Suggestion: It depends:
1. Nature of the load
2. Age of the motor
3. Maintenance of the commutator
4. Type of the DC motor and its associated shaft load
5. Drive, harmonics, common-mode currents
6. Environmental conditions
7. Etc.
It is a good idea to experiment with the voltage range, if in doubt and with so so tech support.
A dc motor is fairly forgiving on the amount of voltage that is applied. One thing about dc motors is that they are versatile. By adjusting the voltage it can serve a wide range of parameters.
Something that you failed to comment on, is the type of motor. If it is a series motor the entire circuit will see the increase in voltage. As voltage increases the current has to increase since resistance is constant, except with the change in heat. As the current rises so will the rise in temperature. This may be your best gauge as to how the motor will perform. Make sure the rise in temperature stays below the rating for the motor.
If the motor is a dual voltage motor with the armature at one voltage and the shunts at another voltage, you have to make sure that you do not allow the voltage of the armature to reach a point that a counter emf is developed. As the armature turns faster, more and more counter emf is developed. The counter emf develpoes within the turns of the armature. Each turn in the armature developes a portion of the counter emf. With the armature circuit being in series this causes the voltage of each turn to add to the next. When this cummulates enough counter emf, the armature will take off at alarming speeds and can cause a catastrophic failure due to excessive speed.
As long as the motor is seeing good commutation it should continue with good commutation at the higher voltage. You may need to check and see what the current density is on the brushes that are in use. Also if you check with the brush manufacturers they do make different designs of brushes to handle higher temperatures.
Why would one want to consider running the Armature volts beyond the Motor rating? If its to run faster, typically a field regulator would be used. However, when field weakening is used, motor torque is also reduced but the Drive then is able to run beyond base speed of the motor.
