Connecting a three-phase motor to a single-phase and three-phase network

Of all types of electric drives, the most widespread are asynchronous motors. They are unpretentious in maintenance, there is no brush-collector unit. If you do not overload them, do not wet them and periodically service or change the bearings, then it will last almost an eternity. But there is one problem - most of the asynchronous motors that you can buy at the nearest flea market are three-phase, as they are intended for use in production. Despite the tendency to switch to three-phase power supply in our country, the vast majority of houses are still with single-phase input. Therefore, let's figure out how to connect a three-phase motor to a single-phase and three-phase network.

What is a star and a triangle in an electric motor

First, let's figure out what the winding connection diagrams are. It is known that a single-speed three-phase asynchronous electric motor has three windings. They are connected in two ways, according to schemes:

• star;
• triangle.

Such connection methods are typical for any type of three-phase load, and not only for electric motors. Below is how they look in the diagram:

The supply wires are connected to the terminal block, which is located in a special box. It is called Brno or Borno. The wires from the windings are brought out into it and fixed on the terminal blocks. The box itself is removed from the motor housing, as well as the terminal blocks located in it.

Depending on the design of the engine, there may be 3 wires in brno, or maybe 6 wires. If there are 3 wires, then the windings are already connected in a star or delta pattern and, if necessary, it will not be possible to reconnect them quickly, for this you need to open the case, look for a connection point, disconnect it and make bends.

If there are 6 wires in brno, which is more common, then you can connect the windings as you see fit, depending on the characteristics of the motor and the voltage of the supply network (read about this below). Below you can see the brno and the terminal blocks that are installed in it. For the 3-wire version, there will be 3 studs in the terminal block, and for the 6-wire version there will be 6 studs.

The beginnings and ends of the windings are connected to the studs not just "at random" or "as convenient", but strictly in a certain order, so that with one set of jumpers you can connect both the triangle and star. That is, the beginning of the first winding over the end of the third, the beginning of the second by the end of the first and the beginning of the third over the end of the second.

Thus, if you put jumpers on the lower contacts of the terminal block in the line, you get a connection windings in a star, and by installing three jumpers vertically parallel to each other - the connection triangle. On motors "in the factory configuration" copper bus bars are used as jumpers, which is convenient to use for connection - there is no need to bend the wires.

By the way, on the electric motor brna covers, the correspondence of the location of the jumpers to these schemes is often applied.

Connection to a three-phase network

Now that we figured out how the windings are connected, let's figure out how they connect to the network.

6-wire motors allow switching between windings for different supply voltages. This is how electric motors with supply voltages became widespread:

• 380/220;
• 660/380;
• 220/127.

Moreover, the higher voltage is for the star connection, and the lower voltage is for the triangle.

The fact is, the three-phase network does not always have the usual voltage of 380V. For example, on ships there is a network with an isolated neutral (without zero) at 220V, and in old Soviet buildings of the first half of the last century, even now, sometimes there is a network of 127 / 220V. While a 660V line voltage network is rare, it is more common in production.

You can read about the differences between phase and line voltage in the corresponding article on our website: https://samelectrik.ru/linejnoe-i-faznoe-napryazhenie.html.

So, if you need to connect a three-phase electric motor to a 380 / 220V network, inspect its nameplate and find the supply voltage.

Electric motors on the nameplate which indicate 380/220 can only be connected with a star to our networks. If instead of 380/220 it is written 660/380 - connect the windings with a triangle. If you are unlucky and you have an old 220/127 motor - you need either a step-down transformer or a single-phase a frequency converter with three-phase output (3x220). Otherwise, it will not work to connect it to the three phases 380/220.

The worst case scenario is when the rated voltage of a three-wire motor with an unknown winding connection pattern. In this case, you need to open the case and look for the point of their connection and, if possible, and they are connected according to the triangle scheme, remake them into a star scheme.

We figured out the connection of the windings, now let's talk about what are the schemes for connecting a three-phase electric motor to a 380V network. The diagrams are shown for contactors with coils with a rated voltage of 380V, if you have coils for 220V, connect them between phase and zero, that is, the second wire to zero, and not to phase "B".

Electric motors are almost always connected via magnetic switch (or contactor). You can see the connection diagram without reverse and self-pickup below. It works in such a way that the engine will rotate only when the button on the control panel is pressed. In this case, the button is selected without latching, i.e. makes or breaks contacts while held down, like those used in keyboards, mice and doorbells.

The principle of operation of this circuit: when the "START" button is pressed, current begins to flow through the coil of the KM-1 contactor, as a result, the anchor of the contactor is attracted and the power contacts of the KM-1 are closed, the engine starts work. When you release the START button, the engine will stop. QF-1 is circuit breaker, which de-energizes both the power circuit and the control circuit.

If you need you to press a button and the shaft begins to rotate, instead of a button, put a toggle switch or a button with fixation, that is, the contacts of which, after pressing, remain closed or open until the next pressing.

But this is rarely done. Much more often, electric motors are started up from remote controls with non-latching buttons. Therefore, one more element is added to the previous circuit - a starter (or contactor) block-contact connected in parallel to the "START" button. Such a scheme can be used to connect electric fans, hoods, machine tools and any other equipment, the mechanisms of which rotate in only one direction.

How the circuit works:

When the QF-1 circuit breaker is turned on, voltage appears on the power contacts of the contactor and the control circuit. The STOP button is normally closed, i.e. its contacts open when pressed. Through the "STOP" voltage is applied to the normally open "START" button, the block contact, and ultimately the coil, so when you press on it, the coil control circuit will de-energize and the contactor will be de-energized.

In practice, in a push-button post, each button has a normally-open and normally-closed pair of contacts, the terminals of which are located on different sides of the button (see. photo below).

When you press the "START" button, the current begins to flow through the coil of the contactor or starter KM-1 (on modern contactors are designated as A1 and A2), as a result, its armature is attracted and the power contacts KM-1. KM-1.1 is a normally open (NO) auxiliary contact of the contactor; when voltage is applied to the coil, it closes simultaneously with the power contacts and bypasses the "START" button.

After you release the START button, the engine will continue to run, since the current to the contactor coil is now supplied through the KM-1.1 block contact.

This is called "self-grabbing".

The main difficulty for beginners in understanding this basic scheme is that it does not immediately become clear that the button post is located in one place, and the contactors in another. At the same time, KM-1.1, which is connected in parallel with the "START" button, can actually be located ten meters away.

If you need the motor shaft to rotate in both directions, for example, on a winch or other lifting mechanism, as well as different machines (lathe, etc.) - use the connection diagram of a three-phase motor with reverse.

By the way, this circuit is often called "reversible starter circuit".

Reversible connection scheme - these are two non-reversible schemes with some modifications. KM-1.2 and KM-2.2 are normally closed (NC) auxiliary contacts of contactors. They are included in the control circuit of the coil of the opposite contactor, this is the so-called "foolproof", it is needed so that it does not happen interphase short circuit in the power circuit.

Between the "FORWARD" or "BACK" button (their purpose is the same as in the previous diagram for "START") and the coil of the first contactor (KM-1) connects the normally closed (NC) block-contact of the second contactor (KM-2). Thus, when the KM-2 turns on, the normally closed contact opens accordingly and the KM-1 will not turn on, even if you press "FORWARD".

And vice versa, NC from KM-2 is installed in the KM-1 control circuit to prevent their simultaneous activation.

To start the motor in the opposite direction, that is, to turn on the second contactor, the current contactor must be turned off. To do this, press the STOP button, and the control circuit for the two contactors is de-energized, and after that press the start button in the opposite direction of rotation.

This is necessary to prevent a short circuit in the power circuit. Pay attention to the left side of the diagram, the differences in the connection of the power contacts KM-1 and KM-2 are in the order of connecting the phases. As you know, to change the direction of rotation of an asynchronous motor (reverse), you need to swap 2 of 3 phases (any), here the 1 and 3 phases have been swapped.

The rest of the work of the circuit is similar to the previous one.

By the way, Soviet starters and contactors had combined block contacts, i.e. one of them was closed, and the second was open, in most modern contactors need to be installed on top of a prefix of auxiliary contacts, in which there are 2-4 pairs of additional contacts just for these goals.

Connection to a single-phase network

To connect a 380V three-phase electric motor to a 220V single-phase network, a circuit with phase-shifting capacitors (starting and operating) is most often used. Without capacitors, the engine can start, but only without load, and you will have to turn its shaft by hand when starting.

The problem is that a rotating magnetic field is needed for the AM to work, which cannot be obtained from a single-phase network without additional elements. But connecting one of the windings through throttle, you can shift the voltage phase up to -90˚ and using capacitor + 90˚ relative to the mains phase. We considered the phase shift issue in more detail in the article: https://samelectrik.ru/chto-takoe-aktivnaya-reaktivnaya-i-polnaya-moshhnost.html.

Most often, it is capacitors that are used to shift the phases, and not chokes. Thus, not rotating, but elliptical is obtained. As a result, you lose about half of the power from the nominal. Single-phase IMs work better with this connection, due to the fact that their windings were originally designed and located on the stator for such a connection.

You can see typical motor wiring diagrams without reversing for star or delta circuits below.

Resistor in the diagram below, it is needed to discharge the capacitors, since after turning off the power, voltage will remain at its terminals and you can be electrocuted.

You can choose the capacitance of the capacitor for connecting a three-phase motor to a single-phase network based on the table below. If you observe a difficult and protracted start, it is often necessary to increase the starting (and sometimes working) capacity.

Or calculate using the formulas:

If the engine is powerful or starts under load (for example, in a compressor), a starting capacitor must also be connected.

To simplify switching on, instead of the "ACCELERATE" button, use "PNVS". This is a button for starting motors with a starting capacitor. It has three contacts, two of them connect the phase and zero, and through the third - the starting capacitor. There are two buttons on the front panel - "START" and "STOP" (as on AP-50 machines).

When you turn on the engine and press the first key all the way, three contacts are closed after the engine untwisted, and you release the "START", the middle contact opens, and the two extreme ones remain closed, the starting capacitor. When pressing the STOP button, all contacts will open. The wiring diagram is almost the same.

You can watch the details of what is and how to properly connect PNVS in the following video:

The diagram for connecting a 380V electric motor to a 220V single-phase network with reverse is shown below. The switch SA1 is responsible for the reverse.

The windings of the 380/220 motor are connected with a triangle, and for 220/127 motors - with a star, so that the supply voltage (220 volts) corresponds to the rated voltage of the windings. If there are only three outputs, and not six, then you will not be able to change the winding connection diagrams without opening it. There are two options here:

1. The nominal voltage is 3x220V - you're in luck and use the above circuits.
2. The nominal voltage is 3x380V - you are less fortunate, since the engine may start poorly or not at all if you connect it to a 220V network, but it's worth trying, it might work!

But when connecting a 380V electric motor to 1 phase 220V through capacitors, there is one big problem - power loss. They can reach 40-50%.

The main and effective way of connecting without loss of power is to use a frequency converter. Single-phase frequency converters output 3 phases with a line voltage of 220V without zero. Thus, you can connect motors up to 5 kW; for higher power, converters that can work with single-phase input are simply very rare. In this case, you will not only get the full power of the engine, but you will also be able to fully regulate its speed and reverse it.

Now you know how to connect a three-phase motor for 220 and 380 Volts, as well as what you need for this. We hope the information provided has helped you understand the issue!

Related materials:

• Connecting a magnetic starter for 380 and 220v
• How to assemble a three-phase shield
• How to choose a frequency converter