3-Phase vs Single Phase Motors

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caratworks

Joined
Jan 6, 2011
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What are the benefits of having a three phase motor power your equipment rather than a single phase motor?


My assumption is that you get
1. Ability to power larger motors
2. Relaiablity associated with a commercial 3 phase motor (theoretically, lasts a lot longer that less expensive single phase equipment available)
3. Smaller motor for same size
3. ??? -Others

Second question, does everyone with a three phase motor use the VFD as a phase converter to power their motor using only a single phase input, or do you have three phase electrical service to your shop.

I just have single phase service, and am trying to plan future equipment.
 
The primary reason most would choose to go with three phase motors is for the ability to have adjustable speed control. Pretty much all of the makers I know use VFD's to convert single phase input to three phase output. There are a few guys who have three phase coming into their shops, but it's not commonly seen in home shops.

--nathan
 
The biggest benni as far as I am concerned is the variable speed. Unless you have 3-phase to your shop the power saving is nullified. Smaller motor yes and cheaper too. Power larger motors well not really you still need the amperage.

I have used static inverters with great success. However like I mentioned the VFD is the main reason for me to switch to 3 phase for the greater majority of my shop tools. I used the VFD as and inverter for a few years and it worked great but you need to de-rate your motor by as much as 50%. I finally went ahead and installed a rotary inverter and it work great. No drop outs like I was getting with the static inverters. I get full power through my VFD now and it still works great. So you can go a far as you want. Now what was the cost? I installed my 10hp rotary for right about $1200. That was new in the box delivered with a controller, all the wiring and switches I needed to set up 5 machines. I already had a VFD and did not include that in the price.

Good luck, 3-phase is worth the effort and cost, IMHO
 
VAriable speed

VFD to convert single phase to 3 phase

NEMA 4 enclosures keep out dust & prevent burnout.

This one will accept 110v 15 amp input and output up to 1.5 HP 220v 3 phase motor.
Very handy and very uncommon trait.
or 2 hp output with 220v input

KBAC-27D

http://www.kbelectronics.com/Variable_Speed_AC_Drives_Inverters/AC_Drives_NEMA_4X.htm

http://www.kbelectronics.com/kbsearch/descriptions/kbda_27d.htm

http://www.kbelectronics.com/manuals/kbda_manual.pdf

http://www.kbelectronics.com/data_sheets/kbda.pdf


Best prices I have found were at Wayne Coe here - It seems the prices have changed, try State Electric and other local dealers.
http://www.waynecoeartistblacksmith.com/Motors___VFDs.html
 
It depends on what you want to do, what kind of equipment you need and cost. In a lot of the country getting a new 3 phase feed from the power company can cost a fortune...OTOH if you are in a commercial area and there's 3 phase available on the pole outside...

We have a fair amount of 3 phase equipment in the shop, and a lot of it is running off 1 phase power through VFDs so we can have variable speed. The exceptions are big machines that would take way too many amps on single phase. If you are going to need a 16" radial arm or table saw, or a 250 amp welder to get your work done three phase would be a great thing to have. A rotary phase converter can get those motors running on single phase power, but it will take a lot of amps...

One advantage to three phase power that has not been mentioned is that a lot of older equipment with 3 phase motors sometimes sells cheap because folks don't have 3 phase, don't want to buy a new motor or a VFD or rotary phase converter or etc... Most small (less than 3hp or so) 3 phase shop equipment will run happily on a VFD powered by a 20 amp 240 volt single phase circuit. If you are handy and electrically inclined, a rotary phase converter can be made from a big 3 phase motor, if you have more $ than time pretty big VFDs and/or rotary converters are not all that expensive. But make sure your single phase service has enough power to run that really big motor before you fall in love with the 20 hp air compressor
 
Thanks for the input.


Are you able to wire a VFD with an A-B-C switch (not sure what the electrical name is called) down stream so you can use the same VFD for multiple pieces of equipment?
 
A lot of equipment period that's 3ph sells cheap, old or new. It's because of the difficulty(cost) or acquiring 3ph service outside of a commercial setting. It's definitely not limited to old equipment.

The advantage of 3ph motors in the sub 2hp range are out-lined well by "12345678910", but it's probably appropriate to clarify that they're outlining specific advantages to a single controller. The vast majority of VFDs in that range are *NOT* inverters, and *NOT* NEMA 4. Although the most ubiquitous in our scene is the KBAC 27D that he's referring to, and carries the aforementioned advantages.

Above 2hp, and that quickly changes, and many of the advantages disintegrate, however, the cost concern for the motor is still very valid if you've got a means of powering that motor.

A KBAC 29 runs roughly $400, so even if you find a scrapyard 3hp motor for $30, so at $430 it's not that economical, however, the motor can still be run with a phase converter, which even more so is likely much cheaper than buying a new single phase motor in the equivalent power range.


Regardless of all that, it's not simply "3ph is better than single ph" *unless* you have actual 3ph service to your building, the major exception being in the case of grinders where you're planning to run them off a VFD for variable control.

The idea that one is inherently better than the other is shallow, if the appropriate qualifiers aren't included in the question. For instance, from a power consumption perspective, replacing a 1-2hp 3ph motor can be less power intensive than using an equivalent rotary or static phase converter, if varaible speed isn't an advantage that counter balances that consumption.
 
So just to clarify, that *MAJOR* advantage of general VFD use is (as the name implies) variable speed with phase conversion. All other *advantages* are generally a limit of circumstances.

If you're using your VFD for the voltage inverter function, it's not because it's an advantage, rather because of a limitation with your power service, and it's generally limited to 1.5hp and below (The KBAC-27D although rated for 2hp motors(220V) will only deliver 1.5hp on 110V service).
 
caratworks said:
Thanks for the input.


Are you able to wire a VFD with an A-B-C switch (not sure what the electrical name is called) down stream so you can use the same VFD for multiple pieces of equipment?
Click to expand...

Yes you can run multiple machines off of one VFD with switching. I run two 3hp and one 2hp machine off of mine, a TECO FM50203. It converts 220V single phase to 220V 3ph.
 
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