Need urgent help with Wye/Delta

Alex Pel · Feb 24, 2006

Hello,

Does anyone have any idea what is going to happen with a 3phase AC motor if it connected in Wye when on the motor junction box was specified Delta connection?

Actually, this situation just happened. I did that mistake yesterday, today I found that motor burnet out. In my theoretical knowledge it is not gonna happen:

WWYE = V2 /R

WDELTA = 3(V2)/R

Wye connection with the same Voltage supply will drawn 3 times less power then Delta, so current will be 3 times less. In this case the motor has 3500 RPM and loaded by a fan.

For me it should produce low airflow but never burn out. What I am missing?

Thanks for any help, company about to charge me $1500 for that mistake.

CroCop · Feb 24, 2006

Looks like you lost $1500.
Good link, about machine tools, but still relevant.
http://www.cncmagazine.com/archive01/v2i07/v2i07i.htm

Lancie1 · Feb 25, 2006

If it is a 9-lead or 12-lead, 6-coil motor, as shown below, then if you connect it in the parallel "star" or parallel wye, but apply high voltage, then it will burn up naturally, because each coil has twice the voltage, and twice the current than if connected in the series star or series wye.

If it is only a 6-lead, 3-coil motor, then in the Delta configuration, with same line volts connected as for Wye, each COIL gets twice the voltage and twice the current and will probably melt down.

Alex Pel · Feb 25, 2006

Well, if Delta increases speed by 1.75 and draws more current by 75% therefore Wye will reduce speed and current by the same values. So still not clear why the motor should overheat?

Alex Pel · Feb 25, 2006

Lancie1

I connected exactly like your Fig. 3

9-6 7-4 8-5 10-11-12

1 – L1

2 – L2

3 – L3

Motor has 12 leads and I known difference between HIGH and LOW voltage.

But on the motor cover was specified Delta connection:

HV

9-6 7-4 8-5

1-12 – L1

2-10 – L2

3-11 – L3

So then I connected in Wye where suppose to be Delta it should be less voltage for winding and less current. Am I right?

keithkyll · Feb 25, 2006

In this case, forget about Delta or Wye. It doesn't matter. There are 2 connection methods shown on the motor plate. One for low voltage (230) and one for high voltage (460). Both connections will run at the same speed with the proper voltage.

Vic · Feb 25, 2006

Alex

You are right when you say that the Wye connection will give a lower voltage and a lower current than a Delta connection at a given motor speed. However, the torque is also reduced to about .33 of the Delta torque. If the load needs the full motor torque at running speed, the motor will never get to normal running speed and the current will be much higher than normal. Remember that the motor current is determined by the motor speed. The lower the speed, the higher the current.

With the motor drawing starting current continuously, the overload should have tripped fairly quckly and saved the motor. Did the overload trip? Is the overload correctly set for the motor FLA. Without the overload, the motor will burn out quickly.

Lancie1 · Feb 25, 2006

Alex,
I think your confusion is that the motor is not a "2-speed" motor as you are thinking, hence your reference to speed.. Instead it is a "dual voltage" motor, designed to run a a single speed, but using either LOW or HIGH voltage. No, you can NEVER swap the voltages on this type of motor. It has been tried many times, and the result is always the same, not good!

keithkyll · Feb 25, 2006

I agree with everything here. The main question is how the motor burned up. If the motor wasn't designed for lower speed, the deviation from nameplate wiring would be the cause. The load is a fan. The load would be very low at half speed.
I suspect an internal short in the motor. It may have gone undetected in the original config, but caused a serious problem when rewired.
If one winding is burned up more than the others, this would prove it.

At $1500.00, I expect a motor that would shut down from overtemp or trip the overload long before total meltdown.

Alex Pel · Feb 25, 2006

Actually there were 4 connections shown:

CONNECTION

LV

12 10 11

| | |

6 4 5

| | |

7 8 9

| | |

1 2 3

| | |

L1 L2 L3

HV

9 7 8

| | |

6 4 5

12 10 11

| | |

1 2 3

| | |

L1 L2 L3

FOR ACROSS-THE-LINE SATRT

12—10—11

6 — 4 — 5

7 8 9

| | |

1 2 3

| | |

L1 L2 L3

12—10—11

6 4 5

| | |

9 7 8

1 2 3

| | |

L1 L2 L3

I chose the last one. What does it mean “FOR ACROSS-THE-LINE SATRT”? I understood that it means start commutation. Start Wye and switch to Delta to reduce a starting current. So I thought to connect in Wye and see how it will work. I checked without load (the fan should blow scrim in normal operation) and didn’t see any problem. At day shift somebody play more and burnt the motor.

Maj. Toxido · Feb 25, 2006

I think even if you make your connection in wye your motor will not burn-out provided that it has an thermal-overload relay on it & ckt. breaker. In my experience i had a problem just like what you have said,my fellow comrades make a wrong connection it should be in delta but he connect it on wye.The thermal-overload keeps on tripping i found out that he make a wrong connection.

By the way across the line or direct on line connection has a high ampere starting & high torque,once you start the motor it will draw a high reading of ampere also big transient in contactor.

I suggest that safety factor & components should have if you are running a 3-phase motor.

Note: Always set the thermal overload with the motor FLA

Hope it will hep!

Alex Pel · Feb 25, 2006

Thanks Maj. Toxido

Looks like the troth, but I don’t understand why? Wye connection should drain less current. Why in your experience the overload keeps on tripping? In my case O/L was set 20 Amps, when FLA is 17A.

BTW “FOR ACROSS-THE-LINE SATRT” which one connection is fist Wye or Delta?

Maj. Toxido · Feb 25, 2006

The motor that I`ve example has an service factor of 1( SF 1 )
that`s why I made the setting of my TOR to it`s full load current.
If you check the name plate of your motor there is a service factor of that motor.

Now in your problem you said that your TOR is not tripping did you consider the service factor of your motor?

Note: If the service factor of your motor is 1 adjust it to FLA
of your motor, meaning your TOR

If your sercive factor is greater that 1 multiply
your FLA with 1.25 this will be your setting of overload

If there is no service factor in the name plate of the
motor it means that consider it 1.

About your question in across the line which comes first wye or delta? it depends in your connection.

If you are using wye, 80% of current is drawn
And if you use delta, 54% of current is drawn

Alex Pel · Feb 25, 2006

Maj. Toxido said:
About your question in across the line which comes first wye or delta? it depends in your connection.

If you are using wye, 80% of current is drawn
And if you use delta, 54% of current is drawn

Can you please explain that does it mean? 80% of what?
You are stating that Wye drawing more current than Delta under the same line voltage. Can you explain why?

Alex Pel · Feb 25, 2006

Just found that confirm my fault but still doesn’t explain why.
http://www.usmotors.com/Service/Bulletins/Issue7-May03.pdf

Need urgent help with Wye/Delta

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