ECN Forum Forums General Topics of the Trade General Discussion Area Icecream Batch freezer running on RPC

Fred...

The general answer is no... if the current coming in is single phase.

The reason is that the incoming AC is rectified into DC and stored in a series of capacitors. (An electron 'tank', if you will.)

The variable frequency alternating current wave form is created out of this DC potential by using the magic of solid state electronics -- which can switch at speeds into the gigahertz.

Such a high speed is not used for VFD logic, though. Megahertz speed is plenty fast enough to generate a sequence of DC pulses -- that when stacked -- create a synthetic wave form -- which is easy to craft between 10 Hertz up to 400 Hertz.

In all VFDs -- they are strictly designed for 3-phase power input -- they have to be de-rated for 1-phase power -- because the rectification side of the system will be starved of power... you're actually trying to get the same output -- power wise -- out of 1/3 of the wave cycles.

This means for actual devices that 2/3 of the rectifier is actually sitting idle.

Then you have to throw in the extreme ripple that a single input phase places in the rectifier.

So a 2.5 hp VFD unit (3-phase inputs) requires that you bump up to a 10 hp VFD unit -- to power the same motor.

(There are no intermediate range units to hand, so you have to jump all the way to 10 hp.)

Even a 5 hp VFD will be starved of current -- and afflicted with nasty ripple -- that makes the rest of the device 'suffer.'

Fortunately, the pricing of a 10 hp VFD is NOT 4 times that of a 2.5 hp VFD. I've seen units (eBay)(Red Chinese) that are only 60% more expensive at 10 hp than they are at 2.5 hp. (side by side -- same manufacturer, etc.)

This works out because most of the expense is in the logical part of the VFD. The actual power flow part of the device is actually pretty simple, and scales cheaply.

Undersizing a VFD for single phase service use leads to short life, and poor performance. It's sort of like running a gasoline engine with two dead cylinders out of six.

With a high torque need at start-up, this is only amplified.