pysics question V=IR overvolting a motor

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colin9876
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pysics question V=IR overvolting a motor

Ive a bit sceptical of the 'More bvattery Amps give more torque' theory because as V and I are linked then really both have to increase in parallel?
Im guessing that overvolting a 24v motor will take more volts AND amps, whilst going directly for a 36v motor will (having a higher resistance) use more volts but not necessarily more amps

Anyway then I thought maybe V=IR doesnt strictly apply to motors, e.g how does a motor use more amps at stall/low speed? even though the resitance of the coil is fixed, amp usage does change!!!
Question - Does the 'back EMF' add to resistance???

colin9876
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Re: pysics question V=IR overvolting a motor

just clarify my question does back emf add to resistance or is back emf contributory to inductance - im a bit confused on those concepts?

p.s. I recently burnt out a 24v motor by upping it to 36v so I dont recommend it! It only lasted about 3 minutes!

antiscab
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Re: pysics question V=IR overvolting a motor

yay physics.

ok, for permanent magnet sychronous motors (which most of the motors we are dealing with are), the equation for motorside voltage is:

V = I*R + rpm*constant

the rpm*constant is your back emf component.
thus, if you increase your pack voltage, the max speed at which you can still push max controller amps through the motor is higher (ie motor voltage is less than battery voltage).

the controller chops the battery voltage, depending upon what the target motorside voltage is (which is determined by your throttle position and a PID current control loop).

although the applide voltage on the motor side looks like a rectangle wave, the motor inductance forces the motor current to be non 0 when the applied voltage is 0, for a *very* short time (short enough that the applied voltage goes back to pack voltage before current falls to 0). this happens 1000's of times a second. what the actual frequency is depends on your controller. my zilla 2k switches at 16.3khz.

the greater the motor inductance, the lower the motor side current fluctuates, and the lower the losses the system has.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

antiscab
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Re: pysics question V=IR overvolting a motor

the other thin about controllers is they trade volts for amps at low speeds.

for instance, on my emax, my controller draws 120A at 54v on the battery side, and at 15kmh gives 350A at 18v on the motor side.

motorside amps is what determines torque. for a permanent magnet motor, the relationship is:

Torque = amps * constant

when you are at speed, and motor volts = battery volts, then battery amps directly affects motor amps and torque.

for instance, my emax battery side is limited to 120A. so at 45kmh, max motor amps is only 120A.

the two examples are simplified a bit because my setup is actually AC (so the current is spread across 3 phases) but the same principles apply.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

antiscab
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Re: pysics question V=IR overvolting a motor

probably part of the reason for your confusion, is that most electric motor text books represent back emf and resistance together as a resistance value that varies with speed.
i find this way to be far less straight forward than just representing the back emf as a voltage and the winding resistance seperately (which is how it is in real life).

re that 24v motor you burnt out, were you using a controller at the time?
did you also increase motor side current?

if the motor was DC, it may have been a commutation failure.
the brushes on most hub motors are neutral timed.
the problem is, the optimal brush advance (or retardation) is dependant on motor current and rpm.
the max variation between actual advance and optitimal advance the commutator can accept is determined by motorside voltage (the higher the voltage, the lower the variation can be).

if you exceed the max variation, you will get arcing between the brushes and the commutator.
this results in pitting of the commutator, and accelerated brush wear.
it also results in increased resistance at the commuator.
i havent seen this type of failure on a hub motor before (actually i havent seen a brushed hub motor before at all).
however i have seen this happen to a car sized motor.
the motor developed a fault while we were hitting it with 2000A, and the brushes, armature terminal and part of the commutator exploded and turned to vapour.

another possibility is the motor side current was beyond its continuous rating for too long.
running at higher voltage, means you can push more amps through the motor at speed.
which is handy seeing as you need more torque to hold a higher speed due to increased wind resistance.
more torque = more amps.
heat (power) = amps^2 * resistance.
so eventually the motor burns out.

if you havent got alot of range (ie not alot of battery on board) then you can push a motor *much* harder, because by the time the motor runs into significant heat issues, your battery is flat anyway.

the motor on my emax is rated to 48v 1500w, that doesnt stop me from running it at 60v 2500w continuous.
just means at the end of my ride, the motor is hot.

likewise, the forklift motors we use in cars we run at a much higher voltage than they are rated to.
we put a 36v kostov 11" series wound motor in a ute, and we run it at 200V at max power point.

whether a motor will withstand more voltage than rated is dependant upon a large number of things.
AC motors are *far* easier to increase voltage and current, since you arent limited to what the commutator can do.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

colin9876
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Re: pysics question V=IR overvolting a motor

Thanks for replies
The motor I burnt out was connected directly with a 20amp foot switch. 24v 250watt motor, had put another battery in series to make it 36v. Serious smoke comming out of it after 3mins! Had been fine when it was at 24v!

causalitist
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Re: pysics question V=IR overvolting a motor

MORE voltage = less effective resistance = more amps pulled.

antiscab
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Re: pysics question V=IR overvolting a motor

only if you are running without a controller, or your controller powerstage has failed short circuit.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

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