Constant speed control motor???

[ozzy0627]

Ok guys need your opinion and reasons....

If you were building a constant speed control motor at relatively low voltage what type of motor would you want to use shunt, series, or compound motor. And the big thing I am looking for is why.

This means that on start of the motor you should have a constant speed and if resistance or voltage is changed to the armature load, the speed should remain at a constant speed when load is changed.

I have done some research on the motors, but am having trouble which to look in the deapest. Can someone steer me in the right directions.

 

[bigben2k]

All things being relative, what kind of load variations and voltages are we talking about here? What kind of rotational speed variations are acceptable? Is this an AC or DC application?

I'm not familiar with these descriptions: "shunt, series, or compound motor". Could you clarify, or provide a link?

 

[ozzy0627]

Rotational speed variations: Anything withing +- 10 percent of ideal speed.

Application: This will be a DC motor

Variations of the load will be: Voltage changed from 0 to 50 volts and resistance changes 1k to 100k


Links to motor I'll have to post later, they're on my other computer.

 

[bluce ree]

application question?

Could you please describe your application for constant speed controlled motors? The application will define what is the most suitable type of configuration and why.

I'm really having some difficutly thinking of why a constant speed motor is necessary. The only thing I can think of is a cutting blade motor or a motor for a wood router. CNC motors are steppers or very high precision servos. While they do have speed control circuitry, usually its either a trapezoidal motion (speed up, constant speed, slow down, target reached.) or a triangular motion (speed up till halfway point, slow down till target reached.) As one of my senior projects, I wrote a 2nd order (closed loop) position control system using a servo & LabView on a PC. I've also done some work with smaller stepper motor driving circuitry.

I have some experience with motion control systems, but I need to know what exactly are you doing with them to give you any advice on which is better.

I'm also not sure on what you're asking. please rephrase your question.

 

[bigben2k]

The load variation is important: what kind of torques are we talking about here?

The motion controllers mentionned by Bluce typically do this. They're meant for smooth robotic movements though: i.e. an arm swinging out to position X: accelerate, maintain, slow down, brake and it's in position. The brake is a voltage reversal.

 

[bluce ree]

We really need specs to answer

Robotic arms are an excellent example.

And you're right, I completely forgot about loading and motor size (rotor inertia is what I'm looking for.) Those are some pretty key elements missing.

The only constant speed with variable load application I can think of is for milling equipment (the actual mill) or for robotic movement via tires/tracks or a mix of the two ala half-track. Moving up hill would require more torque to move at the same speed as on flat land, opposite is true for going down hill.


Is this a commercial project or a hobby? If it's commercial, I understand all the secrecy, but if it's a hobby, dude let us know.


--BR

 

[ozzy0627]

Found what I was looking for.

Constant speed motor: Shunt motor speed varies 5 to 10% even at full or no load. It will not spin out of controll with no load like the series motor because of the shunt field and counter emf. (I was looking for medium torque usage)

Speed controller: Either going to change resistance to shunt field to counter the voltage at the armature (low current loss of power should not be a problem). Or change the voltage directly at the armature.

 

[bigben2k]

I'd have to lookup shunt motor.

Maybe a "constant current" power supply would do the trick here: it's the kind of power supply where the current (amps) is constant, and the voltage varies to maintain that current.

 

[oz2cpu]

I guess you need to be able to construct this your self ?
Just use any DC motor you like, add on the rear side a sensor, optical or magnetical or what ever with an output signal that changes when the RPM changes, if the sensor has pulses or AC output, simply filter it to a DC voltage, OK ??

Now use this "sense" voltage and feed it to an op-amp,
the other input of the opamp is your refecence voltage, where you set the desired RPM, now amplify the output of the opamp to a powerlevel that fit the motor size you use, now you have a closed regulated loop, with constant RPM at whatever load you have.. good luck

 

[R007 3]

Or change the voltage directly at the armature

Not recommended as this is less efficient and will cause the motor to have poor speed regulation(reason for this motor type). Have the voltage constant with this motor.

Place a rheostat in series with the shunt field (the parallel rung) to control the speed of the motor. Since the motor speed varies directly to flux induced into the coil, adding more resistance will weaken the flux and reduce speed.

Good choice of dc motor for what you posted as an application as speed is almost constant even under heavy load and the torque is almost linear with the amount of current applied.

 

[R007 3]

Forgot something...SAFETY!!!

Never open circuit the field rung of a shunt motor that is energized. THis will cause the motor to overspeed instantly due to loss of the field. Will damage motor, may blow it up in your face.

Look at the motor speed calculation:

n = V - IR
--------
K F

Where: n = speed
V = terminal voltage
IR = internal losses
k = constant
F = flux induced into the motor via parallel circuit to the
armature.

This shows that removing the control circuit with the amature energized will cause the motor to try to overspeed into an infinite speed, but the mechanical laws wont allow and BOOM.

You just let the smoke out of the motor yo.