VFD testing: Introduction
Abstract
This new series on VFD testing will guide you through various aspects of testing of variable frequency drives (VFD) and drive systems. We will discuss tests required by standards (particularly IEC), factory acceptance tests, site acceptance tests and system tests, such as full load combined test or string test.
As it is a broad topic, this article provides an introdution, classification and couple of considerations around VFD testing. Follow-up articles in this series will then discuss the specific groups of tests in a more detailed way.
Introduction
VFD testing is an integral part and an important milestone in the execution of any VFD project. Testing shall confirm quality, functionality and performance of the product. Testing at manufacturer’s facility might be the first opportunity when customer physically sees the purchased product. At the same time, testing of variable frequency drives and drive systems is rather costly task requiring resources and dedicated infrastructure. Therefore, it is important to get the testing done right and ensure that it really brings an added value.
Purpose of VFD testing
The purpose of various VFD tests is to ensure high quality of the product(s), demonstrate reliable operation, detect and rectify initial issues if any, validate correct interfaces between system components and confirm system performance that might be part of contractual obligations.
In order to fulfill these ambitious goals a series of various tests can be performed – see next paragraph. Note that the standards define certain mandatory tests while other tests are performed on request or are just optional. Such tests shall be specifically agreed between the manufacturer and the buyer.
Types of tests and their purpose
There are many different types of tests. Some of them test the VFD only while other cover the drive system up to tests of entire drive system together with a driven load.
– Product tests (routine, type and special tests)
– Factory acceptance tests (FAT)
– Tests and certifications based on country of installation
– Drive system tests (back-to-back test, full load combined test)
– Overall system tests (e.g. string test)
– Site acceptance tests (SAT)
1. Product level
(i) Routine tests
Routine tests are tests performed on each single product. It means that every VFD, before being shipped to customer, undergoes these tests. Their purpose is to confirm basic functionality and quality. The routine tests are defined by national and international standards, such as e.g. IEC or IEEE. Note that manufacturers might have their own routine tests and procedures. Renowned manufacturers may include some additional test on top of those requested by international standards.
(ii) Type tests
The principle of type tests si to test and verify the product design, i.e. to confirm design specifications.
As most of the VFDs on the market are standardized to some extend, the type testing usually applies for the first prototype of a product or first batch.
Unless the design changes, the results of type tests are then valid for all the products of the same type.Therefore, type tests are usually not performed on the project VFD. Instead, VFD manufacturer provides the necessary documents and certification proving that the product had been successfully type tested.
(iii) Special tests
Special tests shall be agreed between manufacturer and user. They may require special infrastructure or equipment. Manufacturer may not be able to perform them and a special laboratory needs to be used.
Special tests according to [2] are for instance overcurrent capability test, interphase ripple voltage and ripple current, audible noise test or power factor measurement.
(iv) Factory acceptance tests
Factory acceptance tests are similar (based on) routine and type tests described above. However, each manufacturer has his own test procedure. Note that the scope of FAT may differ from one product to another one. Differences in testing between product variants may occur as well.
(v) Country or industry specific certification and testing
End users in some countries require additional country specific certifications. This means that a VFD designed, manufactured and tested according to e.g. IEC standards is not enough.
Example is for instance certification based on CSA standards for Canada, UL standards for United States, GOST for Russian Federation and certain neighboring countries etc. Either these standards have already been considered in product design and included in type testing or a project specific testing is required. The certification body reviews manufacturer’s design and passed tests and defines whether additional tests are needed.
Certain industries, for instance marine, requires specific marine certification to be done by certification authority (ABS, BV, DNV etc).
2. VSDS level
Next group of tests is performed on a variable speed drive system (VSDS), also called power drive system (PDS) in IEC terminology. Alternatively, these tests might be called integrated system tests. The VSDS typically consists of (but is not limited to) input isolation transformer, variable frequency drive, electric motor, eventually also harmonic filter, output transformer etc.
These tests can be seen as “add on” to the product tests. Assuming that each drive system component (transformer, VFD, motor etc) successfully passed the routine tests and type tests, the VSDS testing shall confirm correct performance on drive system level.
Tests on VSDS level are back-to-back (B2B) test or full load combined test (FLCT). These tests are often cost, resource and time intensive. Therefore, most VFDs will not undergo such tests unless it is a critical equipment, a special configuration or it is explicitly requested by customer from any other reason.
3. Entire system level
Finally this group of tests demonstrates the overall system performance including the driven load. Such tests provide the highest confidence, but require also quite large effort to be performed. At least when we talk about so called string tests, i.e. overall system tests performed before shipping the equipment to its final destination.
Other tests in the system category are site acceptance tests. As the name reveals, these tests are performed after the final installation is finished. They might be linked to contractual obligations.
Continuation of VFD testing series
How will this series continue? We will discuss the testing on product level first (routine, type and special tests) before moving to system tests. The series will be split into few articles to keep the length reader’s friendly and easy to absorb. Therefore, stay tuned. In order not to miss important or interesting updates you can subscribe to our newsletter HERE.
References
[1] IEC 60146-1-1 Semiconductor converters – General requirements and line commutated converters – Part 1-1: Specification of basic requirements, https://www.iec.ch/
[2] IEC 61800-4 Adjustable speed electrical power drive systems – Part 4: General requirements – Rating specifications for a.c. power drive systems above 1 000 V a.c. and not exceeding 35 kV, https://www.iec.ch/
[3] VFD basics (series), https://mb-drive-services.com/category/vfd_basics/
[4] Daniele Buzzini, Maurizio Zago, Testing large ASDS, IEEE Trans. on Industry Applications, July 2013, pp. 1873-1882
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
