Determine voltage of old brushed DC motor
The voltage across a DC motor is made up of two components. A component from the resistance of the coils and a component from the inductive behaviour of the moving coils. When a motor is stalled it behaves like a resistive load. 

Most motors if stalled while supplied by their rated voltage will draw considerably MORE than their rated power (and will overheat if a protection circuit doesn't kick in). 

For a resistive load:

$$P=\frac{V^2}{R}$$
$$V^2 = PR$$
$$V=\sqrt{PR}$$

To get 500W into a 15 ohm resistive load would need about 80V. So that means that this motor is likely designed for a supply voltage of considerably more than 80V. 

I would guess that this motor is designed to run at full power when fed with rectified mains and then there is some control circuitry to reduce power on lower power settings.