Similarly to start a sub diesel engine, having a 1. The above can vary depending on engine condition, ambient temp etc many factors. Working under such severe conditions, sometimes getting overheated, the motors loose some of their insulation, and those motors can draw high current, and at the same time, give less torque, so engines fail to turn, and batteries are drained very fast. Them the field coils or the armature is changed, sometimes the entire motor. When you turn on any electrical motor that is static not moving it will produce a peak on current in order to make that motor start moving, the bigger the mechanical resistance in this case it has to move the whole engine the bigger the peak, then the current goes down to its specs until you release the ignition key.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why do car starter motors draw so much current?
Ask Question. Asked 4 years, 2 months ago. Active 5 months ago. Viewed 44k times. Improve this question. Andrei Grigore Andrei Grigore 1 1 gold badge 5 5 silver badges 11 11 bronze badges. The term "overclocked" doesn't have any meaning for a DC motor. I know I just couldn't think of a better term i am not a native english speaker , so i thought of an analogy with computer CPU's. The word overdriven comes to mind, but I'm not quite sure myself.
After some comments, I understand now your question as "why is the starting current for an electric motor significantly higher than the rated sustained value? I can tell you that this is true for every electric motor to the best of my knowledge.
With this wording, this question may be more appropriate on electronics. I'm voting to close this question as off-topic because it's more appropriate on electronics.
Show 4 more comments. Active Oldest Votes. Update See the comments for better information. Improve this answer. The electrical part of your post is wrong. The "mechanical resistance" is very different from the electrical one. The electrical resistance of a starter motor is certainly lower than what you seem to claim. Furthermore, an electrical motor does not follow ohm's law and even if it did, you would need a low resistance to have a high current.
The resistance in an electric motor is pure losses heat , so manufacturers try to minimize it. This answer kind of misses the point of the question IMO. If it takes a lot of power to crank the car, the starter needs a lot of Watts, not extra Amps. This should not be the accepted answer as it does not explain the initial surge current that the OP specifically asked about.
There is no mention of the back EMF generated by the motor being in motion and the fact that there is therefore no impediment to the high froward current as the motor starts to move. DmitryGrigoryev indeed, it is defined for instantaneous values only.
Show 5 more comments. Dmitry Grigoryev Dmitry Grigoryev 2, 2 2 gold badges 10 10 silver badges 28 28 bronze badges. AndreiGrigore I understand. I just added the last paragraph in case there are people wondering what those big A batteries are for. You're confusing cause and effect.
The motor doesn't draw high current because it "needs" to develop high torque. Rather, a stationary motor is just a coil of low-resistance wire. It draws a high current because of Ohm's law and that high current causes a strong magnetic field which, in turn, generates high torque. DavidRicherby Then you're confusing cause and effect in Ohm's law. The coil doesn't "draw" current, it just can't resist the current that the voltage forces through it. DmitryGrigoryev That's what the phrase "draw a current" means.
Of course the wire doesn't somehow suck current out of the battery: it provides a low-resistance path. In any case, you seem to be responding to the phrasing of my comment, not its actual content. DavidRicherby Well, the motor doesn't "need" to produce torque either, it is just forced to a certain rotation speed by the voltage applied to it, and the only way to develop that speed in presence of friction and inertia is to produce sufficient torque.
Perhaps I'm responding to the phrasing of your comment because I fail to grasp the actual content. Show 3 more comments. Solar Mike Solar Mike Add a comment. Good answer, Paul. But startup is not "normal continuous operation". In the case of a car hopefully the engine then starts and the starter motor is disconnected. A motor operating on V electricity has a V back emf at operating speed and draws a What is the back emf of a V motor that draws 8. The motor in a toy car operates on 6. If it draws 3.
Integrated Concepts The motor in a toy car is powered by four batteries in series, which produce a total emf of 6. The motor draws 3. Each battery has a 0. What is the resistance of the motor? Skip to main content. Search for:. Back Emf Learning Objective By the end of this section, you will be able to: Explain what back emf is and how it is induced.
It has been noted that motors and generators are very similar. Generators convert mechanical energy into electrical energy, whereas motors convert electrical energy into mechanical energy. Furthermore, motors and generators have the same construction. The motor thus acts as a generator whenever its coil rotates. This will happen whether the shaft is turned by an external input, like a belt drive, or by the action of the motor itself. That is, when a motor is doing work and its shaft is turning, an emf is generated.
See Figure 1. Conceptual Questions 1. Glossary back emf: the emf generated by a running motor, because it consists of a coil turning in a magnetic field; it opposes the voltage powering the motor Exercises 1.
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