Servosila-Device-Reference-0xA020192

Micro-Speeding:
Speed Limit
(Electrical
Frequency)
controller switches between Field Oriented Control (FOC) and Direct
Drive Control algorithms. If a commanded speed is less than the limit, the
controller uses Direct Drive algorithm. Otherwise, it uses Field Oriented
Control (FOC) method.
The reason this parameter exists is that Field Oriented Control (FOC)
algorithm does not work well at low speeds. This is so because of a range
of issues related to inaccuracies of sensing at low speeds. Various control
instabilities start manifesting themselves whenever a user issues a low
speed command to a drive operating under Field Oriented Control (FOC).
On the other hand, Direct Drive Control works reliably at low speeds, but
does not work well at high speeds. Thus the need to switch between the
control methods that compliment each other.
However, Direct Drive Control is far less energy efficient than Field
Oriented Control (FOC) in terms of heat generated by the motor. To enable
or disable Direct Drive Control, use a parameter named "Feature: Direct
Drive". If your application does not require operating the motor at very low
speeds, avoid turning on the feature.
NOTE: The value for this limit is automatically computed by the
"Spreadsheet" tool or by an auto-configuration procedure of the controller.
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9 Kickstart: Speed
Limit (Electrical
Frequency)
Hz "Kickstart" refers to a method of starting sensorless brushless motors. The
term "sensorless" means that such a motor is not equipped with Hall
sensors or an encoder.
The controller uses a clever mathematical method called "Sensorless
Observer" to deduce information about the rotor's position from seemingly
unrelated electric current measurements. The "Sensorless Observer"
method is only used whenever controlling a sensorless brushless motor
since no other source of information about the rotor's position is available.
Note that the position of the rotor has to be known to the controller to
properly position magnetic fields inside the motor using Field Oriented
Control (FOC) method, so that the motor produces torque instead of just
heat.
However, a magic behind the "Sensorless Observer" method works only
when the rotor is moving. If the rotor is not moving or not moving fast
enough, the controller does not know the position of the rotor, and thus
cannot properly position the magnetic fields inside the motor. The issue is a
kind of a "chicken or egg" problem. To move the rotor, the controller needs
to know the position of the rotor, but to know the position of the rotor, the
rotor needs to be already moving. This challenge only pertains to
"sensorless" motors, and does not apply to motors equipped with Hall
sensors or encoders that give the controller a usable reading of the rotor's
position at any speed. That's why the Hall sensors are used in the first
place. The "Kickstart" method solves the "chicken or egg" problem by
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