Does Open Delta Service shorten three phase motor life 6

[HVmazz]

I have a proposal to install a replacement (~pump cost = $4,000) 10 Hp water pumping system which is serviced with an Open Delta three phase 240 volt 50 KW utility supply. The old system has experienced three motor burn-outs in the last 4 years. The new motor is rated at 10 Hp, 230 volt, three phase, 28 F.L.A.and has a manufacturers 5 year/10,000 Hr. warranty. I will be adding phase loss/reversal, over, under & unbalanced voltage & short cycling monitoring to the control panel. The manufacturer will honor the full warranty ONLY if "...the 230 volt/3 phase 60 Hz power source is a good dependable supply and NOT originating from an Open Delta power circuit". The utility company engineering department claims this type of service is normal and wants $5,000 to add the third transformer to the service to yield 208/120 three phase which would also require a new motor selection. We also have a small single phase lighting load. The pump manufacturer will make changes to the motor windings to accomodate the 240 Volt Open Delta supply. How much will the pump life be shortened running on an Open Delta source? Should we add our own transformer on the secondary side?

 

[cuky2000]

Determine how much will the pump life be shortened running on an Open Delta source is not an easy task since this is not a deterministic phenomena.

I suggest checking the overvoltage performance of open delta comparing with a three transformers connection including the surge protection used. This probably gives some idea of the investment, risk and guaranty issues.

Perform detail study of this system could cost more that the cost of extra transformer.

 

[wareagle]

I don't think an open delta service in its self will have any effect on the life of the motor. What may happen is the utility may loose a phase and you damage the motor in that way. Since you are providing protection I would not expect to see problems. I would consider using a time delay fuse
with an ampacity very close to the FLA of the motor.
The open delta service is very common and used often by the utilities.

 

[dpc]

If it's running open delta, it has already lost a phase, really.

As long as motor voltages are balanced, the motor life should not be negatively impacted. But as the load increases, the voltages out of the open delta tend to become unbalanced. This will lead to increased negative sequence currents in the motor and possible reduce the life of the motor due to higher temperature.

I'm not a big fan of open delta services for anything over about 10 hp or for anything I cared much about.

 

[jraef]

If it's running open delta, it has already lost a phase, really.

Right! In fact, since you are deriving the 3 phases from 2 sources, if you lose one the entire systems goes down.

The reason (as was explained to me) why some pump mfrs stipulate this is that Open Delta services are usually associated with remote areas where most of the loads are 1 phase residential or farms. The problem then is, the 2 phases that the utility supplies are already at risk for wide imbalance conditions because of variable loading. When combined with an Open Delta 3 phase service which is already bordering on being unbalanced, the risk of overheating the motor is almost 100% sooner or later. The pump mfrs don't want to take a chance on the sooner side.

I did a well pump project with Open Delta service for a winery that was smack dab in the middle of a new residential development (a new trend here in California) and we put phase current monitor relays on everything, set to 30% current imbalance. We could not keep the pumps running; the relays were constantly tripping. We put a Dranetz recording meter on the utility side for 2 weeks and the two primary lines would swing 20-25% hour-to-hour at certain times of the day. We could even predict it based upon commute patterns because when people got home from work and turned down their AC thermostats, the imbalance would spike.

 

[resqcapt19]

Are all open deltas fed with only two ungrounded conductors? I though some were fed with all three phases and they just leave out one transformer.
Don

 

[davidbeach]

An open delta requires three wires on the primary, but one of them can be a neutral forming an open wye-open delta connection. Open delta-open delta requires all three phases but not the neutral.

 

[dpc]

Utilities out west run a lot of what they call "V-phase" taps which are basically two phase wires and a neutral.

 

[itsmoked]

10HP. Cheap VFDs. How about running a 15HP VFD to make balanced stable 3ph for the more from "whatever". This way yo may spend $5-600 on the VFD but that's way less than $5k to the utility. You also get the ability of a VFD on a pump which is one of the most applicable VFD loads. It opens some nice control opportunities and can prevent many motor starts and stops.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Good point, can't believe I forgot that!

 

[itsmoked]

Yeah, YOU, of all people...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[magoo2]

We have a fair number of open delta connections on our system. Sometimes we have all three primary phases available and the customer just has a small amount of motor load. Other cases are rural where it may be a Vee phase arrangement, i.e., just 2 phases and a neutral are available, and it may be miles before you reach the third phase. This latter case tends to make it very expensive to add the third phase when unbalance problems occur.

Compared to a symmetrical bank of 3 transformers, the open delta bank does introduce additional unbalanced voltage to the secondary. I say additional because the circuit three phases are often unbalanced due to load and lack of transpositions. Further, the 2 transformers are frequently different sizes. There are application rules to follow so that the unbalanced voltage is better managed.

Voltage unbalance does shorten motor life simply because the motor runs hotter. Voltage unbalance produces negative sequence heating of the motor rotor. To be a significant effect, the motor would have to be running close to nameplate load. Refrigeration motors are most succeptible to shortened life from voltage unbalance. Some pump motors carry fairly high loads, so they would be affected also. For other motor applications, I don't believe you'd notice any effect on motor life.

The NEMA MG1 standard says the maximum voltage unbalance that a motor is designed for is 1%.

 

[HVmazz]

Thanks for the great responses...
I should have mentioned that this is 10 HP submersible pump
so it is also water cooled. However, the total system cost is at $12,000 now and we do need to protect the investment as best we can.

 

[dpc]

The VFD idea is a generally a good one, but I'd be a little careful about applying it to a submersible pump if it wasn't specified that way. If the leads are long, there are standing voltage wave issues and also, submersible pump motors are not covered under NEMA MG-1 and are not built to any recognized standards. So the motor's winding insulation rating could be an issue.

If there is three-phase primary available, I'd really push for a true three-phase service. If it's V-phase, I'd do what jraef suggested and monitor the negative sequence current.

 

[itsmoked]

How deep is the well?
Can't a filter remove the issue anyway?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[HVmazz]

This application is for a Water Fall on the facility entrance ponds and the pump is only submerged 8 feet but the lead to the control panel is 25 feet.

 

[itsmoked]

I believe 25 feet is inconsequential others can confirm this.

Hey, a perfect use for a VFD. On off hours, you slow the waterfall way down and save money. Or vary it like real ones vary so visitors detect variation from day to day!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[HVmazz]

Sounds like a good idea. I am now looking for a VFD that can handle the 10 Hp Pump and talking to the PUMP manufacturer to spec a motor that will work with the VFD. Should I run the VFD on the Open Delta 3 phase or use a higher power VFD and feed it single phase 240V ?
Thanks again....

 

[waross]

Many open delta systems have been used to supply rural irrigation pumps. In some installations two phases of an existing three phase system were used. In installations only two primary wires were taken to the open delta. The two phase primary systems could be many miles in length. This meant that there would be a neutral current on the system that was approximately equal to the phase currents.
The heavy neutral current and the resulting voltage drop on the neutral would cause phase angle shifts on the system. I have had problems with phase angle shifts on three phase systems caused by heavy neutral currents resulting from severe unequal loading further downstream.
I understand that problems with open delta systems are mostly the result of neutral voltage drops on the system. I understand that open delta works well when fed from a solid, balanced system that has little neutral current.
respectfully

 

[stevenal]

Voltage unbalance can be tolerated by derating. There's a chart in C84.1, probably in the NEMA motor standards as well. Might need up the HP if unbalance is an issue.
Utility is correct, no problem with open delta as long as the unbalance is within the motor's capability.