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Re: Icecream Batch freezer running on RPC [Re: Fredweet] #215387 04/25/15 09:30 PM
Joined: Jun 2004
Posts: 1,273
Tesla Offline

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.

Last edited by Tesla; 04/25/15 09:32 PM.

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Re: Icecream Batch freezer running on RPC [Re: Fredweet] #215388 04/26/15 12:16 AM
Joined: Mar 2004
Posts: 947
twh Offline
Originally Posted by Fredweet
Ok thanks for that info.
Would a 3hp 2.2kW VFD run a 3hp motor?
If the output of a VFD is 11A @ 220v the output is 2420W (Or rounded to 2.2kW) A leeson model is available to convert from 1ph to 3ph with an output of 2.2kW.

However is a 5hp 4.0kW a better choice?

A local motor house supplied a Leeson drive for one of their customers that was rated for single phase input at the rated h.p. output. I specifically asked why it wasn't rated double like the others and that was the answer that I got. You need to check the ratings for yourself.

I have a customer with a VFD that has been converting single phase to three phase for years. It was rated at double the motor rating because the input rectifiers have to handle the input current at 1.7 times the motor load. In fact, I've installed at least a half dozen sized that way - sized by Leeson.

If it's going to be a hard start, get a vector drive. Apparently, they have better torque.

I have two drives that I am responsible for that are hard starting.

One unloads grain from a silo and, when the material freezes, it has to be broken free. I installed a switch to reverse the motor so they could plug it back and forth. The drive will look after the motor and itself.

The other pressures hydraulic lines to test oil wells. When it has to start from a stall to bump the pressure up, it can shut down on an overload fault and has to be manually reset after a few minutes.

There are settings on the drive to allow a higher overload for starting torque. I can't say how a drive compares to a motor across the line because I haven't had an opportunity to make the comparison. If you use a drive, your results aren't guaranteed.

I was put off drives when Leeson refused to warranty a drive because the input rectifiers where blown. I don't give a rat's ass about their excuses, so I supplied the replacement drive and installed it at my own cost. My warranty is good even if Leeson sucks. Still, Leeson drives, at least here, are about 25% cheaper. My deal with the motor house, now, is that they sell the drive direct and supply the warranty.

I did have another manufacturer warranty a couple 50 h.p. drives that were manufactured with the wrong rectifiers, but even then they didn't cover labour or temporary replacement parts.

Make sure you have enough profit that it is worth the risk. If it doesn't work, you are going to own a drive.

Re: Icecream Batch freezer running on RPC [Re: Fredweet] #215389 04/26/15 07:16 PM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
"Vector drive" is sales lingo for a motor that has been specifically designed to tolerate the spiky nature of VFD waveforms.

Cute, no?

Vector drive units don't have better torque -- per se -- they have their wiring -- especially the tap wires -- insulated to a wholly different, higher, standard than the normal NEMA stuff.

Once the windings have wrapped into the magnetic circuit/ taken a couple of laps around the silicon steel -- the magnetic field effects will have shaved off the highest spikes in the AC waveform.

From that point onward, even traditional winding insulation techniques will do fine.

The superior torque sales pitch is due to the fact that the VFD -- itself -- will adjust its frequency towards an idealized pull-in frequency during spool up.

This is to be contrasted with the common induction motor -- that starts out at zero rpms -- and 'sees' a rotating magnetic field racing away faster than the rotor could possibly catch up to. So, during spool up, it -- the rotor -- gets heated up -- and slips very, very, badly... particularly during high torque start-ups. (Traction motors, etc.)

The VFD slows the frequency down -- during the spool-up -- so that the speed of the revolving magnetic field is not so absurdly faster than the rotor.

A vector drive motor can tolerate this condition, too.

Slowing the spinning field down -- dropping the apparent frequency -- increases the delivered torque. (Inverse square law applies)

A VFD is brainy enough to spool up its waveform in tempo with the motor. Such pull-in/ spool up dynamics are adjustable in the field -- see the documentation provided with them.

Vector drive motors and mated VFD have almost entirely replaced DC traction motors. (!!!)

BART, most subways, the major railroads, etc. have all swapped out their DC systems for Vector 3-phase drive systems.

In the case of BART, it takes in 1000 VDC off of the third rail and uses it to power its 3 phase vector drive traction motors. The original power system (DC) was never changed.

It was in this fashion that both old style DC and new style AC could run side by side.

Otis Elevator has done the same thing for their product line. DC is out, vector drive is in.

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