15 years are built per NEMA MG1 Part 31 standards, with insulation systems capable of handling this voltage increase. Many of these motors are also labeled “inverter duty”. If your motor was built before then, and/or heavily used, it is advisable to perform a motor resistance test before connecting it to a VSD.
Finally, if your motor is self-cooled, it is important that it not be run too slowly for any length of time, as the motor will be subject to over-heating. This risk is magnified somewhat by the high-capacitance circulating currents induced in the motor by the VSD. As a consequence, a good rule of thumb is not to run the motor for a prolonged time at less than 50% of full speed. Non-ventilated (TENV) motors can be run down to about 5% of full speed, as they are built to handle additional thermal loads.
So when considering the use of a VSD, determine the following about your hardware:
- Are maintenance programs/inspections indicating a high degree of mechanical wear on connected components (pulleys, bearings, couplings, mounts, etc.)? If so, reduced voltage starting, either by VSD or soft-starter, should be considered.
- Is the motor you want to drive rated “inverter duty”? If not, you can contact the motor manufacturer to determine the voltage rating of the insulation at time of manufacture. At that point, you may also wish to have the motor insulation tested to see if it is suitable for VSD control.
The above are a few of the main factors affecting the selection of a VSD, but the list is by no means inclusive. Please feel to share your thoughts and experiences with other readers in our Comments section. And should you need help or information, please contact us at firstname.lastname@example.org or 866-492-9888. Thanks for reading!