Motors and Watt ratings

Now, I will apologise if what I'm about to say is beyond stupid (I have practically no knowledge of electronics beyond the basics) but here goes -

Plenty of heavyweights, from what I can gather, run on bosch (or equivalent) 750w-1000w motors. I've also heard that the watt rating converts to Horsepower in some capacity. Would it be feasible to use a, say, 2000w Brushless motor (assuming you had a forward/reverse esc with enough amps, voltage etc) in place of those motors to still drive the heavyweight? If not, what is it that makes the Boschs more suitable for the job?

Not that I'm asking for a build, rather just out of interest and to learn!

Thanks in advance

Firstly around 750W rating is about 1hp.

Wattage is the amount of electrical power going through the motor. Different motors will give this power in different ways, some will be high revving with little torque, others will spin very slow with a daft amount of torque. The two motors at the same rating use the same energy, it's just how that energy manifests itself in the real world.

Different motors are designed for different applications, and so are engineered to take advantage of the fact that you can make the same quantity of energy behave with different characteristics.

A 2000W brushless will spin very fast with little torque. A Bosch GPA type motor of the same wattage is designed for massive torque at relatively low rpm. :)

It's 746 watts in a horse power.

Brushless act a little different from brushed motors. Technically you could but you would have to have some serious gear reduction to get it down to a useable RPM. You would also need a closed loop system. Open loop brushless have a torque rpm curve similar to a petrol engine (rises up to a peak then drops off) whereas a brushed electric motor has max torque at zero rpm dropping off as the rpm increases. A closed loop system could take care of this. The closed loop allows the electronics to know the exact position of the brushless motor magnets. I could go on but I won't.

Essentially the reason the bosch 750 is used is because it's always been used. It was once fairly cheap (bit more expensive now) and requires relatively simple electrical knowledge to use.

But wait, there's more! Motors actually have two power ratings; their input power and their output power. The input power is the one usually quoted and its the amount of power drawn from your battery. Output power is the power delivered at the axle and is always lower; the difference is the efficiency of the motor (84% for a GPA) and is radiated as heat. Brushless motors almost always have a higher efficiency than brushed motors, so in theory one rated at 1800 watts would deliver more pushing power than a GPA motor (also rated at 1800W).

Next, we have the RPM issue: All motors have a rating called the 'Angular velocity constant' or Kv for short. This is simply the number of revolutions they do for every volt applied. A GPA has a Kv of 160 so it will spin at 3,840 rpm at 24 volts. Hobby brushless motors are meant to power cars or planes and are designed to have high Kv as that's what the market needs. It would be possible to make a low Kv motor by winding your own from a kit, but there is a good reason not to. The practical way to use brushless motors to drive a heavyweight is just to add more gear reduction.

Last, there is the motor's torque. As Gary just mentioned a sensored brushless motor has a torque curve more like a brushed motor. Unfortunately, the are exactly zero hobby motors with low Kv and sensors :(. Its possible to add sensors to large, low Kv outrunner motors or you can simply gear down a higher Kv outrunner and get nearly the same performance. Its a complicated subject and this guy covers it better than I ever could: http://tinyurl.com/myo7fep.

Ahh, thanks for the help there everyone! It definitely sounds like a complex subject... The Boschs do sound like a good choice then, in that case! The brushless outrunners I thought would be best for a drive would be those NTM 50-60s (the ones that are rated to something insane like 3000w in a motor that's only about 500g) but if the torque isn't at a constant, I can certainly see drawbacks with using them, and what you'd gain in a weight reduction from the sensorless ones you'd lose in the huge gear reductions you'd need...

And Nick, thanks for the link too, I shall give that a good read!

The NTM outrunners are the sweet-spot between the cheapo and name brand motors; everyone who uses them has only good things to say. Gearing down an outrunner shouldn't be too hard compared to a Bosch GPA. The trick is to find a low Kv motor and maybe add one extra stage of reduction.

In a heavyweight situation, the real problem is finding a reversible ESC. There are very few to zero hobby ESCs that can handle the power AND reverse the motor, leaving you with bulky and very expensive industrial controllers. IMHO, brushless motors are viable up to lightweight class bots and then brushed motors take over in heavier classes.

I've only ever heard good things from them, bar magnets occasionally coming loose but I'm told that can be fixed...

That is a huge problem, I've heard of the Toro 8 (or something) that can handle 6S forward and reverse up to about 100A-ish, though I'm not sure that's really going to handle 100kg of heavyweight braking or what have you... Still, from what I've heard, brushless has come quite a long way recently, so there's always the possibility of a few high voltage/high amp hobby ESCs and low KV sensored motors popping up eventually!

Have you looked at the Kelly Controllers site http://kellycontroller.com/? They have some interesting products and at least a few of their controllers are reversible.

RoboteQ has some new brushless motor controllers. Again, they are only for sensored motors so you need to add your own hall sensors to an outrunner. The high price cancels out the cost advantage of cheap brushless motors but Roboteq comes the closest to what a heavyweight needs in a brushless ESC at the moment.

They both seem pretty interesting - again, expensive though... I can definitely see why the jump hasn't been made from brushed to brushless yet, all things considered!