DC 5kw - 48V ---> 10kw - 96V

Sure, you would get 10kW's twice, but not for long. Mikek

PS. I ran a 28V motor at 48V on an electric gokart, but it was not continuously at full power and we didn't drive it for long periods of time, except for the last time, when all that white smoke came out of the motor.

Why not do the scaled down and much cheaper experiment with a 6v toy motor on 12v and report back on the colour of its magic smoke?

Not a chance. You might get away with overrunning a DC motor by a few tens of percent occasionally with cooling in say a model boat or plane. But it will always be life shortening for the motor.

Yes. A DC motor will run at about half speed on half voltage.

About 5kW.

Not without damaging the motor.

At half speed and full torque, you might need additional cooling because the built-in fans are providing less cooling.

My mistake. About 2.5kW Output since the speed is half and torque is the same.

Chances are the core will saturate and the coils will become short circuits for a short amount of time in every commutation phase, turning it into an expensive space heater until it catches fire. Higher back-emf due to higher RPM somewhat reduces the effect, but it might not be enough to keep it alive.

(-> always drive the go-kart at maximum speed and no curves to keep the load torque low)

Ya, using 48 Volts, from a standing start full pedal, the current was just over 250 amps (2.1HP motor ya, right!) By the the time it hit full speed it was down to 40 amps. We used it a couple years before the smoke, The problem was an overheated connection, where a 4 gauge input input wire attached to 8-18 gauge wires. I had a golf kart motor that I installed, the kids liked it better, more torque, but less top speed. It was something about doing donuts! Mikek

Of course, someone has tried... everything.

Motor power is a product of speed (angular velocity, actually: RPMs), and torque.

More volts won't get you higher limiting torque, because a well-designed motor already fully magnetizes the steel (and it's the magnetic field that creates the torque). Also, it doesn't make the shaft fat enough not to self-destruct at a higher torque than it was designed for...

It will get you higher speed. You only get higher power if your load also moves at that higher speed (otherwise it stalls the motor). The startup transient (when the motor is accelerating from zero) is a lot like a motor stall, it takes LOTS of current. That will change to LOTS AND LOTS of current.

The brushes, and the motor's overload protection, are potentially problems. Usually a thermal protector will work correctly at the higher voltage, as will an overcurrent-protection fuse.

Many years ago an uncle had a golf cart shop in Sun City, Arizona. The ol' folks (now I are one :-) were fond of using golf carts to drive to the grocery, etc.

So I made a "soup-up" kit with a starter relay and a long length of copper wire to shunt the field... got 'em up to 35MPH ;-) ...Jim Thompson

They're common here, too. Kids drive them to high school. No license required and a minimum age of 14. When shool gets out, they're everywhere. The golf cart parking at the grocery store is closer to the door than the handicapped parking, too.