ST AN441 Application note

AN441

Application note

Inductive load control with AC switches

Introduction

AC switches are now commonly used as static switches to drive inductive loads such as magnetic transformers, valves, induction motors, etc.

This application note describes the particular points to focus on when such loads are controlled by AC switches like Triac, ACS or ACST. For example, there is an explanation of just when a Triac has to be triggered to reduce the inrush current at turn on.

Typical examples are given for magnetrons used in microwave ovens, transformers for SELV halogen lamps, and universal motors used in vacuum cleaners.

March 2010

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Reasons for inrush current in inductive loads	AN441

1 Reasons for inrush current in inductive loads

1.1Inrush 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

i(t)

R

L

VMAIN

T

u(t)

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

Equation 1

u(t) = R·i(t) + L di(t) dt

Considering the circuit in sinusoidal full-wave mode, with turn on at zero mains voltage, the value of inrush current is:

Equation 2

	URMS 2			R
i(t) =			· L·ω·e− L		·t	− L·ω·cos(ω·t)+ R·sin(ω·t)
	2	2
	(L·ω) + R

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

	URMS 2				R
i(t,t0 ) =			·	(L·ω·cos(ω·t0 )− R·sin(ω·t0 ))·e− L		·t	- L·ω·cos(ω·(t + t0 ))+ R·sin(ω·(t + t0 ))
	2
		2
	(L·ω) + R

Where t0 is the triggering delay at the first turn on

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AN441	Reasons for inrush current in inductive loads

These two equations show that, when the inductive load is switched on in full-wave, the transient current depends on the Triac first turn-on delay with respect to the mains voltage zero point. Figure 2 shows the load current curve for triggering at zero voltage and triggering at the peak mains voltage. This figure comes from PSPICE simulation for a 150 ohm and 5 H load switched on at a voltage of 230 V rms at 50 Hz.

Triggering at zero voltage brings the highest inrush current which can be up to twice the peak current reached in case of triggering at peak voltage.

Figure 2. Inrush current difference according to Triac first turn-on delay

400					400
300					300
200					200
100					100	Mainsvoltage (V)
0					0
0	20	40	60	80	100
(mA)Current					-100
-100
-200					-200
-300					-300
-400					-400

Time (ms)

Turn-on at zero crossing Turn-on at peak mains voltage Mains voltage

Due to this high peak current, two problems may occur at AC switch level.

●High peak current may be higher than ITSM value (maximum surge peak current). In this case the component can be damaged.

●AC switch temperature may exceed the maximum allowed junction temperature (this will not lead necessarily to device failure but electrical parameters are not anymore guaranteed if working temperature is above max allowed value).

Inrush currents have also to be checked to fit electromagnetic compatibility standards. Actually, IEC 61000-3-3 standard make it mandatory to limit inrush currents of appliances connected to the power network to reduce the flickering effect on lighting.

It should also be noted that reducing inrush current helps to increase the reliability of the load and other switches or breakers used in series with the load.

1.2Magnetic core saturation due to remanent induction

In transient operation, the induction can follow a different path and reach the saturation value BS for which the magnetic field H increases very rapidly even for a low induction variation (see Figure 3). At saturation level the magnetic material permeability decreases drastically, down to air permeability. This leads to a lower inductance value. The load current is then mainly limited by the load resistance, and can increase substantially. Saturation then leads to a high increase of the coil current.

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Reasons for inrush current in inductive loads	AN441

Figure 3. Magnetic field H versus induction B (continuous rating)

B(t)	B
	Bs
	Br
t	H
i(t)
t

According to Equation 2, at start up at zero voltage the current is higher (longer time to integrate voltage and so higher induction reached in the 1st cycle) and so there is a higher risk of reaching magnetic core saturation.

Also a second phenomenon can increase risk of saturation. This phenomenon is due to remanent induction. The remanent induction (refer to Br in Figure 3.), corresponds to the point where H equals 0. If a positive voltage is applied from a point where there is a positive remanent induction, the induction will start to increase from a higher initial value, so will reach saturation faster (refer to appendix 1 for further explanation on this phenomenon).

To avoid this phenomenon in circuits controlled by an AC switch, device switch on has to be implemented on the reverse polarity according to previous switch off. Figure 4 shows two different test results carried out on a 200 VA 230 V to 12 V transformer. Curve A shows the current waveform, recorded after a previous identical current waveform. The particularity of this waveform is that the first half-cycle conduction is in the same polarity as the previous one. In this case the transformer reaches saturation very rapidly and the transformer behaves like a short circuit. The peak current is limited only by the series resistance of the transformer.

Curve B shows the same recording but here with the first half-cycle conduction in reverse polarity compared to the last one. These two curves clearly show that saturation is reached in case A due to previous conduction. Then load current can be approximately eight times higher than if care is taken to always trigger the device for an integer value of full-cycle periods.

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AN441	AC switch control with an inductive load

Figure 4. Last turn-off polarity influence on the next turn on

8.75 A

1.16 A

A B

2 AC switch control with an inductive load

2.1Latching current

The latching current IL of a Triac is the minimum value of the load current (circulating through terminals A2 and A1), to keep the device conducting when the gate signal is removed. Diagram a in Figure 5 shows a bad Triac turn on due to too short gate pulse width and Diagram b in Figure 5 shows a good Triac turn on. The pulse width is sufficiently large so that the current i(t) reach the latching current. See Application note AN303 for more information.

For inductive loads, as the current rate of increase is limited by the inductance, care has to be taken to have a large enough gate pulse width to reach IL (refer to Section 2.2).

Figure 5. Latching current of the AC switch

i(t)	i(t)
IL	IL

t	t
IG	IG
t	t
a	b

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