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2 leg control in soft starters


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#21 schow

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Posted 10 August 2006 - 05:10 AM

Dear all,

6 months ago, we had installed a 7.5kW 2-leg sst for a 5.5kW motor, pump application (without input isolation contactor fitted)

Yesterday, I received this complaint, which the customer complaint that the 5.5kW motor was burn and they claimed that the 2-leg sst has heated up the motor and eventually damaged it because there is always 1-phase alive even after the motor stopped.

I doubt it since the motor should not be heated up without any current flowing thru the winding even 1 phase is still live.

Do any one of you have any good technical write out for me to explain to my client telling them the motor damage should not been caused by the 2-leg sst?

Danke!

#22 jraef

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Posted 10 August 2006 - 06:39 AM

Are you absolutely sure of that? If even 1 SCR shorts on a 2 leg soft starter, you WILL damage the motor. If one SCR shorts on a 3 leg version, there is no return path for current. But on a 2 leg design the return path is always present, that hot leg.
"He's not dead, he's just pinin' for the fjords!"

#23 schow

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Posted 10 August 2006 - 09:40 AM

Hi jraef,

Thanks for your useful info but nothing wrong with the sst, we took it back and it run fine with other motor, I am a bit surprise also why the failed motor did not damage the sst.

Any idea or advice?

#24 jraef

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Posted 10 August 2006 - 07:09 PM

The only possible association could be the fact that using 2 leg control causes an unbalanced current to be pulled by the motor during ramping. Unbalanced currents do two things:

1) It increases the negative sequence current (DC component) in the motor, which results in negative torque expressed by the rotor. This is why, as mentioned in earlier posts, a motor being ramped by a 2 leg controller needs more current per unit of accelerating torque than a 3 leg controller; it essentially has to fight the negative torque as well as accelerate the load. Siemens has addressed this issue in their Sirius line of 2 leg controllers by something they call "Polarity Balancing" which reduces (not eliminates) the negative torque aspect of this. No other 2 leg controller has this capability as far as I know, so they will be subject to this phenomenon.

2) The current in the uncontrolled leg is going to be 10 to 20% higher than in the controlled leg no matter what. Even Siemens' super duper feature does nothing to help with this. Keep in mind that this extra current in the one leg will not be adding useful work, it will just be there warming things up a bit more.

More current always means more heat, so a motor being started with 2 leg control is going to heat up more in each start cycle than one with 3 leg control. Generally if the ramp time is short, this is somewhat irrelevant. But if it is a high inertia or high friction load that takes a long time to accelerate, and/or the starting duty cycle is high so the extra heat does not have time to dissipate, this can severely shorten the life of the motor.


"He's not dead, he's just pinin' for the fjords!"

#25 GGOSS

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Posted 10 August 2006 - 10:55 PM

I think you'll find the technology behind the Siemens and AuCom 2 leg soft starters is very similar. In fact scope traces may leaving people wandering if there has been a technology exchange somewhere along the line.

Regards,
GGOSS

#26 marke

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Posted 10 August 2006 - 11:13 PM

Hello schow

I agree with the good comments of jraef and would add that if the motor develops a fault while notor running, i.e. leakage to ground, then this could, over a period of time lead to a motor failure that would take a lot longer if a contactor or three leg controller was used.

If the motor is siting unused for a period of time, then condensation could cause leakage from the motor windings to ground. With one phase live, this leakage current will either dry out the condensation and "cure" the fault, or it will gradually degrade the insulation and result in a failure.

I would strongly recommend a contactor is always used with a two phase controller.

During run, there will be no difference between a two phase controller and a three phase controller in terms of operation and motor loading. The differences are only apparent during the OFF time, starting time and stopping time. If the motor is operating close to it's thermal limits during start due to the inertia of the load, then the motor will be heated more due to the two leg controller and will fail earlier than with a three leg controller.
If the load inertia is very low, and the start frequency is also low, the impact on motor life, of the two phase control relative to a three phase control is minimal.

The bottom line is that the two phase control is only going to significantly accelerate the degradation of the motor if it has start times and start frequencies that are beyond the thermal capacities of the motor, or if there is a laekage problem within the motor and no contactor is used in the OFF state.

Best regards,

#27 jraef

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Posted 10 August 2006 - 11:39 PM

Marke,
Thanks for adding the leakage problem issue, I hadn't thought about that. At first glance I was thinking that would only apply if the leakage were in the winding fed by that uncontrolled leg, but then of course one end of 2 windings is attached to that leg, and the third is indirectly still connected to it with just some extra winding resistance through one of the other windings as well, so it really applies all around. I wrote a paper for someone on this issue and will go back and add that tidbit.
"He's not dead, he's just pinin' for the fjords!"

#28 schow

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Posted 11 August 2006 - 09:15 AM

Dear all,

Thanks for all your advices but can I make a conclusion that the motor was not damaged by the sst but perhaps by some other unseen factors because the sst is not failed and in fact it is running now again after the motor has been rewind. Otherwise I really do not know how to explain this phenomenon!?

Lastly, would like to know is there any different for a 2 leg sst to control says L1 & L2, L1 & L3 or L2 & L3?? Are they all the same? I ask this question because I don't see any different in terms of starting current when I randomly connect the 3 input phases into a L1 & L3 controlled sst but of course the rotate direction would be different.

Regards.

#29 marke

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Posted 11 August 2006 - 09:57 PM

Hello schow

There is no difference. In fact, what you call phase A from the supply in your premises is probably phase b at someone elses. In this country, we have our phases cloured red, yellow and blue. There is no guarantee that red phase is the same phase as red phase in another part of the city, in fact there is no guarantee that the phase sequence goes red -> yellow -> blue. We always joke that you have a 50% chance of the motor going the right way if you rely on the colours. I comisioned a pump station last week which was wired to the colors and the pumps ran backwards.
Many years ago, when I worked for a local electricity company, I built a "radio phasing" device so that we could identify the actual Red phase relative to a standard red phase at HO and that way we ensured that at the district substations all red phases were equal. This enabled us to apply bus ties between substations on a temporary basis.

Best regards,




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