DC Motor output speed?

[Corky]

Hi all,

i presume this is correct but i couldnt find any reading material online to back up my ideas,

A basic DC generators electrical output has a half sinewave as the armature cuts more or less of the field lines per degree of rotation max being at 180 and 360 degrees

so as the opposite to that ie a motor must have an alternating force output?? as theres a more repulsion at 180 degrees than there is at 90.

If this is correct i understand this wouldnt be noticeable, well i presume this just understanding the theory.

any help is much appreciated

Regards, Dale

*****( re-thaught this)- would the output be stepped and never drop as the repulsion at 90 wouldnt slow it but it wouldnt accelerate it much either so it would keep the speed constant?*****

 

[cowboybob]

If I understand your question, does the following help? This is for a brushed, single split-ring commutator DC motor:

https://www.animations.physics.unsw.edu.au/jw/electricmotors.html

Note the motor motion/output result in the animated example displayed when you scroll down a tad.

A motor with a greater number of individual coils in the armature (and more stator magnets and splits in the commutator) would produce a higher frequency sine wave output.

 

[Corky]

Hello again Bob,

yes thats perfect just what i was looking for to back up my original thaughts

cheers mate

 

[audioguru]

The armature of the motor is heavy so its inertia smooths the fluctuations in torque.

 

[Corky]

guru, are you saying the website isnt realistically correct?

Cheers

 

[JimB]

The armature of the motor is heavy so its inertia smooths the fluctuations in torque.

guru, are you saying the website isnt realistically correct?

The animation on the website is simplified to aid understanding.

The half-sine waveform of torque represents the INPUT to the rotor.
When the motor is unloaded, ie not driving an external mechanical load, all of the available torque will be used to accelerate the rotor, and the rotor will increase its angular velocity (spin faster) until the drag from windage and friction matches the torque and the rotational speed will stabilise.

When the motor is loaded, ie the rotor is driving some mechanism, the rotational speed drop and the available torque will be split between accelerating the rotor and accelerating the load.
When the system has stabilised, the torque available to drive the load will vary as the rotor turns, but will not fall to zero because in a practical motor the rotor has mass and that rotating mass will have inertia which will provide torque to the load during the time when there is no torque from the electrical system.

Note that this two pole motor has a BIG problem from a practical point of view, if it is stopped at the zero torque point in its rotation,
it can never start again without an external push.
This is why all practical motors have more than two poles.
Also note that the two pole motor will have very bad torsional vibration, not desirable if we want smooth running.

JimB

 

[Diver300]

When the motor is unloaded, ie not driving an external mechanical load, all of the available torque will be used to accelerate the rotor, and the rotor will increase its angular velocity (spin faster) until the drag from windage and friction matches the torque and the rotational speed will stabilise.

On a DC brushed motor, as the motor accelerates, it generates a back-emf that opposes the supply voltage. As the back-emf approaches the supply voltage, there is less resultant voltage to drive current through the winding resistance, so the current approaches zero.

That is the primary mechanism that limits the speed of a DC motor, and it is also why DC motors take a current surge when starting.

 

[JimB]

On a DC brushed motor, as the motor accelerates, it generates a back-emf that opposes the supply voltage. As the back-emf approaches the supply voltage, there is less resultant voltage to drive current through the winding resistance, so the current approaches zero.

Yes, the current APPROACHES zero, but does not get there because it needs some power/torque to overcome the windage and friction.

JimB

 

[kinarfi]

One of the things I saw on the oil rigs was that with a properly calibrated dial on a gauge is that voltage equals speed and current equals torque. They were operating at 750 vdc and capable of more than 1000 amps. To check the circuit breakers, they would pin the rotary table and go to full speed on the stalled 500 hp rail road traction motor and pop the circuit breakers at 1300 amps. The generators would not even load up because the the phase shift was nearly 90 degrees, the generators, 12 cylinder CATs, ran at 600 vac and could be ran in parallel to feed the SCR bridges that controlled the voltage to the motor.