Induction Motors vs. Synchronous Motors for Lift Irrigation

I am no Electrical Engineer, but for motors that size my guess is that the better efficiency and the Pf corrective ability of the synchronous motor would make the synchronous motor the best choice. Induction motors may be cheaper initially, but total cost of ownership will be higher.

I disagree. Total cost of ownership must include the cost of down time. If you have insufficient expertise to keep the more complex synchronous motors operating, the theoretical energy cost savings can be lost in one day of lost crops when the pump is waiting for a technician to fly in to get it running.

Synchronous motors are often used when power factor from other induction loads must be corrected and/or the speed is low. If you have no other nearby induction motors that need PF correction , then that issue is meaningless to you.

To run an induction motor at low speeds means many poles and small inefficiencies in each pole get multiplied, that's why a synchronous motor often looks better as far as efficiency. But this can often be mitigated by using higher speed induction motors and different pump designs. Your supplier's reluctance to consider this and offer you a choice indicates either a lack of expertise or an unwillingness to consider all the issues.

I would consider induction motors and PFC capacitors. Simple, trustworthy and easy to repair / replace.

Samuel,

Listen to JRaef - he is an expert in this field - his knowledge of electric motors et al far surpasses mine.

Thank you Sir for the lucid explanation.

I had consulted three different manufacturers and all of them had proposed synchronous motors. They came up with reasons such as

(i) Low efficiency

(ii) Induction motors of 10MW capacity are not available

(iii) Irrigation utilities all over the country install synchronous motors instead of induction motors for such capacities

I checked up with motors manufacturers and found that 10MW induction motors are available in the market. I also found that irrigation utilities have installed synchronous motors for capacities of 5MW and above. However, no one has given me convincing answers.

Regarding the efficiency, I found from the efficiency curves, the synchronous motor is only marginally more efficient at full load. Th synchronous motor is more efficient at part load. There is no question of running the pumps at part load. The pumps will be run at full load for a couple of hours and the entire canal is filled up (bunds at intervals are provided along the length of the canal). The gates of the branch canals are then regulated depending on the areas and crops under cultivation

Why have these irrigation departments installed synchronous motors instead of induction motors

I also agree with JRAEF and will use induction motor with individual Pf caps per motorset. Your OP answers your question/plight as well ie. "getting qualified Engineers to run these units is next to impossible". Synchro motors need more specialised care and attention to perform optimally, and then, as you point out, they are not that superior to induction, except in price. My own experience turned out that contractors prefer to quote on the higher cost items because they have their pockets in mind and not your budget. If they add on a percentage to the purchase price, obviously their cut will be higher off the higher priced motor. A synchronous motor is perceived to be more efficient than induction, but only the actual operators will be able to confirm if that is true. The contractor will never tell. Try and call some of the guys in those other irrigation schemes. Check longevity and running costs with them. Please then post those results, i'm curious myself why synchronous became the norm.

Thank you Sir for your reply

I contacted the Engineers in the irrigation departments in states where synchronous motors have been installed. I got vague replies like better efficiency, 10MW induction motors were not available when these projects were taken up, consultant recommended synchronous motors, got full fledged electrical department who take care of the operation, maintenance etc. But I have not got a single convincing answer. But if you verify, large irrigation pump motor sets are being provided with synchronous motors. I pointed out to them that such size induction motors are available now, but still synchronous motors are being installed. No answer to it

Can you explain this?

I am not sure of their reasoning, but i can only assume that they like the fact that a sync motor speed remains constant, regardless of the load and or that they also allow themselves to be used as PF correctors for the whole installation. ( By overexciting the rotor, which is then seen by other "inductors" as capacitors.) Maybe their very good pF is seen by the operators as good value. But bear in mind that the size you are mentioning needs an external excitation, typically some

dc source with its controls. See, more pieces of equipment to look after. Their advantage of precise speed dont seem relevant to me in your application.

Thank you JVRJ for your reply.

Synchronous motors do not need capacitance banks. Induction motors do need capacitor banks to improve the pf. Fine, we will provide capacitor banks and improve the pf (otherwisw penalty is imposed by the electricity boards). This is a simple and reliable arrangement and does not involve any heavy expenditure

Back to the basic question. Why are synchronous motors provided when the cheaper induction motor will do

There are some questions to be pondered.

1) What is the RPM of pump.

2) What happens to the pump performance if speed of motor is less than synchronous speed due to either slip or lower frequency.
3) How the motor will behave under low voltage.

Dear Narendra,

Thank you for your reply. These issues may not affect the performance of the pump induction motor unit. I have referred several text books, but didn't get any concrete matter. If these were issues, why didn't the manufacturers high light them?

I would suggest you to make a comparative study of the two kinds of motors taking into consideration all aspects but not limited to: a)power drawn/kwh from the utility b)down time for maintenance c)initial cost including starter d)running cost/cost of maintenance e)Guaranteed period of life f)warranty period by manufacturer g)impact on pump due to starting current etc Item a)could be reduced by selecting many smaller units instead of a big pump,known as pump/fan law.Even transport,installation,maintenance would be easier

Samuel,

I gave JRAEF a GA on the way through, a common occurance.

The things that you have not specified could relevant. What Voltage is the 10MW per unit supplied at and how stiff is the supply at the point where the motors will be installed?

There are 11kv softstarters and secondary resistor starters are still very applicable at these high power ratings of induction motors.

The rather complex run up requirements for Synch motors is not totally disadvantagous because large pumps generally require some form of soft starting. Units of 10MW also require soft starting for the sake of the supply.

If the supply is not very stiff synch motors are advantagous because the soft start regimes are designed to not bump the system. Briefly the motors are accelerated to synchronous speed and then excitation applied, once the phase voltages are also synchronised then the VCB is closed and load applied. Pumps are a little more difficult to unload than loads such as a dragline MG set, but they can still largely be unloaded. Once the first set is running it helps the supply ride through voltage fluctuations due to its inertia and instantly responding regeneration capacity.

Power factor correction for lead and lag can be achieved to a fine tolerance with synchronous motors. If this brings about a cheaper tarrif, as it should, then the ROI will be more favourable.

The indications for installing Synch motors are:

1 Long run times.

2 Weak supply lines.

3 Savings in tarriff.

4 Wildly fluctuating power factor over the course of the day / week.

There are by the way simple ways to start synch motors, the amortiseur windings will act as a squirrel cage when dropped DOL. The supply line does not like that though and I can't say for sure that a loaded system would start in this way. This is how older dragline MG sets were started (fully unloaded), now they are soft started.

There are other advantages to using Synch motors that don't appear applicable to your application.

The indications

Thank you Emjay for your reply. I will try and reply as best as I can

The voltage is 6.6kV or 11kV. Express 220kV will be provided and substation erected at the pumphouse will step down the voltage to 11/ 6.6kV (for motor supply) and 415V for lighting and misc purposes. Since it is an express line there will be no voltage fluctuations (minor excluding) and power supply will be reliable. (Express lines are provided between the substation and the pumphouse. This line is exclusive for the pumphouse and supply for other purposes or villages are not tapped from this line. This is to ensure reliability of power supply to the pumphouse. This is the norm for all pumphouses)

Motors are provided with soft starters. However, pump motor sets are designed to operate without soft starters. This is necessary because the sets have to work even if the soft starters pack up

If induction motors are provided, capacitance banks will have to be provided to improve the pf. This is a simple arrangement and synchronous motors need not be provided for this reason

Again my basic question. Why are synchronous motors provided and not induction motors

For a motor-pump manufacturer the most important criteria is to match the torque speed characterstics of the pump with the characterstic curves of the motor to be coupled with it otherwise guaranteed valuses will not be achieved during a commissioning performance test.

The manufacturers you have contacted seems to have a particular motor-pump combination design which is time tested using synchronous motors and they are not ready to provide tailor made solution to you due to complexities involved.

There is nothing wrong with selection of induction motor, I have used almost double your consideration capacity induction motors having, capacitors, direct-on-line start (DOL), 11KV rated, for an oil pipeline (450 + 450 kM long) which was to pump oil upcountry in large quantity..

My induction motor-pump combination was supplied by a renowned manufacturer from UK.

Specify pump-induction motor package to various other manufacturers, I am confident that you will get it. But please do not ask seperately for the motor, no matter induction or synchronous one thing for sure you are heading for trouble.

I think that is a good point. Performance is key and you must find a supplier willing to give you what you want and guarantee the performance.

Thank you JRaef,

In earlier days i.e., 7 to 8 years ago, irrigation pump set capacity was less than 1.5MW and were being provided with induction motors. It is only in recent times, unit pump motor set capacities have increased to around 10MW and synchronous motors are being provided. it may be noted that each pump turbine has to be designed separately depending upon the head and discharge. Hence, there is no standard design

Back to my orignal question - why are synchronous motors (instead of induction motors) being provided for lift irrigartion schemes

please refer to my remarks under *10. Or else go to a consultant and get clarifications. If the consultant who recommended syn motor is available show my remarks and ask him why he recommended it.Go through records kept in your office and try to find out whether a report on comparative study made earlier is available.Finally you can ask your employer why they chose syn motor whether they got a cheap quotation or someone got kickbacks(politician?)

Thank you MountK2

Definately the pump and motor set will be bought as a package unit. We can't buy the motor and pump separately and take the responsibility for its performance. The contractor has to supply, erect and commission the pump motor set and guarantee its performance

My only query is, since the induction motor is cheaper, much easier to operate, it should be the first choice for lift irrigation schemes. However, we find that the end users are installing synchronous motors when induction motors will be far cheaper and simpler to operate. Why haven't the irrigation departments insisted on installation of induction motors thereby saving public money? How come installing synchronous motors have now become the norms for lift irrigation schemes?

My advise is to design any electrical system yourself and do some cost estimations, your prospective will definately change.

First of all the Contractor is not designing anything he is just implimenting the design given by the consultant (designer), the Contractor is responsible for the defect in implimentation or site workmanship and not the design unless the contractor happens to be an EPC.

Any product will be cheaper if it is in high demand / mass production and this can be be made available on the shelf or in a minimum time allocation.

Irrigation and remote area pump-motor system designers have to take into account that in most of the cases electric grids are not available that means local power generation, and a standard product befitting all occasions goes into mass production.

What is your concrete basis of assessing large size indution motor to be cheaper? probably you have not taken into account the associated large size transformer and increased capability of grid/generators to cope with associated voltage drops at DOL starting.

The Designer has to look at the matters from all angles besides he has to take into cosideration some 20 - 50 years from now until the project lasts.

To give you a practical example, while I was designing 14 kM long 12 KV transmission line for a 5 MVA puming station, I concluded two viable options:

(1) To select Single Circuit Zebra ACSR conductor or (2)To select two Parallel Circuits at the same pole using Dog ACSR conductor.

After market investegations I found out that while Dog Conductor is available ex-stock, Zebra Conductors is being made available by the manufacturers on confirmed advance order only.

I visualized that if a conductor breaks down say ater 10 years of service, in case of zebra, the production loss and implications by taking into accont time of manufacture and replacement will be enormous.

I selected Double Circuit Dog conductor scheme without hesitation - You can say my design has been non-conventional as you normally see sigle circuits on poles.