DC Motor question

[Squisher]

Can a DC motor function at its stated rpm if the electricity supplying that motor is being throttled by a resistance?

I'm being plagued with a motor burning out too frequently, but I'm being stubborn in not attributing the cause as it being fed electricity through a resistance because I see no loss in rpm until the motor fails. Am I right or wrong?

 

[MotF Bane]

I'm not sure I understand your scenario. Are you saying, for example, that you have a 12V supply, and are using a resistor to knock that down to 5V for the motor to operate, and are asking if that is somehow destroying the motor?

 

[alienb]

Can a DC motor function at its stated rpm if the electricity supplying that motor is being throttled by a resistance?

I'm being plagued with a motor burning out too frequently, but I'm being stubborn in not attributing the cause as it being fed electricity through a resistance because I see no loss in rpm until the motor fails. Am I right or wrong?

Resistor will not drop voltage, but current. Transformers drop voltage. "Electricity" will have to be defined from you.


What exactly is the application here?

 

[PottedMeat]

burning out? are you overloading it?

 

[herm0016]

running a resistor will just make it draw more current if it is in series, the motor will try to draw as much as it needs, and the resistor will draw what ever it does, add them together. how is the motor controlled and what is it used for?

 

[Rubycon]

What kind of DC motor is it?
A "universal" type motor has no speed regulation whatsoever.
There are other types of DC motors that are not actually getting fed DC in order to preserve speed and / or torque at a certain speed, etc.

So in order to attempt to figure this out we need more information, please? :biggrin:

 

[Squisher]

Starting motor on a truck. Motor is fulling functional for months at a time with no lack of rpm or torque, then is dead. I'm reluctant to believe that a DC motor could function as intended, totally asymptomatic, yet all this time is being fed by corroded wiring that is supplying a resistance.

btw-I post in "The Garage" frequently. I believe my question is one step back from that more on a the lines of the functioning of an electric motor.

 

[Rubycon]

Have you cracked it open? Could be a dead spot on the commutator. If it sits right there you have open resistance that will test like an open winding. Sometimes hitting it with a hammer will jog it enough to "catch".

Also bad connections creating a high resistance are bad for ANY motor.

 

[Dead3ye]

Sounds like the solenoid on the starter rather than the starter motor.

 

[PottedMeat]

maybe it's bad solenoid contacts giving high resistance

 

[Rubycon]

I thought trucks used pneumatic starters?

:wub: the sound!

http://www.youtube.com/watch?v=W-VlTKL7U6A

Yeah does the solenoid click at all when it won't crank?

 

[Number1]

Starting motor on a truck. Motor is fulling functional for months at a time with no lack of rpm or torque, then is dead. I'm reluctant to believe that a DC motor could function as intended, totally asymptomatic, yet all this time is being fed by corroded wiring that is supplying a resistance.

btw-I post in "The Garage" frequently. I believe my question is one step back from that more on a the lines of the functioning of an electric motor.

Your old wires may be dropping the voltage down to the point where a slightly used motor cant turn the engine.

 

[Squisher]

Have you cracked it open? Could be a dead spot on the commutator. If it sits right there you have open resistance that will test like an open winding. Sometimes hitting it with a hammer will jog it enough to "catch".

Also bad connections creating a high resistance are bad for ANY motor.

That is what was wrong with the last one. The last time I took it to a rebuilding place where they hand rebuilt it to try to figure out what is going on. This is probably my sixth starter in 3 years. Yes, I'm really good at changing them. I'll be knocking it out right after I post this. Half an hour tops. Solenoid clunks loudly, but no rotation.

People keep telling me to replace all the wiring going to the starter, but like I said in my first post I NEVER hear/feel any lugging or slowness to the motor that would indicate that it is being taxed.

 

[Squisher]

Your old wires may be dropping the voltage down to the point where a slightly used motor cant turn the engine.

But, why would it go from fully functional to non-functional without any slowness.

 

[MaxFusion16]

Resistor will not drop voltage, but current. Transformers drop voltage. "Electricity" will have to be defined from you.


What exactly is the application here?

what? resistor in series will most definitely drop the voltage.

how else do you expect current to flow across the resistor?

there has to be a potential difference across both ends of the resistor, this potential is what causes current to flow. Potential = voltage drop.

 

[Analog]

What voltage, current is this rated at? What load is it under? What environmental conditions is it in? Need more info to help.

 

[bruceb]

Start out by replacing both battery cables and be 100% sure the Ground connection at the engine block is clean and tight. Another possibility is bad contacts in the ignition switch.

 

[Squisher]

Start out by replacing both battery cables and be 100% sure the Ground connection at the engine block is clean and tight. Another possibility is bad contacts in the ignition switch.

I'll be real unhappy if I replace the entire wire loom and it's the ignition switch.

 

[Squisher]

What voltage, current is this rated at? What load is it under? What environmental conditions is it in? Need more info to help.

I don't see how this is relevant. I asked if any DC motor, more specifically the starting motor on a car, would function at its intended speed and torque if more resistance was introduced to the electrical supply. In my mind the motor's speed would vary inversely with the amount of resistance.

 

[Modelworks]

You may have a situation where the motor it is starting is just slightly over the specified torque for the starter. That will make it work for awhile but result in early burnout of the motor. What I would do is connect an ammeter between the starter and power and measure the current . If possible find another truck with the same starter/motor and measure the current for comparison.

 

[edro]

What kind of truck?

Stop trying to back into the solution and start searching for what causes starters to go bad.

Maybe you need a higher power, name brand starter.

 

[Pulsar]

running a resistor will just make it draw more current if it is in series, the motor will try to draw as much as it needs, and the resistor will draw what ever it does, add them together. how is the motor controlled and what is it used for?

What in the hell? Between you and AlienB, I cry for the state of education in this country.

Voltage = Current * Resistance.

If you put a resistor in line, you increase the resistance, and LESS current will flow.

You.... you make baby Jesus cry.

AlienB? Putting a resistor in series will divide the voltage between the resistor and the motor. Of COURSE it will lower the voltage the motor sees.

 

[Mark R]

I don't see how this is relevant. I asked if any DC motor, more specifically the starting motor on a car, would function at its intended speed and torque if more resistance was introduced to the electrical supply. In my mind the motor's speed would vary inversely with the amount of resistance.

It depends on the construction of the DC motor. It may be possible to build a DC motor that maintained speed/torque close to its nominal value, with the addition of a modest resistance to the supply. (The resistance would need to be small compared with the overall power demands of the motor at rated load).

Most universal motors use a "shunt" field construction, where the stator field and rotor are supplied from the same power supply. This gives good speed/torque regulation, but poor speed/voltage regulation (speed is proportional to voltage).

However, by using a more sophisticated contribution to the stator field (e.g. by including a series component of rotor current) the speed/torque regulation can be altered, which could if correctly calculated, be able to compensate for a limited series resistance.

 

[bobdole369]

Think of the application folks:

Starter motor pulls a couple hundred amps for a few seconds. Thus corroded wire CAN drop enough voltage across the corrosion to cause the problem.

Think how a starter motor works, and BTW - Rubycon was right the whole time.

The issue isn't in the motor. It's in the solenoid.

In order for the starter to engage, first the solenoid must actuate fully - usually there is an actual switch involved.

If the voltage presented from the battery across the corrosion were something like 11.1V (say 1.3V dropped across the corrosion) and the solenoid needs at minimum 11V to function, what would happen if the voltage dropped to 10.9V the next day? The solenoid wouldn't engage, the motor wouldn't turn, and at most you would get a series of clicks as the solenoid weakly tries to pull the gear into place. At worst it would hear nothing as the battery discharges through the closed coil without doing any work and just heats up.

This is precisely why you never really hear a 'weak starter' anymore. The solenoid gives out well before RPM of the motor is affected.

 

[pcgeek11]

Resistor will not drop voltage, but current. Transformers drop voltage. "Electricity" will have to be defined from you.


What exactly is the application here?

Wrong. Ohms Law. A resistor in series with a load will drop voltage as all components in a series circuit drop voltage, the total of all these voltage drops equals the source voltage.

E = I X R