In brushless dc machines the current commutation between phases is triggered based on information of the permanent-magnet flux position. In most BLDC drives, either sensors are used to detect this flux position or a self-sensing algorithm is applied to estimate the flux position instead. Over the last decades, different techniques have been developed to detect the current commutation instants from the speed-induced back-emf signal. Several of these methods when used in open-loop speed control show speed variations at an increasing acceleration. These speed variations are often reduced by closing the speed control loop that adapts the current amplitude to generate a torque for stable speed operation. This paper studies the open-loop behaviour of back-emf based self-sensing BLDC drives and gives a criterium for which the average speed acceleration over a time period is zero. The criterium depends on the machine inertia, speed and torque and indicates the robustness of the self-sensing method against speed variations. Fulfilling the criterium reduces the burden on the speed-controller to guarantee stable speed operation. Even in the ideal case where noise on the measured back-emf is absent and small voltages can be measured accurately, the application of a self-sensing method can be hampered due to a lack of robustness against speed variations if not well-designed.