I have a trolling motor that is rated at 12 V. I want to double the speed of the motor.
If I change the armature wiring by using a thinner wire with more turns, will I able to use it on 24 V DC to get more speed?
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If you have a 12 V DC permanent magnet brushed motor, the way to increase its speed by double is to supply it with 24 V, with no changes to the motor. If you rewind it with twice the number of turns of thinner wire, then it will run at the original speed with a 24 V supply.
You will need to keep the output torque similar, as motor heating goes as current squared. You will therefore need to reduce the size or pitch of the trolling prop accordingly. Loading it with more torque will draw more current, and quickly overheat the motor. Running the same torque at twice the speed will allow you to get twice the rated power out of the motor.
But for how long? Doubling the speed will reduce the life of the motor, more or less significantly, in a number of ways.
a) Sparking at the commutator will increase, increasing the wear on commutator and brushes
b) The bearings will wear faster
c) Twice the speed means four times the centrifugal force on the armature components. It may reduce the life to a fraction of a second as it flies apart through lack of strength.
Do you actually want to increase the speed of the motor, or do you want to increase the speed of the boat? It may be better to invest in a motor with a higher rated power, and couple it to a suitable prop.
A boat tends to require a power proportional to speed squared. If you want to double the speed of the boat, you need four times the power. Maybe your motor can run at twice the speed with twice the voltage (with a reduced life and reliability), but running at twice the torque will require twice the current, which means four times the heating. Unless the motor is rated at way over what is needed for its original 5 km/h, then it will not be able to manage 10 km/h, and will overheat. Does it have a ratings plate on the motor?
To paraphrase that famous (and I understand, unscripted) quote from 'Jaws', 'You're going to need a bigger motor'. Or at least, better cooled.