Connecting two DC-Motor in series or in parallel

[Boncuk]

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.

No problem for me. I can live with your statement. Still not got through. Torque alone is no measurement for mechanical work.

My statement was: Doubling mechanical power means four times higher electric power demand, which is represented by the steep curve in your graph.

I guess DC-motors don't behave much different.

Practical work showed different results.

May be they apply for aerodynamic applications only (which I doubt), which also holds true for jet engines. Doubling aircraft top speed requires about four times the thrust - just because aerodynamic resistance increases by the power of two with increasing speed.

Just for comparison: The RF4-E (Phantom) used internal and external (wing and belley tanks) for subsonic flight (max. Mach 0.96) for an endurance of 3 1/2 hours flight time. It used internal tanks for a flight time of 20 minutes at Mach 2.4. (subsonic 1 1/2 hours). (One minute of afterburner operation (English: relight) required 1,700lbs of additional fuel per engine)

Physical laws can't be bent, either using reciprocal fuel engines and jet engines or electric motors - AC or DC.

Boncuk

 

[Hero999]

The steep curve is the efficiency.

Mechanical power is proportional to the speed and torque.

Electrical power is proportional to the voltage and current.

I think I see what you mean though: to increase the mechanical power linearly you need to increase the torque exponentially because the speed is reduced as the torque increases.

Still, I don't see how this makes two motors more efficient than one: larger motors will generally be more efficient than smaller motors.

It's true that aerodynamic friction is exponential but that's got nothing to do with the load characteristic of motors.

EDIT
Here's a datasheet for two different size motors, note how the larger one is the most efficient and they're both more efficient than the small one I posted a datasheet for above.
https://www.electro-tech-online.com/custompdfs/2010/05/0900766b80273327.pdf

 

[swadhingite]

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.

what about the back emf when we connect motors in series?
i have two motors(12v and 800mA) in my bot(football player) which i use to kick the ball. but problem is for this year voltage supply will be of 25v instead of 12v. so what will u prefer. should i convert this 25v to 12v using smps or can i just simply connect two motors in series? will there be any issues ?. just little help please.thank u.

 

[misterT]

what about the back emf when we connect motors in series?
i have two motors(12v and 800mA) in my bot(football player) which i use to kick the ball. but problem is for this year voltage supply will be of 25v instead of 12v. so what will u prefer. should i convert this 25v to 12v using smps or can i just simply connect two motors in series? will there be any issues ?. just little help please.thank u.

If motor is rated 12V, it means that 12V is the nominal voltage.. which means that it will reach the nominal speed without load at 12V. There is no problem driving the motor with 25 volts. Just do not exceed the max current of the motor.. current is what will burn the coils.

It is really difficult to give more accurate advice without knowing the application. But, current through the motor is your enemy, not the voltage. Just make sure that the motor does not stall. Maybe a series resistor is a good idea if you are unsure.

 

[misterT]

If this is just a short burst of current (kicking action) then 25 volts is probably better than 12V.. You get better action with higher voltage. Just make sure that the action is short enough.

 

[swadhingite]

If this is just a short burst of current (kicking action) then 25 volts is probably better than 12V.. You get better action with higher voltage. Just make sure that the action is short enough.

no it is not a short action . Actually those motors rewinds the spring which is used to kick. same is the problem with motors which i use to move the bot so please tell me something about series connection

 

[misterT]

please tell me something about series connection

Just experiment and have fun. You tell me how it worked out.