In a February 2019 Pumps & Systems article, we reviewed how, over the years, the pump industry has been introduced to more advanced and versatile methods to control pump speed.
To achieve true variable speed control, we are looking at the following methods or technologies: fluid drive, eddy current drive, wound rotor motor, adjustable voltage direct current (DC), adjustable frequency alternating current (AC), magnetic drive and steam turbine. Part 1 of this series reviewed fluid drive, eddy current drive and wound rotor motor. This article will review the other methods or technologies.
Electronic Speed Control Drives: Adjustable Voltage DC
The oldest electronic speed control methods are the DC drives, which are also known as DC motor speed control systems. The speed of a DC motor is directly proportional to armature voltage and inversely proportional to motor flux; either armature voltage or field current can be used to control the motor speed. DC motors have become expensive and most DC motor speed control systems have been retrofitted with an AC motor and AC variable speed drive. AC variable speed drives are less expensive, more available and more efficient than DC systems. Many DC drive systems have been replaced where possible with AC variable frequency drives (VFDs).
Adjustable Frequency Drives
A VFD is the most popular method to control the speed of an electric motor driven pumping system.
A VFD is defined as an electronic device used for controlling the rotational speed of an AC electric motor by controlling the frequency and voltage of the electrical power supplied to the motor. A basic drive system consists of an AC motor and VFD managed by a control system (Image 1).
A method of control is required to vary the speed of the drive. This control method can be as simple as an on/off switch and a speed potentiometer controlled by the operator. More complex systems often incorporate a programmable logic controller (PLC).
Larger systems will usually use a distributed control system. This is basically a host computer running a software package that allows the operators to both monitor and control their overall system by one or multiple interface screens.
The drive has an embedded microprocessor that governs the overall operation of the VFD controller. This microprocessor has an operating system firmware that is not accessible to the VFD user. User-defined programming and parameter adjustment is usually done through the operator keypad. This allows the user to customize the VFD controller to meet specific process, motor and equipment requirements.
Unlike the other speed control methods discussed in this article when applying a VFD, the following concerns must be addressed to ensure optimum reliability: added heating of winding (Class F or H Insulation); added winding insulation stresses; reflective wave or voltage overshoot; added chance of bearing currents (insulated bearing, grounding brush, earth ground); added chance of vibration issues; effect on sound levels; large motor concerns; how VFD will be used and key details needed to choose large motors for VFDs.
The nonsinusoidal VFD waveform contains harmonics and peak voltage or current in excess of normal sine wave grid power. On low voltage VFDs, it is common for the motor to see an additional 10 to 15 degree temperature rise. On medium voltage VFDs, motors typically see only a 3 to 5 degree temperature rise.
Additional concerns specific to the motor when applying a VFD: motor torque, speed and temperature; operation above base speed; running current; starting current; motor efficiency; sound levels; motor cable length and grounding.