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15

Introduction

This application note describes how to tune the embedded control loops on the IRMCF3xx series

of control IC’s. This covers the IRMCF312, IRMCF311 and IRMCF343 digital control IC’s for air-

conditioning systems and the IRMCF341 IC for washing machine applications. The control loops

include the ID and IQ current control loops, the velocity control loop, the field weakening loop,

IPM control and bus voltage protection. The note describes the equations used to derive the

control loop constants and experimental methods to tune control constants.

There are 3 main control loops associated with IRMCK3xx series products. These control loops

are current control loop, speed control loop and Field-weakening control loop. The following table

summarizes the parameter dependence of each control loop.

Parameters

Motor Inductance

Motor Resistance

Voltage constant (Ke)

System Inertia

Current

Controller

Yes

Yes

No

No

Speed

Controller

No

No

Yes

Yes

Field-Weakening

Controller

Yes

No

Yes

No

Motor parameters are required (Parameter Configurator) for drive setup. These parameters are

normally obtained from motor data sheet. IR Sensorless motor controller can tolerate +/-10 %

motor parameter error without noticeable performance degradation.

Increased parameter mismatch between motor and controller will result in degradation of torque

per Amp capability. The degree of degradation is dependent on the operating conditions (speed,

load) and motor characteristics (motor parameters and saturation).

Current Controller

The iMotion current controller utilizes Field-Oriented synchronously rotating reference frame type

regulators. Field-Orientation provides significant simplification to the control dynamics of the

current loop. There are two current regulators (one for d-channel and one for q-channel)

employed for current regulation. The q-channel (torque) control structure is identical to d-channel

(flux). The current control dynamics of d-channel is depicted in Figure 1. The motor can be

represented by a first order lag with time constant T = L/R. This time constant involves motor

inductance and equivalent resistance (R: cable + winding). For a surface mounted permanent

magnet motor, the d and q channel inductances are almost equal. In the case of Interior

permanent magnet motor, the q-channel inductance is normally higher than the d-channel

inductance.

In the current control dynamic diagram Figure 1, forward gain A translates digital controller output

to voltage (include inverter gain) and feedback gain B translates current feedback (Ampere) to

internal digital counts via A/D converter. The calculation of the controller gains (KxIreg, KpIreg_D)

is done by using pole-zero cancellation technique as illustrated in Figure 2 where the current

controller is rearranged to give transfer function block C(S). Setting KpIreg_D/KxIreg of C(S) to

the time constant of the motor (T), the controller zero will cancel off the motor pole (pole-zero

cancellation). Therefore, the controller dynamics can be further simplified as shown in Figure 3.

The equivalent transfer function of Figure 3 is a first order lag with time constant Tc. By selecting

appropriate current regulator response (typically 0.5 to 1 msec, entry of parameter configurator,

Current Reg BW = 1/Tc) for a particular application, the current regulator gains can be readily

obtained. It may be noticed that using pole zero cancellation technique, motor inductance enters

into proportional gain calculation and resistance enters into integral gain calculation.

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