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Brushed DC motor health

  1. Sep 27, 2012 #1
    Hello, I am an automotive technician so I work with a lot of 12volt DC motors.

    First, it seems to me that bad connections or wires can cause excessive voltage drops to motors, but I don't understand what the difference is between a good cable and a bad cable. Or what qualifies a good connection vs a bad one. It seems that corrosion develops in electrical connections but I have a hard time seeing how it could develop to a damaging extent inside a tightly bolted connector and metal frame piece. As for wires I don't understand why one battery cable would drop more voltage than another unless there were broken strands that reduced the current carrying capacity by converting the energy to heat at those points. Like a light bulb filament.

    That's the other thing, if I am losing energy that should be delivered to a component it should be manifested in some form elsewhere.

    To my knowledge there is no internal insulation to go bad in stranded wire, and as far as I know copper does not magically lose its ability to carry current over time. I can see that heat could cause increased resistive losses though.

    My big question and current concern is the DC motors. I think that the voltage dropped on the way can cause any electrical component to not function properly but my suspicion is that a DC motor is affected more.

    I don't know and can't find much information about DC motors - mainly AC motors are talked about, but I think DC motors are designed to run at a rated voltage(?) or wattage(?). By reducing the voltage I think the motor will take longer to build enough magnetic force to commutate. Which would lead to increases in current and heat and decreased life span.

    But that brings me back to the question of what is the difference between good and bad components. In the case of motors there are some mechanical and environmental factors like bearings, lubrication, moisture, dust from brushes, and overloads, but what is the electrical wear?

    Do brush springs not push as hard? Slip rings wear? does the commutator short or develop high resistance? Do the turns of copper wire short to each other?

    What happens as it fails? Changes in speed? Current draw? Inductance of each rotor winding?

    Most of my motors are permanent magnet type.

    I don't consider myself a competent technician until I can identify the specific component and part that is faulty, identify the cause of the fault, know whether it was preventable and how, and verify with certainty that it was repaired.

    My tools are oscilloscopes, amp meters, and multimeters.

    Thanks for comments and advice.

    -Andrew
     
  2. jcsd
  3. Sep 27, 2012 #2
    Hello Andrew:

    Well we like to think of wires as being static - no moving parts, nothing to break, but as an auto guy you should know how hostile the underhood environment is. Good conducting copper is a metal which corrodes, the flexible cables have may small strands to make them flexible - but then they shake and vibrate in operation causing breaks, as the cable is terminated it comes in contact with other metals - most junctions of dissimilar metals will also corrode especially in the presence of moisture, salt and dirt combined with heat. So the wires / connectors can degrade over time. The connections themselves - bolted, crimped or clamped connections - water can wick into the joint bringing salt dirt - causing corrosion as well, the fasteners can become loosened -etc - it is just a rough environment.

    When the conductors / wires are degraded - their resistance increases. In the case of a starter motor - which runs for a short period at high current - the weakened spots in the electrical circuit cause the most disruption. Especially since the systems ONLY run on 12V - what seems like a small increase in resistance to the conductors - like 0.05 Ohms - and starter that may want 100A -- Well the voltage drop in the conductor is current x resistance = 0.05 x 100 = 5V --- so the starter motor will not get the full 12 V - in this case 7V may not be enough to turn the motor - or actually the starter solenoid may drop out. Also - note - if this is being caused by a single point in the system - it does generate a lot of heat ( in this case 500W ( 5V drop x 100A) - imagine all of that heat being concentrated in the area of the tip of your little finger- which accelerates the degradation over time.

    As for the motor itself - I am not familiar with the construction of the starter motors today ( I should be!) - but the weakest link is typically the bearings (mechanical) and the brushes which wear out and cause similar increase in the resistance to the wiring example above.
     
  4. Sep 27, 2012 #3
    Thanks for your insights Windadct,

    I'd like to ask two follow ups.

    When a circuit has a bad connection that generates a lot of heat like you mentioned I am curious about how its resistance changes and whether it worsens as a result. I was working on some golf carts with starter motors recently and I got the distinct impression that if I paused about 3-5 seconds between attempts the performance was much better. Actually thinking back it was more like a series of short bursts of cranking was effective whereas holding it in crank was dead.

    I've been taught not to operate a starter for very long or very frequently, so I guess what I'm asking is whether a hot starter will drop more voltage than a cold one?

    And on the subject of starters you mentioned the solenoid dropping out - that would be because the pulling/holding windings don't have enough voltage to keep the solenoid engaged?

    The other question stems from the comment you made about copper corroding very easily but being a good conductor. The power side of most all of my circuits is made entirely of copper but the ground side is mostly made from the vehicle body, frame, or engine/transmission.

    So I wonder how those materials fare when talking about voltage drops. There are a lot of different factors that contribute I know, like painted surfaces, rusty surfaces, wet surfaces all strongly affect the circuit, how does running it through six feet of steel or sheet metal do?

    And I've been thinking for a long time but I'm not sure what I should do to make a good connection and guarantee it will last. I have used sand paper and other abrasives to clean down to flat bare metal before and then torqued the fastener down, but I am also interested in what kind of product I could use to protect from corrosion and to facilitate electrical conduction.

    Some kind of electric grease perhaps?

    Thanks for your help,
    -Andrew

    Oh, and thank you for the math that put the 12V into context. I see that with a LOW voltage system it takes very LITTLE resistance to make a BIG difference.
     
  5. Sep 27, 2012 #4
    Hello Again Andrew:

    Seems like you are on the right track.

    Basically the resistance in the circuit generates the heat based on the amount of current. If this is an "abnormal" condition then it will probably worsen, but this is not a rule. However as you seem to be pointing out - if the system ( or weak link) is hot, at that point of time the resistance also increases. Basically - in typical conductors the resistance increases with temperature.

    So as I mentioned - the weakest part is often a small area - on start #1 it heats up dramatically - but this is a small area. So the longer you wait for start #2 the more the heat dissipates.

    But this is not an absolute rule -example if during starting there is actually a disconnect in the circuit - it is possible to weld the conductors - possibly making a better connection So these problems, esp talking about 100Amps +, can really be a nightmare to troubeshoot.

    As for your solenoid question - yes if the voltage drop the solenoid can drop out - this actually protects the motor.

    The amount of copper (conductor) on the + side is not much compared to the chassis / frame - or negative side - conductor. In the frame there are many paths back to the battery. The wide or broad area through the sheet metal - usually is not the weakest link in the system.

    Corrosion is really like a cancer ( thinking of rusting steel) once it starts it is almost impossible to stop. So for the conductors - if there is any sign of corrosion consider replacing the item, 2nd choice scrubbing the hell out of it, but still a compromise ( I am thinking of scotchbrite - and wire brushes). If you know the material ( conductor) is free from corrosion - the basic battery terminal spray helps (once reconnected) - but it has to be on clean - grease free - conductors.
    Otherwise - the battery terminal grease is OK - it really depend on the situation.

    As for your last line - thanks - sometimes here on PF a simple concept is not accepted as being valid.
     
  6. Sep 28, 2012 #5

    jim hardy

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    dont overlook the condition of the battery. Starting duty works it hard and if the plates are old and short on active area (sulfated) battery voltage drops under load - read your voltage during cranking.
    The ions in the liquid electrolyte get depleted during heavy load, but they can migrate back a little bit during intervals between cranking, making the battery more conducive. So the scenario you described strikes me as warranting a load test on battery.


    Brush springs lose tension
    and brushes will bind in the holders

    but a weak connection will get hot.
    A friend's Corvette ate alternators at rate two a year until we fixed the lug at alternator terminal. Half the strands were broken and the lug ran very hot, heat conducting down the bolt right to the alternator's internal regulator. Took eight alternators before somebody figured out significance of the discolorataion on insulation near that lug. Apparently it came out of factory that way....

    I use "FelPro" copper bearing anti-sieze on my battery terminals and large connections. Nickel variety seems to work for me too, and is same color as lead.
    But that is not its recommended use.

    There are other products made specially for electrical connections, "NoKorode" and "CRC" come to mind.

    old jim
     
  7. Oct 5, 2012 #6
    Hello again.

    I'm sort of tempted to start a new post about the original topic because this one has taken a different direction but instead I will try to bring the discussion back to the original question.

    Now I am not an expert, so I may be wrong, but here is what I am thinking (and I would appreciate clarifications).

    When voltage is applied to a DC motor, it induces a magnetic field in the coiled wire of the winding and that magnetic field is intensified by overlapping the coils on a special iron core.

    As that core becomes magnetized the electrical energy builds magnetic energy which opposes the stator field or stationary magnetic field from permanent magnets.

    A torque is now created from the orientation of the fields and their intersection.

    At this point the motor should turn, provided there is not an excessive load preventing it from doing so.

    As it turns the circuit for that winding loses touch with the brushes because the commutator is sliding out from them.

    Now I'm not exactly sure but I've noticed brushes overlap segments of the commutator so I wonder if the collapsing magnetic field induces voltage into the next winding?

    As far as I know when electricity creates a large magnetic field, especially with a core, it creates Inductance in the circuit. When the current flow stops the inductance produces a back emf or counter emf or a voltage that opposes the one that created it.

    I think it might be proportional to the current but I don't know.

    So I would think the motor would be inclined to arc from the brush to the commutator segment, except for the fact that the inductance is prevented from getting to that level, and for the fact that the brush may be directing some of that back EMF into the next winding.

    In any case even if the back EMF went into the next winding it should be counter productive because it will produce a field of the wrong polarity.

    This reminds me a lot of experiences with electric motors when they've stalled and start making a vibrating buzzing noise. I think that's probably related. I've also seen electric drills arc when over loaded.

    So anyway that counter EMF drops the voltage to the next winding and it takes more time for the magnetic field to build.

    But quickly the motor should reach some kind of equilibrium with the applied voltage, the winding resistance and inductance, and the load being the main determining factors.

    The thing that's absolutely critical to me to understand is what makes a happy motor and what makes a sad motor. What makes it wear out and die prematurely? What kind of conditions will maximize its life span?

    Right off the bat I'm going to say I think overloading is a big one, overheating is a big one, stalling is a big one, and I THINK running it under powered is a big one.

    But I don't KNOW!

    The feeling I'm getting is that if you dont give the motor adequate voltage for its working conditions and load it will rotate too slowly. The current flow will be excessive and the excessive heat will damage everything.

    Yeahhh, I feel like the motor can only handle a set wattage - that it can only dissipate so much heat and withstand so much inductance. And that running it hot may short the windings which would reduce the resistance and the inductance causing both a drop in power and an increase in heat.

    I wonder if scoping the current draw would indicate the health of the windings.

    Well, for the life of me that's my best guess as to what is going on.

    Comments, Corrections, Criticisms, Clarifications are welcome

    Thank you,
    -Andrew
     
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