I want to drive a clock mechanism with a motor. In order to get acceptable accuracy I need a motor that can deliver a certain rpm very precisely, is low on vibrations, noise and power consumption, and has a certain longevity. I don't need "production grade" accuracy, this is just for the learning experience, but I want to come as close as feasible. So far what I found out was that it should probably brushless and be able to operate at a low voltage. Still there is a staggering selection. Which other criteria should I pay attention to?
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\$\begingroup\$ I would say a stepper motor. but you can hardly call it a "mechanical clock". \$\endgroup\$– Eugene Sh.Commented May 14, 2018 at 18:20
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\$\begingroup\$ Ok true it's not powered by a spring so maybe it's not the right word. I considered a stepper motor, but want to have a fixed rpm so that I have to figure out the precise transmission to use for the mechanics. A stepper motor would feel like cheating. \$\endgroup\$– Andreas HartmannCommented May 14, 2018 at 18:22
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\$\begingroup\$ Cheating whom? :) Is it educational project, or you just want make something working? \$\endgroup\$– Eugene Sh.Commented May 14, 2018 at 18:23
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\$\begingroup\$ Cheating myself. The challenge is to get two or three separate hands moving from a single fixed rpm motor using only gears \$\endgroup\$– Andreas HartmannCommented May 14, 2018 at 18:26
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2\$\begingroup\$ synchronous machine and then gears... this is how wall clocks work. generator companies have a requirement to met quite a tight long term average for the supply frequency. Why this way over say some controller and more complex motor commutation? you will not beat the mains for long term timing (well atomic clocks do :) ) \$\endgroup\$– user16222Commented May 14, 2018 at 19:23
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4 Answers
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The challenge is to get two or three separate hands moving from a single fixed rpm motor using only gears.
DC supply.
A stepper motor typically gives you 200 steps per revolution. You could gear that to give the second hand a step per second or half-second. You can precisely control it by microcontroller (so you'll have an accurate clock) and you still have all the gearing to play with between the second, minute and hour hands.
The stepper won't be cheating. It will be a sensible solution to the problem.
answered May 14, 2018 at 18:32
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Use the plate mover motor from an old microwave. These are also available as spare parts for less than $10. These are synchronous motors which rotate at a fixed fraction of the mains frequency, usually about 10rpm. All you have to add is an additional gear which does one round per hour.
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\$\begingroup\$ Good idea, but I guess it's only for AC. I'm looking for DC. \$\endgroup\$ Commented May 15, 2018 at 6:02
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If you want 'a certain rpm very precisely', then your only options for a pure motor are a stepper motor, or a BLDC run as a stepper.
You could use an ordinary motor, servoed with some sort of encoder, but that's just a more complicated way of getting to your exact rpm.
From the comments, you consider that use of a stepper motor is cheating. It is, if you present the clock as pendulum/escapement regulated. It isn't, if you want to build a mechanical time display, that's as accurate as the timebase you choose to employ.
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Depending what you consider a "motor" you could consider one of the solenoid-assisted pendulums. It avoids the sense of cheating as the period is derived from the pendulum, and, as a converter of electrical into kinetic energy is still legitimately a motor.
For example, suspend a magnet on the pendulum, and pulse a solenoid coil when the magnet triggers a Hall effect sensor.
See also Hipp toggle, Synchronome, or a home built derivative, or the much less exotic ATO clock .