Re: DC Motor question

On 28 Apr 2007 02:21:47 -0700, steambuff <martincottrell@xxxxxxx>
wrote:

Hi All,

I have recently finished making a small ride-in landrover for my
grandsons and have a question regarding the electric motor that drives
it. The initial 'road-test' proved excellent but was a little
dissapointing when attempting gentle uphill gradients which caused to
motor to stall so I just wanted to sound out some expert advise before
I decide on how to solve the problem.

I am using a 24volt dc Parvalux motor rated at 200w. I am currently
running the motor from a 12volt battery and the drive is geared down
to run the landrover at a comfortable walking pace.
My questions are as follows:

1. By applying half the rated voltage (12v instead of 24v) am I
reducing the power output to 100watts, or just reducing the motor
speed by half, or both?

2. Depending on the answer to Q1, if I increase the supply voltage to
24v will I need to adjust the gearing to maintain the current walking
pace speed (if the motor will be running twice as fast) and will I
have more power available to take reasonable uphill gradients?

I hope my questions make sense and that someone can help!

Regards, Martin.

It depends on what type of DC motor - series wound, shunt
wound or permanent magnet field. For ALL types maximum power at
12V will be less than half the rated power.

A shunt wound machine will try to run at the same speed
but the efficiency will be dreadful because of the inadequate
field strength.

A series wound machine will be more tolerant of reduced
voltage. The speed will be very load dependent, but at the same
load it will run at a bit less than half the speed.

A permanent magnet machine will run a bit below half
speed but with much better speed regulation.

By doubling the supply voltage you will more than double
and possibly quadruple the output power. This should greatly
improve the hill climbing. The more the than double change in
straightline speed may be a problem and you would need the change
the gearbox ratio or add a power controller.

Jim


.

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