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VFD's on submersible motors

VFD's on submersible motors

VFD's on submersible motors

Has any one had experience applying a VFD to a submersible motor?  

My concern is two different submersible motor manufactures require starting current 3xFLA on a VFD service.  I have many applications that I have applied VFD’s to centrifugal pump loads (variable torque) and I have never seen a starting current on air cooled motors that exceed 110% FLA.  The drives that I typically deal with have a 200% FLA max trip.  The motor requires a ramp time of 1s to 30Hz, which I tested on a smaller air cooled motor and I still could not exceed FLA on start up.  I have adequate velocity flow over the motor at minimum speed.  My concern is that I have a 125hp submersible pump and to properly size a VFD I would require a 300hp drive per the drive manufacturer.

Has anyone run into this or can explain why the starting current is so high on a submersible motor compared to an air cooled motor.

125hp @ 1800rpm
submersible motor
variable torque load
600ft cable run

RE: VFD's on submersible motors

One reason for increased starting current would be that soemthing is increasing the torque on the submersible motor above rated value. Can something be placing a restriction on the impeller? Has the density of the medium being pumped increased?

If the only time you need this current is during start-up, then it may be possible to use some of the short-time overload capability that may be inherent in the drive design as a way of reducing the overall size of the drive you need.

RE: VFD's on submersible motors

I would guess the mfg. knows the motor is going to be started under a heavy load in most applications.  Finding a way to unload the motor at start up might be more expensive than getting the larger drive.


RE: VFD's on submersible motors

Most submersible applications (the big dogs like oil wells) use PAM drives for their applications. You might check out these drives instead. I am not well versed in start up currents and such for different applications but submersibles always have fluid in them (unless it runs out) so they are starting with a high current requirement. The PAM's I am familiar with are made by Centrilift (division of Baker Hughes). Since the amplitude is only varied with these I would imagine they can provide more current at start-up. I hope some of this helps.

RE: VFD's on submersible motors

This sounds a bit strange to me and so do the explana-tions.  A centrifugal or turbine vane pump full of water is a normal condition and starting torque should be minimal.  Even if the submersible pump has a large standing head of water above it, it should start with minimal torque and current.

On the other hand, if this pump happens to be a positive displacement type or the liquid being pumped tends to take a "set" when stationary, then, of course, large starting torques and currents are possible.

One way to hedge your bet a little with hard-starting loads is to specify a good sensorless vector drive.  If the chosen model is capable of building an accurate motor model before running, it will generally produce more torque per amp than open loop V/Hz types.

RE: VFD's on submersible motors

Sorry, I'm confused as well. It is not a condition of the pump motor that holds the current at 110%,it is the VFD that has been set to this. A value 3 x FLC is high for starting, and is what you would set a soft start for. If the pump is started at 110% full load current then it of course has to accelerate the pump and the water column. The manufacturer may have worked out that this gives such a long time to start that the motor/pump will be damaged. You need to ask him the design basis of the 3xFLC fiqure

RE: VFD's on submersible motors

Hello dk18

There is some confusion here. Yes it is typical for submersible pumps to require a start current of around 300%, and it is also typical for air cooled pump motors to require a starting current of around 350%.

In your application, you re looking at using the submersible pumps on a VFD, and in this case, you will not see a high start current.

The start current is high when there is a large difference between the frequency of the supply, and the rotational speed of the rotor, i.e. the motor is operating at high slip.
When you use a VFD, this should never happen. I like to describe this as "You never 'start' a motor on a VFD, you run the motor from zero speed" The frequency of the supply applied to the motor begins at zero Hz and is ramped up from there, so the slip is never high and the "start" current never occurs.

I would not expect to see any significant change in the current characteristics of a submersible pump on a VFD compared to a surface pump on a VFD.

Best regards,

Mark Empson

RE: VFD's on submersible motors

Marke is right, and to further that thought my suspicion is that the 300% current reference was meant for Reduced Votlage starting such as solid state soft starters or autotransformer starters. Many submersibles use hydrostatic pressure for lubricating the bearings which require a minimum pump output immediately in order to perform. If you were to start out with too low of current (again on a RVSS or RVAT starter), you may cause incremental damage to the bearings every time you start. The equivalent issue when using a VFD for submersibles is minimum speed and the amount of time you must not exceed in getting there. It could be that from their experiences, they think that even with a VFD you will need to apply that much current to achieve an acceleration rate fast enough to satisfy their design requirements. I would ask them for clarification though, I too think this is excessive given the way a VFD works, and maybe they just don't understand. Even if you wanted Locked Rotor Torque from the motor (the absolute MOST is can deliver from a standstill), the VFD should be able to deliver that with no more than 220% current, and usually around 150% depending on the application.

"Venditori de oleum-vipera non vigere excordis populi"

RE: VFD's on submersible motors

Hello dk18,

As indicated by both Marke & jraef, I expect that the supplier of the submersible motors is confusing two different technologies ie soft starters and variable speed drives.

If you start the submersible via a soft starter, the starting current is likely to be in the range of 2.2 to 3.5 x FLC dependant upon both motor and starter design characetristics.

If you start the submersible via a variable speed drive, starting currents in the rane of 1.0 to 1.2 x FLC can be expected.

You should note also that subject to submersible motor design you may need to accelerate the pump to operating speed within a certain period of time. That is of particular importance in submersibles that contain a thrust bearing. The rapid acceleration acts to lift the rotor section of the pump thus allowing the thrust bearing to lubricate before the rotor returns to its correct position.

Manufacturers of submersible motors who make use of thrust bearings in their designs typically state:

a) When using with reduced voltage starters, starting time and stopping times are not to exceed 3 seconds.

b) When used with a variable speed drive minimum (starting) frequency must be greater than 30hz ie for 50hz applications.

Hope the above helps.



RE: VFD's on submersible motors

If the motor is required to accelerate from 0% to 30% speed in 1s (mentioned in the original post), it may be possible that the VSD requires a 300% overload.  Acceleration time is dependant on acceleration torque (VSD current) and system inertia.

It is also interesting to note that there will be 600ft of motor cable.  This will effictively "rob" the motor of VSD current due to the cable charging current required by the cable.

Although it does not seem reasonable to request 300% overload from the VSD, it may actually be required!

RE: VFD's on submersible motors

This start current is normal even in the Area of around 7-8 Times FLA is common on very large pumps. We are testing a 1800Kw pump that at D.O.L. start requires 1400A start current
at 6300Volts, Back that out through the transformers to the tune of about 20,000 A at 450V at the gen's. If you run through a VFD then the start current will be more in line with the actual running current ar the 125% max VFD current setting s. THe other option would be to electronic soft start, if you do not plan on using the VFD for anyother reason other than to start the Pump.

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