Controlling Progressive Cavity Pumps 

Mono Pumps are a form of positive displacement pump and from a control perspective should be treated like a liquids conveyor rather than a pump. The Moving Cavity pump displays very different characteristics to the standard centrifugal pump commonly used for water pumping and is able to cope with a wide range of liquids from water to highly viscous liquids and high solids content.

The Mono pump is a Progressive Cavity pump which moves fluid by means of a moving cavity along it's body.

The Progressive Cavity Pumps comprises a rotor mad up of a steel helix and a stator made up of a steel tube with a helical rubber lining. The wavelength of the rotor is exactly one half of the wavelength of the helix in the stator. Rotation of the rotor causes the cavity to move depending on the direction of rotation.
The Progressive Cavity Pumps comprises a rotor made up of a steel helix and a stator made up of a steel tube with a helical rubber lining. The wavelength of the rotor is exactly one half of the wavelength of the helix in the stator. Rotation of the rotor causes the cavity to move depending on the direction of rotation.
There are a number of different stator designs for different applications. Common designs are "Equal Walled" stators and "Unequal Walled" stators. These stator designs are used to cope with different types of fluids and solids sizes.
The interface between the rotor and stator is an interference interface and is lubricated by the fluid being pumped. If there is no lubrication, the friction increases dramatically. Operation of the pump under dry conditions causes a rapid deterioration of the stator and should be avoided.
If the "pump" is not used for a period of time, the rotor - stater interface can dry out and this results in a very high start torque requirement to get the rotor spinning. Once the fluid has lubricated the surfaces, the friction drops and the torque requirements reduce. The Progressive Cavity Pump is self priming provided that the inlet is immersed in the fluid to be pumped.
Typically, the "dry" start torque of a Mono pump can be in the order of 160 - 180% depending on the design of the stator and rotor. In order to start the pump under dry conditions, you must provide this level of torque. Under normal start conditions, the start torque requirements are much lower.

Direct On Line Starting

There is rarely a problem starting a Progressive Cavity Pump using Direct On Line starters as full torque is applied to the pump. There can however be issues if the Locked Rotor Torque of the motor is less than 180%.

Soft Starting

Starting a Progressive Cavity Pump with a soft starter is very much like starting a conveyor with start torque requirements in the order of 40 - 60% except under dry start conditions when 150 - 180% start torque may be required. This can easily be achieved using an MSF soft starter by setting up a torque ramp with an initial start torque of 40% and a final start torque of 180%. The torque will ramp up until there is sufficient torque to start the pump rotating.
A alternative way is to use two parameter sets with one parameter set programmed for normal starting conditions and a second parameter set programmed for dry starting conditions. The operator would need to operate a switch to select the normal start or the dry start.
Make sure that the motor is capable of supplying the required torque. The motor should have a Locked Rotor Torque of at least 180% and preferably greater than 220%.

Speed Control

Controlling a Progressive Cavity pump with a Variable Speed controller can be problematic if it is treated as a pump. The Mono pump should be treated like a conveyor carrying liquids rather than solids.
Select a drive that can develop at least 180% torque at zero speed. Some drives will need to be upsized along with an upsized motor, others such as the VFX can develop this torque provided that the drive is rated appropriately. Many drives on the market can develop full torque from 5 - 10 Hz upwards, but struggle to develop the high torque at zero speed unless they are operated in closed loop vector mode.
The start torque can be increased by using a lower speed motor. For example, if the application calls for a 4 pole motor, use a 6 pole motor with it operated at 50% over speed for normal operation. This will increase the low frequency torque by 50%.
The drive must be set up as though it was controlling a loaded conveyor. DO NOT enable field weakening on a Mono pump. Field weakening is applicable only to centrifugal loads such as centrifugal pumps and fans and can be used as a means of reducing the iron losses in the motor when full flux is not required.

DO NOT USE a PUMP Rated drive for a Mono Pump.