Author Topic: Braking small DC permanent magnet motors.  (Read 516 times)

Offline Pete W.

  • Hero Member
  • *****
  • Posts: 789
  • Country: gb
Braking small DC permanent magnet motors.
« on: July 25, 2018, 06:01:18 AM »
Hi there, all,

I'm currently renovating a device that controls a couple of small, low voltage, permanent magnet motors via foot switches.  Each motor has its own section of the power supply, delivering symmetrical positive and negative voltage rails with respect to a common 0 volt rail.  Then each motor channel has a pair of foot switches, single pole, closed while pressed, one for 'up' and the other for 'down'.  The switches are interconnected so as to prevent connection conflicts.  (As soon as I can, I'll scan and post my intended circuit diagram.)

My dilemma is whether to wire the system so that when no motor movement is required, i.e. both foot switches are in the NC position, both sides of the motor are connected to the relevant 0 volts.  This would rapidly bring the motor to rest - however, I dimly remember a theory that the current that would flow through the short-circuited armature would, in time, demagnetize the motor's field magnet.

The motors are proprietary and obsolete and no spares are available.  They are incorporated in mechanisms for which mechanical modifications are effectively 'out of bounds'.  So I am anxious to avoid anything that would shorten their lives.

I could incorporate a resistor in the circuit but this would take space and would require adding mounting tags in an already crowded enclosure.

Has any Modder out there trodden this path before?  I welcome any informed comments.
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest design change-note!

Offline awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6593
  • Country: gb
  • East Sussex, UK
Re: Braking small DC permanent magnet motors.
« Reply #1 on: July 25, 2018, 06:24:01 AM »
Surely the current flowing during braking is not much more, or certainly of the same order of magnitude, as when accelerating from stopped, so braking short circuited should do no more demagnetising than in the accelerating case  :scratch:

I'd be tempted to find a similar 'expendable' DC permanent field motor and do a few experiments logging the accelerating and braking currents. If the braking proves to be vastly bigger, try again with a resistor and adjust it's resistance to a compromise between effective braking and current flowing.

Andrew Mawson
East Sussex

Online PekkaNF

  • Hero Member
  • *****
  • Posts: 2142
  • Country: fi
Re: Braking small DC permanent magnet motors.
« Reply #2 on: July 25, 2018, 07:22:07 AM »
That actually might hae some truth in it, but it requires two things:

1: braking current must be higher than operating current: a) operating current is limited and there is considerable inertia mass that causes excessive current on braking if the braking current is not limited (I.E. dead short). Usually this is more of the commutator overstressing problem with most modern motors

AND

2: motor has permanent magnets that have low coercivity i.e. they magnetize and demagnetize easy. That is property with many AlNiCo magnets. I think I saw one very old servomotor that used those magnets. Ferrite and most modern motors do not use alnico. Alnico has many wonderfull properties, but it is sensitive to "demagnitization" then again, it is possible to dial in very predictible flux (force) into them or even reverse the direction with external magenetic field (often coil). Motors with alnico magnets are not that common on mdern equipment or consumer market, they are more of a rarity. One pretty sure way to know it is this case if the manual tells to instal yokes to "short circuit" PMs if the rotor is removed from the motor.

https://www.dextermag.com/products/permanent-magnets/alnico-magnets/

Pekka

Offline PK

  • Sr. Member
  • ****
  • Posts: 370
  • Country: au
Re: Braking small DC permanent magnet motors.
« Reply #3 on: July 25, 2018, 04:42:49 PM »
You can limit the braking current with a resistor yes?