Phase Converter or VFD

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Shaneyj

Shaneyj

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Katy, Texas
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I will be helping a friend install a Northfield 12HD Jointer in his shop. He is like a giddy school girl about this thing. Apparently, its the Cadillac of jointers...
Anyways... I've been researching his options for power supply.
Here is the set-up
His shop (converted garage) is 120/240 single phase service, 200A. Also the point of service for the rest of the house.
Jointer Specs:
Louis Allis motor
3 phase 220/440
8.4/4.2 amps
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My friend's information gathered from Northfield rep: use a rotary phase converter, not a vfd. The rep seemed to think that a vfd could cause damage because the motor on the jointer needs to run at max rpm. He added something about having to rewind the motor in the event of failure because the shaft of the motor is integral to the shaft of the cutter head.

This information is second hand. I'll be calling the Northfield guy tomorrow to get information.

My research: found a few forum topics of guys asking same type questions that I have and it seems the consensus is to use a RPC instead of a VFD, without a reason why.

Do any of you guys have input about this? Is there a reason a RPC would be preferred over a VFD, from an operational standpoint?
 
VFD pros: You can adjust the frequency of the feed thereby the speed. VFD cons: programming, and what the guy said in the post above.


Rotary Phase converter Pros- Work very well in the application.

Rotary Phase Converter cons: You have to start up the Converter and then go start up the machine. And reverse that order when shutting down the machine. Otherwise you will single phase the windings of the motor and you will also end up in the rewind shop if you do that.
 
RPC's are capable of powering other machines, a VFD is good for one motor.
RPC's should be sized a 1.5X the largest motor or 2X for hard starting loads.
RPC's are pretty robust if you get a quality unit, but require extra space and wiring, and can be noisier.

VFD's are not suitable for dirty environments without a proper enclosure to exclude the grime. You need to thoroughly read the manual and perform the programming
The VFD output van be conditioned with a reactor, to lesson the risk of high DC pulse voltage, from damaging non rated motor windings.
VFD's can be obtained that are designed to be supplied from single phase, or most 3 phase unit's can be oversized to compensate for operation on single phase. It's not a good practice to repeatedly power up and down a VFD input, damage can occur to the recharge circuit.

There can be cost difference depending on shop equipment that need 3 phase. VFD is likely more expensive if you don't need speed control.

MTW
 
I concur with Action Dave. A VFD might well kill the motor. If you decide to use a VFD you should also install a load reactor at the least.
 
What kind of use is it going to see? Is this for nearly daily production or a hobby shop?

Nearly daily production - one may consider getting three phase service as he very well may be interested in other three phase machines.

Hobby shop - maybe even consider a static phase converter. The thing may seldom if ever need all three horsepower in a hobby shop.
 
Jreaf on here was key on info, when I was building one 10yrs via another forum

I probably would steer clear of the static idea, from what I remember you only get 2/3 of the hp rating. We had one hooked up to a mill, and my dad always thought the bearings in the spindle we shot because it chattered. It ran like a top on the rpc.


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Great input. Thank you all.
This is a hobby shop. Pretty serious, but still hobby. Operational maybe 2-5 times per week.

Sent from my HTC6545LVW using Tapatalk
 
Fears of motor damage are IMHO exaggerated in this case. At 220V applied to a motor designed to be capable of 440V, plus the likelihood that the distance from drive to motor is less than 25ft, the winding insulation is going to be more than adequate to handle being run on a VFD. The big danger on applying VFDs to old motors is that in the past, motor winding insulation only needed to be at least 1-1/2x the RMS AC line voltage, so 1000V rated insulation for the magnet wire on a 575V motor was fine for Canada, meaning it was MORE than fine for a 440 (480) volt designed motor, so that's what motor mfrs used.

But once VFDs came to be and then they kept getting smaller/cheaper/faster by using higher speed transistor technology, a phenomenon called "Reflected Waves" and/or "Standing Waves" started showing up with a vengeance. Without going into the physics too deeply, the high speed pulses of DC that make up the PWM output (which is the "magic" of a VFD) starts to make the wires look like capacitors (conductors separated by insulators), so the "capacitors" create an additional charge on the wires that flows along them, until they meet an impedance change, i.e. the motor terminals. Some of that extra charge gets reflected back the other way until it hits another impedance change, the VFD terminals, where some of it reflects again. This reflects back and forth, building as it does, until spikes of voltage between phases build to as much as >2x the voltage. Since the bus voltage on a 480V drive is around 650VDC, you start seeing spikes as high as 1300V, often even higher, depending on how long the cable distance is and what the "carrier frequency" is in the VFD transistor firing scheme. NEW motors designed as "inverter duty" will use newer insulation that now starts at 1488V (a NEMA standard), which resists the damage from these issues. SO... if you have and old motor with 1000V insulation, and 1300V spikes, it's only a matter of time before the insulation fails. Alternately, in the past, a FEW motor mfrs used winding insulation on straight 220 (230) volt motors that was rated for only 600V, and since the DC bus on a 230V VFD will be around 330VDC, again, the standing wave spikes being 2x that value might exceed that rating.

But in THIS case, you have a motor designed for 220/440V, which means the insulation they would have used would be AT LEAST 1000V, so that is MORE than enough to survive even the worst case (750VDC) spikes that might come off of a VFD running at 220V. Where people run into problems with this concept by the way is that they discover that (on the used market) for small sizes, 480V rated VFDs can be less expensive than 240V rated VFDs, because there is not as much call for them. So people who have 240V single phase find out they can get a cheap used 240-480V transformer, step up to 480V input to the VFD, and run the motor at 460V. BUT, they then end up with this voltage spike problem and destroy the old motor.

There is another potential issue regarding current flowing from the stator to the rotor and damaging the bearings by causing EDM (Electric Discharge Machining) like effects on the bearings and raceways, ultimately damaging them. This too is caused by capacitive effects as well as "common mode noise" in the system, created by the high speed DC pulses coming from the VFD. This issue is also more rare on 230V systems, and can be easily fixed by simply paying CLOSE attention to ALL of the drive installation requirements regarding grounding. If you REALLY want to be safe, you can inexpensively add what's called a "shaft grounding ring" to the end of the motor that gives the voltage an easier path to ground.

If you are still worried, get what’s called a DV/DT filter for the output of the VFD, but it might cost as much as the VFD. A DV/DT filter would pretty much guarantee you not needing to worry about either problem. To be honest though, I've installed maybe a couple of hundred 240V VFDs on older machine tool retrofits with no filter, no shaft grounding rings, and no problems.

But one other truth already mentioned: One VFD/One motor. If you ever plan on running MORE than one 3 phase motor off of a single phase service, then the RPC starts to make more sense.
 
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jminer99er said:
Jreaf on here was key on info, when I was building one 10yrs via another forum

I probably would steer clear of the static idea, from what I remember you only get 2/3 of the hp rating. We had one hooked up to a mill, and my dad always thought the bearings in the spindle we shot because it chattered. It ran like a top on the rpc.


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Static converters are best for constant level loading situtations. If you have a varying load you need to change capacitor values for optimization at low load vs high load for a given machine.

That said, I haven't ever seen a jointer that likely utilizes full 3 HP capacity often at all. Unless you know they are going to use it at maximum width capacity, speed and depth of cut comes into play as well, chances are this thing never gets loaded more then what a 1 hp motor could do, or only for very short duration when it does. I'd still say a static converter is likely fine on this machine.
 
Jraef

i think I learn more about VFD’s reading your posts than I do in an all day class from one of the VFD manufacturers.
 
I just got home and showered up after putting in an American Rotary 20 hp rated phase adder machine and wiring it up to a couple of existing compactor machines. Works great. They used to have 2 10 hp models made by another brand hooked up in parallel to each other feeding separate 3 pole breakers in a panel acting like a pv combiner which had a third 60 amp breaker mounted in it that was the main breaker for a remote panel out by the machines , which is fed by a piece of 4 conductor #2 ser alum cable. I had to mostly rip out everything and redo it from scratch, except I used the old #2 ser cable over again cause the new rotary adder can handle those two compactor machines with ease. No more sync combiner setup. I did have to go to the controllers out at the machines to move all control loads off of the derived phase as the machines wouldn't run correctly until I did that. And over the years the ten or twenty people who messed around inside those control cabinets moved control wire feeds all over the place so tracking them down took some time. Some were on B phase which meant I couldn't just switch b and c since a bunch were already on c and that is the derived phase. Mikey can straiten out anything though, except there were 4,651 mosquito's attacking me the whole stinking time. It's right next to a canal.
 
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