Reasons for inrush current in inductive loads

AN441

2/16

Doc ID 3579 Rev 3

1

Reasons for inrush current in inductive loads

1.1

Inrush current due to inductive load behavior

Many inductive loads are controlled in full-wave mode. This is the case of valves, pumps,

compressors, etc. For these loads, the inrush current greatly depends on turn-on delay at

start-up.

We do not consider, in this section, the effect of magnetic circuit saturation that could also

lead to inrush current increase. Refer to Section 1.2 for this point.

A typical inductive load, controlled by an AC switch, can be simulated using a standard RL

circuit (Figure 1).

Figure 1.

Inductive load control with Triac

According to Figure 1, the AC load current i(t) is define by Equation 1.

Equation 1

Considering the circuit in sinusoidal full-wave mode, with turn on at zero mains voltage, the

value of inrush current is:

Equation 2

In case of a delay applied between zero voltage and Triac triggering in the first half-cycle

(assuming following cycles are in full-wave mode), the value of inrush current is:

Equation 3

R

T

V

MAIN

L

i(t)

u(t)

dt

(t)

di

L

(t)

R·i

u(t)

+

=

()

()

()

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

ω

+

ω

ω

−

ω

+

ω

=

−

t

·

·sin

R

t

·

·cos

·

L

·e

L·

·

R

·

L

2

U

i(t)

t

·

L

R

2

2

RMS

()

()

()

()

()

()

(

)

()

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

ω

+

+

ω

ω

ω

−

ω

ω

+

ω

=

−

0

0

t

·

L

R

0

0

2

2

RMS

0

t

t

·

·sin

R

t

t

·

·cos

·

L

-

·e

t

·

·sin

R

t

·

·cos

·

L

·

R

·

L

2

U

)

t

i(t,