AN1172

Inductive loads on/off control

7/23

Pumps and Fans are, for the most part, induction or permanent magnet motors. Their series

inductance is in the range of one Henry, and their winding resistance equals a few hundred

Ohm. Their power factor is low. Hence, after switch turn-off, the reapplied voltage across it is

high and appears with a high rate of increase (as described in Equation 1 where L and cos

ϕ

are the inductance and power factor of the load, V the mains RMS voltage and C is the ACS

capacitance value).

Equation 1

Figure 6. shows that the (dV/dt)c rate for an ACS402-5 die without any snubber, controlling a

230 V 220 mA pump, is lower than 10 V/µs. The measure will be similar with an ACS108-5

die because it presents the same capacitance value as an ACS402-5.

Equation 2 shows that the current rate of decrease is almost half the RMS current (0.44 ratio

for a 50 Hz mains frequency and 0.53 for 60 Hz).

Equation 2

To summarize, it can be said that the worst case commutation appears with pumps or fans.

In this case, the stress that ACSs must withstand is:

Equation 3

Figure 6.

230 V 220 mA RMS pump switch-off

()

)

F

(

)

H

(

6

)

V

(

)

µs

/

V

(

C

L

10

sin

2

V

c

dt

/

dV

−

≅ϕ

3

)

Hz

(

)

A

(

RMS

)

ms

/

A

(

10

f

2

I

2

c

dt

/

dI

-

≅π

()

⎩

⎨

⎧

≤

≅

µs

/

V

10

c

dt

/

dV

I

5

.

0

c

dt

/

dI

)

A

(

RMS

)

ms

/

A

(

dV/dt = 8,7 V/µs

Iout (10 mA/div)

Vout (50 V/div)