Does the brushed DC motor and Permanent magnet Dc motor need position feedback in the control system

[mitardraskovic]

Hello guys. I am an electrical electronics engineering student. I have two questions. if you answer my questions ı'll be happy. Thank you from now.

Does the brushed DC motor need position feedback in the control system? why?

Does the permanent magnet dc motor need position feedback in the control system? why?

 

[davesparks]

Does a simple dc motor need feedback?

No, a simple DC motor doesn't 'need' feedback, just give it a suitable power supply and it will sit there spinning all day long.

 

[static zap]

Since compound is mentioned ,was thinking "get" characteristics for a series , (and cost where it may be worth while protecting from unwanted conditions )
..Design is usually Costed , rather than perfect word -Fool proof..
( Spread bet --Yep Xmas Punters !)

 

[ruston]

Deleted . Took to long to post.

 

[PEG]

You lot are getting real cynical...he may just be trying to better himself...and get out of that Delight factory,where the merest discrepancy in chocolate thickness,earns you fifty lashes...

 

[marconi]

Does the brushed DC motor need position feedback in the control system? why?

If you want a DC brushed commutated motor to control a position variable then yes it does need a position feedback sensor and signal to a closed lo0p control system. The reason is that the stator and armature magnetic fields are permanently at right angles to one another when axis through the commutator brush pairs is parallel to the stator field:

In this arrangement to Torque is the vector product of the Stator Field (A) and Armature Field (B) = T = A x B and of magnitude AB sinQ, where Q is the angle between the fields.

See for more on vector product:

Vector Product of Vectors - http://hyperphysics.phy-astr.gsu.edu/hbase/vvec.html

So once the Stator and Armature fields are crosses by some angle greater or less than zero there will be a torque on the rotor one way round or the counter-way. Using a commutator there is no null position when sinQ = zero - there would be if slip rings were used or the axis of the brushes was changed to be at right angles the the stator field. Think about this or make a simple motor:

Does the permanent magnet dc motor need position feedback in the control system? why?

No it does not normally need a controlled loop control system with position sensing. Why? Because the armature now is a permanent magnet mechanically fixed to the rotor so as the rotor turns so does the direction of the armature field. The stator field depends on the positions of the coils - let us assume for now they are at 12, 3, 6 and 9 o'clock positions. For the simplest method of stator drive, the coils would be energised individually to produce stator fields with magnetic field axis up, right, down or left - depends on the direction of the current through the stator coils. If they were all reversed then it would be down, left, up and right - you get the idea.

The torque on the rotor through the interaction of the stator and armature field is as before S x A and of magnitude SAsinQ - the difference is that the armature field direction is free to rotate and will do so until sin Q is zero. Thus, the position of the rotor is determined by the stator coil distribution and how they are energised by current. In this was the rotor can be made to step by an increment (in my case 90 degrees) from one position to another just by providing appropriate stator currents to the coils. Thus in some applications a stepper motor can be used for open loop position control without a position sensor.

Merry Christmas

Marconi

 

[Deuce]

You lot are getting real cynical...he may just be trying to better himself...and get out of that Delight factory,where the merest discrepancy in chocolate thickness,earns you fifty lashes...

Many people want to better themselves, the question is, how to do so effectively.

Its a competitive world, and if people's responses aren't what the op hoped for then their next step in 'bettering themselves' is to come back with an improved question. Or accept the fact they didn't try as hard to seek knowledge as someone else might.

Its an electricians forum, we all know laziness equals danger in the real world... Its actually disrespectful for the op to expect anything from such a lazy question.

Hence, I doubt we will hear anymore from them. Although I remain hopeful we will, and in a somewhat more structured manner. I personally feel it's very rude to put less thought into a question than you expect others to put into answering it, that's my bugbear..

 

[marconi]

The image of the split-ring single loop commutator dc motor I included in my last post does have a rotating armature field so there could be a position (depending on the momentum of the rotor ) - when the axis of the coil is aligned with the stator field and torque reduces to zero; but then the current reverses in the armature coils which causes further torque to maintain rotation in the same direction.

For a multiple split-ring commutator connected to multiple armature loop coils as commonly used in a dc motor the armature field is fixed in space as determined by the axis of the brush pairs.

 

[marconi]

For a bit more engineering/mathematical rigor the torque T in a PM motor is:
T = - S x A sinQ

The minus sign shows that T is a restoring one driving the rotor to a null position ie: as Q increases T acts in the opposite direction. It is this effect which enables the motor to be used in open loop control.

Something for you to think on: Does a brushed dc commutator motor need velocity feedback for a constant speed/angular velocity application?

Similarly, does a dc PM motor require velocity feedback. If it does not how might the rotation speed be controlled?

 

[oracle]

I'm not really sure about the ethics of postings, but I do try to answer questions raised and solutions offered concerning my posts.

This OP has not responded to anyone other than to repeat his questions so the critics have a point.

 

[CACJL]

I have installed motors with feedback, I only use brushless 3 phase induction motors controlled by an inverter. I would not touch a brushed permanent magnet dc motor with a barge poll.

 

[Lucien Nunes]

I would not touch a brushed permanent magnet dc motor with a barge poll.

Neither would I, it seems an odd choice of tool for servomotor maintenance. And, FWIW, I do have a barge pole and I do maintain brushed PM servomotors.

 

[ElectroChem]

Neither would I, it seems an odd choice of tool for servomotor maintenance. And, FWIW, I do have a barge pole and I do maintain brushed PM servomotors.

Curious to know what sort of brush/commutator lifespan you see on those? I maintain some brushed DC motors, but they are RPM tacho feedback not servos.

 

[oracle]

Curious to know what sort of brush/commutator lifespan you see on those? I maintain some brushed DC motors, but they are RPM tacho feedback not servos.

Ever had one of these "Run away" or overspeed due to a loss of tacho feedback?

 

[Lucien Nunes]

In average use, years for brushes, decades for comms. Brush motors often outlast the driven machine if they are looked after. E.g. we've just retired a 40-year old CNC machine on which AFAIK neither axis motor has ever been touched, just had brushes replaced as needed. I think it had new Z axis brushes in about 2012, motors were so old that Fanuc couldn't decode the part number at first.

Life depends on how the drive is used. This one rarely went above about 25% of rated axis torque so they didn't have a hard life. Brief peak currents can pit the oxide film and accelerate wear, so rapid reversals, chatter due to feed backlash etc can consume brushes much more than steady feeding.

Also had brush spindle motor (it has worn out two of those in its lifetime) and tachogenerator feedback. Many electronic servo drives have a tacho sanity check that will stop the motor if the tacho is out of range relative to the motor armature volts or computed speed from an encoder if fitted. The Mitsubishi drive in this machine would flag an error if the TG was open-circuit, before even allowing the motor to start.

 

[freddo]

Ever had one of these "Run away" or overspeed due to a loss of tacho feedback?

Yes saw that about 14 years ago on a washing machine, the tacho magnet fell off the motor shaft and the machine was damaged beyond repair.