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Advanced Applications

·Start with the PID gains still on their factory settings - Pgain =1, no integral or differential action.

·Select fast ramp up and down times as otherwise these will limit closed loop performance.

·Make sure the scaling parameters match the feedback signal and the expected range of the transducer.

·Increase the Pgain (P2280) until the system starts to oscillate, possibly looking at the value of the feedback if the physical effects are not obvious. Reduce the value of P2280 to 35% of that where oscillation started.

·Increase the integral gain P2285 until the system oscillates again. Reduce the value to 50% of that where oscillation started. This quick setting method will give good results in most applications. More precise setting methods would normally involve using an oscilloscope to look at the sensor signal response to step changes in the setpoint.

·Note that systems such as fan cooling may require the motor to be ‘off’ most of the time. Set P220=1 in such cases to avoid excessive DC current heating of the motor.

8.2Braking and Slowing down using Inverters

8.2.1What happens when a motor is stopped?

When the output frequency of the inverter is reduced, the motor will slow down. If the output frequency of the inverter falls rapidly, the motor may no longer ‘motor’, but may act as a generator.

Positive

Torque,

Current

Normal Operating

Point - Motoring

 

Reducing output

 

Frequency

 

Speed, Frequency

Negative

New Operating

Torque,

Point - Generating

Current

Figure 8-2 Graph showing the Motor acting as a Generator

If the motor and load have a high inertia, the motor will take longer to slow down so generation is more likely to occur. The generated energy returns to the inverter (i.e. regenerates) as a negative current. This is known as regeneration. The current is returned to the DC link, but cannot return to the supply because of the blocking action of the input rectifier. Therefore the current charges the DC link capacitors and if the DC link voltage becomes too high the inverter will protect itself from overvoltage by tripping. If the inverter trips there is no more motor flux, so it no longer regenerates and comes to an uncontrolled stop. However, there are several possibilities to control braking and stopping using an inverter.

MICROMASTER Applications Handbook

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