Slow down AC hoist motor with PWM or locate a DC hoist?

Discussion in 'The Projects Forum' started by lukehoist, Jan 19, 2015.

  1. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    I need to control the speed of the lifting and lowering of a load with an electric hoist. The plan is to have a load be lifted to a specified height, then have it lowed back down in a cycle. I plan to use a load switching timer to do this.

    The time it takes to lift or lower need to be controlled. I know pulleys are an option but not for this project. The travel of the cable will be about 30 feet vertical. Many of the electric winches state speed of 30 fpm. I want to slow this down so it takes about 45 min to and hour to travel the 30ft.

    I have not bought the hoist yet since the ones I find are all AC. Does slowing down an AC motor with a PWM work?

    I wish I could find a DC hoist. There are many DC winches but not hoists with mechanical brakes for overhead lifting.
     
  2. mcgyvr

    AAC Fanatic!

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    What is the load?
    Got a price range?
    There are variable speed hoists out there.. too pricey?
     
  3. MaxHeadRoom

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    You may have a problem finding a variable speed hoist
    Other than DC, another alternative is get a 3phase motor version and fit a VFD.
    Max.
     
  4. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    The load is light. ~100 lbs. I hoped to use a cheap Harbor Freight or Northern Tool electric hoist. They are all AC. There are cheap (under $150) DC winches but not hoists. The cheapest DC hoist is the Warn DC350 for about $500.

    For this project I could spend up to $350 for the hoist.

    I came to the forums to ask questions about wiring the cycle control sequence of lifting and lowering. Now I am stuck trying to figure out how to slow down the motor before I can even get into a control system.

    Eventually I want to set a timer. When turned on, the load lifts to the top. The time this takes would be controllable (at least 45 min for 30 ft of travel if not more). Then it would wait at the top for a set period, say 20 min. Then the timer would switch the operation and the load would lower to the bottommost position. It would then hold there for say 30 min. Then the cycle would repeat unless switched off. I think this will require some sort of limit switches but have to at least find a hoist that will work first.

    Excuse my ignorance but I assume those inexpensive hoists linked to above are not 3phase and I am not sure where to get a 3phase hoist. I noticed PWM ac controllers on Amazon. Would it burn the motor up if used?
     
  5. shortbus

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  6. lukehoist

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    Jan 19, 2015
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    Thanks. I looked at those but in the instruction manual on page 6 it says "22. Secure load after moving. NO LOCKING MECHANISM."

    It does say "Automatic Load-Holding Brake." Not sure what that means if there is not a locking mechanism. If the power fails does it release the load? Would the winch be able to be automated to power in and power out without having to flip or reset a ratcheting mechanism?
     
  7. shortbus

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    Don't know how the brake works. But do know, NO winch should be used as a hoist.
     
  8. Bernard

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    Aug 7, 2008
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    As a start: What is the load weight. Use worm drive for load holding. Use DC motor for speed control. That will be a really big gear reduction- best to make your own lift.
    What part of the world are you in for possible parts suppliers.
    Pie in the sky thought: 115 V AC Garage door opener with worm drive, US $ 9.95 driven by DC inline 1.6 RPM 12 V motor, 78 in-lb. est. 24:1 reduction US $ 18.99.
     
  9. tcmtech

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    Nov 4, 2013
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    I think you will have a hard time getting any normal power winch to move that slow. Reducing the motor speed 60:1 over normal is not an easy thing to do and maintain reasonable control.

    Either it would have to be done with a lot of small starts and stops or a specialized high reduction drive system like others are talking about with a continuous duty rated motor will be needed.
     
  10. Reloadron

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    Jan 15, 2015
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    I worked with several units that were variable speed hoist. We used them for specific test and the speed was critical. The units I remember were Yale manufacture using 480 Volt 3 Phase service with VFD drives. The units were actually small by standard rated for 1 ton but we never went over about 500 Lbs. Those units were on a tray and suspended from a 5 or 10 ton capacity crane. Depending on how accurate the travel needs to be will play a roll in the cost. We were doing inches per min. I would suggest you really define your needs and contact a few crane and hoist vendors to get some ideas. If you need accurate I would think a VFD. Anyway, I would check with some companies that specialize in these things, depending on your specific needs they may not be inexpensive.

    Ask suppliers if their hoist can be driven with a VFD. A VFD can be had for single phase AC for under a few hundred bucks.

    Ron
     
    Last edited: Jan 19, 2015
  11. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    That much I have figured out so far. I will definitely use a hoist designed for overhead vertical loads, not a winch.
    I am in Ohio. Didn't even think of a garage door opener. I thought it would not be long enough for the 30' travel. If I understand you correctly I would modify the garage door opener by attaching a DC motor to it? Then add a 24:1 reduction? If so I will research how to put this together. I can weld and fabricate but do not have experience with motors.
    It does not have to have much control at all. I do not care if it uses lots of on/off start/stop functions as long as the motor doesn't overheat. You can tap a hoist remote to move the load just a bit. If there was a way to automate the on/off switching to stretch it out to ~45 min travel time that would be good.

    This does not have to be a precise continuous control. It could even go on for 20 seconds then off for 3min on for 20 seconds off for 3 min ....

    That sounds like it would be perfect but if the hoist is industrial the unit might be too heavy for my intended mounting frame. As you said, it will probably be expensive and I do not need that type of precise control. This is basically picking up a heavy curtain of fabric. Not exactly but good enough for explanation. The total weight of the fabric will be 60 - 100lbs, It will be attached to the same frame as the hoist so not all of the weight will be lifted.

    Once I get the circuit control design then it will go through its raising and lowering cycle 9 hours a day for about 4 months.
     
  12. Reloadron

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    Jan 15, 2015
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    Oh OK, not as critical as I was thinking. :)

    This being the case I would be looking at some of what you mentioned. Northern Industrial Tools Electric Hoist — 220/440-Lb. Capacity or something along those lines comes in around $120. Wolf Automation has plenty of single phase VFDs for under $150. You are only looking at 3/4 to 1.0 HP motors on these small hoist. What you don't get are features like brake and clutch which you really may not need, as long as the motor can be VFD driven. Someone real familiar with these hoist could likely help on that note.

    Ron
     
  13. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    Thanks for the supplier tip. Always difficult to find when moving into unknown territory.

    So the VFD would control just the speed or would it control the cycle timing as well? I am going to read about the VFDs.

    I also wondered if a Time Delay Relay like this would work to do the 20 second on 3 min off technique.

    Hopefully one of these methods will work so I con move on to figuring out how to set up controls to run through the cycle. Electronics are not my specialty at all but I can usually follow along with guidance.
     
  14. MaxHeadRoom

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    Jul 18, 2013
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    Not sure what a PWM AC controller is but Normally 1ph VFD's do not have a good reputation, as they are reported to drop out of run under load, the best is a 3ph motor, the VFD itself can be connected to 1ph supply.
    For your application I would tend to look at one of the 12vdc hoists, they are around 360:1 ratio with double planetary gearing, a electric brake would be easy to fit if it comes without a brake, they can be had around here for around $120.00 for a 500lb hoist.
    A PWM drive could be used for rpm control.
    Max.
     
  15. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    You found a 12v DC hoist for $120.00? I would buy that in a heartbeat. All I can find in DC here are winches for pulling not hoists for lifting. Do you have a link for the DC hoist? I would be interested.

    Also, thanks for the clarification about VFD's and PWMs. VFD for AC and PWM for DC.
     
  16. tcmtech

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    Nov 4, 2013
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    If it was me and I was working with such low weights and speeds I would be tempted to take normal hand crank boat winch and replace the hand crank with a chain and sprocket drive attached to a small DC gear motor.

    Given the reduction ratios and power involved I would suspect that a common automotive windshield wiper motor driving a chain and sprocket assy on a boat winch would do just fine.
     
  17. Reloadron

    Active Member

    Jan 15, 2015
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    A VFD will only control the motor speed, however, all my experience with the things have been on 3 phase 480 volt motors. MaxHeadRoom brings some new information to the table. I wasn't aware of single phase VFDs having any issues but, again, I haven't used single phase units. So you may want to heed the words of Max. I simply don't know and apparently Max has experience with single phase units or a strong working knowledge of them.

    As to the timing. Timers like the one you linked to and a host of others come in some basic flavors. Repeat, One Shot, Delay On Break, Interval and Delay on Make to name a few. Most of them in for example Interval mode will be On for a preset period the Off for the same period. Not what you want. You want different On and Off periods from what I read. There are timers that will do this. Timer Relay, 10230 hr, 8 Pin, 10A, SPDT, 120V Macromatic among others make them. Looking at things you need to control another option is if you know anyone good at programming a micro-controller. Once the hoist scheme is figured out you can note everything you want to do. Then get more suggestions. For example if you can find a DC Motor winch that will work, drive it with an H-Bridge and let a micro controller run things. Curtain up trips a limit sensor and timing begins, then curtain down trips another sensor and timing begins.

    Ron
     
  18. MaxHeadRoom

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    Jul 18, 2013
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  19. lukehoist

    Thread Starter New Member

    Jan 19, 2015
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    Thanks, I am keeping that in mind for Plan C. Since there are so many complete setups for relatively little money I want to try to modify something off the shelf first. I know sometimes that makes more headaches but I think it will work here. If not, then I will try to fabricate a setup like you describe.

    Shortbus mentioned above that NO winch should be used as a hoist. I realize a winch should not be used as a hoist for big loads but as Max and tcmtech said, I am not lifting much weight. Page 3 of the manual says, "Never use as an overhead hoist, or to suspend a load." but on page 7 it describes the brake as "Self-locking gear train (Rated Load Holding)".

    I wonder if my use case is an exception? ALL of the "fabric weight" will be positively anchored to the lifting frame. Since it is curtain-like, the motor will lift the lowest point of the fabric up to the highest point of the fabric, which is attached to the frame. The rest of the fabric will hang down the the midpoint. It doesn't sound like much weight but it starts to add up so I don't want it to fall but it would not fall to the ground. The hook and cable would cause more damage by falling.

    The winch that Max linked to has a "Self-locking gear train" which to me sounds like a positive mechanical brake that would work even if the power goes out. It that incorrect? That Warn winch powers in and powers out. I would not ever disengage the clutch. Seems like it would work with a PWM to slow it down.

    Although the manual says, "This winch is rated for intermittent duty. It should not be operated with the motor slowed down to a low RPM. When the motor approaches stall speed, a very rapid heat build-up occurs which may cause motor damage."

    Now need to check how a PWM affects the duty cycle. I may be wrong but I think slowing down the rpm is different than using a PWM. To slow down the rpm you lower the voltage but with the PWM you apply full voltage in a pulse so it does not damage the motor, right?

    Am I missing something? Is using a "pulling winch", with a self-locking gear train, to lift fabric that is anchored to the ceiling still a bad idea?

    I know some people that work with Ardrinos and I can keep my head above water with basic computer code so that might me an option. I have not used an Ardrino before.

    From what I gather it could handle all of the logic that would otherwise be built with a series of Timer Relays. The relays are ~$40 a piece and I would need at least two. An Ardrino is ~$50.

    In this case would the Ardrino completely replace the remote control provided? Seems like it would get input from the two high and low limit sensors and it own internal timer. Then based on that information would send a signal to the PWM that would move the motor at the desired speed. If any of the component can't be switched directly by the Ardrino then a relay need to be in the middle. In the right ballpark?

    If the DC winch mentioned by Max or a similar one will be "reasonably" safe then I can get to collecting an H-bridge, Ardrino, wires and coffee. On further research the Ardrino looks like it can be programed to act like a PWM.
     
  20. Bernard

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    Have you given up the one hour requirement? Warn winch might be in 26 RPM range or maybe 13 ft /min.
     
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