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Starting current

Starting current - how does it occur and what do you have to consider?

Starting current: guidebook, background information and valuable tips

In electrical engineering, the starting current (or inrush current) is the maximum instantaneous input current drawn by an electrical device when it is switched on. It occurs when a device is first turned on because the initial impedance of the device is very low, and the current can be many times higher than the continuous or rated current of the device. Starting current can be problematic in power supplies under certain circumstances because it can cause circuit breakers or fuses to trip.

There are several factors that contribute to starting current, including the capacitance of the device, the resistance of the circuit, and the voltage at the time of switch-on. Inductive loads such as motors can also cause inrush current.

To limit the starting current, there are technical solutions such as a soft start relay that slowly ramps up the voltage to the device, or current limiting resistors that limit the amount of current that can flow into the device. Selecting devices with lower inrush current ratings or designing circuits with lower capacitance can also help reduce starting current.

Why is the starting current that important for the power generator?

The term "starting current" comes up regularly in connection with emergency power generators. In addition, we know from our day-to-day business that especially newcomers to the world of mobile power generation often have a hard time with this topic or that there is a certain lack of knowledge in general. Non-compliance with a few important rules, however, can quickly become a major problem for any power generator, as in the worst case, capital damage is imminent. However, we want you to enjoy your DENQBAR product for as long as possible! For us, this is reason enough to pay a little more attention to the topic of starting current with this article.

Why is starting current so important?

Certain consumers (which ones, we will go into in more detail in the next section) need a multiple of the actual nominal power when starting. These power peaks are only called up for a short time, but they have to be handled by the power generator.

Conventional gensets, e.g. simple converters, can still handle these peaks relatively well. These devices do not have such advanced electronics as, for example, the more progressive inverter power generators. The simpler design of the converters is more resistant to overloads, but these devices cannot deliver a clean and harmonic sinusoidal voltage in power grid quality as the inverter power generators can. In general, the following can be stated: Excessive starting currents can definitely bring generators to their knees. Serious damage cannot be ruled out either.

That is why it is essential to look profoundly at the starting currents of the consumers. With the information on this page, the search for a sufficiently powerful generator will be much easier. But what types of consumers are there at all and do the dreaded starting currents occur with all of them?

Does starting current occur with all loads?

Basically, consumers can be divided into three groups:

Resistive loads

This class can include, for example, radiators, construction spotlights or hotplates. They are defined by the fact that the power consumed (watts) is converted into brightness or heat. That is why they are also called active power consumers. For the power generator, this class is the least problematic, because with resistive loads, the output power is exactly equal to the input power from the power generator. Or to put it in a nutshell: resistive loads have no starting current.

Inductive loads

The next large class are the so-called inductive loads. These include, for example, electric hand tools (chain saw, drill, compressor, circular saw, etc.). Here things get a bit more complicated. These devices have their own built-in motor and this first has to "get going" - and this is where the notorious starting current comes into play.

Let's imagine a bicycle that has to accelerate quickly from a standstill. This requires a little extra power in the calves. However, once the bicycle has gained momentum, it is noticeably more pleasant to ride. The startup behavior of an inductive load is similar.

Starting current - resistive and inductive loads

Therefore, before buying a power generator, it is important to be aware of the following points:

  • What is the rated power of the loads I want to connect?
  • Do these loads require a starting current during startup?

This information can be found in the respective manual. If necessary, contact the manufacturer, who will be able to provide the exact parameters. Tip: Allow for a little "room to grow" in the performance of the generator. 20 to 30 % are optimal. On the one hand, this protects the engine (= prolonging the product service life), and on the other hand, the operating noise of the generator is significantly reduced.

Capacitive loads

This is probably the most critical consumer category, but fortunately only very few will come into contact with these loads. Examples are discharge lamps or flashlight devices that have a charging function. To satisfy their appetite for electricity, it already requires special equipment: a Barber-Colman regulator. "Normal" power generators, which are mainly available on the market, will not be able to provide the required starting current of a capacitive consumer.

Can the starting current be calculated and how long does it last?

That would be nice, of course, but unfortunately it doesn't quite work that way. The individual consumers and their motors are built too differently. As a rough value, however, 2 to 6 times the actual rated power has become common.

In the following we would like to give a few concrete examples from our daily business to give a better picture of the dimensions:

  • Metabo chop and miter saw KGS 216 M, rated power 1500 W, 3 times starting current, therefore 4500 W for a short time
  • Gardena domestic water controller 4000/4 electronic, rated power 800 W, approx. 3.5 times starting current, therefore 2800 W for a short time
  • Makita chain saw UC4020A, rated power 1800W, 3 - 3.5 times the starting current, accordingly, for a short time up to 6300 W

But how long must a generator be able to handle these power peaks? The duration of the starting current differs greatly from consumer to consumer, so that unfortunately no exact specification is possible at this point. As a rule, it is "only" a fraction of a second; we are talking about milliseconds. Even in extreme cases, the motor reaches its target speed after a few seconds.

One way to measure the starting current is to use a clamp meter that can measure both AC and DC current. The meter is placed around one of the power supply lines and the current can be measured while the unit is turned on. Another method is to use a special inrush current meter that can measure the current waveform and calculate the peak value.

It is important to know that the starting current can be very high and can damage the meter if it is not designed to handle these high currents. So make sure that the meter you are using is suitable for the high currents and it is best to consult a specialist for this.

Is there any danger or problems with the starting current?

The best advice here is: getting enough information in advance is the key! This is the only way to make the right choice of a sufficiently dimensioned generator. The most important key data are the rated power of the consumers to be connected and whether a starting current occurs (inductive loads). In this case, studying the product manual or asking the manufacturer will help. With this information, you are on the safe side and an appropriately powerful emergency generator should be found quickly.

Starting current with and without limiter

Modern devices, such as Inverter generators, usually come with an overload protection. If the power demand (e.g. starting current) is too high, the fuse is triggered and switches off the generator before major damage occurs to the sensitive electronics. However, the overload protection should never be deliberately overloaded in the long term to avoid damage to the device.

The option of starting current limiters (also called soft switches or soft starters) should also be mentioned at this point. These devices limit the current peaks and thus protect the electronics of the generator from fatal overloads. You can quickly find them at specialized dealers and relevant online sales platforms.