Variable Speed Drive for Pump Control


Thread Starter


Hi all,

I am going to design closed loop system. my process variable is the flow rate, and i would like to control it using variable speed drive. the pump we use is 3 phase, and it has manual variable speed indicator so i can change the speed manually. it's from Mono Ltd.

please i need your help. still i need a variable speed driver for this pump to adjust my output according to the flow rate comes in? or i don't need to? if i do need variable speed driver, please could you help with which one i can use (specification).

please let me know if you need more information

thank you
A basic requirement for using this pump in a closed control loop is that you will need to be able to adjust its speed by the controller. You say that you now adjust this speed manually. You are missing an important piece of hardware: the actuator that can translate the controller output signal to the pump speed adjustment. I was not able to find such a device in my quick search of the Mono Pump website, but perhaps a Mono Pump sales person can help you.

In any event, the relationship between rotational speed of this pump and flow rate is linear, so you should have no problems in controlling flow, if you are successful in finding an actuator.
Thanks dick caro,

i think no need to actuator as my output controller (PLC) will send the signal directly to the pump wire to change the frequency so that will change the flow (control it) based on the signal comes from the controller. so its like bypass the the manual adjustment. that's what i think and please correct me.

so i still looking for the proper specification for my pump VFD

I have used those pumps to control flocculant in a mining application.
You don't need the manual speed adjustment but you can just leave it set to one spot like a coarse flow adjustment.

I have also seen them with a Raise/Lower speed adjustment (small split phase motor) on the gearbox.
is it possible to use VFD for this type of pump that has manual variable speed (hand wheel)?

I think you answered your own question - YES. All you need is to replace the pump 3-phase motor starter with a VFD sized for that pump motor. Leave the hand-wheel manual adjustment in the maximum flow position, and connect the PLC output (usually 4-20mA) to the VFD configured such that the 4-20mA signal will become the motor speed setpoint. Then 20mA output will be the maximum speed of the pump motor, and 4mA should be zero flow.

>is it possible to use VFD for this type of pump that has
>manual variable speed (hand wheel)?
Check this VFD manufacturers website:

P1000 series is a variable torque control VFD designed to be used with variable torque load applications such as Fans & Pumps. Pick the VFD with the suitable HP and input voltage for your application and connect it to your PLC as dick caro mentioned in his post.

These VFD's also have an internal PID closed loop control function. So you can also connect your Flow meter/transducer signal to the Analog Input of this VFD and have the VFD control its speed based on your desired Flow set point which you can set on the VFD's display keypad. There are application examples in the above mentioned website.

Good luck!
OK, do <b>NOT</b> use a VFD that is designed for Variable Torque pumps, they are purposely under sized (to save money) knowing that you do not need full torque capability for a centrifugal machine until you are at full speed, and even then it is virtually impossible to overload the machine if applied properly.

A Mono pump is a "Positive Displacement" progressive cavity type pump. The VFD must be capable of making the motor deliver deliver full rated torque immediately and continuously, including the ability to be briefly overloaded in order to deliver Break-Down Torque (200% of normal) for a few seconds. For that you need a "Constant Torque" or "Heavy Duty" rated VFD.

As to what brand, find a supplier near you with a good reputation, inventory on hand and a knowledgeable specialist. Since you have already displayed that you lack experience with applying a VFD, you need someone who can help you and be available for you until you get up to speed and experienced. Also pick a major brand name, smaller companies in that industry tend to come and go quickly, so if you buy something and 3 years later you need it repaired, you have a paper weight if the mfr no longer exists. MOST importantly, do NOT buy the cheapest thing you can find if the operation and protection of that pump is important to you. Cheap drives are being made in China and sold on eBay at unsustainable prices that are absolute garbage and will cost you more in down time than you save on hardware.

as you mentioned there's internal PID controller in VFD. so can I used to replace the PLC? I mean I connect my transducer to VFD and control the flow rate (no need for PLC to control the machine?)

thanks and looking forward to hear from you
If your VSD has an analog input, your flow transmitter's 4-20mA signal can be connected as the measurement signal for the PID controller in the VSD, and you do not need a separate PLC for this control loop. However, you will need a way to input the setpoint and the tuning constants for the PID and tell the VFD to use one of its PID function blocks to do this control loop. Theoretically you can use the front panel of the VFD to do all this, but almost all VFDs have software to operate on a Personal Computer (PC) to set up and configure the VFD operating software. Very frequently, the PC will connect to the VFD using an Ethernet port and cable. Ask the VFD vendor about their software support. This may become a much larger project than you anticipated.

Dick Caro