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Join Date: Feb 2022
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Soft Starter

02/02/2022 11:50 AM

I have noted on 2 Nos. soft starters (1 No. Danfoss compact soft starter 30 kW and 1 No. Solcon 132 kW) that there is output voltage 400V when there is no start signal order. However, the motor does not start. Only when the start order is active, the motor start.

In the Solcon Manual, i noted this warning '
''When the RVS-DN is connected to mains full voltage may appear on the
RVS-DN’s output terminals and motor’s terminals, even if control
voltage is disconnected and motor is stopped. ''
Does someone have an explanation?
Secondly, on a Solcon soft starter, there is an alarm Wrong Connection at startup. We checked the motor and cable, both are ok. If someone has experience with solcon, an advice to resolve this problem would be much appreciated. Thanks

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#1

Re: Soft Starter

02/02/2022 8:14 PM

I think this is just the way it works....but I would check it out anyway...

https://forums.mikeholt.com/threads/failing-soft-starter.125407/

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#2

Re: Soft Starter

02/02/2022 11:28 PM

Only two phases are controlled by the thyristors. The third phase feeds through directly and "floats" the motor WRT the ground when stop. You will get almost zero volt if you measure the line to line voltage at the motor.

When you start, the thyristors ramp to full on, increasing the line to line voltage at the motor.

Soft starters are NOT a galvanic switches. A proper contactor should be placed in series to isolate the motor when not in use. It is always live even when stop.

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#3

Re: Soft Starter

02/02/2022 11:50 PM

The instruction manual probably tells you to check the phase sequence, since the device does check for phase sequence faults. Try swapping a couple of the incoming conductors and see if that fixes it.

While there may be line voltage on the motor windings with the main switch on, you are not measuring phase to phase, you must be measuring line to ground. Leakage current or capacitive coupling through and around the semiconductors would not be unusual to find. You would never work on the equipment without the supply disconnect open and tested. Depending on the application, an isolating contactor might be prudent, if unexpected operation of the motor due to massive semiconductor failure might risk damage to people or property.

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#4

Re: Soft Starter

02/04/2022 10:09 AM

It would be a curiosity if the <...output voltage 400V when there is no start signal order....> were to take place with the motor disconnected, and it were to disappear when it was connected. The posting says nothing about this.

Any ideas why?

<...If someone has experience with solcon, an advice to resolve this problem would be much appreciated....>

These people can help. Try a phone call.

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#5
In reply to #4

Re: Soft Starter

02/04/2022 11:12 AM

He should have been measuring the line to ground voltage. Without a proper galvanic disconnect means, the motor is always floating at one of the line voltage.

400V is a strange value though. It should have been 230V (Europe 400VAC), 277V (USA 480VAC), and 377V (Canada 600VAC). He was probably measuring on the wrong side of the starter.

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#6

Re: Soft Starter

02/08/2022 4:43 AM

Leakage current. As I said in the above linked thread from another forum 6 years ago, it’s because there is a snubber circuit around the SCRs and a digital multimeter will read a voltage through them because it presents no real burden on the circuit. If you used an inductive voltage tester it would not read anything. All soft starters are like this.

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#7
In reply to #6

Re: Soft Starter

02/08/2022 7:49 PM

Don't you think that the motor winding would prevent you from reading a phase to phase voltage as high as 400V? There’s something wrong with the OP description, the voltage at most should be line to neutral unless there’s a ground fault on the circuit. Of course, if this is a 690V motor, that explains everything. They are reading line to ground, a bad assumption 400V was line to line....

And no response on possible phase rotation fault....

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#8
In reply to #7

Re: Soft Starter

02/08/2022 9:44 PM

Agree. GA

I didn't have Japan at 660V to 690V in my list.

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