1. Berger Lahr Servo Motor
2. Berger Lahr Servo

What you have dear friend is resonance. Frequencies at which the motor is incapaable of producing enough torque to rotate the shaft. There are two time constants which limit the operation of stepper motors. One is mechanical and the other is electrical. The mechanical time constant produces a resonance at between 200 and 300 Hz.

(steps per second). The electrical time constant limits the maximum speed to about 5 KHz.

Berger Lahr type WS5-5.28100 Stepper Motor Controller 600VA 230 - 240v 50/60Hz or can be set for 115v. Made in Germany.

Berger Lahr Servo Motor

The way around the first area of zero torque is to ramp the velocity up and then back down. What you are after is a trapezoidal velocity profile. Once you've achieved that you can experiment with more exotic concepts in the motor control menagerie. There is no known way around the second area of zero torque that I know of. One more thing. The frequency of the first resonance will change depending on things like friction and the inertial load on the motor shaft.

Berger Lahr Servo

So any testing you do to locate the resonance should be done under realistic load conditions.