In constant slip-speed control, the drive system is designed so as to accept a torque command input and hence the system demands an additional feedback loop requiring the use of speed sensor. The method is highly robust with respect to changes in machine parameters and results in high efficiency of both the machine
itself and the inverter at the cost of somewhat sluggish response in closed-loop speed control situations.
itself and the inverter at the cost of somewhat sluggish response in closed-loop speed control situations.
The constant slip-speed control is inherently a current source based control strategy which offers the advantage that as the current is readily controlled and limited, the drive becomes extremely robust. However, this comes at an expense that the control strategy requires phase current feedback.
One of the simplest strategies for current control operation is to utilize a fixed slip-frequency, defined as :
where ωs1 is the slip speed in rad/sec.
ωr is the rotor speed in rad/sec.
which suggests that many different optimization of machine performance can readily be obtained by appropriately selecting the slip frequency, including achieving the optimal torque for a given value of stator current (maximum torque per ampere) as well as maximum efficiency.
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