Specifying an Automatic Metering Pump

When a specifying an automatic metering pump, we must specify both the variable speed motor and the controller.  Let’s take a look at the system to help you make the motor and controller choice that will satisfy the needs of your application.

When you want to specify a metering pump the first two elements to define are matching the wetted end materials of construction to the liquid being pumped so the pump can safely and economically handle the liquid and determining which pump model will deliver the range of flow that you need for the application.  In order for the Madden pumps to operate automatically Madden pumps use a variable speed motor and a controller.  Here is what you need to know for specifying a motor and controller for an automatic metering pump:

• Madden diaphragm metering pumps come equipped with a manual output adjustment knob located on the top of the pump for easy access.  The Madden pumps can be adjusted over a 10:1 turndown range by using the adjustment knob.  So for a 60 gallon per hour pump, for example, the manual output range would be from 6 to 60 gallons per hour.   But changing the pump output manually requires someone to determine that a change in pump output is required and then to physically change the output knob setting.
• But what about controlling the pump output automatically?  The Madden metering pump output will change very linearly with changes in the drive motor rpm.  So, if the pump motor is running at 1,725 rpm for a 60 gallon per hour pump, by reducing the motor speed by 50% to 862 rpm the pump output will drop to 30 gallons per hour.  To change the motor speed you need to have an electronic variable speed drive and a matching variable speed motor.
• There are two basic variable speed drive systems: AC and DC.  The AC system requires an inverter duty variable speed motor, and an AC variable frequency drive (VFD).  The VFD can change the normal 60 Hz current frequency down to 6 Hz, and by changing the voltage with the frequency being supplied to the motor the motor speed is changed.   VFD’s models are available for 1/60/115-230 and also for 3/60/230 or 460 factory power supply.  Models can also be used with 50 Hz power.  The KB Electronics VFDs Madden supplies come with a NEMA 4X water tight enclosure and an installed 4-20 mA signal follower.  An option for the controller is a remote/local selector switch.
• Now here is the most difficult concept for most folks to understand.  All AC power variable speed inverter duty motors are actually 3 phase motors.  The inverter duty motor rating means that the motor can operate with a variable power frequency which will change the motor speed.  If the supply power available to power the VFD controller is single phase such as 1/60/115, then the controller will convert the output power going from the controller to the motor to 3 phase.  So we will always supply an inverter duty 3 phase motor to operate with a controller that is plugged in to single phase factory electrical power.  There are no truly single phase variable speed motors, but that doesn’t matter to the pump user because his single phase powered controller is designed to work with the 3 phase inverter duty variable speed motor.
• The DC system requires a 90v DC variable speed motor, and a DC variable speed drive (VSD).  The VSD can change the DC voltage from 90 volts down to 9 volts, and by changing the voltage being supplied to the motor, the motor speed is changed and with the change in motor speed the output of the pump changes.   VSDs are available for 1/50-60/115 and 1/50-60/230 AC electrical factory power supply.  The KB Electronics VSDs are also NEMA 4X water tight enclosure with an installed 4-20 mA signal follower and an option is a remote/local selector switch.
• To control the VFD or VSD a 4-20 mA control signal is required to tell the VFD or VSD to change motor speed and pump output.  An instrument or a computer can be used to generate the 4-20 mA control signal by tracking a value of a plant system operation.  For example, an flow meter can measure the waste water flow from an operation and increase or decrease the value of the control signal it is sending to the VFD.   The VFD will need to be programmed to respond to the changing signal so it can vary the pump motor speed.
• Whether AC or DC variable speed systems are used, it should be noted that Madden pumps operate best with stroking speeds from 15-230 strokes per minute.  Speeding up a pump or slowing it down outside that range can impact the pump performance.
• Variable speed motors are standard totally enclosed motors, but we can also supply optional washdown duty and explosion proof motor enclosures to meet any special requirements for the environment where the installed pump will be.

If it seems confusing to specify a motor and controller for an automatic metering pump contact us, we’ll be glad to assist.  For more information on the KB Electronics VFDs, click on this link.  For additional help in specifying a metering pump for your application, consult the Madden Pump Selection Guide on our website, call us, or send us an email inquiry.  We are always quick to respond.