# Three-phase motor Delta/YY connection diagram and windings troubleshooting

I am trying to troubleshoot this 1.4 kW three-phase motor and I'm unsure how its internal windings are interconnected or whether they are shorted.

On the plate it says "Delta/YY 400 V", and I'm not sure what YY means. Most articles online describe delta and star (Y) configurations. But what is YY? Star-star?

The motor has 6 wires coming out of it, color coded in pairs of red, green, and white.

I assumed the pairs with wires of the same color are the two ends of a winding, but then this wiring diagram on the motor got me confused.

"niedrige Drehzahl" means "low speed", and "hohe Drehzahl" means "high speed". But how is the Delta configuration the "low speed"? I thought Delta was the high speed, with more power.

And how is that even a Delta configuration with 2U, 2V, and 2W left disconnected? Either the diagram is incomplete, or the motor already has some connections done internally, or something else.

Measuring resistance with a multimeter I get these values:

Between 1U and 2U -> 11.8 Ω, 1V and 2V 21.4 Ω, 1W and 2W 11.8 Ω. If these were to be the ends of each winding, the resistance should have been the same and close to zero, right?

What is also weird to me is the resistance value between wires of different colors, which I assumed correspond to different windings, and should have an "infinite" resistance between them, i.e not be shorted.

But I get between U1 and V1 19 Ω, U1 W1 19 Ω, V1 W1 19 Ω, and the same respectively for U2, V2, and W2.

The motor spins fine in both configurations, but without any load attached. I didn't test it under load, and I'm not sure if I should.

How can I find if the windings are damaged? Is it working just because somehow a magnetic field is still rotating even with damaged windings? Does this motor have more than 3 windings? Is this wiring scheme common? What is YY?

Shorts between the turns of a mains AC powered induction motor windings reveal themselves quickly as smoke - it's like a shorted transformer.

Leaks between different windings and the motor body are revealed by using an insulation meter,"megger" is a traditional tool for it.

Your motor is a dual speed motor. The motor can have either 2 fully separate stators for different speeds, only one is used at a time. Or there's 2 stators used in the same time, but their windings are ingeniously interleaved so that the number of poles is doubled if the windings of the second stator get reversed voltage. Doubling the number of the poles halves the synchronous (=no load, no friction) speed. Your motor is an example of that 2nd type.

For more data search for dual speed induction motors. Heres's a randomly picked advertisement http://www.donbassmotor.com/documents/Multi-Speed_Three-Phase_Induction_Motors.pdf (Disclaimer: they do not pay to me)

It contains also the connection explanations. Here's a screenshot from the linked ad. You have only 6 free wire ends, so a half of the connections are made internally and they cannot be changed.

This technology was new in the same era when landline telephones and spark transmitter radios were the state of the art in electronics.

It's a Dahlander two-speed, pole-changing motor.

The motor windings are internally connected as follows:

The motor is externally connected in delta for low speed.

The wiring is as follows:

The motor is externally connected in double-star for high speed.

The wiring is as follows: