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Current draw of a motor, under different situations

  1. Mar 9, 2012 #1
    So, I'm kind of in a debate, and before I say something that makes me look like a clown :redface:, I thought I'd check with the group.

    Someone said that a "dying or low battery in car or motorcycle will destroy the starter motor....because the motor will respond to reduced voltage by increasing the AMP draw....thus causing too much heat".

    I don't think this is true....well, not exactly. (BTW- I also think heat is a function of WATTS not amps per say.....am I wrong?) I don't think that a motor cares how many volts are coming in, so long as they are adequate to allow the motor to spin at its designed speed, and also are not too much. However, if LUGGING (trying to get up to speed but can't), then I think the amps are not increased per say....but they do stay at "peak" or "startup current" (due to the load) for an extended period. I guess what I'm saying is that I THINK it's not the low voltage that kills a motor (via heat from extra current)....it's the LOAD that has increased because the motor is not able to spin at full speed, thus increasing the amps (no higher than they were with a "good" battery.....but keeping them there for extended period) ??

    So, if a motor runs on 10-12 volts, and spikes at a current draw of 200amps for 1 second and then drops to 30 amps once up to speed......IF the battery is getting low and is only at 8 volts, then, the motor still won't draw more than the 200amps (right or wrong?)...but it may continually pull that kind of current (the 200a), since the motor is "lugging" and unable to attain speed. Am I out in left field on this one?? :confused:

    Maybe someone can take what I said.....and make it more clear in terms of amp draw of a motor. Hope that's slightly more clear than mud....but I think maybe it isn't?:blushing:
  2. jcsd
  3. Mar 9, 2012 #2


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    In General, motors are desinged for their specific voltage and amperage ratings. Anytime you lower the voltage....you generally have problems with starting the motor....etc. And yes it damages over time. If you lower the voltage....you will also lower the amperage.....the motor does NOT like this.

    I don't think that lowering the voltage raises the amps. V=I*R. The "R" or resistance of the starter motor is set.

    The watts of the motor (P=I*V) is determined by the voltage and amps. It will only deliver the specified watts if the voltage and amps are the ones specified.

    So if you say a motor has 100 watts for example.....you can't say if I connect half the voltage to it, it will automatically double the amps. Doesn't work like that. The actual voltage and amps given to the motor determined by the RESISTANCE of the motor will determin your power (P=I*V) or ((V^2/R)=P) or (I^2*R=P)

    The first of the formulas are first and formost determinded by V=I*R. It rules it....it owns it. The other two formulas above have R in them.....these you can use.

    Take the second one ((V^2/R)=P) for example.
    You just said you went from 12 volts to 8 volts.

    Lets say for the sake of the argument.....the ohms (resistance) of your motor is 1 ohm.
    (12^2)/1=144 watts.

    Lower it to 8 volts....
    (8^2)/1= 64 watts.

    Not the same!

    The resistance of the motor can certainly change with different loads....but again it is the resistance controlling all the variables.

    Get it?
  4. Mar 9, 2012 #3
    Psparky, I think OP is right and you showed why amps does not increase when voltage is dropped.

    But you agree with him that the starting motor is damaged, and I didn't catch a reason why. Is there a reason why it would be damaged? I don't know much about engines/starting motors to understand why.
  5. Mar 9, 2012 #4


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    I'm not sure who I was backing up....I was just pointing out my beliefs....
    Some of the things max wrote were not quite right.

    I'm not sure why lower voltage kills motors....but it certainly does. Motors hate when you change the specified current through them. And they simply respond by dying.

    This is exactly why you hook a capacitor in parallel when correcting power factor. The motor has no idea the capacitor is there.....the motor receives the same voltage and current. It loves that. However, the coal burning power plants sees a different load.....one with a better power factor and burns less coal simply because of that capacitor shifting the current and voltage closer together.

    Someone else will come in and correct me and add some further things soon enough....but I'm on to the right track....I believe.
  6. Mar 9, 2012 #5
    Oh, sorry I just was referring that he is correct that a motor will not draw more current just because the motor voltage is dropped, which is what his friend told him happens.
  7. Mar 9, 2012 #6


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    I agree with this statement. However, I don't agree with some of his other statements.
  8. Mar 9, 2012 #7


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    All that being said...when a motor is trying to start.....it is trying to build up a magnetic field. Now if for some reason the magnetic field does not get built up (not enough voltage for example).......the motor will behave more like a dead short.........In this case the motor may actually pull way more amps. Why? Because the resitance of the motor has gone way done....almost to a short.

    Transient is tricky business...

    Again though...it is based off the "resistance". So come to think of it.......there may be some valid points in saying that the motor pulls more amps based off a short circuit effect.....not because of the "watts" of the motor.

    So to answer the question above...how does it kill the motor....I think the answer is that it fries it from too much current due to the short ciruit effect. (magnetic field not being built up)

    My originial response was based more off of steady state.....my second response is more off of transient.

    In starting a motorcycle....is that transient or steady state? Hard to say....depends how long you are cranking.

    I dont know man....just throwing some stuff out here so we can all learn. I have learned just by writing all this crap! It has raised more questions than answers! Some one else please help!!!!
    Last edited: Mar 9, 2012
  9. Mar 9, 2012 #8
    With regard to a DC motor, the voltage across the motor, minus IR losses, is proportional to the speed or RPMs of the motor. The current through the motor is proportional to the torque produced by the motor. Normally the back EMF created by the speed of the motor opposes the voltage being applied and limits the amount of current the motor draws and thus it's torque. As more and more load is applied to the motor, the motor slows reducing its back EMF allowing more current to flow, producing more torque.

    With a lower applied voltage to the motor not only will the motor be turning slower, which reduces the back EMF and allows more current to flow, the losses such as the internal battery resistance, the cable loss and the IR drop in the motor become more important. This may mean that the motor may not have the speed or torque to turn over the engine and stall. When the motor stalls there is no longer any back EMF and all the current goes towards heating up the motor windings and that can damage the motor.
  10. Mar 9, 2012 #9
    The damage will mostly occur if the motor does not start - or sits there not moving with current flowing. At a low voltage you are more likely to run the motor too long in this stalled condition - or it takes longer to get to speed. With a good battery = the motor starts quickly - ideally on the first try. Repeated attempts to start the motor are the problem.
    The Resistance of the motor is only part of the equation - in a DC - permanent magnet motor, the motor provides Back EMF as it comes to speed ( actually generates torque). When you first energize the motor a large amount of current flows (limited principally by it's resistance) and decreases as it begins to apply torque to the load. It is the BACK EMV x Current which defines the power delivered to the load - if it was just the winding resistance - it would not be a motor - it would just be a heater.
  11. Mar 9, 2012 #10


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    Are we talking about electric cars or motorcycles? My car doesn't have a motor...
  12. Mar 9, 2012 #11
    In looking at the equations and equivalent diagram of a DC motor you can learn a lot(everything?) about its behavior. For car/mc applications the motor is probably a series or compound dc motor (the way the field winding is connected). Try to google it.

    As stated by psparky, extensive heating is the main cause for breaking a dc motor. And as you know, heat is a result of current where as cooling usually comes from a fan connected to the motor shaft. Thus cooling is proportional to the motor speed.

    As seen in product data sheets, without external cooling, the motor cannot supply nominal power below some rotor speed over a longer time span, due to heating.

    With the equations, related to current, speed and voltage and heating you can probably find out for your self what is happening at low voltage.
  13. Mar 9, 2012 #12
    Regardless of what type of car you have it is difficult to believe it doesn't have a motor. Do you crank start it?
  14. Mar 9, 2012 #13


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    The OP did not clearly specify what sort of motor he was interested in, but there was the implication that we were discussing an electric starter motor.
  15. Mar 9, 2012 #14


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    I agree that scenario is likely or possible. A sluggish starter is likely to be engaged for much longer than intended, so the motor (not rated for continuous use) is likely to overheat. And because it is not turning as fast as designers intend, the reduced back emf allows higher armature current and this can overheat the windings and brushes.
  16. Mar 9, 2012 #15


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    Slightly stronger than an implication, by my reading of OP:
  17. Mar 12, 2012 #16
    How to calculate induction motors formula ?
  18. Mar 12, 2012 #17


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    Hi Dhende. It is best that you start a new thread with your topic, and you are likely to get a lot more responses that way. Induction motors are not used as starter motors in cars, which is what this thread turned out to be, despite its misleadingly general topic.

    When you post your question in its own thread, I think you should explain which equation you are wanting to derive, too.

    Good luck!
  19. Mar 12, 2012 #18
    If the motor has permanent magnet, then the torque will be directly proportional to Armature Current. (At the start when the motor is at 0 speed, there is no Back EMF so Starting Current = V/R ).
    So, the starting Torque decreases in direct proportion to applied voltage.
    Once started the motor will slowly pick-up speed and hence back Emf increases.
    At any given speed, motor torque is proportional to Armature current I = (V-E)/R
    where E is back-emf which is directly proportional to motor speed.
    So, as the speed increases the motor torque decreases (because E increases and I decreases), and at equilibrium (i.e. final speed), the motor torque exactly equals the load torque.
    If the applied Voltage is reduced, the starting torque will be less. So, the motor picks-up speed slowly. But the steady state speed will also be much less because,
    At steady state,
    I = (V-E)/R and we need same I as before to produce same torque (remember Steady State = Motor Torque = Load Torque). But we have reduced V, so a reduced E (i.e. reduced Speed) will be providing the required I.

    I checked some maths (including motor Torque, acceleration and speed pick-up times) and found that, in either case, the Timer-Integral of Current is the same, i.e., whether you apply higher voltage or lower voltage, the heating effect on the wires is the same.

    BUT, a big BUT, if the reduced Torque due to Reduced Voltage couldn't accelerate the motor to its steady state speed (i.e. it stalls), then It would draw high starting current all way long, and will Damage the motor. Although the starting current in this case is lower than the starting current due to regular applied voltage, this current may be still many (10) times higher than the steady state current on regular voltage.

    I don't much know about starter motors and how long does they generally need to operate, so I was mostly talking about Generic DC motors in Generic Applications.
  20. Mar 12, 2012 #19


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    the person is correct that the low voltage may damage the motor. I have found a very upsetting misconception with power, voltage, current, and resistance relates back to P = EI, and V = IR... people tend to assume that power is a constant, so then they assume voltage and current are inversely proportional but this isn't the case, unless you use a drive or something to correct the amount of power. motor's draw current (at a fixed voltage) based on ONE variable, the motor winding impedance. if the motor is say 24 volts, it will draw locked rotor current at start-up. This current will decrease as the motor speeds up, because of counter emf. This is good, the motor would burn up if it drew locked rotor current for longer than a little while.

    situation 1: if you increase the voltage to a motor, it will draw MORE locked rotor current.

    Situation 2: if you decrease the voltage to a motor, it will draw LESS locked rotor current.

    if the low battery voltage in situation two doesn't allow your motor to draw locked rotor current sufficient enough to overcome the locked rotor torque requirements for turning your engine... the motor will not develope counter emf and will draw a reduced version of rated locked rotor current for extended period of time.

    This current draw will be LESS than rated locked rotor current, but greater than it's nominal operating current. It could be enough current (and thus heat) at a specific voltage to ruin the motor.

    So... he .she isright in a way... but let's say the battery was nearly dead... and could only supply 4 volts. The motor would not draw a inversely proportional amount of current to "compensate" for the reduced voltage to attain the same power, it isn't a smart mechanism

    When I was trying to get a grasp on this, I asked some one about motor starting... and why motor's draw such a high starting current. They said... "it's like moving a shopping cart loaded, it's harder to overcome the friction at first, but when you do, it becomes easier, you need more work at the start" But....... I am much smarter than a motor... I know I have to overcome the friction, the motor does not know what it has to overcome...

    The motor tries no harder to move the engine shaft than it does my hand if i hold it, it exhibits one locked rotor current and torque for a specified amount of supply voltage.
  21. Mar 13, 2012 #20
    Hi. Very new to this forum. was just passing by and read most, not all reply.
    I am a sparky by trade and believe the phrase "lower voltage will kill the motor" came from the use of AC powered devices. where the r turns to z. on an induction motor (lawn mower, power drill ?, furnace motor, fridge/freezer motor) when the less than designed voltage is applied to the device that is under load, the induction motor will try to = the situation by drawing more amps to make up for loss of voltage and over heat the windings. it still wants/tries to do the work called for and will destroy itself.
    i belive in fact any motor, with not enough "energy" applied to it will over heat when trying to move a load. stalled I mean. $.02
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