To operate your pump at optimum efficiency, you may have to periodically adjust the flow rate in the slurry system. But in a slurry system, you can't vary flow by using valves as you do in a simple water-pump system. Instead, you have to use the pump itself to control the flow.
To vary pump speed, you need to vary the motor speed, and the most popular method of doing that is to use a Variable Frequency Drive, which consists of an alternating current (AC) motor and a Variable Frequency Controller (VFC), an electrical device that varies the frequency of the electrical power coming in to the motor.
Here's how varying the frequency of the current will change your pump speed.
An AC motor is constructed of a rotor, which spins, and a stator, which remains stationary but contains several poles that create a magnetic field that also spins and runs the motor. AC motor speed depends on the number of motor poles, the frequency of the power applied to them and a factor called slip.
To determine the speed of a motor, you would use the following formula:
120 x Frequency (Hertz) / Number of Motor Poles = RPM
The frequency of AC current may be 50 or 60 hertz, depending on where you are operating. Most of North America and part of South America use 60 hertz power. Most of the rest of the world, with a few exceptions, use 50 hertz. Maps can be found online. Thus, a 4-pole motor operating at 60 hertz will operate at 1800 RPM:
120 x 60 Hertz / 4 Poles = 1800 RPM
NOTE: When you're using this formula to calculate motor speed, you may need to account for slip — that is, the difference between the rotor speed and the speed of the rotating magnetic field in the stator. A typical AC induction motor has approximately three percent slip, so an 1800 RPM motor actually operates at about 1750 RPM.
Using the formula above, you can see how you can change the speed of the motor in two ways:
- change the number of poles (which is impractical during pump operation)
- change the electrical frequency (which is relatively simple when you use a VFC)
Do you have specific questions on how to control the flow rate in your pump? Contact our Technical Services Team.In addition, pump users who work internationally need to be aware of the changes that may occur when they use AC power supplies in countries outside the United States. For example, a pump that delivers 100 horsepower (hp) in the United States will not deliver the same horsepower in another country. Why? Because many countries deliver AC power at 50 hertz — not 60 hertz as in the U.S.
To calculate the difference, you can use the following formula:
100 hp x 50 hertz/60 hertz = 83 hp
This difference is important because if you truly need 100 hp, you'll need to use a bigger motor.
To determine actual motor speeds when using a VFC device, always consult with the equipment manufacturer.
