About the relay we put in a word
- He was killed by the starting current!
The most frequently used component for switching circuits is the good old electromagnetic relay. However, choosing the right relay is often not an easy task. Well, let's try to cover this issue.
And what is the problem? Well, we calculated the current in the load and took the relay with the appropriate rating. But such a calculation may be mistaken: the whole problem lies in the starting currents.
Short-circuit starting currents
Let us arm ourselves with an oscilloscope, a Hantek CC-65 current clamp for it, a HWPT07 voltage transformer for galvanic isolation of the oscilloscope from the circuit being measured (it is not necessary, but convenient to look at where the voltage sinusoid switches) loads.
For convenience, the measured currents are normalized to the conditional “rated” current of the device, which is determined by the formula as the power indicated on the device divided by the mains voltage (230V) and multiplied by the root of two (current peak value) -
"Purely resistive load, what are the starting currents?" - Asks the reader.
But no, the spiral of the lamp is cold; it has much less resistance. The 95 W incandescent lamp has a resistance of 40 Ohms, which gives an estimated starting current up to 320 V / 40 Ohm = 8 A, that is, 13 times more than the rated current!
We see that the starting current exceeds the nominal value by 8 times, the heating time of the helix is less than one half-period, and the peak duration is approximately 2 ms.
Warm floor. Kettle, heaters email. boilers
The temperature coefficient of nichrome spirals in heating elements is very small, and the starting current is close to the rated current.
The exception is self-regulating cables for underfloor heating. They have a semiconductor heating element, its starting current can be more than 2 times.
LED and compact fluorescent lamps
“A-ha-ha, what are the currents of a 10-watt light bulb!”
Such small-power lamps contain a rectifier bridge with a capacitor. That is, it is a purely capacitive load, and the starting current must be very large. As a rule, to reduce it, manufacturers put a resistor in front of the bridge.
Let's look at the charts:
It can be seen that everything is very good with Ikeev’s lamps. But with other LED lamps, the starting current exceeds the nominal value of 150 - 200 times, and the duration of the peaks is ~ 100 μs.
“The inductive load has zero starting current! This is inductance! ”
Well, at the moment of contact closure, the current really grows smoothly, but then:
1. if the moment of short circuit hits zero voltage, then the current surge is twofold (for a purely inductive load);
2. until the engine reaches the rated speed, the current exceeds the rated current several times; the more powerful the engine, the greater the excess.
Similar to the LED lamps at the input of these power supplies is a diode bridge and large capacitors. To reduce the starting currents, manufacturers put NTC thermistors, green (sometimes black) and round:
In a cold state, they have a noticeable resistance, which limits the starting current. When the power supply operates, the thermistor heats up and its resistance decreases (20–30 times), almost without interfering with the flow of current. But after turning off the power supply for some time (up to 1 minute) the thermistor remains hot and cannot limit the starting current. Therefore, it is highly desirable to wait 10 to 30 seconds after turning off the power supply before turning it on again. Below are graphs with repeated switching on in ~ 15 s (with fast switching peaks even more):
How to live with this knowledge?
The relay documentation can indicate several currents:
nominal current (Contact rating current) and maximum switching current (Max. Switching current) or inrush current (Inrush current), etc. Also, in “ordinary” relays, the starting current is often not indicated. About unusual write below. That is, if “10A” is written on the relay, then it means that, by default, it also has an inrush current when switching does not exceed 10A. Perhaps it can be multiplied by 2, but this is not accurate.
If the maximum starting current is 10-20A, and the LED light bulb has a starting current 100 times the nominal, then it is very sad: it turns out that only 20-40 W of light bulbs can be switched.
So with conventional relays, you either need to limit yourself greatly in selecting the load and lower the power, or be prepared for the fact that the contacts will often weld and the relay will have to be changed. For loads with large starting currents, it is better to use special relays.
And now a minute of advertising about the wonderful, most excellent relay modules of our production in the world with Modbus RTU control of the WB-MR3LV / I and WB-MR3LV / S series and their versions with HV inputs, as well as the WB-MRPS6 modules. In them we put the relay HF115F-I and HF115F-S of the manufacturer Hongfa (the largest in China and the fourth in the world manufacturer of the relay).
The distinction of the HF115F-I relay is special contacts from AgSnO2, and the HF115F-S also has a special design of two pairs of contacts when the first pair (tungsten contacts, high pulse current) closes a little earlier than the second (low contact resistance, high DC).
The photo shows the contacts of the relay HF115F-I (left) and HF115F-S (right).
Starting current HF115F-I - 120A / 20ms, which allows switching incandescent lamps with a total capacity of up to 2 kW.
And HF115F-S - 165A / 20ms and 492A / 1.5ms, 800A / 25mks, that is, up to 3 kW for incandescent lamps and up to ~ 600 W for LED lamps.
Photos of these remarkable relay modules:
And there is also a WB-MRWL3 relay module with a HF161F-W relay: its feature is a large rated current, which allows it to work with the 16A circuit breaker and use socket groups for switching.