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teddillard
09 October 2010, 0834
A friend of mine who does a really popular gardening blog, The Gardeners Eden (http://www.thegardenerseden.com/), was asking me to put together a post on electric farm, yard and garden tools. Thus, The Electric Gardener.
http://evmc2.wordpress.com/2010/10/09/the-electric-gardener/

My question is, if you're converting, say, a splitter to corded AC, is there some easy way to spec a motor to replace a 10 hp gas motor? I get that you need the right RPM, but how about AC motor horsepower conversions?

teddillard
09 October 2010, 0847
Well, I found this:
“Electric cars are driven by large electric motors usually rated between 3.5 and 28 horsepower. For those accustomed to gas engines, this may not seem like much power, but the rating systems used for gas engines and electric motors are so different that the numbering system is almost meaningless. Gas engines are rated at their peak hp, electric motors are rated at their continuous hp. The peak hp of an electric motor is usually 8 to 10 times its continuous rating.”
Here:
http://www.evconvert.com/eve/electric-car-motors

So, maybe a 1hp AC 110V motor that spins at 3000 RPM would do OK? Faster would be better, I figure a gas splitter at full load has the throttle wide open, probably running at 5-6000 RPM.

edit- Funny- as I look through what's available, the farm and garden manufacturers are now starting to rate the electric motors in peak HP rather than constant... I guess to make them look better compared to gas.

frodus
11 October 2010, 0816
You're dead on.

SOME motor manufacturers will give you a peak rating, but most of the motors (especially 110VAC) are rated continuous because they are run continuous. If you need 10hp continuously to drive your splitter, then you probably should get a 10HP motor. If its just once in a while, then you might be fine with 5hp.

BaldBruce
11 October 2010, 1257
The hydraulic aplication you referred to is interesting in that it is one where the AC motor excels!. Most people recomend an electric motor that can have less than 1/2 the HP of the gas verson. This is due to the tremendous torgue that the electrics have. Most gas driven hydraulics have to be sized to handle the starting torque needs which are a major problem for the ICE. Check out sites such as this http://www.hydraulicspneumatics.com/200/TechZone/ManifoldsHICs/Article/True/6405/TechZone-ManifoldsHICs

teddillard
21 October 2010, 0422
So, say I got a 575V 5hp 1750RPM Balcor motor (for $20). If I run it at 110V, will it run at a correspondingly lower RPM? I'm pretty sure this is a 3-phase motor, not sure how, or if, I can even use that at 110V... hep me mr. wizzrd.

frodus
21 October 2010, 0817
It won't work well, if at all.

The AC Motor stator magnetic flux is equal to the stator voltage to frequency ratio. If you use the same frequency, but lower voltage, you have lower flux, and lower torque, thus lower HP. If you decrease 80%, you have 80% less flux.

And I found some more info here:
http://www.reliance.com/prodserv/motgen/b7093.htm


While NEMA standards for fixed speed AC motors allow for a 10% voltage variation from nominal, it is important to recognize that at 10% lower than nominal flux, performance including the nominal HP rating will vary. For example, it may require 10% more current than nominal to deliver rated HP. While this additional current is almost always available from the incoming line it may not be available from the variable frequency controller. Users that are familiar with static DC drives and their characteristics in low line conditions may be unpleasantly surprised to find that AC variable frequency controllers often do not provide the same rating capability at low line conditions. Operation of an AC motor at lower than nominal flux levels will result in increased slip and rotor heating which is self compounding and may lead to a thermal runaway condition. High efficiency AC motors designed for sinewave operation are often particularly susceptible to poor performance when the controller output voltage is low, since they usually employ low flux density designs at nominal terminal conditions.

Its not that it won't work at all, its that it will work very poorly.

I hope that helped.

teddillard
21 October 2010, 0840
I'm detecting a theme here. Read the label, and don't try to cheat. Similar to what I was told a long time ago in photography: believe your light meter. :D

Thanks Travis, I think I may actually be learning...

frodus
21 October 2010, 0846
I'm learning too actually. I knew that it probably wouldnt' work, but didn't know the exact details well enough to explain.... so I researched it this morning.

Its good that we discuss this type of stuff because in the end, we all learn.

Yeah, probably best to use the motor within 10% of its specs, although frequency can be varied so 60hz doesn't really matter a ton. just don't use it at too high of a frequency or that will give it issues as well.

teddillard
21 October 2010, 0926
shhh. You're spilling the Great Secret of Teaching. ;)

BaldBruce
21 October 2010, 1323
Not only won't this work, you'll be lucky if all it does is blow a fuse! A three phase motor is not happy running on only one of it's phases. (Don't ask me how I know that one for sure!) As Travis pointed out, AC motors do not behave like DC motors where you can just lower the input voltage to obtain lower speeds. They use a combination of frequency and voltage to vary power delivered and speed of rotation. Those fancy AC motor driver controllers are varying both simultaiously to deliver the required torque you are asking for.

FYI, they make two differnt kinds of single phase to three phase adapters. static and rotational. Both are large and very expensive. The industry uses them only when everything else has been ruled out. Best to stick to the simpler motors designed for the voltage and frequency you have available.

(Ted, I couldn't agree with you more - I learn something every time I teach.. Which is almost every hour with all the young engineers I work with!)

DaveAK
21 October 2010, 1603
Who did you black out this time Bruce?

BaldBruce
21 October 2010, 1750
That IS the problem when you screw up with really big power equipment! At least part of the city knows you screwed up......
Actually that miswiring job was many years ago and only cost the Government a new 8HP fan motor. That's also how I learned about putting a fuse in each of the three legs of a three phase circuit instead of the minimum of 2 necesary. Murphy will always dictate that the unfused leg is the one some idiot (me) will misswire and then fry the motor from the excess current draw.