Subject: Drive isolation transformers. VFD definition: History the short version. February 2012

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1 February 2012 Subject: Drive isolation transformers This paper is designed to provide BASIC information on the application of transformers designed to manage secondary loads comprised largely of alternating current motor starting and speed controls. VFD definition: A VFD (variable-frequency drive) is an electronic device used to start and/or control the RPM of an alternating current motor by controlling the frequency of the power supplied to the motor. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variablespeed drives (VSD), AC drives, microdrives or inverter drives. History the short version Prior to variable frequency drives, motor speed control was generally accomplished through voltage adjustments with DC motors. With the development of electronic circuits in the 1960 s with the development of thyristors, a solid state device similar to a diode in the 1980 s, it became possible to power AC motors from high power converters. Today thyristors rated as high as 4000V and 4000A are now available, allowing even the highest rated AC motors to be supplied with a variable frequency from a solid state electric converter. Pacific Crest Transformers 300 West Antelope Road Medford, Oregon Tel : (541) Fax : (541)

2 Prior to VFDs, AC motors were started across the line which resulted in very high inrush currents since the motor rotor was idle. To alleviate this problem, reduced voltage motor starters were developed. These devices applied a starting voltage of approximately 33% of the operating voltage initially to limit the current. As the motor began to speed up, the current dropped at which time the voltage was ramped up to about 66%. The current, of course, increased markedly, but would decrease once again as the motor RPM continued to increase. Third stage would then kick in which provided full voltage to the motor bringing it up to full operating RPM. Why used (benefits) Variable frequency drives can provide significant energy cost savings where motor-driven applications do not require full speed. VFDs gradually increase the motor speed which decreases the mechanical and electrical stresses, which in turn translates to reduced maintenance and repair costs. In some cases the savings can offset the cost of the VFD in as few as several months. Fan and blower applications are prime examples of applications where such savings may be available. In addition to merely starting motors, VFDs offer the significant advantage of controlling motor speed. Controlling speed allows for precise control of conveyors, assembly lines, and a host of processing applications. A VFD also provides the option of variable or constant torque. Markets Motor starting and speed control market opportunities are widely available within the industrial industry. Each transformer varies in base and K-factor load requirements requiring specific designs for each to insure maximum performance and product life. 2

3 In each of these markets, there exists opportunities to provide transformers for new installations, upgrades, as well as replacements. Above are just a few of the industries in which continual drive isolation transformer requirements are to be found. The loads however, require that specific load requirements be obtained in order to insure that the proper design parameters are identified. These transformer requirements are not suited for off the shelf products. K factor Although a technical dissertation on K-factor is beyond the scope of this paper, it needs to be touched on in order to understand the reasoning for design modifications made to accommodate the impact that the VFD has on the load side of the transformer. ANSI C : IEEE Recommended Practice for Establishing Transformer Capability When Supplying Non-sinusoidal Load Currents, details the subject of transformer K-factor rating. The K-rating relates to the transformer s capability of supplying 100% of the 3

4 fundamental 60 HZ load plus load side harmonic content, while operating within its operating temperature limit. Harmonic content is comprised of voltage and current frequencies in addition to the fundamental (60 HZ) which result in waveform distortion. The electronic circuits utilized in variable frequency drive equipment result in harmonic loading. The following graphic provides an example, albeit simplistic, of this condition. Note the fundamental (60 HZ) sine wave. The wave form is sinusoidal. Now note the 3 rd harmonic load (in green above), which in itself is also sinusoidal. Multiple sine waves however are additive. With this being the case, the actual sine wave (voltage or current) that the secondary side of the transformer sees is the resultant non-linear wave as depicted above in red. This distortion may require that the transformer design be modified to accommodate the additional load. Failure to do so could lead to accelerated aging of the transformer insulation system which translates to reduced operating life. 4

5 Here is a K-factor general use chart. Note that a K-20 rating is generally specified for VFD (aka variable speed drive) application. Transformer design considerations In addition to kva and voltage, there are several factors that are considered when designing isolation drive transformers which may or may not be supplied with the inquiry. These include but may not be limited to: conductor (copper, aluminum) insulating fluid (oil, silicone, envirotemp) environment (corrosive, indoor, outdoor, mobile, etc.) duty cycle (the percentage of time a transformer is energized and loaded per its rating) K-factor 5

6 Unless requested otherwise, a K-20 rated transformer would be provided if the load was solely for VFD duty. For compound applications however, duty cycle and K-factor specifics would need to be provided which could result in additional modifications to core and coil assembly to accommodate the additional load requirements. Pictued above is an example of a transformer designed and built for a specific drive isolation installation. The electrical characteristics meet the load requirements for the VFDs it will supply power to. The tank is designed to interface with associated switchgear. Since the installation is to be in a petrochemical environment, all guages and monitoring equipment are terminated in explosion proof terminal boxes. Pacific Crest Transformers has been serving the industrial market since It prides itself in providing transformers designed for specific applications rather than mere off the shelf products. Transformer load requirements vary wildly and PCT has the experience to insure that the right questions are secured prior to providing a propopsal. 6

7 PCT posseses the engineering expertise to address today s complex load requirements. The products are designed to properly manage each installations specific loads efficiently while aliviating concern over transformer failure or overheating. PCTs core and coil technology includes robust propriety clamping and cooling methods, which insures the effective management of inrush currents, heavy duty cyles, and harmonic load profiles. -Pacific Crest Transformers- "First in Response, First in Delivery, First in Quality 7

8 Rectifier/VFD/Motor Duty Configuration 1) kva 2) Voltages High (Delta or Wye) Low (Delta or Wye) BIL kv kv 3) Phasing 4) Harmonic Profile a. K Factor b. Attach Harmonic Load Data (out to 31st harmonic) 5) Required Standard (IEEE/IEC) # 6) Dedicated Motor Starting Across-the-line or Soft Start Full Locked Rotor Current A Duration of Starting Curve (Time) 7) Duty Cycle/ Loading Profile a. Peak Current A b. Peak Duration (Sec.s/Min.s/Hrs) c. Frequency/Intervals (per Hr, Per Day, etc.) 8) Overvoltage/Under Frequency Magnitude % 9) Overvoltage/Under Frequency Duration (Sec.s/Min.s/Hrs) 10) Rectifier Circuit # 11) DC Current Component % Pacific Crest Transformers 300 West Antelope Road Medford, Oregon Tel : (541) Fax : (541)