As we discussed last week in Part I, defining your application and its specific characteristics is critical to ensuring a cost-effective and successful drive installation. In addition to the mechanical considerations we discussed in Part I, there are several electrical line and load-side issues which should be factored in when deciding on the right drive (or no drive) for your process. Because there are many factors involved we will cover supply side issues for AC drives this week, and load side next week. Future posts will discuss DC drive applications in detail.
Line (Supply) Side Considerations:
- Voltage: Modern variable frequency drives (for AC motor applications) are rated to accept a nominal voltage with a typical +10/-10% tolerance. Outside of this range, the drives will usually trip out to protect themselves. In the case of an over-voltage condition, the risk is to the DC bus, which typically runs at about 1.2x the incoming voltage level. Once that level exceeds about 1.4x of the upper end of the incoming voltage range, the drive will trip out on OV. (Note that unlike AC drives, due to the nature of their output electronics DC drives up to 500V can operate at lower incoming voltages; however, their output voltage reduces accordingly.) Output voltage, although tightly controlled by the drive, essentially peaks at the incoming voltage level, so in effect you get out what you put in. With a few special exceptions, drives will not replace step-up/down transformers.
- Phase: Nearly all drives provide a 3-phase output, most often fed by a 3-phase supply. But drives are also specified for their ability to convert single-phase voltage to 3-phase, often instead of rotary or electronic phase converters. However, they do so at a cost – their DC electronics must be beefed up to handle the single-phase input. In essence, this is because the DC link capacitors are charging only 1/3 of the time they would if fed with 3 phase, so in order to smooth the DC bus ripple and supply the inverter with the proper voltage level they need to be higher capacity. Also, single-phasing results in higher line currents, so more conducting capacity is needed. Generally, an AC drive operated on single phase will need to be de-rated about 50% from its 3-phase rating, effectively doubling the size of the drive needed. You will find lower-power drives rated “out of the box” for single-phase input; these are simply drives manufactured to handle the higher capacity. Such drives are difficult to find at ratings above about 3kW.
- Impedance: In cases where the upstream line impedance is too low (a.k.a. “stiff”), as for example with a drive system connected very closely to a main switchgear, the impedance may need to be increased to reduce the stresses on drive electronics caused by high current rate of change (i.e. di/dt). This is often accomplished by inserting line reactors on the AC line or chokes on the DC bus. Many modern drives come equipped with line reactors, typically providing 3% impedance; if more is needed, optional reactors can be obtained. These reactors have the added benefit of reducing harmonics on the supply line, the generation of which is inherent to most drive designs and which can cause operational problems for sensitive upstream equipment. (More on harmonics in a later post…)
- Over-current protection: Drives provide extensive, user-programmable motor protection. However, they have limited built-in protection for their electronics, and proper installation requires the use of high-speed, low “I-squared t” over-current protection. Typically class J/gG fuses are specified to limit current and clear it very rapidly, before sensitive electronic components can be damaged. In some cases, drive manufacturers have tested and approved circuit breakers for this purpose. However, they tend to offer less rapid response, more challenging coordination with other protective devices, and, if set too tightly, trade nuisance trips for the convenience of not having to replace the unit once tripped. In any case, it is always advised to use manufacturer-recommended devices and ratings.
Next week we’ll move to the down-stream (load) side of the drive to discuss issues regarding motor supply and protection. In the meantime, please feel free to share your thoughts, questions, concerns, etc. in the Comments section. And as always, you can contact me with any questions, comments or application needs you may have at firstname.lastname@example.org. Thanks for reading, and please join us next week!