Inside the delta vs DOL with soft start.
Inside the delta vs DOL with soft start.
(OP)
Not too long ago we had a star-delta motor controller blow apart but did not hurt the motor, at least from megging and ohms measurements. I read this forum with enthusiasm and followed your collective advise and asked the boss to order a soft start. He ordered a soft start controller to handle the motor FLA. He also wanted to connect the soft start in "inside the delta" configuration. I said that this was not neccessary since we could DOL with the soft start since it was rated for motor HP. I also told him that engineers at this site generally frowned on that configuration. Now for my question, the motor is a 12 lead motor that was being used in star-delta. I have been installing the soft start and intend to wire the motor to the soft start controller just like it would be if it was connected to M1 and M2 in the old star-delta configuration during run. In that:
Output soft start controller,
Phase 1 tied to T1, T7, T6 and T12.
Phase 2 tied to T2, T8, T4 and T10.
Phase 3 tied to T3, T9, T5 and T11.
That is how the motor was connected to the line when both M1 and M2 are energized and the motor is up to speed and in run. This is a little late in asking but is there anything wrong with my design?
Output soft start controller,
Phase 1 tied to T1, T7, T6 and T12.
Phase 2 tied to T2, T8, T4 and T10.
Phase 3 tied to T3, T9, T5 and T11.
That is how the motor was connected to the line when both M1 and M2 are energized and the motor is up to speed and in run. This is a little late in asking but is there anything wrong with my design?





RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
1.) By a full size soft starter does that mean that if the motor has a FLA of 560A and the soft starter has a rating of 570A that we should be able to use the standard delta connection and not "inside the delta"?
2.) Is delta 2 with two parallel windings the same as one delta winding during start?
3.)In the old star/delta the transistion time was set to 6 seconds. I have set the new soft starter to 6 seconds also. The default start time of the new ABB soft starter with built in bypass contactor was 10 seconds. Do I need more time when the compressor is connected?
I have started the new ABB 570A soft starter with the 500HP (480VAC) high speed uncoupled motor and the max current it pulls is 890A before the current starts falling rapidly and then the bypass contactor pulls in. In the old star/delta connection current would ramp up and pull 1200A when in star and still pull 1200A in delta before current started falling. Coupled or uncoupled. 4.) Have I forgoten anything?
RE: Inside the delta vs DOL with soft start.
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1)
When I looked at the ABB documentation I found that the 570A softstarter had dual ratings:
315kW (~400HP) for "in-line" connected
540kW (~725HP) for "inside delta" connected
The different HP rating's are probably why your boss is asking you to wire your 500HP motor "inside delta" instead of "in-line".
2)
The ABB literature also illustrates "in-line" is the same as the "delta2" wiring shown above, and for "inside-delta" one of the two windings is connected to the soft-start while the other winding bypasses the softstarter (it's connected to the line voltage upstream of the softstarter).
3)
The factory settings are appropriate starting point, then you can experiment to find what works best for you. The timings are adjustable so that you can tweak it to fit your conditions.
4)
It is perfectly normal and expected for a softstarter to significantly reduce starting currents.
However, you should wire the softstarter to the motor "inside-delta" to avoid exceeding the softstarter's ratings.
Best of Luck!
RE: Inside the delta vs DOL with soft start.
When a soft starter is wired normally (DOL as you refer to it), if one SCR shorts, there is no immediate risk of damage because there is no return path for current flow through the motor windings. You need at least 2 shorted SCRs in opposite phases to present a risk. In a soft starter wired inside the delta, if ANY one SCR shorts, you can lose the motor in short order because 1/2 of the current path through the windings is always present in that configuration. So the prudent course is to use what is referred to as a "fault contactor" in the circuit that opens up if there is any kind of fault on the circuit, i.e. shorted SCR, thus isolating the motor windings. Now the cost and space savings of using the soft starter rated for 58% of the motor FLC is offset by the cost and space required for the Fault Contactor and additional wiring.
There is also a theoretical need to use SCRs rated for a higher PIV rating when used inside the delta compared to a standard line connection because the potential peak voltage is higher. This is ignored by most soft starter manufacturers who promote this configuration because it would add cost, but technically using standard rated SCRs in starters wired inside the delta make them MORE prone to shorting. This argument has been ongoing in the industry, unbeknown to many smaller manufacturers who simply copy what other people do.
Harmonics during soft starting are generally considered insignificant, but when the soft starter is connected inside the delta, the non-sinusoidal nature of the current is exaggerated and THD is TWICE that of a standard connection. So for the same amount of starting torque, more of the current is being wasted as heat in the motor or conversely you get less torque-per-amp during start. So if your application requires a long start-up ramp or frequent starts, using an inside the delta configuration will result in more thermal stress in your motor.
The connection pattern of a standard line (delta) connection is somewhat simple, just get the rotation correct. On the other hand, connecting inside the delta is extremely specific, the pattern must be followed exactly, otherwise the motor windings can be damaged almost immediately upon energization. There is little room for error, yet almost nobody is familiar with exactly how to do it (other than following the written instructions explicitly). For the average end user, this can present a significant risk of future problems during an emergency repair.
Bottom line, inside the delta is not worth the risk IMHO.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Inside the delta vs DOL with soft start.
I just opened the book that came with the ABB PSTB 570-600-70 to page 3.6 and in the description it states that this soft starter is rated for 500HP "in line" connection and 700HP "inside the delta" connection. (you scared me a little because I can no longer connect inside the delta due to physical restraints) I was at home when I wrote the last post without any documention and was concerned because the last time I started the uncoupled 500HP motor, as it was coming up to speed the soft starter was vibrating some.
In the motors application it will either be left on a long time or shut off a long time. The boss proveded me with 3/0 cable to make soft starter connections. The terminals on the soft starter can only accept three cables per phase. Is this enough copper? I've checked the code and it seems on the edge to me.
RE: Inside the delta vs DOL with soft start.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
You are right to question a mechanical lug on the extra flex cables at the motor, that is a concern to be sure. I always opt for high compression crimp lugs on the cables. But you can still use a connection block, just use one with studs instead of mechanical lugs like this (appropriately sized of course);
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Inside the delta vs DOL with soft start.
jraef, Thank you for the picture.
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
I've always done softstarting in the main motor leads but have been tempted to try inside delta once or twice. Not any more!
Thanks again, DickDV
RE: Inside the delta vs DOL with soft start.
Two conductors per phase in one conduit and one conductor per phase in a second conduit will not have the same impedance per conductor. The impedance imbalance will result in current imbalance.
RE: Inside the delta vs DOL with soft start.
What davidbeach is referring to is that in ferrous metal conduit, the fields around the conductors interact with the conduit itself to create impedance in the conductor. In the conduit with 2 cables per phase, you will have more interaction than in the conduit with only one per phase. So in each phase you will have 2 cables with slightly more impedance that the 3rd. Remember that current flows in the path of least resistance, and impedance is the same as resistance in this case, so more current will flow in the 3rd (single) cable and overload it. If you did this with PVC conduit it would not be an issue, but metallic conduit is a problem. You can read the NEC, article 300.3.B.1 and 310.4, but to be honest, this issue is not well covered IMHO (I'm looking at the 2002 NEC, maybe they amended it in the 2005). This paper gives a better perspective however.
http://www.ul.com/regulators/ode/0703.pdf
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
respectfully
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
RE: Inside the delta vs DOL with soft start.
If your line is more balanced that your load, it is sometimes reflective of connection problems or some mechanical problems in the machinery. A thorough thermal scan of all connection points can be enlightening of any hidden connection issues that may not be enough to see visually. Mechanical harmonics get reflected back to the motor as torque requirement fluctuations, and the motor may not react as fast as that harmonic so current harmonics are created, which can be additive to the point of creating current imbalance. That, combined with minor deviations in winding resistance and flux penetration in the motor can add up. "Rolling" your conductors would be a good idea to start with, followed by a vibration analysis IMHO.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Inside the delta vs DOL with soft start.