# How to connect a three-phase motor

I am trying to fix an old three-phase drill.

The motor has 3 wires coming out of it, which connect to the three phases.

Connecting it to the power does nothing, and so I tested it with a multimeter, and saw that two of the wires coming from the motor are shorted together, but not with a good connection (~25ohm).

I am not educated on electricity beyond your common layman knowledge, and definitely not on why three phases are needed or how they are used (or indeed, what this term even means, beside the obvious 3 wires)

Therefore, my assumption at first was that this short is a fault somewhere inside the motor.

Then upon reconsidering, I realized that if the three phases are completely separate, and there is no 0/ground going to the motor, then how can the circuit be closed?

Is this short indeed a fault? how is there a closed circuit when the only lines going into the motor are power lines?

Thanks :)

/Edit

Given the useful answers and comments, I can only assume something inside the motor is bad. This is because 1) Nothing at all happened when it was connected to electricity, not even anything bad. 2) The multimeter shows there is only a physical connection between one of the three pairs. I will hopefully be able to test this further and supply photos tomorrow. Thanks!

/After further testing

It seems I was misled, and the three phase socket in the wall didn't even have any power running to it. Whoops!

With actual power feeding into the motor, it sort-of tries to spin, with A LOT of resistance, and eventually after a few seconds manages to spin very slow. It gets very hot.

Because there is only a physical connection between one of the three pairs, I am guessing this means that only one of the phases actually does any work.

I'll try perhaps to fully open it, although I don't believe I have quite the right tools for the job.

Thank you a lot for the answers and explanations, at least I have some basic information about this subject that I knew completely nothing about two days ago :)

/Conclusion

The motor was sent to be fixed, and indeed the windings got ruined and had to be remade.

A big thank you to you all on educating me :)

• That sounds like a star connected motor, and what you're measuring is the normal resistance of the windings... Dec 26, 2017 at 0:06
• Have you tested for continuity between one wire and the body of the drill - if one wire is connected to the body then it is not 3-phase...Some pictures may help... Dec 26, 2017 at 0:13
• Does the motor plate say it is 3 phase ? Do you have 3 phase power available ? What colors are the wires ? How is it that you are working on this ? Dec 27, 2017 at 0:29
• @Optionparty it does indeed, check the newly attached photos. There's a story behind why I have this in my hands, but it's long and irrelevant :) Dec 27, 2017 at 3:01
• Please check the power outlet if it has three 0°/120°/240° live phases connected to L1, L2, L3. I guess it has 0°/N/180° connected to L1, L2, L3 instead, which would explain the quick overheating and very slow spinning. Dec 27, 2017 at 3:25

If it is really a three-phase motor then the following applies.

Figure 1. Top: three-phase motor windings connected in star (Europe) or wye (North America) configuration. Bottom: delta ($\Delta$) configuration. Source: Electronic Project Focus.

You should get the same resistance reading between L1 - L2, L2 - L3 and L3 - L1.

Three wires does not mean three phases. For example, it could be a single-phase motor with live (L), neutral (N) and earth (E).

Photos and a geographical location would help. (That's why it's an option in your user profile.)

• The current geographical location is Israel, however I honestly have no idea where it was originally bought from, it's very old. I will supply photos when I am next to it again! Regarding the resistances - only two of the wires had any connection between them with said resistance, the other two pairings had no connection at all. The overall wire that I replaced was connected to a 3-phase socket, but had only the ground and 3 terminals connected, not 0. Dec 26, 2017 at 0:37

Three phase motors have only three "hot" power wires connected to the windings. There is no neutral connection. An earth/ground wire should be connected to the frame of the motor, but sometimes that is not done. When a motor is part of a machine, the ground wire may be connected to the machine frame rather than the motor frame. Each of the three wires serve as a "return path" for the other two. The symmetric phase displacement among the phases makes the three wire connection a balanced symmetric system.

If you have connected the motor the same way it was originally connected to the same or an equivalent source and nothing happened, there are several possibilities. If "nothing" really means nothing, no sound, no tripped circuit breaker or blown fuse, no motor rotation, there must be a complete lack of connection. There may have been a prior failure that completely burned open all of the internal motor connections. The external wiring may not be making any connection.

Three-phase motors can be internally either wye (star) or delta with only three wires brought out for external connection. It is probably more common for six or more wires to be available for connection options. If you received the motor with a three-wire cable connected to it, that connection is appropriate for the original power source. Don't change that without labeling everything and understanding what you are doing.

Any information about the motor rating and the power connection marked on the motor or the original machine might be very helpful. The information that you have provided thus far strongly suggests a failed motor.

• Would the order in which the power wires are connected to the internal motor wires matter? I had to change also the three phase plug itself, which was very old and looked different to modern ones, and did not give any distinction between the wires, no sign, symbol, nothing. Only the zero and ground were marked. There was no sign of life whatsoever. Dec 26, 2017 at 1:45
• The order of wires determines the direction of rotation. If you get the motor to run, but it runs backwards, swap any two wires. If there is a zero and ground marking with just three wires, this is not a three-phase motor. If the marking is just on the plug, it may not be a proper plug for three phase, but the motor could still be three phase.
– user80875
Dec 26, 2017 at 1:46

I appears as though the photos are a later addition to the above comments, but that is most definitely a 3 phase motor. The nameplate indicates it is a single voltage design (400V), meaning whether it is connected in Delta or Star (Wye) is irrelevant. YOU would only bring in the three wires to the three terminals they give you. This motor is rated for 400V 50Hz, hopefully that's what you are attempting to give it (you didn't state that). A chart I have on World Voltages indicates that the 3 phase standard in Israel is 400V 50Hz so it should be fine, but make sure. If you only give it 230V 3 phase, it should still spin normally with no load on it, but it would be weak when under load.

The fact that you do not read continuity from any pair other than one would indicate to me that you have an open winding inside of the motor, hence it failing to start. Take it off and send it to a motor shop to be sure, but expect that the cost to fix it will likely be more than the cost to replace it with a new one.

Before hooking up the new one (or the repaired one), make sure you have the correct voltage though.