Motors & Drives Information

Questions concerning various applications where variable frequency drives and AC motors often arise. Discussions with and answers from various users indicate that the operational behavior of some applications are still a mystery. This article will clarify some of the concerns raised by typical users.

Typical Question

I am an electrician in a can manufacturing plant. Requirements for cooling water used to be around 100gpm for our ironers. There have been several modifications, and we need to increase the flow. Our superintendent wants to know if we can regulate the flow of a bigger pump and motor with a variable frequency drive (VFD). He wants to replace a 100gpm pump and a 5hp motor with a 200gpm pump and 10hp motor and regulate flow by changing pump speed. Is this something you should do with a centrifugal pump?

Discussion Group Answers

"It is best to have the pump Torque-speed characteristics properly aligned with the motor Torque-speed characteristics to establish appropriately integrated motor-pump set. You select your operating points on the pump curves and then determine the maximum motor rating you need. You then select the VFD for the motor."

"Check your requirements. If you need 5 hp at 100 gpm, then depending on your system requirements, it is probable you will need more than 10hp for 200 gpm. Centrifugal pumps have a power/speed relationship that is cubic, the power requirements of the pump reduce dramatically as the speed is reduced. The reduction in speed also affects your pump delivery. In general, variable speed control offers excellent pump and process control."

"Precise control of pump processes is possible with VFDs. Pressure in water can be maintained to closer tolerances. A VFD is an electronic controller that adjusts the speed of an electric motor by varying frequency/voltage and then the amount of power supplied. That allows the motor to continually adjust to work just hard enough, rather than running full speed all the time."

"Depending on the actual power requirements and a number of other issues, if the present pump is not heavily loaded, you might try just adding a drive and running the pump motor in excess of 60 Hz. Due to current limiting and maximum available voltage, the maximum torque will fall off as the speed increases, but you might not actually be using all 5 Hp in the present installation."

"Using VFDs you can control the speed of the motor and hence the flow of the pump. The advantage lies in the fact that power decreases by the cube of the speed decrease. Thus a 5% (0.95) speed decrease will cause the power requirement to decrease by cube of the decrease (0.95x0.95x0.95 = 0.86).

"This is provided that the motor is suitably matching the pump (10hp for 200gpm @ a certain head) and also make sure that the motor you buy is an inverter-rated motor. But you have to check the economics behind this, since the power decrease in a 10hp motor would be lot less than the investment done in buying an inverter and an inverter-rated motor, even if you are considering a couple of years for payback."

Clarifications

We live in a fixed frequency world. All standard pumps and motor are designed to operate at a constant frequency (50 or 60 Hz). However, variable frequency drives allow fixed speed motors and connected loads to operate at other than those standard frequencies.

Changing the frequency and associated voltage allows the motor to change its operating speed. As long as the voltage-to-frequency ratio is constant, the torque is reasonably constant. In most cases, the motor's operating speed range, for constant torque, is 3 or 4 to 1. In centrifugal pump applications, the torque requirement deceases rapidly as the speed decreases and the motor's operating speed range can expand to 10 to 1 or more. Operating speeds from 180 to 1,800 rpm are not unusual for AC motors used in centrifugal pump applications.

Hammering in low flow situations using valves for flow restriction are normally not a problem when variable frequency drives are used to directly control the motor/pump speed. Most variable frequency drives reduce the volts-per-hertz ratio as the speed deceases. This results in cooler operating motors.

Since less horsepower is required as the speed deceases, the demand for torque also deceases and the volts-per-hertz ratio can be reduced. Typically values are 115 volts at 30 Hz for a 460-volt, 60Hz-rated motor. An unknown fact is that input current to the variable frequency drive is less than the typical no load rated current of the AC motor. Also, where some line voltage unbalance is present, some phases may not indicate any current. This "unbalance current" presents no problem but may disturb some plant electricians.