# Phase Convertor or Drive ?



## erics37 (May 7, 2009)

Not necessarily, but I've installed a lot of drives and zero phase converters, for what that's worth.


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## HARRY304E (Sep 15, 2010)

dronai said:


> Have Drives replaced Rotary Phase Convertors ?


No ,Rotary Phase Convertors create 3 phase from a single phase source.

I do not know of any VFD's that can do that.


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## dronai (Apr 11, 2011)

HARRY304E said:


> No ,Rotary Phase Convertors create 3 phase from a single phase source.
> 
> I do not know of any VFD's that can do that.



Read up Harry


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## Wirenuting (Sep 12, 2010)

dronai said:


> Operating a motor contactor or disconnect between the VFD and the AC motor, while the VFD is running, can cause severe damage to the VFD. Such devices should only be operated when the VFD is in a STOP mode. In cases where such a device is required, an "early-break" auxiliary set of contacts on the device should be interlocked with the VFD's External Fault input or Stop input so that if the device is opened while the VFD is running, it will shut off the output of the VFD. If wired to the VFD's Stop input, the stop method must be set to Coast.
> 
> After reading this, I'm thinking of the conveyor E stop, or the motor disco at each motor. I have had to open both while the belt was running. The disco was to LOTO for a pm. Why didn't this cause damage to the drive ? The "early break" on the aux was bypassed on some the motor starters in the MCC.


Getting early break contacts installed is like pulling teeth around here. 
I'm the only one who does it. 

My coworkers just keeps replacing drives because, "They went bad".


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## dronai (Apr 11, 2011)

erics37 said:


> Not necessarily, but I've installed a lot of drives and zero phase converters, for what that's worth.


I've installed one rotary phase converter for a large pump. The last time I spoke with a pump tech, he told me they are just using drives for that now.


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## dronai (Apr 11, 2011)

Wirenuting said:


> Getting early break contacts installed is like pulling teeth around here.
> I'm the only one who does it.
> 
> My coworkers just keeps replacing drives because, "They went bad".


Some of the guys training us, were the ones bypassing the aux, to get the system running without a reset of some sort. They left the aux wires off and capped. But I didn't see a drive fail from this ?


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## HARRY304E (Sep 15, 2010)

dronai said:


> Read up Harry


Good stuff....:thumbup:


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## Wirenuting (Sep 12, 2010)

dronai said:


> Some of the guys training us, were the ones bypassing the aux, to get the system running without a reset of some sort. They left the aux wires off and capped. But I didn't see a drive fail from this ?


I'm sorry, I should have said,
Our maintenance guys just pop the local disconnect open to service the units. 
They never look for the drive or stops. 
Thats how we keep loosing drives. 

I always install the aux on the disconnects to save the drives and I will clean then also.. Funny how much dust builds up in them.


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## JRaef (Mar 23, 2009)

Wirenuting said:


> Getting early break contacts installed is like pulling teeth around here.
> I'm the only one who does it.
> 
> My coworkers just keeps replacing drives because, "They went bad".


If you are using a "Safety Switch", meaning the knife blade style, UL98 listed, the aux contacts that are attached to the handles are always Early Break, meaning the aux. contacts open before the main contacts open. It's part of the UL98 requirements.










But if you are using the cheap little UL508 listed rotary disconnects where the aux. contacts are attached to the main contact block, that is not the case.


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## youngapprentice (Mar 31, 2012)

from what ive read vfd's are the way to go if your just converting for a single motor, otherwise rpc's can supply more than one load depending on the idlers size.


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## JRaef (Mar 23, 2009)

dronai said:


> Have Drives replaced Rotary Phase Converters ?
> 
> 
> Also:
> ...


All VFDs are capable of making a 3 phase output from a single phase input, but some do not let you. You have to check. The reason is, different designs use different ways of powering the PC boards inside of the VFD. Some take the power from the line to a power supply board, some tap into the DC bus with a DC-DC converter, which is more expensive to make. Those that tap off of the line then can end up not getting power if you connect to the wrong terminals. Some mfrs do this on purpose, because when you convert 1 phase to 3 phase, the VFD size must be double the size of the 3 phase motor. So if you have a 10HP motor, you must use a 20HP drive.* So to avoid having a problem on phase loss of the diodes trying to pull too much current, they use the separate power supply in order to kill the VFD control power if a line phase is lost. That can also prevent you from using it as a phase converter, if the mfr forgot to put in programming to allow you to defeat that protection.

RPCs are still a better plan if you have multiple 3 phase motors that must run in a facility at the same time, for example a machine shop with a mill, a parts washer and dust collector. You would have to buy 3 separate VFDs, or 1 RPC and a small 3 phase panel. Sometimes that makes more sense.

Re: opening a disconnect on the output of a VFD. The damage potential is incremental. When contacts open under load, like a contactor or a disconnect switch, the air gap acts like a capacitor at first, so the voltage across the gap actually increases significantly right before the dielectric of the air finally stops it. During those moments, you are seriously stressing the semiconductor material that makes up the transistors on the drive. On a microscopic level, you are punching little holes in the layers, allowing current to flow when you don't want it to. At some number of occurrences you reach a point of no return and the transistor can no longer stop the current flow, so it shorts. It will not likely happen from doing it once, or twice, or 3 times. You cannot predict when it will happen, only that it will at some point. The more you do it, the closer you are to that failure point. So safety contactors do it, but the concept is, better to lose the transistor than the arm... All that you need to do to avoid this is to tell the VFD to shut off an INSTANT before the contacts open, hence the Early Break aux contacts.

That said, a lot of the newer drives are now a little more advanced and the VFD can detect that the switch is opening and disable itself really fast. I know that on the latest generation A-B drives, we no longer warn people not to open a disconnect down stream. 

* Small VFDs, 230V 3HP and under, often do not need de-rating because they just build them to handle the added current and DC ripple from single phase input. It doesn't really cost that much more at those sizes.


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