VFD's on submersible motors
VFD's on submersible motors
(OP)
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
460V/3ph/60Hz
174FLA
submersible motor
variable torque load
600ft cable run
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
460V/3ph/60Hz
174FLA
submersible motor
variable torque load
600ft cable run





RE: VFD's on submersible motors
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
Barry1961
RE: VFD's on submersible motors
RE: VFD's on submersible motors
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
RE: VFD's on submersible motors
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
http://www.lmphotonics.com
RE: VFD's on submersible motors
"Venditori de oleum-vipera non vigere excordis populi"
RE: VFD's on submersible motors
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.
Regards,
GGOSS
RE: VFD's on submersible motors
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
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.