Siemens A1 User Manual
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Advanced Applications
MICROMASTER Applications Handbook
49
· Start with the PID gains still on their factory settings - P
gain
=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 P
gain
(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.
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.
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.
the time. Set P220=1 in such cases to avoid excessive DC current heating of
the motor.
8.2
Braking and Slowing down using Inverters
8.2.1
What 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.
the output frequency of the inverter falls rapidly, the motor may no longer ‘motor’,
but may act as a generator.
Speed, Frequency
New Operating
Point - Generating
Point - Generating
Positive
Torque,
Current
Torque,
Current
Reducing output
Frequency
Frequency
Negative
Torque,
Current
Torque,
Current
Normal Operating
Point - Motoring
Point - Motoring
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.
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.