Some regions frequently suffer from the devastating effects of earthquakes, as we have regrettably seen recently in both Ecuador and Japan. When it comes to AC drives, though, it’s not only seismic activity that they’re exposed to. As a matter of fact, your AC drives can be exposed to high vibration levels during transport, storage or operation, like when they’re mounted directly in a compressor or in the bridge area of a ship, for example.
So, are your AC drives going to survive in these high vibration or seismic conditions? If you’ve been following this series, you may have guessed that the answer is yes… as long as they are designed to survive. This is a bit of a heaver topic as we’ll dig into some of the technical details of the testing procedures. But, by the end, you’ll have a better idea of what to look for as well as what steps we take to ensure you always have the best AC drive for your applications when vibrations are your main concern.
Vibrations literally come in all shapes and sizes. Short, sharp vibrations typically fall under the classification of a shock. What we think of as typical vibrations last longer and aren’t so large, but even they can do quite a bit of damage if they go unaccounted for. But even these types of longer vibrations vary in how they present. For example, with seismic events, such as large earthquakes, surface vibrations can have amplitudes in the range of centimeters. This is several times larger than the vibrations seen in machinery, which is more typically in the range of millimeters. Additionally, seismic events create ground motion which moves simultaneously in all directions in a random fashion, which adds some additional challenges for your equipment.
Preparing for seismic events
In order to ensure that the AC drives will operate and continue to operate in these high vibration environments, Danfoss Drives ensures that we’re running rigorous testing processes to meet certain specifications. For example, certain arrangements and panels of VLT® HVAC Drives, VLT® AQUA Drives and VLT® AutomationDrives are tested according to ICC-ES AC-156 for seismic conditions. By testing for three main factors, we can determine the suitability of these products to be mounted in any kind of building on any floor that can be impacted by seismic events.
The three main seismic characteristics of the tested equipment in this procedure are:
- SDS (Spectral response acceleration at short period)
- Ip (Importance Factor)
- z/h (Height factor ratio)
Values of SDS of 2.6 g or higher, Ip=1.5 (for critical care facilities) and z/h=1, assure that the component can be mounted in any kind of building on any floor. For further details consult Danfoss Drives.
When it comes to vibration, we employ two different types of vibration tests which, in combination, allow several Danfoss Drives products to be suitable for applications with high vibration levels. The tests used here are Sinusoidal vibration tests (according to IEC 60068-2-6 standard), which are short-term tests used to determine resonance frequencies and amplification. The vibration effects are generated by periodic forces such as rotating, pulsating or oscillating forces similar to the vibrational forces originating from ship’s motors, compressors, piston motors or other similar applications. After the resonance frequencies are determined, endurance tests are carried out in each direction (X-Y-Z), both at the resonance frequencies and by sweeping frequency ranges that use differing acceleration curves that are based on the application and mounting location. Ship classification companies, for example, can require acceleration levels of 4 g (four times the acceleration of gravity) for products mounted directly onto main motors and/or pipe systems, but only 0.7 g for the rest of the products.
Functional and reliability testing
Random vibration testing (according to the IEC 60068-2-64 standard) is also done, either as short-term functional testing, or long-term to determine fatigue of mechanically weak components (reliability testing). Vibration levels are applied in each direction (X-Y-Z) according to a frequency dependent Power Spectrum Density level (PSD) curve (up to 100 or 1000 Hz), empirically built up through knowledge of the particular application. This PSD curve is normally named by its total gRMS value (square root of the area under the curve), which must not be confused with, or compared to, acceleration levels at sinusoidal vibration. This gRMS can vary from 0.7 gRMS in a normal wall or floor mounting to more than 7 gRMS for direct installation on motors or compressors.
It’s all quite well defined from the position of an AC drive manufacturer, but how does that help you select the right AC drive for your application?
Part of the solution relates to another IEC standard, IEC 60721-3-3, which defines different mechanical conditions classes (M classes), depending on stationary (sinusoidal) and non-stationary vibrations (including shock), from 3M1 (0.1 g vibration / 4 g shock) to 3M8 (5 g vibration / 25 g shock) levels. Identifying the right products is made a bit easier when you can refer to this particular IEC standard so you can know what to expect.
By now it should come as no surprise that Danfoss Drives has some of our products that are rated to top classes so they provide optimal behavior in your high-vibration applications. Both VACON® 100 X and VLT® Integrated Servo Drive products are rated as 3M7 class products which means they can withstand 3 g vibrations/ 25 g shock. Additionally, VACON® 20 X and VLT® DriveMotor FCP 106 are rated as 3M6 class products, which means they can withstand 2 g vibrations / 25 g shock.
Special requirements for marine applications
For your Marine applications, VLT® HVAC Drive, VLT® AQUA Drive, VLT® AutomationDrive, VACON® NXP Liquid Cooled and VACON® 100 INDUSTRIAL all carry additional marine type approvals and so are compliant with their correspondent vibration testing requirements. These marine type approvals often require additional mounting hardware that helps dampen vibrations in the AC drives, so it’s important to check if that’s required for your AC drive of choice for these applications.
As you can see, there’s quite a bit of variation between shock, vibration and seismic environments. All of these have their own unique challenges when it comes to selecting and installing your AC drive and making sure that it has the longest functional lifetime possible. Knowing what conditions can occur and looking at the specifications of your AC drive will make sure that your selection is always the best selection for the job.
Check back regularly with us here at FocusOnDrives.com for regular updates on the best ways to ensure that your investments in AC drives are always the safest investments around. Next time, in Part 7 in the “It’s a harsh world…” series, we’ll look at Drives vs. Dust, fluff and other solid particulates where we take a closer look at what impact these elements have on your AC-drive lifetime and operation. Additionally, let us know in the comments what environmental impacts give you the biggest challenges and how we can help you overcome them. Regardless of whose name is on the label, we’re here to help! In the meantime, you can find out more about all our products here.
Authors: Abraham González Ponce, Application Knowledge Manager, Application & Service Products, Danfoss Drives
Jake Roeder, Global Product Marketing Manager, Danfoss Drives