Motor Trips While Shifting from Star to Delta

You need to go to a solid state soft starter....

https://ab.rockwellautomation.com/Motor-Control/LV-Soft-Starters

https://cr4.globalspec.com/thread/110726/Induction-Motor-Trips-While-Shifting-Star-to-Delta-Connection

If the installation is "brand new" then what does the installer/designer say?

In problem solving terms, there has never been a successful operating state, so there is no reference to return to.

New to me implies warranty, or incomplete testing during handover.

One small question, are you from the installer company or the recipient?

Hi,

We are on site and the its undergoing testing and commissioning. Contractor is still liable to provide a workable solution but even with manufacturer (ABB) help he is unable to do so and asking for advice.

I am not at site but from the design office and not involved day to day on the job . I originally had a VFD on the pump which i find out now they value engineered :-) ..

I too think so.

OP to check the timer settings to ensure that delta relays are not engaging before the star relays disengage.

OP states it works fine w/o motor connected...

The old way of doing this would be a closed transition Y∆ starter. Damned expensive if you tried to build one today. I’ve worked on them, they are frightening to see in action.

Go for a soft start.

In my youth in the UK as an electrical apprentice undertaking installation and electric motor repair/maintenance then closed transition SD starters were frequently encountered complete with Manual Control. Yes, frightening alright.

They also provided a steady stream of repair and rewind jobs

In today's world, I would agree just use appropriate soft start.

From what installs I have seen in UK, soft starts and star-delta are used as default, instead of some good rules of thumb making for clean DOL installs, this since 1963 observation down in Kendall plant near Bristol. A 1000kVA 5.75% impedance distribution transformer will start a 200hp motor across the line in an industrial facility with no objectionable flicker, easily.

You’ve not seen many UK industrial installations then?

1500HP DOL is squeaky bum time when it starts for the first time.

It's squeaky bum time in US as well. Definitely get some flicker, even when at different distribution voltage than lighting.

My personal experiences seeing what small industrials (in rural Cotswold/Bristol) apply was last in 2007-2008. Many star-deltas at what I thought was ridiculously small HP ratings. When starting 20,000 HP wood refiners, we used a typical main power transformer (20MVA captive transformer) as a reduced voltage starter, Central Maine Power couldn't take the starting current in Rumford Maine without browning out all of the towns in the area...

That 1500 isn't at 415 volts, though?

The time relay that would make one second delay between the Y - D connection often helps with this problem.

Good luck !

I assume that the starter is an open transition type where there is a short period during transition from Star to Delta where the main contactor is closed but both the star and delta contactors are open.

In this condition, power is applied to one end only of the motor windings and, although there is no current flow, the motor now behaves like a generator due to the residual magnetism in the rotor core.

The instant that the transition begins, the motor loses torque and begins to slow down, the frequency of the induced voltage is now controlled by the speed of the rotor, which will necessarily be out of phase with the supply voltage.

When the power is now reconnected in the Delta form, you are effectively connecting mains power to an unsynchronised generator - resulting in a very high current spike and torque transient which, depending on the instantaneous phase difference, can be high enough to trip overloads or breakers, and even damage equipment.

The reason that you are having success when there is no load attached is likely because the motor is not slowed sufficiently to produce a phase difference of such magnitude that will cause the current spike to trip the safeties.

Extending the star time period does nothing to alleviate this problem.

Altering transition time - if you could - is problematic as the phase relationship can vary with motor load.

Moving to a Closed transition type starter can overcome the problem, but has its own disadvantages with resistors etc. being involved.

The resistors, together with a fourth contactor, allow a current to flow in the windings during transition with the aim being to overcome the induced voltage and thus maintain phase relationship. The resistors have to be suitably sized to achieve this.

The only really satisfactory solution is to go to a solid state starter which is likely cheaper than a closed transition unit in any case, and certainly simpler to install and maintain.

I would echo comments from contributors 4 & 5. If you are relying only on the break before make contact in the change-over timer then it is likely that the star is dropping out slowly and is still connected at the instant when the delta closes creating a pulse of short circuit conditions. Check if the star coil is fed through a N/C auxiliary pole on the on the delta contactor and that the delta coil is fed through a N/C auxiliary pole on the star contactor. That wiring arrangement guarantees that both contactors cannot be closed simultaneously.

"That wiring arrangement guarantees that both contactors cannot be closed simultaneously."

Even that can fail, if the lighter-duty auxiliary contact switches to closed faster than the heavier-duty star main contacts open, and the delta relay closes very quickly.

I agree with several other posts; now that soft-starters are so inexpensive, I see no reason for using a star-delta starter, on any new installation.

The point of wiring in the way I outlined is that the coil of the delta is not energized and therefore does not start to close until the star is open. If the auxiliary contact is the fourth pole on the same armature as the main contacts it is constrained to operate at the same time. If it is mechanically slaved from the main armature, usually a block of 2 or 4 auxiliaries plugged onto the top or side of the contactor then it will operate fractionally later than the main poles. Adding a separate relay for the N/C auxiliary is very poor design as it introduces an additional coil that reduces the reliability of the whole system. Having designed, built and maintained hundreds of control panels over a career of 45 years I have never encountered the scenario that you describe.

If the bean counters cost engineered out a VFD on a 90Kw condenser water pump then they are clearly only looking at capital cost and do not understand the energy saving opportunities and long term cost savings available by using a VFD in this application. What makes you think that such short sighted idiots would shell out for a soft starter after they have already paid for a star delta set?

OP states it works fine w/o motor connected...

The shorts that sometimes occur on transition from star to delta occur through the motor connections. No motor connections=no shorts.

I don’t understand how, each coil of the motor has a reactance. These people are talking about cross the line short circuit because the starter is miss-wired or miss-timed internally. That is not going to change if you pull off the 6 motor leads. (?)

Yes it does, because some of those leads are connected to both contactors. If the Delta Contactor closes before all sections of the Star Contactor have opened, there is a direct short across one or more main lines. I'm referring to old machines. I'll look to see if I can find a diagram...

There are only 6 wires, each connects to one terminal. All other wiring is part of the starter, we have to assume the OP did not compromise the starter wiring while testing the installation?

Sorry, I looked, but couldn't find the circuit diagram I had in mind. This was an old machine I worked on years ago. I don't have time to create a new drawing right now.

Looking at the diagram, if you have the motor coils correctly identified (polarity and phase) and connected to the starter, it will work ok unless a problem with the starter wiring. Motor disconnected will catch all starter wiring and timing problems, where a phase to phase short circuit might occur.

Other problems like rolling of the phase connections on one of the contactors will require motor windings to test, so that might be your concern? That mistake would result in effectively single-phasing the motor, should not be tripping the two upstream breakers. All other problems you are experienced with where the motor must be connected stems from incorrect identification of the motor leads, my guess?

Thank you for that drawing! Precisely as shown, if that Star contactor closes while the Delta contactor is still closed (or vice-versa), you have a direct short across all three phases of the incoming mains.

It may not be common, but it is not rare for relays (contactors) to have contacts fuse closed (lack of preventative maintenance). We had some fairly large motors; at least one of the contactors did just that, blowing three 600Amp 480V fuses (50+ year old main switch panel, no circuit breakers). It was pretty spectacular!

Yes, that’s what I was trying to tell a couple of the other sleuths, that sort of trouble root was eliminated by the OP testing the starter with no motor connected. I think the OP was also trouble shooting from afar, so good details on the equipment is already lacking.

"that sort of trouble root was eliminated by the OP testing the starter with no motor connected."

Not necessarily. If the person doing the wiring considered those three wires that connect the Delta and Star contactors as part of the motor wiring (they do connect to the motor), they may or may not have been connected when testing the the assembly "with no motor connected".

Still, only 6 wires connected to the motor. Each has a reactance between them, so no short circuit possible. Remove the 6 wires, you still can overlap the star and delta contactors, as you noted earlier. The wires cannot connect the start & delta contactors together and connect to the motor, more than 6 connections?

Is ensuring discharge valve of the pump is closed prior to giving start command to motor not an option??

Seems simpler solution considering it is standard practice in industries.

It’s possible the breakers are tripping on locked rotor/overload, but that would take some time, and really, with the OP mentioning that both the main and motor breaker trip together, it sounds like it’s right away, not time overcurrent. The overloads should really be tripping first for your postulation, not that we couldn’t have 3 things wrong with the installation...

What is the design pressure rise through the pumps, please?