stepper acceleration rate - limits?

[Jonathan]

I'm running a stepper via an EM806 driver and I'm generating the pulses with a PIC via PWM. The frequency is modified by changing the PR2 register

So strictly speaking you mean pulse frequency modulation, not PWM, as the pulse width (quite rightly) is fixed.

The motor stalls sometimes during acceleration, which got me into thinking that maybe the speed is increased too quickly.

So have you tried changing the speed (via PR2) very slowly, to verify your supposition?

I'm guessing the answer is no, but is there an easy way to calculate dynamic torque or an acceleration rate to guarantee the least torque drop during speed ramp up?

Torque that the motor can output, i.e. it's rating, or the torque required by the load? If it's the former then you can make some assumptions to get a reasonable guess. If you know, or can calculate, the load inertia and load torque then it's just basic physics to work out the torque required from the motor. Find out what the load inertia is, recall that torque is inertia multiplied by angular acceleration, then pick an acceleration such that the required torque never exceeds the values indicated by the torque-speed characteristic of the motor.

The motor ramps up from 60RPM to 240RPM within 1500ms

So change in speed is 180RPM in 1.5 seconds. For torque calculations you'll need the angular acceleration, .. Now if you know the inertia of the system, i.e. motor and load (taking into account any pulleys etc), then the torque required for this acceleration is simply . where J is the inertia.