electric motor "duty cycle" question

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electric motor "duty cycle" question

Postby Shoeshine » Thu Jan 26, 2012 12:43 am

Hi All,

Not sure if this is the best place to post, but a lot of the mechanically savvy folk seem to hang on this thread.

Can any gear-heads out there tell me what the rated duty cycle of an electric motor means in real terms? ... or more probably point me in the direction of a good read?

I am planning a largish kinetic sculpture, where one element needs to be raised and lowered in a semi-continuous cycle. I have seen a lot of ATV style winches on sale for decent prices. In theory they sound great. Good gearing ratio, i.e. torque/RPM, and they are already 12v which is easy for me to power as the idea currently is to power the whole contraption off a golf cart battery cell bank. I can also automate the movement easily with an arduino motor shield and relays.

The problem is that for the most part they are rated to 5-20% “duty cycle”. When I Google duty cycle I get a pretty common run down of on time/down time, but not a lot of explanation of why. I am assuming this is a function of heat? For most of these winches I am not even close to the rated weight. Does this increase the duty cycle? Would added cooling like a heat sink or fans help?

I need around a 50-60% duty cycle if I am understanding this correctly.

Or, am I totally off-base in understanding how this works? Are there better sources for motors that I should be considering?

Anyway, any input welcome,
Thanks in advance
Shoe
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Re: electric motor "duty cycle" question

Postby MikeGyver » Thu Jan 26, 2012 12:56 am

I would have to google search and im sure I would find the same definitions as you have so I'm no help on that part but. How about an Electric scooter motor? It runs off 12v and is meant to run for longish periods of time, no Duty cycle that I know of. The only problem I can see would be if you need a lot of torque.
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Re: electric motor "duty cycle" question

Postby motskyroonmatick » Thu Jan 26, 2012 6:44 pm

How about a 12 volt linear actuator if you don't need 360 rotation.

http://www.wholesale-electrical-electro ... 77389.html
(random site and product for descriptive purposes)

If I understand duty cycle correctly you get more duty cycle with less work performed by the machine. I know my welder works that way. When I have it really turned up I can't weld continuously. When using less power I get more run time between automatic duty cycle shutdown or I can weld continuously with no duty cycle interruption.

If you used one of these actuators at a much lower load than it's max rated then it might work well and possibly have less sound than a winch motor and gears.
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Re: electric motor "duty cycle" question

Postby Shoeshine » Thu Jan 26, 2012 9:44 pm

Good suggestions, thanks

The scooter motor might be a possibility I would then just need to find a low rpm/ high torque gearbox to tie it to. That was my initial excitement on the ATV winch because it already has a gearbox integrated that is right about where I need it.

nice thought on the linear acctuator but I am afraid I need about 4' of linear travel.

appreciate it
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Re: electric motor "duty cycle" question

Postby gyre » Sat Jan 28, 2012 9:49 am

There are forums I linked to on the electric bike threads.
Vis for Voltage might be especially useful.

Reducing heat can be effective.
There will be a lot of wind, but lots of dust too.

There are 4 foot actuators and ways to amplify movement.
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Re: electric motor "duty cycle" question

Postby BlackRockCityPimp » Sat Jan 28, 2012 11:44 am

Knowing how many pounds you intend to raise, distance raised, and if you are using one lift point (center) or several (outside corners) would help alot.

As far as duty cycle goes you pretty much gleaned the meaning from what you read and posted. And yes its usually to keep from over heating and killing motors n such.

Let us know how much ya gotta raise and how far it goes.
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Re: electric motor "duty cycle" question

Postby some seeing eye » Sat Jan 28, 2012 12:53 pm

It can be hard data to get, but you should consider lifetime as well. Those winches are cost reduced for a lifetime of just a few tens of hours. You would see that in the bearings, gears, brushes and mechanical fit/strength.
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Re: electric motor "duty cycle" question

Postby Shoeshine » Sun Jan 29, 2012 12:11 am

thanks for the thoughts so far...

to be specific the element in question has'nt actually been built yet... but I would estimate no more than ~200lbs and about 48" of linear travel on a fulcrum about 6' from the lift point.

as usual, this unfortunately needs to be a cost vs. wow factor decision. I am hoping to find a solution that is possible with relatively inexpensive componants or somehing that can be had from a salvage yard or the like.

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Re: electric motor "duty cycle" question

Postby uski » Sun Jan 29, 2012 4:29 am

Duty cycle generally exists because of two things :
- Temperature (motors emit quite a bit of heat)
- Wearing of the bearings/gears/whatever with regard to the rated life of the device (i.e. if the device is rated 5 years at duty cycle 20%, it'll only be 1 year at 100%, assuming you can cool the device properly)

Yes, running a motor at less than the given load will increase the acceptable duty cycle. However, please note that the temperatures on the playa are high, and I think it will be good to provide shade and proper cooling and/or heatsinking to the motor. Some motors have an integrated fan.

Always use a mechanical thermal cut-off switch, in addition to any other security device you may have in place. Always make sure that the temperature of the windings NEVER EVER exceed 150°C / 300°F or windings will be damaged. This translates to a much lower case temperature for the motor; I'd get a 100°C / 260°F thermal fuse (such as http://search.digikey.com/us/en/product ... ND/1014759 ). Be careful, this is a fuse, once it's melt it's over, so get a few spares and add a temperature cut-off software switch in your adruino (put a digital temperature sensor on the motor case and switch off the motor when temperature is close to the one of the fuse).
Some motors have an integrated thermal fuse, take a look.

Hope this helps :)
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