How insidious extensions and carrying do not allow power tools to work

Surprisingly, a fact. Yesterday, a good friend of mine, a homemade friend of mine, called me to see why the circular did not start. He says that before she worked perfectly, then a neighbor took her for a while, and now the circular does not start. The peculiarity of the machine was that it installed a two-kilowatt three-phase electric motor, included in a single-phase network according to the "triangle" scheme, with two blocks of capacitors - working and starting.

To determine the malfunction, we first measure the resistance of the motor windings. The resistance of the windings is usually tens of ohms. In this case, the resistance very quickly changes from zero to the maximum value. This is affected by the effect of capacitors. While they are charging, the resistance drops to zero. As the capacitors charge, the resistance increases, and when the capacitors are fully charged, their resistance equals infinity, so the ohmmeter shows the resistance of the motor windings.

After making sure that there are no short circuits and leaks to the ground, we connect the voltage. Turn on the power for 1-2 seconds, so as not to burn the windings and wiring, because starting currents of powerful engines are very large. For these seconds, measure the voltage on the motor windings.

In my case, the voltage turned out to be 160, 120 and 108 volts on three windings, respectively. Of course, this voltage is simply not enough for the operation of such a powerful electric motor.

We are looking for where the tension went. There are no leaks on the case (we measured the resistance between the machine case and all the terminals in advance, and made sure that it is equal to infinity). Dirty and burnt contacts, of course, can create resistance, so we measure the voltage after the starting contacts and in front of them. In my case, the voltage turned out to be the same - 160 V. Moreover, before starting the voltage before the contacts is 230 V, and during starting it drops to 160 V.

It turns out that when we turn on the machine, we "drop" the voltage throughout the house. This is very dangerous for household appliances. We go into the house and measure the voltage in the socket before and during the start of the machine. We get respectively 230 and 210 V. Sagging, of course, is, but not critical. For household appliances - safe. So where did the 50 volts needed to start the machine go?

And they were lost in the extension cord. The wire used for the extension cord was stranded, thin and very long. Its resistance is 5 ohms.

Let's look at how this resistance affects voltage drop. As we remember from the school physics course, the voltage drop during the series connection of consumers is defined as the product of the current in the conductor by its resistance.

When you connect a 100-watt light bulb to the extension cord, the current in the circuit is 100/220 = 0.45 amperes. The voltage drop in the extension cord will be 0.45 * 5 = 2.5 volts. As you can see - this is not scary either for the bulb or for the extension cord.

When connecting a powerful consumer (machine, heater, etc.) with a power of 2000 watts, the current in the circuit is 2000/220 = 9.1 amperes. The voltage drop is 9.1 * 5 = 45.5 volts. Those. we supply 210 volts to the extension cord, and remove only 160.5 volts from it. For an electric motor, this voltage is not enough to work (but enough to burn the windings is enough). But the two-kilowatt heater will give out much less than the calculated power (about 1.1 kilowatts).

BUT! The loss of voltage in the extension cord is not lost without a trace! The extension cord is very hot. Which can lead to melting of the insulation of the wire and internal circuit (or even to a fire of the insulation).

Be CAREFUL and CAREFUL!

Read also on this topic:Why it is dangerous to use tees and extension cords in an apartment

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Comments

[#1 Maniac_Schekatillo]

And at 160.5 V, the current in the circuit will be 12.5 A. And if the extension cord is made of a section, for example, 1 mm2, then such a current is almost maximum for it. Those. the conductor is already warming up. And if he is also wound into a coil, then he will fuse into a single ball, do not go to your grandmother. And sooner or later you will feel the incomparable aroma of burning insulation with nothing.

Therefore, you must always remember. that the longer the extension cord at the same load, the larger the cross section it should be. And if the load can also increase significantly - then extinguish the light, throw a grenade ...

In other words - do not use extension cords thoughtlessly.

By the way, I just saw the right article on this subject - How to calculate the cable for the extension cord.

[#2]

Good day. Very informative article! Just can not understand why the motor winding can burn out at low voltage? Shouldn't it be the other way around?
Thanks.

[#3 MaksimovM]

Ross, for the same power consumption - the lower the voltage, the higher the load current flowing through the winding and vice versa. For example, with a single-phase network voltage of 220 V, the load current of a consumer with a power of 3.5 kW will be approximately 16 A, and if the voltage is 160 V, then the current with the same power consumption of 3.5 kW will be already 22 A. Thus If the motor rotor winding is not designed for such an excess of current, then it may burn out.

[#4]

MaksimovMIs this true for all consumers? For example, can a simple computer fan burn out if less voltage is applied to it?

And then - the article says that the engine did not start, because there wasn’t enough voltage, but if he nevertheless ate more amperes, why didn’t he start?

[#5 MaksimovM]

Ross, I didn’t mean that he would burn out right away. Each appliance is designed to operate at a specific voltage. If the mains voltage systematically goes beyond the limits of permissible values, then sooner or later one or another electrical appliance will fail.

And at the expense of the engine, I think it was about the fact that with a reduced voltage the electric motor will not work in normal mode, that is, it will provide the declared characteristics, for example, there will be a lower speed of rotation.

[#6 Evgeny]

Good afternoon. I have an induction cooker. She worked for about four years. But in the last month, instead of two kilowatts, it gave out 1.3. I bought a new one, but it also gave out 1.3. kilowatts, on the stove there is an opportunity to see. I didn’t know what to do. I started to google and found your article, it turned out that the children changed the extension cord. He returned his back and lo and behold! Kilowatts are back. Thanks you!