AN414

Application description

Doc ID 1876 Rev 2

9/14

Figure 7.

Voltage Compensation (Dynamic performance: motor current ripple

reduction (1 A/Div, 2 Ms/Div)

3.4

Motor power limitation

The purpose of the power limitation is to keep the motor within its safe operating area in

case of excessive loading (avoid overheating and hard brush switching). The motor current

is evaluated every 3 milliseconds by measuring the peak chopped current in the IGBT

emitter.

If we call d the duty cycle of the PWM signal, Ud the motor DC supply voltage and Imot the

average current in the motor, the electrical power fed to the motor is:

P = Vmot x Imot = d x Ud x Imot

If we want to limit P to a maximum value Pmax (300 W in our example), we must keep the

duty cycle below a maximum duty cycle "d_max":

d £ d_max with d_max = Pmax / (Ud x Imot)

The program repetitively measures Imot and Ud using the A/D converter of the ST6265C.

Then it extracts d_max from a table addressed by Ud and Imot. It then compares d to d_max

and decreases d if it is larger than d_max.

The power limitation presents "smooth" transitions between "limited" condition and

"non-limited" condition, in order to avoid oscillations or jerks.

Imotor av = 2.5 Amps ; Vmotor = 105 V

Uac = 230 V ; C = 100 μF

2 ms/dv ; 1 A/dv

0 Amps --

0 Amps --

ΔI = 2 Amp

ΔI = 1 Amp

MS19695V

1