As we discussed last week, every motor does not need an adjustable speed drive, despite what industrial controls salespeople and literature may tell you.
Certainly, in some applications a drive will reduce your motor-related energy costs by 10 – 60% and recoup your investment within 24 months[i][ii][iii][iv]. That’s why This week, we are focusing on contraindications: five situations where a drive will fail to reap the promised benefits and/or may harm your motor assets.
1. This motor’s load is fixed and unrestricted.
Drives produce the greatest energy savings where the application currently uses mechanical means — such as gearboxes, throttling or bypass valves, dampers, and hydraulic or magnetic couplings — to regulate load or flow to match demand. However, if your load doesn’t vary, and if you’re not currently using one of these mechanical devices to reduce load/flow, then you’re probably not going to see the energy savings you need to justify installing a drive on this application.
2. You need this motor to operate at very low or very high speeds.
Drives enable users to match a motor’s operating profile to their demands; however, operating outside that manufacturer’s speed-torque specifications comes at a cost: heat. Heat is one of the quickest ways to kill a motor. Bearing grease in overheated motors becomes less viscous (runnier), accelerating bearing wear and leading to premature failure. Heat also degrades the epoxy protecting the stator windings, leading to premature winding failure.
3. You’re planning to install this drive on a motor that is more than 15 years old.
Motor manufacturing and performance standards have changed greatly over the past twenty years. Inverter-duty motors, and many (but not all) general-purpose motors manufactured since 2001 use insulated bearings, shaft-grounding brushes, and high-voltage winding epoxy to protect motors from the transient voltages and induced shaft currents that some drive technologies create. [For more information on using drives on older motors, see Mike Howell’s recent Plant Engineering article.] Many repair shops can retrofit older motors with these features. However, because efficiency standards have also increased over the past 15 years, you may redeem substantial energy savings simply by replacing this older motor.
4. You have power quality issues.
Power quality problems include voltage variations (sags, swells, and interruptions), transients, waveform distortion, frequency variation, amplitude fluctuations, phase imbalances, and low power factor. Certain types of drives can fix certain power quality issues; however, drives can also exacerbate other power quality issues. For example, the ubiquitous pulse-width modulated variable frequency drive may be great at improving power factor, but many models are susceptible to harmonics, transients, and voltage sags.
5. You cannot place the drive within the manufacturer’s maximum tolerable distance of the motor.
Particularly when using pulse-width modulated variable frequency drives [PWMVFDs], it is essential to minimize the distance between motor and drive, keeping it well within the manufacturer’s maximum tolerable distance (varies by model; generally, 100 to 250 feet). I’ve personally seen motors fail in as little as four weeks after the site failed to heed this specification.
PWMVFDs switch on and off 3,000 to 15,000 times per second. Sometimes, due to resistance in the cable running from drive to motor, the leading edges of two switched waveforms “reflect,” or collide, causing the motor to see voltage transients. Additionally, it is possible for PWMVFDs to switch at the resonant frequency of the cable, causing “gain,” or voltage amplification (i.e., delivering 700 Volts instead of 460 Volts). Moreover, long cable lengths can pick up harmonics induced by radio frequencies and nearby high-current lines. If you absolutely cannot locate your motor within the manufacturer’s maximum tolerable distance, spend the extra money on shielded cables and low-pass filters.
Next week we’ll conclude this three-part series with ways to optimize your motor-driven systems without a drive. For assistance prioritizing which of your motors would benefit from a drive, and to find out how to reduce your motor-related energy expenses by up to 30%, check out www.motorsatwork.com or email me.
Nicole Dyess is the Director of Client Solutions for Motors@Work, which provides cloud-based energy management solutions for maintaining and operating motors and motor-driven systems at their peak efficiency and lowest cost.
[i] Siemens, “Variable Frequency Drives and Energy Savings” (2010).
[ii] Honeywell, “VFD Energy Savings and Payback Calculator” accessed 2016.
[iii] Rockwell Automation, “Energy savings with variable frequency drives: Invest in energy management with intelligent motor control solutions,” (2007).
[iv] ABB, “Energy efficiency: Using drives to control motors can lead to big savings,” accessed 2016.