Project - AC 3 phase motors

[Dannbr]

Hello all,

I am working on a project and wanted some input on what others thought about motor choices and such. I am building a cart to move boxes weighing over 3000 pounds. It is a battery powered device that will be remote controlled. The motors needed must provide 66 ft-lbs of torque each. In order to up the torque worm drives will be used. In choosing between AC and DC motors, I went with 3- phase AC motors with VSDs over DC motors mainly because the price of DC motors is high. Also DC motors would require 90VDC or 180VDC for 2 HP motor. My main concern is motor type because of the high inertia load. I am currently pricing blower style AC motors. Would these be sufficient or would I need to use something more heavy duty? Also, finding an inverter to go from 12V or 24V to 230VAC are pricey. what do you think about DC vs AC in this case? Any suggestions would be helpful

Thanks

 

[waross]

We had a similar question a while back. Search the site. Most of your questions have been answered as well as some issues that you may not have considered.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

AC 3phase induction motors that are available off the shelf will not accept less than 200VAC in most cases. That means your DC bus Voltahe must be in the neighborhood of 280VDC at a minimum, so allowing for reasonable battery discharge Voltahe drop, you likely need to start at around 350VDC, let's call it 360V to make the math easier.

Using 12VDC batteries then, you need 30 of them in series to get there, then you must look at the power (kW) required and the time you want it to run to calculate the total number of batteries in parallel needed.

Or, you could use passive Voltahe doubler circuits to boost the DC Voltahe level, expensive but no power electronics. The reason you find it difficult to find a cheap alternative is because the only viable one is to use a DC chopper based power supply to boost the voltage to where the VFD can use it. So for the most part you will have a VFD and transformer ahead of your VFD. The point is, you are attempting to use standard parts that were not really designed for this task. For sure it has been done, in fact I'm embarking on a project to retrofit an old truck to be powered by a VFD later this summer, I plan on using the series battery method, I have plenty of room for them under the truck bed. But that truck wou
D make for an impractical "cart".

For something like a cart, I would forego that concept and find a simple DC motor and drive that will give you what you want. There are plenty of simple versions available off the shelf, old golf carts are a great resource to experiment with. I have seen some on Craigslist around here that are beat up and the batteries stripped, but the motors work, there was one a few months ago that sold for $50.

If you are hell bent on avoiding DC motor maintenance, you can look at brushless (permanent magnet) DC motors, but the price premium may have you thinking about just how much maintenance you might be willing to perform.

"Will work for (the memory of) salami"

 

[mikekilroy]

66#-ft is ripe for planetary gearbox, or as bare minimum, helical gearbox - I would NEVER consider a worm gearbox for a battery power system! Why give up potential 97% efficiency for your worm that may eat 1/2 the power thru it?

torque without speed at that torque has little meaning so if you want more meaningful inputs, give us rpm you require also.

http://www.KilroyWasHere<dot>com

 

[Dannbr]

Hey mike,

Thanks for your input, I am shooting for around 200 RPM to the wheels. The planetary gearbox is much more efficient, I will be looking into it.

 

[mikekilroy]

2.5hp. So nice 10:1 (max single stage ratio available - aka, lowest cost) planetary means nice little 4" square by about 6" long 7#-ft 2000 rpm 5-20amp motor.

google AC brushless synchronous motors (aka 'servo motor') - there are many good reasons they are used on most hybrid, full EV, and cart thingies like you describe.

They can run on lower voltages easy too; maybe as simple as 10pc, 12v, 20amp-hour batteries - assuming you only need to run for about 30-60 minutes between charges.....

http://www.KilroyWasHere<dot>com

 

[ScottyUK]

jraef,

A passive voltage doubler with DC input ? Are you sure?

 

[jraef]

No sorry, you're right ScottyUK, I misrepresented that (even to myself) and then spit it out too quickly without thinking about it. A voltage doubler rectifies to DC, but then uses an AC input back into the DC through caps to double the voltage (it's hard to describe unless you look at the circuit diagram). So if all you have is DC to put into it, it won't work, you are right. <slaps forehead>

"Will work for (the memory of) salami"