Thread: Phase Perfect inrush current

I was talking with a Phase Perfect rep today and he indicated that the single phase input line would have minimal inrush current on it while starting a load as long as it's within the 4 sec overload rating. I'm kinda skeptical because one leg is a pass through. Although, I might buy it if all three legs were manufactured and there was enough capacitance on the DC buss to handle the motor start. Has anyone with a PP checked the inrush current on the input side while starting a motor? And is it really reduced by any significant amount?

I haven't put a meter on mine, but my 70A single-phase feeder breaker is not big enough for a 220V 10HP inrush surge of three-phase 140-180A and yet I don't pop it when starting a 10HP motor downstream of the PP.

The trick is that the "pass through" isn't passive. The PP can actually source current back out onto the L1 & L2. The brochures for both the previous DPC generation and the current PT generation includes "Clean power fed back to power grid during regenerative load situations". (See page 19 in either version.) So the internal DC bus does affect the "pass through" legs as well as the manufactured leg. The block diagram in the brochures suggests that it doesn't, but it must.

Edit: Found this more substantial statement on the PT330 page:

REGENERATIVE POWER CAPABILITY
Phase Perfect converters handle regenerative power by passing clean, balanced power back onto the single-phase line when three-phase loads are in a generating mode. Most other power switching converters waste regenerative power by burning it up with braking resistors. Putting regenerative power back onto the line saves electrical power and even allows Phase Perfect to operate continuously as a three-phase to single-phase converter.

3-phase to 1-phase converter? Must be sourcing current onto L1 & L2.

Thanks. That makes a little more sense.

I'm trying to find a way to run 3-phase equipment in an off-grid system. There are inverters that can be paralleled or configured to generate 3-phase power, but the inrush current is still above the surge rating of multiple inverters. If a PP can sufficiently buffer the single phase line from the inrush, then it would be much cheaper to use one inverter and a PP, rather than using a cluster of inverters. If I can can get some 'real world' numbers it'll help me figure out if a system like that is feasible.

By far the most effective way to reduce inrush current is to put a VFD on every load motor, and soft-start them with whatever parameters it takes to meet your power capability. If its feasible to use all single-phase-input VFDs maybe you don't need a 3-phase system at all.

The PP itself when powered on has a pretty hefty inrush current. The lights blink when turning on the PP, but not when soft-starting a 20HP motor fed downstream of the PP, on its own VFD.

I've thought about using VFDs on the motors, but I'm not sure how an inverter would handle the harmonics generated by the VFDs. I'm sure line reactors would be a necessity at a minimum.

After more searching, I found this link which states:

There is an increase current above FLA because it takes real power to accelerate the rotor from standstill to several thousand RPM instantly. Generally, when a digital phase converter starts a three-phase motor across the line, the current drawn from the single-phase line is equal to that which would be drawn from any one line if the motor were started across the line on three-phase service.

If that's the case, then I'm getting close to where I need to be. I still may need a VFD on the larger motors, but the PP should provide clean power back to the inverter, eliminating any issues of running a VFD off of an inverter.