Back to basics: VFDs and harmonic mitigation

Variable frequency drives (VFDs) have problems with application and operations; learn how to mitigate common issues.

10/17/2017


Leaning Objectives

  • Understand the issues involved with hThis article has been peer-reviewed.eating in motors and motor bearings.
  • Learn about voltage stresses on motor insulation and harmonics in the input current lines.
  • Know how electrical engineers can design systems to mitigate these motor problems.

 

Figure 1: (A) shows the circuit of a 3-phase pulse-width-modulated (PWM) inverter. (B) shows the Line-to-line output voltage VAB of the PWM inverter. Courtesy: Syed M. PeeranFrom the time of their entry in the field of electric motor control in the early 1950s, variable frequency drives (VFDs) have continually evolved and been improved upon at a rapid rate. There are two primary reasons for this frenzied growth and refinement. The first is that adding variable-speed operation to most industrial apparatuses, such as pumps, fans, blowers, cranes, hoists, elevators, etc., results in considerable energy savings. The second is that VFDs offer the most efficient method of controlling the speed of ac motors.

Even though the basic technology has been in existence since the early 1950s, the HVAC industry, together with the water, wastewater, and chemical industries, did not promote widespread use of VFDs until the mid-1980s—a campaign that was initiated by a large electrical manufacturer. Since then, there has been fierce competition among VFD manufacturers to improve their products and make them more acceptable. Consumer education on the technology and the accompanying engineering issues also has risen significantly.

As with any emerging technology, several problems and issues have surfaced in the application and operation of VFDs. Four main issues are:

  • Additional heating in motors.
  • Motor bearing problems.
  • Voltage stresses on motor insulation.
  • Harmonics in the input line currents.

VFD designers and manufacturers have addressed these issues in different ways, some of them proprietary. Engineering impacts of the above four issues have been mitigated but not fully resolved. The present trend is that most of the VFD designs have now fallen into two main categories: the multipulse front-end type and the active front-end type. In other aspects, most designs offer the same features in control, instrumentation, diagnostics, protection, automation, and auxiliaries. The older current-source inverter (CSI), the line-commutated inverter (LCI), and the voltage-source inverter (VSI) designs have now faded out.

Figure 2: An input transformer and rectifier of a 3-phase, 6-pulse converter is shown. This is the circuit of the line converter of a 6-pulse VFD. Courtesy: Syed M. PeeranAdditional motor heating

Heating, in addition to that caused by I2R losses, in the stator of the motor is due to the non-sinusoidal currents forced into the winding by the VFD inverter. In the older CSI and LCI designs, the motor currents were significantly non-sinusoidal. Motor heating due to the harmonics was an issue of concern.

Motors were often derated to allow for the increased heating. However, with the modern pulse-width-modulated (PWM) VFD-driven motors, the motor currents are very nearly sinusoidal except for a small high-frequency component. This component, whose frequency is in the range of 4 to 16 kHz, is due to the PWM switching of the power devices in the inverter. The high-frequency component of the current causes additional local heating due to the skin effect in the stator winding conductors and in the stator iron teeth due to eddy currents.

Some manufacturers offer output filters to suppress the high-frequency under the trade names of “Sine-wave” and “Matrix” filters. Such filters essentially have series-inductor shunt-capacitor configurations. Output filters are normally housed in the VFD enclosure. Separate output filters also are occasionally used. The series inductor causes additional power loss and heat in the enclosure, lowering the overall efficiency of the VFD a little bit. The benefit gained in caparison with the additional cost and loss of overall efficiency is dubious in many cases.

Motor bearing problems

Motor bearing problems of electrical origin existed even before the advent of VFDs for motor control. It is impossible to constrain the magnetic flux to the stator and rotor iron and the air gap. There is always some flux produced by the stator winding, which links the shaft at the non-drive end (NDE) to the drive end (DE) of the shaft. Because of some magnetic dissymmetry between the NDS and the DE of the machine, there is a net 60-Hz induced voltage, which drives a small current in the axial direction in the shaft. This current finds a path through the shaft, through the two bearings, and through the stator frame. The current causes additional heat and can cause accelerated wear of the bearings. The path of the current is broken by insulating one of the two bearings, usually the NDE bearing. This insulation can be added for a small cost.

A second phenomenon caused by the VFD has been responsible for accelerated wear, minute arcing across the bearing lubricant, “fluting” of the bearings, and eventual bearing failures in many VFD-driven motors. The phenomenon is due to electrostatic induction of a high-frequency voltage between the shaft and the bearing race, causing the film of lubricant to break down electrostatically in the form of a tiny arc. The PWM inverter of the VFD impresses high-frequency pulses on the motor windings. Stray capacitances between the stator winding and the rotor and between the rotor and the stator frame, which normally have a negligible impact upon the operation of the motor, produce a high-frequency voltage across the film of bearing lubricant.

Figure 3: Input transformer and rectifiers of a 3-phase, 12-pulse converter are shown. dc outputs are connected in series or in parallel. Courtesy: Syed M. Peeran

In particular, the winding-to-rotor capacitance and the total capacitance between the rotor and the grounded stator frame constitute a capacitive voltage divider that produces a voltage across the bearings. This voltage discharges in the form of a tiny arc, which causes a small current known as the electric-discharge machining (EDM) current to flow across the bearings. This discharge causes accelerated erosion of the bearing surfaces and eventual destruction of the bearing. Induced voltage, which can cause arcing, is in the range of 5 to 30 V.

One way of avoiding damage to the bearing is to provide an alternative path for the bearing voltage to discharge. Such an alternative path can be provided by a shaft-grounding brush or a ring of conductive fibers that ride on the shaft to continuously ground the shaft. The grounding brush is a copper or a carbon brush riding on a machined surface on the shaft. Because the brush is subject to vibration and wear, it is an item that requires regular maintenance. Excessive vibration and incomplete contact would defeat the purpose of providing the brush.

A ring of conductive fibers, marketed as the Aegis ring, takes care of the vibration problem and is a good solution provided it does not need replacement too often.

Another way of reducing the bearing damage is to provide a rate of change of voltage (dv/dt) filter in the output of VFD. The filter reduces the bearing’s high-frequency voltage.

Voltage stresses in the motor insulation

When driven by the VFD, there are two ways in which the motor stator winding insulation can be stressed over and above the level normal as compared with non-VFD-driven motors:

  • The voltage-doubling action of the PWM pulses at the motor terminals.
  • The standing-wave phenomena when the cable length approaches the critical length.

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