Since the introduction of variable speed electronic controls for DC and AC motors, users have noted a troubling problem; voltage potential is sometimes induced on the shaft of the motor.  The resulting current can discharge through gearbox bearings, through the motor bearings, or through the encoder or tachometer attached to the motor.  This discharge causes the premature failure of motor bearings and/or encoder bearings. This paper discusses the phenomenon of encoder and motor damage due to motor shaft currents through encoders, and proposes several remedies and guidelines .ProblemBoth AC and DC motors controlled by electronic speed controls can experience “shaft…

By Adalberto José Rossa - Formerly with WEG Automacao, now with Infineon This is the third and last part of a series of technical articles which studies the stray currents in installations with typical variable frequency drives (VFDs) and three-phase AC Induction or Permanent Magnet motors (PM motors). In the previous parts the CM circuit modeling and main influencing parameters, the measurement of the CM currents and the main effects were discussed. In this third part the mitigation techniques are presented and general conclusions based on the materials presented in the three parts are presented along with techniques to reduce…

By Adalberto Jose Rossa – Formerly of WEG Automacao, now with Infineon - with contributions from Mark Zawadzki from International Paper Read the first in this series here. Introduction This is the second part of a series of three technical articles, which studies currents in installations with typical variable frequency drives (VFDs) and three-phase AC Induction or Permanent Magnet motors (PM motors). In the first part the circuit modeling and main parameters influencing the stray currents were presented. In this second part the measurement strategies for CM currents are presented as well the main effects of the common mode currents…

By Adalberto José Rossa - WEG Automacao, with special thanks to Mark Zawadzki from International Paper for the text revision and valuable ideas and suggestions. This technical article studies the stray currents which appear in installations with typical variable frequency drives (VFDs) and three-phase AC Induction or Permanent Magnet motors (PM motors). These currents flow in electrical power cables through stray capacitances to the electrical ground system. Part 2 will discuss the stray current measuring strategies and the effects of the common mode currents and part 3 will deal the counter actions to mitigate the undesired effects. The entire series…

In some drive applications, several motors are connected to the load at various locations. If the motors are mechanically coupled together through the driven machinery, load sharing must be considered. In this type of application, torque can be transmitted from one of the motors through the mechanical elements of the driven machine to the location of the other motor or motors. If too much torque is transmitted from one motor to other parts of the machine, that motor will be overloaded.This article will discuss only applications that can be handled by several motors connected to one adjustable frequency drive. We…

Third in a series from blogger David Sellers Senior Engineer – Facility Dynamics Engineering This series started with a post that looked at drive and motor efficiency vs. load characteristics, including improvements that have over the years in drive efficiency.  Then, I did a post that looked at why folks involved with HVAC systems want to vary the speed of motors in the first place and what some of the benefits are.In this post, I’ll take a look at what happens when you combine a drive with a motor to serve a fan system.  In general terms, what I am…

In my last post, I took a look at the results of some research I did regarding variable speed drive efficiency and motor efficiency.  In this post, I’ll take a look at why designers, in particular, building system designers, are driven to apply variable speed technology in their systems.  This, combined with the previous post, will lay the foundation for the next post, where I will look at what happens when you apply a VFD to motor serving a centrifugal machine in an HVAC system.  Its mostly good news, but there are a few surprises.Variable Speed and Building System LoadsFor…

By Dick Orndorff, Square D / Schneider Electric, Raleigh, N.C.Specifying AC drives in Division 16 of electrical specifications is critical to optimizing the cost efficient operation of an industrial plant. Understanding and accepting this assertion requires familiarity with how AC drives can be applied, including their compatibility with motor control centers (MCCs).In the past, AC drive designs of Variable Voltage Inverters (VVI, 6-step adjustable frequency), adjustable voltage and wound rotor controllers, were nearly always specified in Division 15 of mechanical specifications, because of motor coordination issues. Today, with the modern pulse width modulated (PWM) adjustable frequency drive, motor coordination is…

Recently, Variable Frequency Drive (VFD) manufacturers have seen a trend to move the VFD specifications from the mechanical portion of the specification (section 15XXX) to the electrical portion (section 16XXX). While this trend may stem from concerns about harmonics, it may also be influenced by certain VFD manufacturers who have a vested interest in having the drives specified in section 16XXX. Another reason for this trend may be that mechanical department personnel at consulting firms are often not comfortable with the harmonics issue. It is the writer’s opinion that specifying VFDs in section 16XXX can cause unnecessary concerns. This article…