Small Electric Motor

Hello, klebe.

Your question is lacking a few pieces of information. Can you tell us:

Electricity for this eclectic motor, 12VDC, 24VDC, 120VAC (60Hz)?

The rise rate of ± 1 in/sec... what about descent rate? Same? At the end of a rise or descent cycle, is there to be a pause? How long?

How do you anticipate suspending this 7 lb weight? Fish line comes to mind.

Have you worked out how to mount this motor to the ceiling? Or, is it some type of framework? How small should it be? How large can it be?

Will this be installed outside? Subject to extreme temps, wind, snow?

Will it simply run 24 Hrs a day? How long should it last before it needs some attention?

Assume we can suggest appropriate equipment: will you be assembling this?

Hello

the electric motor mus to be 120 volts,the descent rate must to be same the rise.It is better no pause .About suspending the 7 lb weight fish line ok but I prefer something more visible ,like white thin cord.The motor can mount to the ceiling by screws and any simple framework,I don't care about the size but minimal it is better.Must to install inside and must run 6/8 ours a day.Yes I will be able to assemble all thing

Thank you

Carlo

We can help with the sums for the motor/gearbox and whatever, but you'll need to do a risk assessment for the installation. If a kid had a 7lb weight dropped on their head from the ceiling, their parents would get a bit upset.

The last system I designed (used for radiological instrument calibration) ended up with no-one being permitted to enter the room while the machine was operating - the cost of guarding all the moving parts with Safety light curtains, mats etc. was prohibitive.

Not trying to put you off, just being realistic. It's something you need to consider (for insurance etc.).

please see the picture, do you like it?

I'm sorry but the picture is very small

Carlo

Since the weight will be cycling up and down, the motor can be anywhere. A counter weight will make it work better, take less energy (smaller motor).

Depending on your mechanical abilities, this can be all bought stuff or found stuff.

I would use the drive out of a microwave oven turntable, (dead microwaves are all over) it moves about the speed you want. The chain used for hanging ceiling lamps is available by the foot, and pulleys can be made out of wood.

`I would use a small dc gearmotor, use a gear with 2 snubber tensioning gears and Roller chain #40 with counter weight. 2 whisker switches, a pair of motor starters as relays and a DC motor controller /DC power supply.

eclectic adj. 1. Selecting or employing individual elements from a variety of sources, systems, or styles: an eclectic taste in music; an eclectic approach to managing the economy. 2. Made up of or combining elements from a variety of sources: "a popular bar patronized by an eclectic collection of artists, writers, secretaries and aging soldiers on reserve duty" (Curtis Wilkie) You must mean electric, right? Why do you want to do this, Carlo?

Your project sounds simple, so if you understand the basics you should be able to figure out the detail yourself.

First issue is the power you need - and that's about 0.001 HP or 0.8 watt multiplied by a factor or at least 2 or 3 to allow for friction and losses - so power of itself will not be an issue. I would guess that almost any motor will be big enough. and it might be that if you use a bit of secondhand equipment that it will be 100 times more powerful than you need! It will be inefficient but it will be cheap and will work.

You can check motor size by looking at its claimed power, or multiplying its rated, volts x amps. This is because power= watts = amps by volts

If the motor is specified in terms of revs and torque, then power = torque times revs - see the comment on units below. Be careful though that the two figures are not max revs and max torque because you won't get them both at the same time and in some motor types that is a big difference.

When calculating power be sure that you use either meters, seconds, and Newton for force. You can get N of force by multiplying Kilograms by 9.8. If you work in the imperial system its feet, seconds and pounds for force. Mass is normally expressed as pounds in the imperial system, but more correctly should be expressed as "slugs" and multiplied by 32.2 to get pounds. You won't be doing any dynamic calculations that involve acceleration so just using Force units, lbs or N in each system will work fine.

When comparing torque and speed figures you just need to member that the product of the two is always a constant for a particular situation. If a motor is rated at 1200 rpm and 1 inch-oz of torque, then with a gearbox you will give you 600 rpm and 2 inch-oz of torque OR 2400 rpm and 0.5 in-oz of torque, minus a bit because of friction in each case. It doesn't matter what the units are, rev/sec, ft-lbf for torque, whatever, so long as you stay consistent. It's only in converting to or comparing with power figures that you need to be careful about units.

Small motors suggest that you will either use a 12 volt or similar motor with either batteries, or more likely a 12 volt supply DC (Direct current) or AC (Alternating Current) from a device connected to mains AC power - in your case 120v. A simple (suitable) transformer will give you 12v AC from 120v AC. You will need a rectifier (often combined with a transformer) to get a DC voltage from an AC voltage. Just make sure this stuff (called a power supply) is big enough to provide the current that is needed, with a suitable safety factor (say 1.5 or 2).

Then there is the matter of speed - both up and down, and that could be an issue if you want a slow descent speed. Some motors, the AC induction type for example, run at essentially constant speed regardless of load. The speed of others, like the DC series motor or the DC/AC Universal Motor, vary almost inversely to the torque. This may not be too big an issue if the load is always the same (as it is in this case). It will just be a matter of getting the settings right with adjustments to voltage and gearbox ratios or the like.

But on the matter of speed there is a big potential problem and that is of the right speed when going down. If you use a stepper motor you can control speed in either direction quite easily, but that involves a more complex control system, and unless you can buy one off the shelf (and you can), you are not likely to want to go that far. Most other motors, unless they are configured as a brake, will essentially let the load fall to the ground, and at a higher velocity than you would want.

One cheap solution to the over-run or reverse direction speed control issue is to use a worm and worm wheel gear box. These typically have speed reductions of around 50:1 and with a large winding drum would be suitable for your application. They work well as a safety device too. If the power stops, the motor stops and the winding drum does too, regardless of load. One downside is that they are not efficient and you will lose about half your power in friction - though this is not likely to worry you in this application.

The only other issue is how to arrange a reversing switch. This can be arranged mechanically by attaching the weight to a continuous loop that is as long between centres as the up and down distance you want. Alternatively you can arrange the mass, or a lump on the lifting line to nudge a switch that swaps over the current connections to the motor. There are a number of ways to do this - and you'll probably go for a relay arrangement that locks into one direction that when the relevant relay/switch point is bumped by the moving trigger point on the lifting line (or something equivalent).

A typical element of the relay device/s above will be such that when a switch is activated (albeit temporarily) by the trigger point it will cause a current to flow that will move a relay into the on position and that will provide a current that will both drive the motor and hold the relay on. A bump of the switch at the other end will do the same thing (in the reverse direction) but also switch off the previously locked in current loop. All this would be easier to describe with a picture, but a search of the web will probably give you a diagram to follow.

Good luck.

Mmm..bit embarrassing. Power calculated from revs and torque should include a 2 x pi factor or in other words 6.28. This is to do with converting the "revs and radius" part of the equation into velocity because power equals force x velocity.

So 1 HP = 746 Watt

= 746 Newton m / sec

= 550 ft lbf /sec

= 2 x 3.14 x revs/sec x ft lbf

Carlo,

I have read many workable ideas to the problem you have stated, but I want to suggest that you follow the KISS (keep it simple stupid) principle. If you can arrange a continuous operating belt or roller chain for example running between the top and bottom heights desired, it is trivial to attach the 7 pound "thing" to a chain link or a part of the belt. Drive the pulley or sprocket at one end and the "thing" follows up and down at the constant speed chosen. I heard your choice that a cloth tape as better than fishing cord and I think that a fiber reinforced flat rubber timing belt may be similar enough, since it is not oily like the roller chain. You can find gear motors, timing belts and pulley wheels from many sources-do a Google search. Remember if you don't plan for safety, the worst possible outcome will occur first! I would at the very least, have the "thing" shutoff when you are not present unless you can make the moving parts inherently safe.

Luther M

Carlo<

I have seen a simple setup much like this. Use a small stepper motor, attach a small roller chain running to a shaft or pipe. Then simply wrap a 1/4 " or 1 Cen. diameter rope around the pipe. The stepper motor will wind the rope on the pipe, then reverse at the pre-determined stopping point, and allow the motor to act as a brake gently lowering the 7 pound weight to the lower position. A relay wired properly will reverse at both ends of the travel.

This was done to elevate a heavy hot air balloon basket from a trailer, lift toward the ceiling in a garage, then lower it to the trailer when desired. The rope simply winds around the pipe. Speed can be controlled by the size of the pipe which, when larger, creates a faster assent/decent rate. The chain is not required to be heavy at all. Hope this idea helps.

Moses G

The simplest way to control the speed while going up and down is with a cam arrangement. Since it has to go up by only 50 inches, this is likely to be simple.

But this is provided yoy have the space for putting a handle (arm) of 50 inches diameter on motor.

So one rotation takes about 100 seconds (50 seconds up, 50 seconds down). This comes out to be 0.6RPM.

This can be easily achieved through DC Motor + gearbox (since your power requirement is so low, instead of going for AC motor, DC must do)

Klebe,

I am in favor of a small reversible DC gearmotor and pulley arrangement. They are easily obtained as would be a variable DC power supply to adjust the speed of operation. In addition, you need two reversing switches, one at the top and one at the bottom. The object being lifted could trigger the bottom switch. A light nylon cord could be used to lift the object with a triggering device such as a block or disc was attached 50 inches from an upper switch when the object was resting on the bottom switch. The upper switch could be mounted to a bracket near the motor.

The bottom switch may be more difficult to devise. If the object is metal perhaps it could close the gap between two metal strips mounted on a small wood strip or a small microswitch mounted in the base of a thin platform.

I hope its an idea you can work from.

tommm

dear friend, u can tell me what is the mining of " 7 lb"? i thing it is lower than 7 Kg. if it is that, u can use desk jet motor witch is use for move head, it has hi torque and move both side, ( clock ways and anti clock ways). through the programmable controller IC u can control the speed. subhajit my mail address is subha_rya@yahoo.co.in. u can contact me through this mail address thanks subhajit

A DC gearmotor from a hobby shop would work well. 12 volts DC and 7 or more inch-lb of torque, 30 rpm is readily available. Power supply could be a plug-in power pack. Two stops on the rope (maybe just knots) could work as your top and bottom limits and toggle a reversing switch.

Thank you to all for the help.Mr Thompson,seem to me interesting your idea about the two stops on the rope if I well understand the knots should hit the switch and reverse the motor rotation.Do you know were I can find this kind of switch.Thank you

Carlo

Radio Shack (The Source) or any electronics supplier will have double-pole, double-throw switches which will work. You would need to attach an extension to the handle in order to reach the rope. I recommend using spring wire for this so that the switch does not get broken by the rope pulling on it.

Also I forgot to mention that I changed the weight and it isn't anymore 7 pounds but only 3.5 pounds

Carlo

Buy a HOIST - available in hardware stores

Here's an idea which will give you control over a smooth operation, and height of travel, also smooth transition from up to down.

Use a wheel/pulley with an arm attached to a pin on the pulley. Attach the other end of arm to a fixed point. suspend weight from opposite end of the arm. The ratio of the length of the arm to the diameter of the circle of the pin attached to the pulley will set your lift height.

A small motor achieving approx 1/2 rpm of the large pulley (chain or v-belt drive). (this might be the most challenging, getting a 1/2 turn per minute). A washing machine motor would be cheap, big enough, quiet, but may take some effort to get slow rpm on the large pulley.

Sorry, forgot to add the diagram... :)

Thank you for the idea but I want to keep more minimal possible

Carlo