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I understand that most car starter motors' power is rated between 0.5kw to 1.5kw. Doesn't that mean that they are supposed to draw 40-120 amps? (500 w / 12 volts etc.. )? Yet when they start, they draw hundreads of amps for the split second they run. Why does that happen? Are the motors "overclocked" during that period?

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    The term "overclocked" doesn't have any meaning for a DC motor. "clock" means an alternating signal with a fixed frequency. – Dmitry Grigoryev Aug 21 '17 at 8:20
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    I know... that is why i used quotes. 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. – Andrei Grigore Aug 21 '17 at 8:25
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    The word overdriven comes to mind, but I'm not quite sure myself. – Dmitry Grigoryev Aug 21 '17 at 8:57
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    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.stackexchange.com. – Quentin Aug 21 '17 at 14:42
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    I'm voting to close this question as off-topic because it's more appropriate on electronics.stackexchange.com as it's about the current draw of electrical motors in general. – Leliel Apr 20 '18 at 1:18
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It takes a LOT of power to get the rotating assembly - crank, pistons (or rotors), etc. - moving. For reference, try turning your engine over with a breaker bar on the crank. It's not super-easy (though some of that is due compression).

All the parts in the rotating assembly - crankshaft, connecting rods, pistons, valves, camshafts, timing chain - add up to a very, very heavy piece of metal that must be moved by a fairly small electric motor (starter) to start the car. Not only that, they have to get moving pretty quickly for the combustion cycle to take over. That takes a lot of power.

You can work backwards from your numbers using Ohm's law (V=I*R) and the definition of power (P=I^2 * R). The significant factor here is resistance, which is huge in this context.

So, the short answer: Metal parts are heavy, and take a lot of energy to move. This is one reason that things like lightweight alloys and composites are so important in high-efficiency designs: by reducing the weight of the moving parts, we reduce the energy required to move them. All of that surplus goes to the output, making your car / bike / jet pack / spaceship faster.

Update

See the comments for better information.

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    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. – Quentin Aug 21 '17 at 7:01
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    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. – Dmitry Grigoryev Aug 21 '17 at 8:25
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    @DmitryGrigoryev with a (roughly) constant voltage source (car battery), more power equals more current (P=u*i). so lots of mechanical power needed => lots of electrical power needed => lots of current needed (if low voltage DC source such as a car battery is used). – Quentin Aug 21 '17 at 9:26
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    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. en.wikipedia.org/wiki/Counter-electromotive_force – uɐɪ Aug 21 '17 at 10:48
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    @DmitryGrigoryev indeed, it is defined for instantaneous values only. I begin to understand better the original question, which I understand now as "why is the starting/peak current for an electric motor significantly higher than the rated (sustained) value ?". With this wording, this question may be more appropriate on electronics.stackexchange.com. – Quentin Aug 21 '17 at 14:36
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All electric motors consume more current at startup compared to steady state. Check out the label on your fridge for example (or look at this one): the max current on the label is 2-3 times higher than the value you'd obtain from power to voltage ratio.

The reason behind this lies in the properties of electric motors. Approximately, such motors have torque proportional to the current and speed proportional to voltage. When the motor starts, you need much more torque to get it running than you'd need in steady state to keep it running. Hence you need more current.

By the way, a lot of cars have even more powerful starters (e.g. a Landcruiser has a 2.5 kW one). That's over 200 A in steady state. Multiply that by 2 or 3 to get the startup current, and you'll get around 500 A that the battery must be able to provide.

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    @AndreiGrigore I understand. I just added the last paragraph in case there are people wondering what those big 600A batteries are for. – Dmitry Grigoryev Aug 21 '17 at 8:27
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    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. – David Richerby Aug 22 '17 at 5:46
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    @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. – Dmitry Grigoryev Aug 22 '17 at 10:08
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    @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. – David Richerby Aug 22 '17 at 13:14
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    @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. – Dmitry Grigoryev Aug 22 '17 at 13:17
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A characteristic of electric motors is that they produce the highest torque when stationary, coupled to this is a very high initial current 400 to 600A for cars and commercial starter motors can exceed 1000A.

Once they start to rotate the current demand goes down - remember that the pinion / flywheel ratio is 10 to 1 or more so when the engine is being turned at 500rpm then the starter is doing 5000...

2

Consider the following model of an electric DC motor

  • Va = 12Volts in a car
  • Ra = the ohms resistance of windings, cables,battery etc.
  • La = inductance (consider it to be zero in first approximation)
  • Ia = current through motor
  • Vc = Voltage electromagneticall induced into motor (proportional to rotation speed wa)

The rated power of a motor is conventionally defined as the availble output power (≈Vc*ia) at some combination of speed and torque. Under normal continous operation the input power (=Va*ia) will be a bit higher than the output power.

But startup is not "normal continuous operation".

As a first approximation we can treat the inductance as zero. The current drawn by a DC motor then depends on three things, the supply voltage Va, The resistance of the windings Ra, and the "back EMF" Vc which in turn depends on the motors rotation speed. Power delivered into the back EMF (= Vc * ia) is mostly delivered to the load while power delivered into the winding resistance ( =iaiaRa) is wasted as heat in the windings.

Due to intertia in both the motor and the load the initial rotation speed is zero, hence initially the current in the motor is limited only by the winding resistance, the motor draws far more current than normal and all of the power entering the motor is wasted as heat.

Gradually as the load and motor come up to speed Vc increases, thus V_Ra decreases, thus Ia (= (Va-Vc)/Ra) decreases as well and the motor transitions to normal continuous operation. If the engineers did their job right then the motor should reach a safe operating speed before it overheats.

In the case of a car hopefully the engine then starts and the starter motor is disconnected.

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A typical starter motor is an induction motor which can produce high torque on starting. It has a stator coil and a rotor coil. The stator coil is composed of many turns of copper wire that are fixed to the inside of the motor housing. The rotor coil is composed of many turns of copper wire that are fixed to the rotor shaft. When the starter is turned on, the 12 Volt (V) car battery sends current to the starter motor. At this instant the resistance (R) of the motor is just the resistance of the copper wire that makes up the stator and rotor coils and is therefore low (less than 0.05 Ohms). The initial starting current (I) is therefor high (greater than 240 Amps; From Ohms Law I=V/R =12/0.05). this is the peak starting current and will only last a fraction of a second. As the starter motor rotor begins to turn, the electric fields of the stator and rotor coils will interact to produce a "back EMF" which is an internal voltage which opposes the input voltage from the battery. The starter motor's motion will be opposed by the mechanical force required to turn the engine over until it starts. Starter motors are matched to the engines that they are required to turn, so that they would only be required to turn the engine for a few seconds. The current requires by the starter motor during these few seconds will drop to about half of the peak current mentioned above.

  • Good answer, Paul. Welcome to Motor Vehicle Maintenance & Repair. This is known as several things: Inrush Current, Startup Current, Locked-Rotor Current... and it is the reason there's a hefty wire straight to the battery from the starter. – Bevan Jan 20 at 5:05
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During the starting, the starter motor draws so much power that the voltage collapses somewhat (caused by the internal resistance of the battery). By this, the nominal power P = U I of the starter corresponds to a current I that is higher than what you compute with U = 12V (e.g., if the voltage is drained down to 6V, the current is twice as high to have the same power). Also, note that the power corresponding to the loss of voltage and the same current produce heat in the battery...

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    Good batteries should lose not more than 25% of their voltage rating during cranking. A battery going down to 6V may not be enough to start the engine. – Dmitry Grigoryev Aug 21 '17 at 14:24
  • Also, there's nothing in the starter which limits it to nominal power. If you stall the engine (with clutch and brakes) and turn on the starter, it will try to deliver perhaps twice or 3 times its nominal power, not for long of course. – Dmitry Grigoryev Aug 21 '17 at 14:30

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