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Connecting two DC-Motor in series or in parallel

Discussion in 'General Electronics Chat' started by misterT, May 12, 2010.

  1. misterT

    misterT Well-Known Member Most Helpful Member

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    The original thread about making a perpetual motion machine by connecting two electric motors together was moved to trash because the whole idea and discussion was stupid. But seriously speaking, what do you get when you connect two electric motors (DC-motors) together? This is my opinion:

    If you connect two motors together mechanically (ie. you connect the shafts of the motors), the result is equal to one motor with different characteristics. If you connect them electrically in series, you will have a motor that can take more voltage (it can spin faster) than either of the original motors (if they can take it mechanically), but the torque is limited to the weakest one. If you connect them (electrically) in parallel, the result is a motor that can deliver more torque than either of the original motors, but the speed is limited to the weakest one.

    Motor is just another component, like capacitor, you can connect them in series or in parallel to get the characteristics you want. Connecting two motors together you wont have a generator-motor -pair. You will have one generator or one motor, depending on the application.
     
    Last edited: May 12, 2010
  2. Hero999

    Hero999 Banned

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    I don't see the point mainly because Two smaller motors will generally have more losses than one large motor with double the power rating, then there's the higher cost involved.
     
  3. misterT

    misterT Well-Known Member Most Helpful Member

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    I can see that most people do not get the point the original poster is making. One distracted reply will generally destroy the whole discussion.
     
    Last edited: May 12, 2010
  4. dave

    Dave New Member

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  5. mneary

    mneary New Member

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    If the two motors aren't matched, (i.e. have different RPM/Volt) then the result is two motors fighting. The outcome will be determined by the actual rpm/V of each, the DC resistance, whether they are opposing or aiding, losses in the coupling, gear ratios, current availability from the power source, and of course the load they are attempting to drive. If one motor is sufficiently more powerful it might even destroy the smaller one. The "weaker" motor might have some influence on the output, but it would never prevail.

    What is "weakness" anyway? Is it 1/strength? Is it measured as rpm/V, DCR, or what?
     
  6. Boncuk

    Boncuk New Member

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    Hi misterT,

    I have a close, but somewhat mentally limited friend who wanted to build a "perpetuum mobile". Since I had an electronics shop I also had all the necessary parts to make his dream come true. :) (For myself, just to get the kick out of the experiment.) :D

    I combined the main elements, battery, motor and generator according to his "supervising" advice.

    I also did not forget to add a volt-meter to the battery, indicating battery voltage accurately to 0.01V and an am-meter to indicate battery charging current.

    Starting the "system" battery voltage dropped from 12.5 to 12.2V upon starting the motor coupled mechanically directly to the generator, which wasn't a problem since the generator would charge the battery as soon as it had achieved nominal rpm.

    Yes, indeed, there was charging current indicating the battery was being charged, but simultaneously battery voltage dropped more and more. (How come? Who stole the valuable energy? :))

    After some hours the show was over, indicating a battery voltage of 9V (damn low and deadly for a lead-acid battery) and decreased motor rpm - hence decreased generator rpm.

    I honestly wish you all the best inventing your "perpetuum mobile", but I'm afraid this thread will also have it's final path into the trash can.

    My final conclusion: Even in space (no gravity) and a motor and generator without mechanical bearings (power loss) you'll not get the desired result. Electric current produces heat in every consuming element and that's the least you can afford building a "pertetuum mobile". Heat subtracts from the driving power gained via the generator, not to talk about electro-chemical losses within the battery.

    My final advice: Forget about a "perpetuum mobile" and concentrate on something useful and doable.

    Regards

    Boncuk
     
  7. misterT

    misterT Well-Known Member Most Helpful Member

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    Hi Boncuk,

    This thread was intended to be about "What do you get when you connect two motors in parallel or in series". Not about "perpetuum mobile" or any other nonsense. The question is: If I have two 12V motors with two different torque and speed constants, what are the characteristics of the series or parallel combination of these two motors? What is the torque constant of the combination? What is the speed constant of the combination?

    Good story anyway, thanks for telling it :)
     
    Last edited: May 13, 2010
  8. Boncuk

    Boncuk New Member

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    Hi Hero999,

    it depends very much on what you want to do using two motors.

    Generally spoken a motor running at 100% mechanical power will consume 100% electrical power. One motor running at 50% mechanical power will consume 25% of electric power. Thus two parallel motors running at 50% mechanical power will consume 50% of the power delivering 100% power of one.

    The electric power demand increases at the power of two doubling the mechanical power.

    Very much to my customer's appreciation and to Woods of Colchester Ltd. I offered parallel fans for each project I was concerned with by planning and execution.

    Not only electric power was preserved, but the twin setup took care of emergency backup as well. (Upon failure of one fan the remaining one took over the entire load - variable pitch propellers).

    I was a believer in Woods of Colchester products with a variable pitch fan constisting of a few mechnical parts, most important a diaphragm being blown by a low pressure compressor (max. 1.72 psi) varying pitch angle by blowing air into the hub or bleeding it via electro-pneumatic valves.

    NOVENCO (Norway) tried to copy Woods' technique (not patented) and were bankrupt after five years. The aluminum cast blades separated from the steel core causing heavy damage in production halls working their path through machines (and workers) like shrapnels.

    Next to follow was FLÄkT (Sweden), which fusioned with Woods 2003. I oracled a bad future for both because of totally different design and now both of them seem to be bankrupt as well. Woods is probably supported by GEC, but I doubt this will go on longer.

    Boncuk
     
    Last edited: May 13, 2010
  9. Boncuk

    Boncuk New Member

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    Hi misterT,

    parallel or serial, who cares? The outcome will always be BS. :D

    Regards

    Boncuk
     
    Last edited: May 13, 2010
  10. Gary B

    Gary B New Member

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    Not having bothered to build this to test it, the following is pure theory.
    I think that it will act as a single motor of composite design. In other words as if the total number of windings in the field coils and rotors had been wound on a single shaft. The rule is at some supplied constant voltage the fewer turns, the faster it will go. The larger the size of the wire making those turns, the more torque available.

    In reality something like this occurs with the traction motors of Diesel-electric train engines. Only one motor to the axel but, each power truck effectively connects the motors together through the tracks. As the engineer starts and gathers speed his throttle is switching the motors windings through different combinations of series and parallel at each step along the way.
     
  11. Boncuk

    Boncuk New Member

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    Hi Gary B,

    living in Bulgaria you certainly know about the legendary "Citroen 2CV" also called "deux CV". There was a version called "2CV Sahara", equipped with two identical engines.

    Engineers had a hard time to synchronize both engines for absolutely same speed and torque since they were coupled to the main drive shaft.

    Without proper synchronization one must push the other, loosing power. The same applies if they run at different rpm.

    Electric motors won't behave differently.

    Regards

    Boncuk
     
  12. Hero999

    Hero999 Banned

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    Sorry, that's nonsense, the higher the mechanical load, the more power the motor will consume.

    Efficiency normally increases as more loading is applied up to a point but beyond that, it drops as even more load is applied.

    I suggest you read some data sheets.
    http://www.rapidonline.com/netalogue/specs/37-0445.pdf
    http://www.rapidonline.com/netalogue/specs/37-0441.pdf

    DC motors shouldn't need synchronising.

    Providing they're the same type, the back EMF generated at a certain speed should be nearly the same. As long as the shafts are connected together, they should current share pretty well when connected in parallel and have nearly exactly the same voltage across them when connected in series.
     
    Last edited: May 13, 2010
  13. Gary B

    Gary B New Member

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    Sorry, I work for the US Department of State so haven’t been around here enough to know about local cars. I think you are talking about DC motors in the car. AC motors will phase lock with each other if fed from the same power source. In either case, I wasn’t talking about efficiency but, the behavior of the system.
     
  14. Hero999

    Hero999 Banned

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    The only type of motor which needs to be phase locked is the synchronous motor, normal induction motors will be fine, synchronised or not.
     
  15. Boncuk

    Boncuk New Member

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    Sorry, you obviously didn't read thoroughly. I didn't tell anything different.
     
  16. Hero999

    Hero999 Banned

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    I reread your post, sorry it doesn't make any sense to me.
     
  17. Gary B

    Gary B New Member

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    When one connects multiple AC motors to a common power supply you get a servo action between the motors. If just the field coils are powered, turning the shaft of one will cause the shafts of all the others to turn the same amount. Apply power to the armature winding and they all turn together. A friend used to run a “Himalaya” ride at carnivals and told me of this behavior.
     
  18. misterT

    misterT Well-Known Member Most Helpful Member

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    About the efficiency curve of electric motors:

    The efficiency describes the relationship of mechanical power delivered to electrical power consumed. At constant applied voltage and due to the proportionality of torque and current, the efficiency increases with increasing speed (decreasing torque). At low torques, friction losses become increasingly significant and efficiency rapidly approaches zero. A rule of thumb is that maximum efficiency occurs at roughly one seventh of the stall torque. This means that maximum efficiency and maximum output power do not occur at the same torque.

    So, if you have just one kind of motors in stock, putting two of them in parallel can be more efficient than using one motor at maximum power.
     
  19. Boncuk

    Boncuk New Member

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    OK Hero999,

    may be my English isn't clear enough. I'll retry: (not my native language)

    An AC-motor running at 100% mechancal load will consume 100% electric power.

    It will consume 25% electric power at a a mechanical load of 50%

    Paralleling two motors at 50% mechanical load each, the total electric power demand is 50% (2X25%) of that a single motor requires to deliver 100% mechanical work.

    Mechanical power follows a straight line (linear) while electric power follows a line to the power of 2.

    For fan operation this is quite decisive when planning air handling units. Doubling air volume with fans means four times higher dynamic pressure, hence twice the mechanical load, also four times higher energy demand than using a single motor (fan).

    Hope this is a bit clearer now. If not - just forget about it. I don't how to express it better. :)

    Sorry for the misunderstanding.

    Regards

    Boncuk
     
    Last edited: May 14, 2010
  20. BrownOut

    BrownOut Banned

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    Here is the problem. You're saying that η=P(out)/P(in) is twice at 50% load as at full load. That part just doesn't hold true.

    As for synchronizing two motors, I've thought about that when I had a stash of motors that were about half of the output I needed. The way to combine the outputs is to use a differential, similar to the rear axle of a car. Each motor would connect to the "axel shafts" and output taken from the "drive shaft" ( sorry for the terminology, but I don't know a better way to state it ) The output would then be the average of each motor's speed, and the power would add.
     
    Last edited: May 14, 2010
  21. Hero999

    Hero999 Banned

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    Boncuk,
    You're talking about AC motors, this thread is about DC motors.

    Anyway, it's not true, the current is directly proportional to the torque.

    Attached is a graph from one of the DC data sheets I linked to previously.
     

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