APPLICATION NOTES

TOSHIBA

Application Examples

• Pumps
• Sawmill Applications
  • Canterlines
  • Log Merchandizer

Progressive Cavity Screw Pumps

In the oil patch a typical method of pumping oil out of the ground is via the use of a Progressive Cavity pump, commonly called a “screw” pump as the rotor of the pump resembles a corkscrew, and effectively augers the oil up out of the ground. This pump is at the bottom of the hole and is connected to the surface by a rod. This rod can be 2000 to 3000 feet long depending on well depth, and is 7/8" to 1-1/4" diameter. At the surface, the rod is coupled to an electric motor. The ratio of the sheaves and/or gearbox is such that the rod runs slower than the motor and torque multiplication is provided to the rod.

The rod acts like a spring with a driving force on one end (the motor) and a resistance (the screw pump) on the other end. As such, the rod will have several turns of potential energy stored over its length when running. When the motor stops, this potential energy is released, causing the motor and polished rod combination to spin backwards.

Depending on the well depth and the resistance of the load, the stored energy in the rod can cause the motor/polished rod to reach potentially dangerous speeds. While there are mechanical brakes to prevent this from happening, the ASD should also act as a secondary safety to limit the speed that the motor achieves in this backspin mode.

Normal start-stop commands for the ASD are from a pressure switch mounted by the well head. During normal operating conditions, the pressure switch will close allowing the ASD to ramp up to set speed. Speed search is required on all starts as the motor may be back spinning as per above.

If the pressure in the line becomes too high, the pressure switch will open and stop the ASD until the pressure drops to an acceptable level. Stopping should always include DC injection braking to limit the back spinning effect for 20 seconds. The desired result of the braking is not to hold the rod still, but to allow it to unwind slowly rather than snap into the reverse direction without any resistance.

The torque required driving the rod could occasionally rise to levels great enough to break the rod. This can be caused by swelling in the rotor of the screw pump, sand getting into the pump, etc. When encountering this scenario, it is desirable to attempt to work through this overload, as it may be temporary. If this overload condition continues, then the ASD will automatically decrease the output frequency to the motor and stop the drive, latch it off, and annunciate the stop condition as an over torque. Again, on stopping, the ASD will DC inject the motor at a low level to retard the backspin. An operator must initiate a restart under this condition.

Speed is set in different ways depending on the end user. Some use potentiometers, some use the keypad, some use the Hampton Power remote monitoring software.

DC injection braking (line items 146 & 147) has been successfully used to control backspin and Overtorque trip level (line item 171) to limit the torque to the rod to prevent rod breakage.

Canterlines

Log Merchandizer

Merchandizer Application

Merchandisers are on the input side of a sawmill and are used to cut logs to the desired length. Typically, the logs are scanned and the subsequent information is fed into a computer to determine a “solution” of how to optimally cut the log - how many lengths and how long each of those should be.

There are various styles of merchandisers.  Merchandising requires that either the log be positioned and / or the saws be positioned. In this particular case, the saws are mounted on carriages and the carriages are positioned.

Issues surrounding this application include providing enough torque to meet the customer’s acceleration and deceleration rates plus accuracy and speed of getting the carriages into final position. The existing “machine” had limited information available so a field visit was required to determine the application parameters so that drive and motor sizing plus optimal gearbox ratios could be calculated to produce the required torque. The customer had a preference for a specific position controller and issues such as interfacing the controller to the ASDs had to be determined.

There was a significant improvement in responsiveness that reflected the customer’s initial specifications. Both speed and positioning accuracy were dramatically improved by the conversion from hydraulics to AC adjustable speed drives.