# Help needed with motors for musical machine project

Discussion in 'The Projects Forum' started by George Higgs, Sep 18, 2012.

1. ### George Higgs Thread Starter New Member

Sep 18, 2012
2
0
Hello,
I need help sorting out a motor/controller/power supply situation for a musical machine I'm building.
I'm looking to buy a motor for a machine I'm building.
I'm looking at this motor: http://radionics.rs-online.com/web/p...633D424F544826
It's rated at 41 watts, 14.5 volts and 6.9 amps. It goes from 0  6000 rpm. I can't figure out why that is, if power = volts x current, I figure the amps should be much lower, but obviously I missing something in my thinking.
In any case, the thing is, and the reason the amps are pertinent, is that the only speed regulator I can find has a continuous limit of 3 amps (http://radionics.rs-online.com/web/p...633D4E4F4E4526), and a peak of 5. The guys at radionics (the online shop) said this wouldn't do, but I'm thinking that because my shaft speed is going to be set at a constant 1920 rpm(I'll never go above this), I am using under a third of the 6.9 amps current the motor could potentially demand, therefore keeping well under the 3 amp limit of the speed controller.
The other issue is a 14.5 volt power supply (the motor is rated at 14.5 volts) which I'm having trouble finding. The larger question that I really had is about how the motor/speed regulator/power supply relationship basically works. Is the motor connected to the speed controller, which is connected to the power supply, so essentially the motor only gets what the speed controller gives it? In that case, does the power supply have to be 14.5 volts, or can't it be below that.
Thanks very much.
-George Higgs

2. ### wayneh Expert

Sep 9, 2010
14,846
5,331
A motor draws a LOT more current when it starts from a stop, is loaded heavily, or is loaded so much it stalls. The amp rating likely reflects the stall (or maximum) current. The wattage rating with its calculated current is more likely a continuous rating under a recommended load.

You can supply a motor less voltage (eg. 12V would be fine for that motor), but it loses torque as the voltage drops. Using PWM to control the average current is a better way to control speed without losing torque.

I agree that the motor controller is a bit undersized for the motor. It might work great for quite a long time, especially if the manufacturer has conservatively rated it, or not more than a few minutes if it's ratings are optimistic. It's too expensive to find out the hard way how long it will last. If you can find the manufacturer, you might ask them for an opinion.

Or just start shopping for "DC motor controller". I found this, this and this, for instance.

Last edited: Sep 18, 2012
3. ### George Higgs Thread Starter New Member

Sep 18, 2012
2
0
Hello wayneh,
I had incorrectly thought that this regulator used PWM.
Is PWM hard or expensive to undertake with analogue circuitry?
-George

4. ### wayneh Expert

Sep 9, 2010
14,846
5,331
No, you were right, it does use PWM. I hope I didn't mislead you to think it didn't. It's problem is the peak current rating being less than your motor, otherwise it would be fine.

PWM is not hard to do, especially if you just need one direction (polarity). All you need is an oscillator with an adjustable duty cycle, and you use the output of that to drive a MOSFET as a fast on/off switch to control your motor. Many folks use a 555 timer IC for their signal generator, but you can also use an op-amp or comparator in a simple circuit to make one.

I use the IRF540N MOSFET as a generic switch for many tasks and it would be fine for your motor. You use a MOSFET as a switch to ground, which is connected to its source pin. When the gate pin is raised about 10V above the source, the MOSFET is fully on and current can flow into the drain pin, typically connected to the low side of your load - your motor. The high side of the motor is connected directly to V+.

Hopefully someone can provide us a link to a schematic, if you want to pursue building your own.