how to choose an electric motor and speed controller?

Hi,
I`m going to design turning table (50-60cm diameter) the spinning disc is made from aluminum so its weight won`t exceed 10kg. it will spin by the mean of an electric motor (maybe gears will be used also). I want the spinning speed to be variable where I can control it from 0 up to 300 rpm (gradually or by steps where each step is 50 rpm) by them mean of speed control or whatever. The feeding power will be used is the normal home socket which is 220-240V 50Hz.

My questions are:
1- What is better to use and easier to implement? AC or DC motor?
2- What will be the power of my motor? and how many amps are enough to turn it at maximum speed? (let`s say the load on the table won`t exceed 20kg)
3- Whatever has been chosen AC or DC, what is the best and easier way to control the speed? How will i control the speed?


Thanks.

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-max-2 years ago

Oh, also, you can vary the speed of a DC brushed motor with PWM, as many motor drivers do. I think the easiest way to make this work would be to use the inner workings of a drill. Get a cheap cordless drill on craigslist and take it apart and use the already built and designed motor driver, motor, and gearbox do to the rotating. For power, you can use a power supply that can deliver the rated voltage at like 10A. It might not be a nice quiet thing though, but probably by far the cheapest solution!

iSulley (author)  -max-2 years ago

Hi,

Thanks for replying.

Do you think the inner workings of a drill will be able to spin the table of 60cm diameter with load of 20kg on it at 300rpm as maximum speed easily without heating the motor?

And what kind of feedback control loop can i use to check and control the current speed?

iceng iSulley2 years ago

Handrills that have the most torque, plug into the mains and are universal motors (means AC or DC).

They also have a extreme speed vs Tq curve which is hard to control and will overheat at lower speed.

serv_torque_speed_env2.png
-max- iceng2 years ago

For some reason I thought they were induction motors. Don't know what I was thinking, but yeah, good one's do have more powerful motors.

-max- iSulley2 years ago

Yeah, you'd be pushing it pretty hard, but I do not really know the math and science behind it. It can probably be made to work though with enough cooling. Does the turntable rotate easily with that much weight on it at that speed? I am going to guess that maybe a bigger motor will be better suited. Maybe you can get an old meat grinder or saw or something with a bigger universal motor in it.

Well lots of things can be used for speed control. You can go the completly mechanical approach and use a centrifugal switch, or have a rotary encoder and microprocessor figure out the speed, or a tachometer that connects to a motor controller as a feedback control to precisely regulate speed. I do not know where to get one, but unless you really need to precisely control the RPM, I would not bother. Put so many watts of power into the motor, and it will go only so fast.

iceng2 years ago

Try to find an old treadmill. It has all of the things you want in a rotary machine.

iSulley (author)  iceng2 years ago

hi,

that seems to be easier. it makes the implementation more easier.

Thanks a lot

Although a tretmill will give you what you need in terms of motor and speed control, you have to be aware that there is no breaking.

10kg empty and up 20kg loaded is quite some force once spinning at 300rpm.

I suggest to use support rollers for your disk and a flexible drive shaft to make alignment easier, I tend to use the flexible steel "drives axels" from lawn trimmers.

If you need breaking try to get a standard DC motor treatmill, you can add a directional switch on-off-break.

When on break you can use a powerful resistor across the motor terminals to stop the motor much faster.

If you can't find a good resistor substitute with a 100W or 250W halogen lamp (the kind for work lights).

iceng iSulley2 years ago

An interesting way to look at making something like a potters wheel is from the standpoint of power.

In America standard outlets are limited to 15 Amps @ 115 VAC which is 1725 Watts and at 746 watts per HP that is 2.3 Horse_Power.

Now small motors are only about 70% efficient and that leaves only 1.6 usable mechanical HP.

There are available commercial wheels for sale out there which means a 1.5HP should your upper bound limit for the drive.

BTW I think 8 amps is the biggest big hand drill i know of which is 920 Watts about 1.23 input electrical HP and at a best 60% efficient series AC motor is actually only delivering 3/4 HP and getting hot enough to burn the motor insulation in a matter of minutes.

-max-2 years ago

A DC brushed motor will be probably the easiest way to vary the speed and torque by varying the voltage across it. The RPM and torque and voltage/current are all interrelated as well as the internal design of the motor, so a feedback control loop like from some rotary encoder is needed for more precise control of speed.

You can also use a variac on AC brushed motor, or more specifically, a universal motor. They will run off of AC, and controlling the "speed" of them can be done by varying the voltage and current. Again, like before, a feedback control loop may be necessary for precise speeds.

AC induction motors run at fixed speeds, and the speed can be changed by varying the frequency (so something not easily done without a $$$ motor controller.).

Perhaps a large RC brushless motor and ESC will work well. The brushless motors found in the RC hobby are basically low voltage 3 phase AC motors with a inverter or driver called an ESC to power them. Not the cheapest solution probably, and I think it is possible to program the ESC to run the motor at a constant speed, since it has the ability to measure the speed of the motor based off of the EMF kickback.

But for the absolute best precision, you can use a stepper motor which is designed to rotate a small amount in controlled increments fast enough to appear as continuous. Again probably an expensive solution for something that does not require such precision. But that may be what you need, I do not know.