In our October 2016 e-newsletter we mentioned that “…by limiting the amount of inrush current to motors, VFDs can also extend motor life.”
This inspired some conversation so to provide more information we looked at what Jerry Muehlbauer of Marathon Electric had to say on the subject. In a Q&A he address the subject as follows:
Q: Can a motor handle more starts/hour on VFD power?
A: Yes, provided that the VFD is properly tuned and its output frequency is steadily increased (or ‘ramped up’) from zero to the set point and the motor is not ‘line-started’, which causes extremely high (6-8 times rated) motor currents. When a motor is line-started the enormous heat generated in the motor windings limits the number of starts/hour (motor must cool down between starts). Line starting could be compared to starting your car while on jack stands: Put it in gear, start the engine, place a brick on the gas pedal, and then push it off the jack stands. This results in uncontrolled acceleration & huge mechanical stress on the transmission and engine. How many times can you do this before something breaks? Across the line (ACL) starting produces similar thermal and mechanical stresses in the motor and driven load. According to NEMA MG1, the life of a motor is basically determined by the number of ACL starts.
When the VFD ramps up the output frequency, inrush current is eliminated and mechanical stress is greatly reduced, permitting the motor to be repeatedly started without significantly shortening insulation life. Acceleration rates as short as 1/2 second (for small motors) or 1-2 seconds (for larger motors) are still long enough to avoid high inrush currents. In the automotive example, this is comparable to starting the engine, putting the transmission in gear, then driving away at the selected acceleration rate.
There are a number of authors who have discussed whether a variable frequency drives (VFDs) are helpful in limiting the amount of inrush current to motors and thereby extending motor life.
Joe Kimbrell, product manager for Automation Direct, in his article published in march of 2014 states that “Controlling starting current can also extend motor life because across-the-line inrush current shortens life expectancy of ac motors. Shortened lifecycles are particularly prominent in applications that require frequent starting and stopping. VFDs substantially reduce starting current, which extends motor life, and minimizes the necessity of motor rewinds.”
WEG’s publication “Choosing a Variable Frequency Drive or Soft Starter based on your application need” also discussed this issue in the first paragraph:
“When accelerating an AC motor to full speed using a full voltage connection, a large inrush current may be required. Additionally, the torque of the AC motor is mostly uncontrolled and can shock the connected equipment, potentially causing damage. Variable Frequency Drives and Reduced Voltage Soft Starters and can both be used to reduce inrush currents and limit torque; thereby protecting expensive equipment and extending the life of the motor and coupling devices. Choosing between a variable frequency drive and soft starter often depends on the type of application, the mechanical system requirements, and cost (both for initial installation and over the lifecycle of the system).”
Today, VFDs are used to provide precise control of a wide range of processes and process variables in manufacturing applications.
By allowing motors to run at less than full speed, VFDs can save energy — often 30% or more. But they also offer other benefits. They can improve equipment and machinery uptime, reduce switchgear and cabling costs, and enable more precise process control. As a result, they can improve productivity, quality, and profitability. But VFDs can cause unplanned motor bearing failures — often in as little as three months.
VFDs Improve Process Control…
When used to control conveyors — belts or overhead lines — VFDs allow line balancing for optimized production and minimal idle time. With their rapid acceleration and deceleration capability, VFDs can also minimize transfer times between production cells. And by providing non-emergency stop and start control, VFDs can also be used to gently slow the speed of inclined conveyors.
For processes that could run at full uncontrolled speed (such as extrusion), VFDs offer greater control by eliminating variations in line voltage or frequency that could cause unwanted variations in the product itself.
VFDs also improve the efficiency of machines that control process variables in industrial production processes such as the speed of heating blowers and cooling fans, air compressors, assembly lines, quench lines, filling equipment, wire drawing equipment, etc.
Rather than throttling or braking fixed-speed motors, VFDs control motor speed by controlling the amount of power to the motor itself. And by varying the amount of power to the motor according to precise time curves, VFDs can provide soft starting and stopping — smooth ramp ups and slow downs of machines to reduce wear-and-tear and extend machine life.
And by limiting the amount of inrush current to motors, VFDs can also extend motor life.
And yet, the most impressive benefit from the use of VFDs may well be the energy savings they produce. According to a U.S. Department of Energy study, approximately 25% of industrial motor system energy usage was for pumps, and another 14% was for fans. Rather than diverting, redirecting, or throttling the output of motors that power these devices, VFDs provide a way of precisely matching motor output to load. And the benefits of VFDs extend beyond manufacturing systems to plant/facility systems such as HVAC and air handling systems.
But VFDs Damage Motor Bearings
But despite all the control and energy saving benefits they offer, VFDs can damage the motors they control. VFDs produce voltages on the shafts of the motors they control — voltages that can discharge through motor bearings, causing electrical discharge machining (EDM) pitting, frosting, and fluting failure.
AEGIS® Rings Protect Motor Bearings from VFD Damage
To protect process and facility system motors and prevent crippling, costly downtime, VFD-driven motors need proven long-term bearing protection from damaging VFD-induced shaft voltages.
Proven in millions of installations worldwide, AEGIS® Shaft Grounding Rings channel damaging shaft currents away from bearings and safely to ground.
AEGIS® SGR Rings are available in solid- or split-ring versions, and also as uKITs with mounting brackets that simplify installation on motors with shaft shoulders or end bell protrusions.
For medium-voltage motors and large low-voltage motors with shafts to 30” in diameter, AEGIS® PRO Series Rings also come in solid- and split-ring versions, and may also be mounted on universal mounting brackets.
AEGIS® solid rings are designed for installation on new motors before they are put into service, and split rings are ideal for field installation on in-service motors because they eliminate the need to decouple motors from attached equipment.
In addition, a large and growing number of motor manufacturers now offer motors with AEGIS® Rings factory installed.
For more information on the types of bearing damage VFDs can cause, click here.
For an application story on how AEGIS® Rings solved the ongoing problem of bearing damage to a 1000 HP transfer motor at a pulp and paper plant, click here.
For a list of manufacturers that offer motors with AEGIS® Rings factory installed, click here.
Learn About Electrical Bearing Damage and How to Prevent It… While Enjoying Lunch on Us!
Lunch-and-learn presentations should be educational, not simply promotional. To benefit those who commit their time to them, lunch-and-learns should address topics that are not well understood and should provide the latest information on them as well as solutions to problems. And we know from experience that shaft voltages and the damage they can cause to motor bearings result in unplanned, unwanted, and expensive downtime. So, learning why they occur and how to prevent them is critical to design engineers and to plant maintenance people.
The problem is that the damage caused by VFD-induced shaft currents manifests itself as physical damage. And all too often those who deal with such damage assume it was caused by poor shaft alignment, inadequate lubrication, or other physical factors.
When a variable frequency drive (VFD or inverter) is used to control a motor, it alters the waveform of the power to the motor. VFDs change balanced sine wave power into a series of unbalanced positive and negative pulses that create capacitively coupled common mode voltage on the shaft of the motor. Without a low resistance path to ground, this voltage will discharge through the motor’s bearings. These voltage discharges cause electrical discharge machining in the form of pitting (tiny fusion craters in metal surfaces), frosting (widespread pitting), and fluting (washboard-like ridges in the walls of the bearing race resulting from the operational frequency of the VFD).
Without proven long-term bearing protection, these discharges can destroy motor bearings — often in as little as 3 months!
At AEGIS®, we have been educating motor users, motor repair shops, plant maintenance personnel, specifying engineers, and contractors on the causes and prevention of electrical bearing damage for years. And through the years we have come across more than a few products that claim to protect against electrical bearing damage, but don’t actually live up to these claims. We know what works and what doesn’t. And because we have tested them, we can tell you where and how they fall short.
We have also developed best practices for diagnosing electrical bearing damage and preventing it in motors large and small, low or medium voltage. We have even detailed these best practices in a 56-page Bearing Protection Handbook that discusses in detail the causes of electrical bearing damage, how to diagnose it, and step-by-step best practices for protecting motors from it.
Yet, despite these efforts to inform and educate motor users, we still find many who are unaware that the problem even exists or are unsure why or how to address it.
So if you would like to learn about the causes of electrical bearing damage and best practices for preventing it, we have a 1-hour presentation on the subject. And we would welcome the opportunity to make this presentation to as many of your people as you would like — from a handful to a room full — at whatever time you like. We’ll even bring lunch or other refreshments.
To request a Lunch-and-Learn training at your facility, click here.
VFD-induced shaft voltages damage motor bearings and shorten motor life
If you’re using variable frequency drives (VFDs) or inverters to control motors, the motors are at risk of electrical bearing damage that can dramatically shorten their lives. VFDs
induce harmful voltages on motor shafts — voltages as 40 volts peak — that can destroy bearings in as little as 3 months!
Through electrical discharge machining, VFD-induced discharges can blast millions of pits in metal bearing surfaces. These discharges burn and contaminate bearing grease, drastically reducing its effectiveness. They also result in fluting, bearing failure, and costly unplanned downtime. And while most motor manufacturers offer “inverter-duty” or “inverter-ready” models, these motors have inverter-rated insulation to protect their windings, but nothing to protect their most vulnerable components — their bearings.