Saturday, August 7, 2010

Variable Speed Control for a DC motor. Please help.?

I have purchased this DC motor (link is below) and want to ';hook up'; a variable speed control to it. The problem is, I don't enough about electronics to figure out what speed control I need to buy. I found many on the web from 15-40 dollars.


The motor can take up to 120 vdc and 5.5 amps. I would like a speed controller that can give it this power or as close to it as possible, for under 40 bucks. I don't mind soldering and connecting wires by following a circuit diagram so kits are fine. Also, it would be nice if the speed controller used ac from a wall outlet so I dont have to buy a power supply.


I would appreciate if someone with some electrical engineering knowledge could post one or two links of speed controls that would suit my need. Thank you very much in advance!


http://www.surpluscenter.com/item.asp?UI…





How about this one???


http://www.surpluscenter.com/item.asp?UI…Variable Speed Control for a DC motor. Please help.?
I agree that the controller should be fine.





I would interpret the motor specifications as follows:





Cont. duty: Continuous duty. The motor can be operated continuously within the limits of the other specifications.





12/120 VDC: Minimum and maximum voltages or voltage range.





10,000 RPM at 120 VDC: The maximum RPM, the no-load RPM when 120VDC is applied.





5.5 Amps: The maximum continuous current.





50 in.-oz. torque: The maximum continuous torque.





Current is proportional to load torque. The current vs torque curve displayed under the “More specs” tab show 4.5 amps for 50 in-oz and 5.5 amps for 65 in-oz, so it is not clear whether the actual maximum operating point is 4.5 A/50in-oz or 5.5 A 65 in-oz.





Speed at any given load is proportional to the applied voltage. The speed vs torque curve displayed under “More specs” shows speed vs torque for 110 volts. For 0 to 50 in-oz of load torque, the speed with 110 volts applied would be about 9000 RPM with no load and 6800 RPM with 50 in-oz of load.





For operation at lower voltages, the speed at any given load torque would be reduced in proportion to the voltage reduction. For example, with a load of 50 in-oz, the speed would be about 6800 RPM with 110 volts applied. If the voltage is reduced to the minimum, 12 volts, the speed would be about 11% of the 6800 RPM or about 750 RPM at 50 in-oz load.





Horsepower is Torque (in-oz) X Speed (RPM) / 1,008,400. That means that the maximum continuous horsepower at 6800 RPM and 50 in-oz would be about 1/3 Hp. If you overload the motor to the 8 amp controller maximum, you could get 95 in-oz and the speed at 110 V would be about 4900 RPM. That would give you nearly 1/2 Hp. That would probably overheat the motor pretty quickly.Variable Speed Control for a DC motor. Please help.?
from the specs, it can take 15 amps. And the stall or start current is bound to be a lot higher, which they don't spec.





The one shown is not nearly powerful enough. You need one that can handle 30 or 50 amps. Which means you can't plug it into an ordinary AC outlet, you need a much more powerful connection.
Since the controller you link to has built in overload protection, and a 8A continuous output, I would think it would work fine with a 5.5A rated motor, yes, if you overload the motor into its intermittent operating range, the supply will either shut down or current limit, but in the continuous operation range it should be fine, note that this is not a AC induction motor, if you accelerate the load slowly you can start a PMDC motor with pretty modest current.





';A motor controlled by this board must start at or near zero speed and then the potentiometer can be turned up for higher motor speed, the board will not allow the motor to be switched on at high speed.';

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