Motor basic design questions

[YoSamES]

I've been reading and reading, and can't find a description in one place of what changes occur in motor design as the motor changes from 24v to 36v to 48v, from 250w to 350, 500, 750 etc. I read about fast and slow winding, RPMs at the specified voltage, etc. I realize that sometimes the same exact motor is given different specs. And I realize that if you go from 10a x 24v input to 10a x 36v on the same motor, you've changed the watts from 240 to 360. But I keep seeing motor specs that make me think there are actually different designs for say a 24v/250w motor and a 36v/350w motor.

So generally speaking:
-What is different design wise between otherwise identical motors of 24 and 36 volts?

-What is different design wise between otherwise identical motors of say 250w and 500w?

Hope these are not stupid questions.

 

[spinningmagnets]

The power specs on the advertisements are very murky at best, don't trust any of them. The two most common ways to have different power levels (for what looks like the same exact motor) is to change the controller settings to allow more volts, or more amps (or both). If the cheapest available controllers are not adjustable, then they will send a different controller to match the advertised wattage (brushless motors can operate on a wide range of voltages).

The Kv of a motor (in rough terms) is how many RPMs it provides per volt that you apply. Some sellers simply use the same motor for the 36V kit and the 48V kit, which means the motor will spin about 30% faster at the higher voltage.

If you are selling kits in a place that has speed limits (like 20-MPH in the US), then you can have the same motor, but each is wound with a differing thickness of wire so that they both provide 20-MPH from either 36V or 48V.

Swapping-in a different controller that allows higher amps will make the acceleration feel more powerful, but more amps also adds more heat, and small motors (like the 250W you mention), are very sensitive to heat spikes.

Concerning Kv, there is a certain amount of airspace between each tooth on the stator for winding copper wire. You can fill that space with a few turns of thicker wire, or more turns of thinner wire. more turns will spin slower, and fewer turns will spin faster (if both are at the same voltage). This is further complicated by turning wire "in hand". This means that several very thin wires that are attached at their ends...will act like a single fatter wire. Some motors are would "in hand" because a bundle of thin wire is easier to hand wrap around the stator teeth. If someone hand-winds 6 motors an hour, thicker wire is harder, slower, and hurts your fingers after a long day.

Hope this helps...

 

[footloose]

Definitely not stupid questions in my book.
I've wrestled with the same questions for a long time.
Hope some experts can enlighten us all!

Here is my take, assuming two visually identical motors.

Difference between 24V and 36V rated motors: the internals of 36 volt motor will have more windings on the armature (eg a 'slow winding'), in order to keep the RPMs of the two motors more or less in the same ballpark.

Difference between 250W and 500W rated motors: only the sticker on the outside, primarily to comply with regulations of different countries and for marketing purposes.

EDIT -- See spinningmagnets has already provided a great explanation of the differences! Thank you SM.

 

[major]

So generally speaking:
-What is different design wise between otherwise identical motors of 24 and 36 volts?

-What is different design wise between otherwise identical motors of say 250w and 500w?

I'll take a shot at this. But emphasize _generally speaking_.

-What is different design wise between otherwise identical motors of 24 and 36 volts?

If identical means the same RPM torque characteristic and motor size, then the 36V motor would have 50% more turns/coil with wire which is 33% smaller in cross sectional area.

-What is different design wise between otherwise identical motors of say 250w and 500w?

If identical means the same speed, then the higher power motor will have more active material (copper, steel and magnet) and greater surface area.

 

[dogman dan]

Where it gets sticky, is when the motors are not identical. In general, many kits are just 36v kits that have a controller that can be pushed to 48v with no problems.

So the motor is indeed identical. But in general, a 250w kit won't have the identical motor of a 500w kit. In general, the very low wattage kits have a smaller size gearmotor. These will melt if you push them.

In other cases, for example the kits from Yes.com, I'm told that the 48v 1000w kit does indeed have a different motor than the lower wattage 36v kit. Different winding perhaps, and thicker wires to power it. The 1000w kit definitely has a bigger, higher power controller too.

But once you have truly identical motors, then the only difference is the controller that is used with the motor. That could be anything from 200w to 3000w. Some will sell a wildly mismatched controller with a motor. If the only thing they will tell you is a wattage, then you may be doing business with the wrong vendor.