Wiring a DC motor

[rasen58]

Can I connect my DC motor straight to a 9V battery? or do I need a resistor in between?

I've tried it and the motor spins, but will having no resistor ruin the motor?

Also, when I connect the two wires to complete the circuit, I see small sparks, is that normal?

[Acme]

A spark is not unusual. The motor may burn out if its voltage rating is significantly lower than 9volts. If you know what device it came from and the voltage it used, you may get some idea what to expect and whether or not a resistor is called for . If you don't know the device & its voltage, look for markings on the motor which may give the voltage rating or allow you to do a web search and find the voltage rating. Have fun!

[Sensei]

I plugged DC motor (from old cassette Walkman or so) to 5.5 V 4 AA batteries that I had at hand.

Without anything Ampere meter is showing 2.4 A.

With DC motor Ampere meter is showing 0.210 A, voltage drop on DC motor is 1.5 V (4 V remaining).

With DC motor and 210 ohm resistor, 0 A, and motor doesn't want to run.

With DC motor and 4.3 ohm resistor, 0.19 A, voltage drop 1.5 V. Motor runs but slightly slower than without resistor.

[Ten oz]

Resistors in series simply add resistance to a circuit. The more resistance you have the more current required. So if your concern is about providing to much power to the motor a resistor in series won't improve your situation. Placing resistors in parrallel decrease total resistance http://physics.bu.edu/py106/notes/Circuits.html

 

It seems like to me that you merely want protection? Figure out the what current (amps) your motor is rated for and put a fuse inline with it. That will prevent the motor from overheating because as the temp increases resistance increase which draws more current (amps) and blows the fuse.

[John Cuthber]

"Resistors in series simply add resistance to a circuit. The more resistance you have the more current required."

Nope.

Arguable, the more resistors you have, the more voltage you would require.

"So if your concern is about providing to much power to the motor a resistor in series won't improve your situation.

Yes it would. The difficulty is calculating the value of the resistor you need- especially since this depends on the current drawn my the motor and that, in turn, depends on the load.

 

"That will prevent the motor from overheating because as the temp increases resistance increase which draws more current (amps"

Nope, when it gets hot the resistance rises (slightly) and it draws slightly less current.

But the resistance of a fuse is small and so it doesn't affect the current much until it gets hot enough to melt (the hint is in the name). Once it melts there's no longer a wire to carry the current.

 

Why post nonsense?

 

On a related note, Sensei,

when you say "Without anything Ampere meter is showing 2.4 A." do you mean that you connected the meter directly to the battery without anything to limit the current?

If so, you risked damaging the meter.

You should never try to measure the current from a voltage source.

[Ten oz]

"Resistors in series simply add resistance to a circuit. The more resistance you have the more current required."

Nope.

Arguable, the more resistors you have, the more voltage you would require.

"So if your concern is about providing to much power to the motor a resistor in series won't improve your situation.

Yes it would. The difficulty is calculating the value of the resistor you need- especially since this depends on the current drawn my the motor and that, in turn, depends on the load.

 

"That will prevent the motor from overheating because as the temp increases resistance increase which draws more current (amps"

Nope, when it gets hot the resistance rises (slightly) and it draws slightly less current.

But the resistance of a fuse is small and so it doesn't affect the current much until it gets hot enough to melt (the hint is in the name). Once it melts there's no longer a wire to carry the current.

 

Why post nonsense?

 

On a related note, Sensei,

when you say "Without anything Ampere meter is showing 2.4 A." do you mean that you connected the meter directly to the battery without anything to limit the current?

If so, you risked damaging the meter.

You should never try to measure the current from a voltage source.

Ohm's law states that voltage is equal to current times resistance. http://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law

 

I made the mistake in my origanal post of multiplying rather than dividing. My finally advice was correct however. Motors have amperage ratings. It he or she is looking to protect the circuit a fuse will do it. Fuses blow on amperage not voltage. They are for over current protection which is what it seems like is needed in this case.

Edited June 7, 2014 by Ten oz

[John Cuthber]

Using a fuse is a good idea.

But this

"because as the temp increases resistance increase which draws more current"

is still wrong.

As the motor gets hotter it will draw (slightly) less current.

 

But, as the load on the motor increases and the speed decreases, the current will rise.

You might need to protect the motor from the high currents that flow when the motor is stalled.

However that runs you into a different problem- the motor is always "stalled" when you first start it.

So, a fuse that would protect the motor from a long term over current, would blow every time you started the motor.

 

It's complicated when you do this with big motors.

With small motors and batteries it's not an issue- the internal resistance of the battery and the motor will limit the current as long as you don't use too high a voltage.

[Ten oz]

Using a fuse is a good idea.

But this

"because as the temp increases resistance increase which draws more current"

is still wrong.

As the motor gets hotter it will draw (slightly) less current.

 

But, as the load on the motor increases and the speed decreases, the current will rise.

You might need to protect the motor from the high currents that flow when the motor is stalled.

However that runs you into a different problem- the motor is always "stalled" when you first start it.

So, a fuse that would protect the motor from a long term over current, would blow every time you started the motor.

 

It's complicated when you do this with big motors.

With small motors and batteries it's not an issue- the internal resistance of the battery and the motor will limit the current as long as you don't use too high a voltage.

As I admitted I messed up the math in my initial post. However a fuse is the way to protect the motor and the way to make sure you are using the right size fuse is to check the current rating of the motor.

 

Wattage (work) is equal to the product of Voltage and amperage. In this case Voltage is constant so amperage will increase when the motor works harder. So a fuse will protect it.

http://www.atlantechsolar.com/calculate_volts_amps_watts_conversion.html

Edited June 8, 2014 by Ten oz

[John Cuthber]

You seem to have ignored the problem of starting current.

Also, there are not many cases where a circuit powered by a 9 volt battery needs a fuse.

[Ten oz]

You seem to have ignored the problem of starting current.

Also, there are not many cases where a circuit powered by a 9 volt battery needs a fuse.

Fuses come in various time delays which covers starting current. I agree that a 9v system (assuming it is just a single battery) doesn't need current protection. It just seems like the question as asked is about how to protect the circuit. Circuit protection for a motor is current protection.

[John Cuthber]

The people who make, sell and buy things like this

http://en.wikipedia.org/wiki/Motor_soft_starter

seem to think a fuse isn't up to the job.

I happen to agree with them.

[Ten oz]

The people who make, sell and buy things like this

http://en.wikipedia.org/wiki/Motor_soft_starter

seem to think a fuse isn't up to the job.

I happen to agree with them.

I assume this post is sarcasm? In that case it is brilliant and clearly I should stop posting while I am behind. No use in putting a fuse on something small of a single 9v battery. You are right.

 

At the off chance it is not sarcasm .........this form of protection is for AC motors operating at much high power factors than a 9v DC motor. As perviously stated, perhaps that is your point.

[Enthalpy]

AC motors operating at much high power factors than a 9v DC motor.

Nonsense.

http://en.wikipedia.org/wiki/Power_factor