Frequently Asked Questions (FAQ)

Q. Is IPC’s Universal Power Converter AC-link the same AC-Link used by Princeton Power Systems (PPS)?
A. No, the Universal Power Converter (UPC) technology is completely different, with the link being truly AC, having no DC offset in voltage or current. Additionally, our link is dynamic, always oscillating, while the PPS link is quasi-static, in that it may be stopped intra-cycle in order to reduce power output. The UPC AC-link reduces power output instead by reducing the charge transfer on each power cycle. Also, the UPC AC-link produces two power transfers for each link cycle, whereas the PPS AC-link has only one power transfer for each link cycle. Other important features of the UPC converter lacking in the PPS AC-link include full buck-boost capability, true soft switching, input-output isolation, and much higher minimum link frequencies which allow for greatly reduced I/O filtering requirements, which improves efficiency and reduces weight and costs.

Q. Is the UPC a Resonant DC-link converter?
A. Although the UPC has a superficial resemblance to resonant DC-link converters, it is not such a converter. Note that those converters have line filter inductors between the switches and the input/output, as these are voltage source converters, whereas the UPC has line filter capacitors, as it is a current sourced converter (an AC current source, not a DC current source as is usually done). Also, although the UPC link has “partial-resonant” intervals, and the link is an inductor in parallel with a capacitor, the link is not operated resonantly. The link waveform is not sinusoidal as the resonant DC link waveform is. Most importantly, the UPC may be operated without a link capacitor, as it only serves to provide soft switching, whereas the resonant DC link converter cannot operate without its link capacitor. Resonant link converters suffer from only being able to control switching at discreet intervals given by the resonant frequency, which causes poor PWM waveforms and I/O filter ringing and power loss. The UPC controls current in precisely controlled amounts within each power cycle so that it has a near perfect current PWM waveform, producing no harmonics below the power cycle frequency (which is twice the link frequency, with two power cycles per link cycle). The standard resonant DC link converter has link voltages about twice the peak line-to-line voltage, while the UPC converter’s peak link voltage is equal to the peak line-to-line voltage.

Q. Can the UPC operate over the full power range without problems?
A. Yes, it can transfer power in either direction at any level commanded by the controller while maintaining high link frequencies, and therefore the input/output filters may be very small since they may have a resultantly high resonant frequency. Minimum link frequency occurs for maximum current at zero output (or input) volts, and is only about 30% lower then full power link frequency. Link frequency at or approaching zero power is about three times higher than the full power link frequency.

Q. Does the UPC need any external filters to meet IEEE-519?
A. No, all motor drives and power converters using our technology need no external filter to meet the stringent harmonic limits of IEEE 519.

Q. Does the UPC have any voltage buck or boost restrictions?
A. Only as given by component ratings. The UPC may supply or sink full rated current with any voltage ratio between input and output, and may interchange input and output since the topology is perfectly symmetric.

Q. Can the UPC handle all power factors?
A. Yes. As per the previous question, the UPC can buck or boost with any I/O ratio, within component limits, so it has not problems supplying a purely reactive load, inductive or capacitive. It may also draw or supply current with any power factor desired, to or from any voltage. For example, a wind generator running at 50% of the utility voltage may be operated by the UPC with unity PF, and supply that power to the utility at any power factor requested by the utility, all with very low harmonics at all power and voltage levels on both input and output.

Q. With 12 AC or 24 DC switches, how can the UPC be economical?
A. Since soft switching is used which allows for slow switches, and the switches see no more than the twice the input line-to-neutral voltage (a little more than the peak line-to-line voltage), converter power per switch is comparable to standard power converters. And costs of power semiconductors have fallen dramatically, leaving most of the cost of a conventional IEEE 519 compliant motor drive in the large input filters. Most importantly, due to the relatively high frequency current source of the UPC, I/O filtering requirements are minimal.

Q. What is the link frequency range?
A. For low voltage drives and converters (less than 600 VAC), the link frequency at full power will be about 7 kHz, although much higher frequencies are possible. The power cycle frequency is double this, or 14 kHz. Full current but zero voltage (or zero power factor) output voltage will result in about a 30% drop in link frequency. Link frequency increases as current is reduced, with the link frequency increasing by 3 to 4 times as power approaches zero at full output voltage. At zero power, the link is purely resonant.

Medium voltage drives and converters will have link frequencies of about 1.5 kHz, due to the much longer turn-off and reverse recovery of high voltage switches, but since these converters are high power, the link inductors are still relatively light and have low losses since air core inductors become more efficient as they increase in size, and self-shielded air core toroidal link inductors become advantageous at power levels over about 500 kW.

High minimum power cycle frequencies allow for low input filter requirements, since the I/O ripple frequency is the same as the power cycle frequency, which in turn is twice the link frequency. The L-C of the input filter must have a resonant frequency that is lower than the lowest possible ripple frequency. With a minimum ripple frequency for low voltage drives and converters of about 10 kHz, the L-C filter resonance can be over 3 kHz, which allows for very small input reactors. In fact, these reactors may be smaller than the smallest input reactor that’s useable for a standard motor drive.