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?

 

[Lucien Nunes]

Those questions don't make sense without context, and they also look like study questions which we won't answer for you although we'll help you answer them yourself.

If you mean position feedback to the drive electronics, then the difference lies in the method of commutation. How is a PM motor commutated? What governs the relationship between rotor position and which coil(s) are energised?

If you mean position feedback to the servo system itself, then the application dictates the answer. A lathe axis-drive and a fuel feed auger could both be part of a closed-loop control system, but only one requires positional feedback.

 

[oracle]

As noted above context is required, since you are an electronics student I suppose you are asking about DC motors, stepping motors and servo motors?

 

[Lucien Nunes]

the speed will be directly proportional to the armature current.

I think you meant to say voltage, not current!

But the OP asks about position feedback, not velocity feedback...

 

[oracle]

I think you meant to say voltage, not current!

But the OP asks about position feedback, not velocity feedback...

Thank you, edited and rephrased!

 

[mitardraskovic]

Thank you for your help. I want to explain my question again.
Is the position feedback required in the brushless and brushless DC motor control system? why?. That's the question

 

[Lucien Nunes]

How would you answer this question?

 

[Spoon]

Thank you for your help. I want to explain my question again.
Is the position feedback required in the brushless and brushless DC motor control system? why?. That's the question

Welcome to the forum mate.
If you give us your thought on the answer then we will give ours. That way you will learn more.

 

[PEG]

Hi,you have not explained the question,just re-asked it

More detail regarding the type of motor,and it's use and control,would be needed to provide an answer.

At the moment,we are at the "Do i need to wear a jumper?" stage...

 

[Silly Sausage]

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?

Are we suffering a poor computerised Turkish-English translation problem here?

Can you give us the FULL question?

 

[mitardraskovic]

The question of these electrical machines is 1 course. precisely this is the question of the teacher. You can think of as simple dc brushed and brushless motors. Does a simple dc motor need feedback? we usually use a compount motor.

 

[Deuce]

Thank you for your help. I want to explain my question again.
Is the position feedback required in the brushless and brushless DC motor control system? why?. That's the question

This isn't a complete question at all. There are literally billions of brush-less and brushed motors in everyday use without any position control. There must be more to this question than you're sharing.

But in any case, if you don't KNOW that you haven't asked enough to form a complete question, then what's the point in worrying about the right answer even if we could magically offer it to you? You still wouldn't know anything that you could apply in the real world.

I would suggest you hop over to the industrial electrical forum, that's where us automation geeks lurk... We're all too arsey to give you an easy answer, but some education might be forthcoming - if you learn how to ask a question

 

[PEG]

So as not to seem a miserable bunch...i am going out on a limb,and will suggest the question might be "Why would position control be required,on a ---- type motor..."

A reasonable answer to this,would be,that a device or machine,may have the requirement to start/finish a cycle type process,at a precise or pre-determined position.

There are a multitude ways of doing this,many combinations,and reasons why one method would be better than another,on the same motor.

This is why you would have to either frame the question within detailed parameters,or furnish us with the full picture.

Nevertherless...a happy Christmas

 

[Deuce]

So as not to seem a miserable bunch...i am going out on a limb,and will suggest the question might be "Why would position control be required,on a ---- type motor..."

A reasonable answer to this,would be,that a device or machine,may have the requirement to start/finish a cycle type process,at a precise or pre-determined position.

There are a multitude ways of doing this,many combinations,and reasons why one method would be better than another,on the same motor.

This is why you would have to either frame the question within detailed parameters,or furnish us with the full picture.

Nevertherless...a happy Christmas

I agree, but sadly the question appears to be not why, but what position control.

I am happy to be part of the aforementioned miserable bunch but the truth is, I see a student seeking knowledge and I do want to help.. It's just this particular student seems to have the question ahead of exam time, yet despite this apparent advantage has only bothered to post half of it here

I suppose the 'one size fits all answer' is 'electronics'. Frankly with enough tech you could pick up a 100 year old motor and have it stop in any position you wish, so long as there are step sensors in place and you have a rough characteristic of the motor, the electronics and programming can easily do the rest, even if reverse voltage braking is required, it's all possible.

For me the biggest problem is that the student is apparently less interested in posing the question accurately, than we are in answering it. I feel the student needs to pick up their game a little, unless they just want a pass, not an actual skill

And yes, Happy Christmas to you all

 

[Silly Sausage]

For me the biggest problem is that the student is apparently less interested in posing the question accurately, than we are in answering it. I feel the student needs to pick up their game a little, unless they just want a pass, not an actual skill

And yes, Happy Christmas to you all

I suspect he's spread bet every electrical forum on the web grasping for an answer. I've got a vague, hazy memory when I used to look at books, if you remember those things.

 

[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.

 

[Lucien Nunes]

I can just imagine it getting up and walking across the room. Same thing happens if the triac goes S/C. Washing machines don't have quite the same level of monitoring and interlocking as machine tools though.

Here's the drives cabinet of that lathe of about 1978 vintage. Top left is the spindle drive, IIRC 7hp 0-320V DC out. Lots of hardware trimpots and status indicators on the front. LEDs include overspeed, underspeed, overcurrent, AC supply out of range, tach error, motor temp, heatsink temp, field loss etc.

Field loss on a wound-field shunt motor is another problem that can cause massive overspeed. In the good old days of rheostatic starters for brush motors, the hold coil was often series-fed from the end of the shunt field, so that the starter would release if the field circuit opened up.

To the right of the spindle drive are the DC axis drives. They don't look very chunky but they are quite capable of eating their 32A 400V supply fuses without harm, if something goes disastrously wrong that stalls the axis motors, like an endstop overrun.

 

[gnuuser]

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

yes and laughed my arse off when the conveyor flung the cooling pies against the wall
the manager was sh!tting kittens until he found the emergency shutdown button)

back to the original post I agree that the question should have been thought out properly before asking.