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wave commutation, but some manufacturers use other means to express Ic. Typically, servo motors are rated to establish maximum continuous capacity with heat distribution of internal winding losses that’s even throughout the motor. This means during the rating process, electrical cycles within the motor move at a speed fast enough to make for even distribution of heat due to internal losses … but slow enough to ensure jXL and core losses are essentially zero within the motor. Recall that jXL is inductive reactance. This physical test speed is usually around 1 to 4_rps (revolutions per second) for motors but may be slower or higher — as a function of the motor’s pole pairs. Most manufacturers define continuous Torque (Tc) and resulting current (Ic) at this or similar speeds. The published continuous torque and current specifications are also often identified as stall torque (Tc_stall) and Stall current (Ic) regardless of the type of commutation and resulting current units.
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Note the difference between stall as it’s used in the context of controlled closed-loop dyno tests (for determining a servo motor’s maximum continuous capability) versus our definitions for stall, stalled, and stalling as they describe overload conditions exceeding an induction motor’s maximum capability —whether the rotor has stopped moving or not. Also consider what the servo drive’s output is doing, when a servo motor is held under some load with little or no movement. Because we are using sinusoidal commutation
for our example, the controlled threephase ac output is effectively at a standstill — presenting a continuously PWMed non-moving three-phase output with a value (appropriate to the servo motor’s commutation position) equal to what would essentially be a frozen signal at that position location. This may be envisioned in your mind’s eye as an instantaneous snapshot of the moving sinewaves. The main difference between the two major commutation methods is that: • Six-step or Block (unmodified trapezoidal) commutations only allows current flow through two of the three motor phases at any given time — for 2-ON 1-OFF at all times • Sine-wave or sinusoidal commutation allows for current flow through three motor phases at the same time (when
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