Hi, I am building a small ply platform for someone to stand on on castor wheels with top and bottom board, rubber on floor side and smooth board where the fixed castor wheels will travel. The speed should be between 1 and 2rpm plugged to a main socket and a max weight of 100Kg to move horizontally in circle. I'd need a switch for on/off making sure that the person on top of this platform doesn't fill any jerking especially at start, so smooth start and continuous/constant speed. Something not too noisy either. I looked at rotisserie electric motors but the maximum weight to be lifted vertically is around 45kg. Would those motors do a good job for twice the weight to be moved horizontally? If not what motors can you suggest? most most appreciated. Tony
I guess you would need a gear box. Most motors like more revs (more like 1000 rpm...). If you use mains it is often 1750 rpm.
If you use a worm wheel, your load will not immediately be dropped upon a power failure. Might be safer if you lift people or other breakable stuff...
It's not clear what the build will be. Try and draw a picture!
Is it a pure lifting matter or a horizontally moving matter?
It's about turning a small platform with 100kg on it, not lifting vertically.
Motors from a motability scooter or golf trolley , with gearbox
Information on estimating required motor torque and power for wheeled robots:
I’m still not understanding the goal of this project…
Lateral motion is mentioned, along with lifting.
On top of this requirement, if a person is standing on the platform, the design has to consider the ‘inverted pendulum’ effect of moving a mass that is located above the moving platform.
This isn’t a simple problem to solve in a single paragraph.
What’s the application? Drawings ?
Make a prototype without a motor and measure the torque required to turn the platform at 1 to 2 rpm. To give yourself some margin for changing conditions, multiply that torque by 1.5 or 2.
That's the torque your drive train must deliver to the platform.
The required motor torque will depend on the drive train you select, whether belt drive (low noise), chain drive, gear box, etc.
How do you intend to FORCE the device to actually move in a circle? castor wheels like to go in a straight line, hate to go in circles.
Do you understand torque? A wheel or gear on a motor is essentially a lever. If you have a weight on a chain and a gear, a gear that's half the diameter can lift twice the weight. And of course it moves more slowly.
You'll need to know the torque and speed of the motor, and if you are lucky the specs will include a torque-speed graph.
A similar thing happens when you "gear down" a motor (or do the same thing with pulleys... You get slower speed and proportionally more torque.
If you need to lift or move a weight fast you need a more powerful motor. If you don't need speed you can slow it down and use a smaller motor.
So it is actually not just a switch you want, but a complete speed controller.
As always on these fora, you really should explain your purpose as it does affect the best approach. 
OK, Yes it's to put a person on the platform and turn the platform so he/she can be drawn at different angles and needs to be plugged in the mains. I won't use castor wheels as I found a heavy ball bearing circle thing on ebay, so lower friction and 1rpm is mainly to avoid too much jerking at start and stop. Once I receive that ball bearing circle thingy, I'll put legs on it and a ply platform on top, but how do I measure the torque? I presume I would need to climb on top and apply a force but how do I measure it. I could maybe use a rechargeable drill on low to high settings and you guys could maybe deduct what torque the drill is giving? If a drill could give 1rpm I'd use one but I'd need to invest time and money in a gear box, I'd rather not.
The platform could end up bigger with the person seating or laying on a bigger platform. As i am thinking of having the motor right in the middle I'm not sure that the size of the platform would make any difference as the ball bearing circle can take up to 150 kilos with weight spread on 3 point on a 450mm circle radius with rigid ply or similar. The first platform will be 500mm diameter and only to stand on.
Nice little project 
Stay away from mains motors, not a good idea.
Plenty of low voltage dc motors available with gearboxes would be much safer and practical.
One way would be to attach an appropriately sized fish scale to your fully loaded platform at radius "R" and pull in a tangential direction. Slowly increase the pull and note the force "F" at which the platform starts moving.
The starting torque, F x R, is likely to be less than the torque to keep the platform moving.
So, it would probably be on the safe side to design your system to deliver F x R x 1.5 or 2.0 to the platform.
edit to add: as @Paul_B rightly points out, there are likely to be other sources of torque, the magnitudes of which may be less predictable than the starting torque. Including a mechanism to limit the torque on the drive train may be prudent. The mentioned friction rim drive would be one way to achieve that...
You obviously have two options for this.
Either you have a geared motor driving the centre of the turntable, which (gearing) will need to be particularly sturdy as it will be subject to substantial torque as the subject moves or is moved on and off, or you provide a "rim drive" by a rubber wheel attached to a geared motor.
I vote for the rim drive with a rubber tire on the wheel attached to the motor shaft. You can easily cover the motor and wheel so stuff stays out of the way. Also can adjust the speed by changing the diameter of the wheel. In addition, the turn table does not have to be perfectly round or smooth.
Might be nice if ball bearing thingy can be driven from inside. Than there is far less chance that things (long dresses and such) get eaten by the drive train.
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