A Review of Variable Speed Drive Technology: Part 2

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.

Image 1. Basic drive system (Images courtesy of the author)

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.

Image 2. Components of a typical steam turbine driver

Maintenance

The VFD must be in a clean, filtered environment. Heat the sinks, vacuum away dust and do not use compressed air. If heavily soiled, used a light natural fiber brush. Do not use a synthetic brush. For Type 12 enclosures with cooling fans, replace the air filters as necessary.

Conduct a thermal scan, paying special attention to the connections. Check the cable lug torque. Use a megger to measure insulation resistance.

The typical life expectancy of a VFD is five to 10 years. After 10 years, most drive OEMs will discontinue the manufacturer or replacement parts. In addition, VFD technology continues to change, making older drives obsolete.

Magnetic Drive

The principle behind a magnetic drive or coupling is like that of an eddy current drive. The magnetic drive replaces the physical connection between motor and load with a gap of air. Motor torque is transferred to the load across an air gap. Varying the air gap between the magnets and conductor changes the strength of the magnetic field and, hence, controls output speed.

A few features of magnetic drive technology:

• No-contact power transfer. This eliminates vibration, reduces noise, tolerates misalignment, provides overload protection, extends motor and equipment life and reduces overall maintenance costs.
• Energy efficiency. The application and load requirements will determine the efficiency.
• Quality. Technology improves product quality and optimizes process rates.
• Soft start/stop. Reduces the motor’s startup power demands and the resulting brownouts, alleviates paying for peak power, allows downsizing of motors, increases motor life and reduces maintenance.
• Simplicity and ruggedness. System can be maintained in-house and used in harsh conditions.

Steam Turbine Drive

Steam power is one of the oldest technologies in the industrial sector providing power through the industrial revolution and into the 20th century. However, there are some limitations when applying a steam turbine.
First and foremost, you need steam and you need a source to generate the steam. This requirement limits the use of turbines in certain applications.

The most notable pumping application for steam turbine drivers is powering boiler feed pumps.

A steam turbine is powered by the hot gaseous steam generated by a boiler. The steam enters the turbine through a nozzle that controls the speed of the steam (Image 2). The turbine is fitted with blades mounted on a shaft that is coupled to the driven component (pump) that turn as the steam blows past them. The blades have tight clearances that contain the steam, maximizing the efficiency of the turbine. The steam expands and cools as it flows through the turbine blades.

Image 3. How an impulse turbine works vs. a reaction turbine

The rotating element (shaft and turbine blades) are contained in the turbine’s outer casing.

This casing must handle the high pressure and temperature produced by the steam.

There are two types of steam turbines:

• An impulse steam turbine features a jet of steam from a fixed nozzle that pushes against the rotor blades driving the blades forward (Image 3). Pressure drops take place in the fixed blade (nozzle).
• A reaction steam turbine does not use nozzles. The rotor blades are configured to form convergent nozzles using the reaction force produced as the steam accelerates through the nozzles. The steam is directed onto the rotor by the fixed vanes in the stator (casing).

Efficiency

The efficiency of a steam turbine driver or any engine can be defined as its ability to convert the input energy into useful output energy, which is expressed in Equation 1. Steam turbine efficiency or isentropic efficiency is the efficiency that compares the actual output with the ideal isentropic output to measure the effectiveness of extracted work.

Efficiency (ɳ) = Output/Input
Equation 1

Maintenance

Steam turbines require the correct steam pressure at the turbine inlet and high steam quality in order to ensure optimum reliability.

One guideline for maintenance to keep in mind is that the steam must be of highest quality. Also, steam supply lines, valves and casing should be insulated to prevent loss of latent energy. The steam supply lines must be properly configured for condensate removal, correctly sized drip pockets and steam traps.

Additionally, excessive forces to the turbine flanges should be mitigated. They include pipe dead weight, thermal expansion, thrust and spring rate caused by different expansion joints.

Impact as a result of improper installation will result in premature bearing failure, nozzle degradation, turbine blade failure or distortion and premature coupling failure.

Which speed control is best? It depends on the application and business model. Also, consider your issues and concerns—such as energy efficiency, reliability, uptime, production concerns or environmental.

Read part one of this article here.

To read more content from columnist William Livoti, click here.