Question:Can it happen that a motor operated with the purely controlled method 'V/f characteristic control' can achieve a higher maximum torque than when operating the same motor in the current-controlled motor mode 'servo control'?
How could this be justified?
Answer:
In fact, under certain circumstances, a higher maximum torque can be achieved with V/f characteristic operation than with servo control. This was demonstrated by tractive force measurements on a test rig with the possibility to change the load in a way suitable for comparative measurements. With the motor current set at the same level, a slightly higher torque was achieved in V/f characteristic operation.
This torque behaviour can be explained by the different philosophies of the two methods:
V/f characteristic control:
This purely controlled method is used with simple frequency converters. The motor is fed with an AC voltage and its 'voltage amplitude' and 'frequency' are set. As a result, and depending on the current load of the motor, the motor current is adjusted. The motor 'automatically' finds an optimal balance between the excitation and effective components of the motor current, so that maximum torque is produced.
The small advantage in terms of achievable torque is offset by disadvantages in terms of dynamic drive behaviour. In the case of rapid load or setpoint changes, e.g. through superimposed control loops, the possibilities for control intervention in V/f operation are significantly more limited than with servo control. In particular, the control of the motor current takes place in a relatively slow control loop.
Servo control (current-controlled method):
Servo control is a current-controlled method and therefore requires corresponding sensors. The hardware of a servo inverter is therefore correspondingly more complex. Servo control, however, offers much faster intervention options, because in addition to the voltage amplitude and the frequency, the phase position of the current can also be adjusted. In particular, the possibility to intervene on the phase position enables very fast current and thus torque changes. This in turn is a prerequisite for dynamic drive behaviour, which is necessary if the superimposed speed and position control loops are to be / must be dynamically adjusted.
The servo control is based on a motor model to emulate the real motor and controls the magnetising and the effective components of the motor current independently of each other, which significantly improves the dynamic characteristics.
With regard to the achievable torque, the motor parameters entered are of great importance. The servo control imposes an operating point on the motor, which results from the motor model. If there are deviations between the set and real motor parameters, this operating point can be more or less next to the optimum operating point in terms of torque.
In particular, the parameter 'rated motor speed' has a great influence here.
If this value deviates in the range of 5 to 10 rpm from the actual rated speed, then a greater reduction of the achievable torque is plausible.
The steeper the torque characteristic curve of the motor, i.e. the lower the nominal slip, the greater the influence.
Recommendations for measurements and empirical optimisation for servo control:
- Vary the rated motor speed in small steps and simultaneously observe the torque behaviour.
- Possibility to determine the rated speed: Load the motor in V/f operation with rated torque and measure the speed. Then set this as the rated speed.
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