Cheap, easy ideas for motor speed control. Maintain full torque.

[fastline]

I am building a device that I need some level of speed control. I have tried to use a variable voltage control on a shaded pole motor and the torque loss makes it nearly impossible to get much variance without stalling. I know that cap start motors will not work but I think an AC shaded pole with a real variable frequency system would work.

I realize maybe the 'right' way would be a real VFD with a 3ph motor but this is a rather mundane device that does not warrant much expense.

Then I started thinking about maybe a brushless DC motor setup but just not sure. I really don't have an accurate estimate for torque requirements but due to the gear reduction design, it won't take much.

Basic parameters might be approx 1/4hp, final drive or the 'pto' variable from 40-150rpm, continuous long lasting design. I am not sure a brush DC motor is really right for this.

I have a 1/6hp AC shaded pole motor that I might use but it really comes down to how to speed modulate the motor with variable frequency.

 

[BobW]

I don't think you'll have much success by controlling voltage only. You'll need to control frequency as well.

In a constant torque scenario for an induction motor, one generally keeps the voltage proportional to frequency, so that at half the rated frequency, you have half the rated voltage. That's a bit of a simplification, because it doesn't account for fixed losses, but it should be a reasonable starting point.

 

[MaxHeadRoom78]

DC brushed motor and a KB or Baldor drive, they can be had on ebay quite often.
Or a $5 $10 PWM controller off ebay.
Max.

 

[Tony Stewart]

Rotary disk speed control is possible or shaft encoder to set speed and variable phase Triac.

Torque available rises from loading at 90% speed at 100% torque to 200% under peaking at 70% RPM then drops rapidly to 100% torque at 40% max speed with another 3rd harmonic hump peak at 100% torque at 20% speed then drops to 40% at stall speed. Efficiency is only 20~40% for this lowest cost AC motor.

 

[dr pepper]

A dc brush gear motor is by far the easiest and cheapest, the only reason not to have a brush motor is longevity, a simple circuit can be built for torque/speed control
If you really want bldc then how about a r/c esc (electronic speed control) you can use these with just about any motor that runs at low voltage.

 

[Tony Stewart]

The essence of any good design starts with clearly defined specs on what you need.
Torque range, speed range, inertia or load, cost, R&D effort & time, power source options.
Then step load response starting and applied load. RPM vs step torque

From this you can select power, budget, engine and a control system, then controller, feedback and detailed design.

Anything else I missed can be added. Without this systematic approach, you will have too many surprises, which are inevitable. Murphy's Law.