Typical examples of inrush current reduction

AN441

10/16

Doc ID 3579 Rev 3

3.3

Induction motor

Induction motors or universal motors are today widely used in home appliances for

applications as such as vacuum cleaners, washing machines, blenders, and fridge

compressors. The typical load power is in the range of 100 to 3000 W. These motors

present a very high inrush current due to the inductive current in case of turn on at zero

voltage.

This inrush current is also increased in the case of a universal motor as the back EMF is

zero at zero speed. For zero speed the line current will then be limited only by motor winding

inductance and resistance, but not by the back EMF that can reach 10 to 50% of line

voltage. So applying the whole line voltage at motor start-up will cause a much higher

current than applying the same voltage when the motor is running at full speed.

A better solution is to implement a soft-start. This consists of progressively increasing the

conducting angle of the AC switch.

Figure 10.

Over current at Triac turn on for universal motor

Table 1.

Halogen lamp test through a transformer

Turn-on moment

Output power

(12 V halogen lamps)

Inrush current

Transformer 1

150 - 300 VA

ZVS

275 W

17.52 A

1.26 A

ZCS

275 W

8.65 A

1.26 A

Transformer 2

100 - 210 VA

ZVS

200 W

20.32 A

1.3 A

ZCS

200 W

5.32 A

1.3 A

Transformer 3

200 VA

ZVS

200 W

7.48 A

1.12 A

ZCS

200 W

2.83 A

1.12 A

-80

-60

-40

-20

0

20

40

60

80

0

100

200

300

400

500

600

700

800

Time (ms)

With soft start

Without soft start

I(A)