What are the limitations for the observer + PLL for a low speed PMSM?

[DTard.1]

In my application, I am driving a low voltage, low speed PMSM with a custom power electronics board. Using motor control workbench, I have a configuration where I can reliably start the motor and it spins up to speed.However, the reported speed thorugh the console seems lower than actual speed. For example, the console reported speed is oscillating between 100 - 150 RPM, but the measured frequency of the current waveforms, as well as the observer electrical angle indicate a frequency close to 20 Hz. The motor is a 10 pole motor; therefore I expect 240 RPM at 20 Hz.

I am in the process of trying to independantly confirm the speed using an encoder, but this test setup is ready yet.

Once question I have is my operating speed too low for the observer? Is there some information on the observer available that defines its ideal operating range?

Finally, is there a way to set the motor control to run open loop?

[GMA]

Dear DTard,

We are sorry for responding so late and we are trying to improve our presence on the forum. If your point is still pertinent, it is quite difficult to give a firm value of a minimum speed limit as too many parameters are to be taken into account. Internally, a speed reliability, based on quadratic error, is computed in "STO_PLL_CalcAvrgMecSpeedUnit()" function. 
"Open Loop" feature is described in "Open loop description and usage" chapter of documentation "User Manual".