Advanced Applications

 

 

 

International English

 

 

Ramp - up time for

Setpoint

 

 

 

 

 

 

Setpoint

 

 

 

 

 

Setpoint Source

P2257

P2260

 

 

 

 

 

P2253

 

 

 

 

 

 

 

KK

+ Ramp down time for

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Setpoint

 

 

 

 

 

 

 

P2258

 

 

 

 

 

 

 

 

 

 

P Gain

I Gain

 

 

 

 

 

 

P2280

P2285

PID in upper limit/lower limit

 

 

 

 

 

 

 

 

 

Transducer

 

 

P53.10/P53.11

 

 

 

 

 

 

 

 

 

 

 

Type

 

 

 

 

 

Maximum

y

P2271 Error

Kp

Tn

y

 

Feedback Source

feedback threshold

+

P2273

 

 

 

 

P2267

 

 

 

PID Output

P2264

x

 

 

 

x

 

 

 

 

r2294

KK

 

-

 

 

 

 

Minimum

 

 

 

 

 

 

 

 

 

 

 

 

feedback threshold

 

 

 

 

 

 

 

P2268

 

 

 

 

 

 

PID Output UpperLimit

P2291

PID Output Lower Limit

P2292

P2293

Accel/Decel time of PID limits

Figure 8-1 A Typical Closed Loop Controller

Figure 8-1 shows how the closed loop controller is implemented in the MM420.

Note..

That all setpoints are now expressed in percentage. When controlling a Setpoint, the frequency is not the desired quantity.

The setpoint usually derived from a fixed frequency has adjustable ramp rate that can be set before being compared with a feedback signal. The feedback signal, which is usually a 0-10 V or 4-20 mA signal derived from a transducer connected to the analog input, is also pre-processed with threshold limits. The difference between the set point and the feedback signal (the error) is then amplified and filtered using gain and integration constants. These are usually known as the P and I terms. In this case a differential term (D) is not included as it is rarely needed. Before the P and I processing the error signal is optionally corrected (“transducer type”). This correction is to determine whether to speed up, or slow down the motor to reduce the feedback signal. That is, increasing the speed of a vacuum pump will decrease pressure; increasing the speed of a compressor will increase pressure.

After processing with the P and I terms, the resulting signal is passed to the inverter which increases or decreases the output frequency as required, subject to certain limitations, such as acceleration and deceleration times.

8.1.3Setting up a feedback control system

Some general points.

·Remember that once closed loop operation is enabled values such as the setpoint are displayed in percent of full scale.

·If possible run the drive open loop first, to check, in particular, the sensor feedback voltage or current.

·Check P2271 value for the correct ‘sense’ operation. Set P2271 according to the sensor/actuator types. If the feedback signal reduces as the motor speed increases, select P2271=1. Otherwise, use P2271=0.

48

MICROMASTER Applications Handbook