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

[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.