Bearing Protection for VFD-Driven, Explosion-Proof Motors Improves ReliabilityBy Shaquila | Published on Sep 15,2015
In plants that process combustible materials, these motors avoid electrical damage often caused by energy-saving inverters.
A new line of explosion-proof, inverter-duty motors provides a solution to the problem of electrical bearing damage caused by energy-saving inverters (also known as variable frequency drives) in petrochemical plants and other facilities that process combustible materials.
These motors are designed with longer and tighter fitting joints (also called flame paths) that, in the event of an explosion in the motor, cool the escaping gases enough that their energy is insufficient to ignite when they exit the enclosure. These new explosion-proof motors with bearing protection meet the requirements of National Fire Protection Agency (NFPA) Division 1 and Division 2, Class I and Class II while protecting the bearings from inverter-sourced shaft voltages.
First of its Kind
Safety standards prohibit the external mounting of potential arc-producing devices (including conventional grounding brushes) in Division 1 or Division 2 hazardous locations. However, installation in a Division 1 motor enclosure is permissible, as long as the previously described flame path is not compromised. This internal-mount design is approved by Underwriters Laboratories (UL).
The new National Electrical Manufacturers Association (NEMA) premium explosion-proof motors carry an ingress protection (IP) rating of IP 54 as published by the International Electrotechnical Commission (IEC) for severe-duty motors regularly exposed to water spray, chemicals or particulates. Available from 3 to 50 horsepower, these stock motors include a Class F insulation system and are name-plated 60/50 Hertz, 190/380 volts at the next lower horsepower.
Types of Hazardous Locations
The NFPA standards define Class I areas as areas where flammable gases or vapors are or may be present in sufficient quantities to produce ignitable or explosive mixtures. Class II areas are those where combustible dusts, such as grain or coal dust, are or may be present.
A Division 1 location is one in which ignitable concentrations of flammable gases, vapors or dusts may be present in ambient air, either continuously or intermittently, during normal operations or maintenance. Division 2 locations are those in which flammable gases, vapors or dusts may be present intermittently but are normally confined and can escape only if there is a rupture of the vessel or system in which they are contained. Explosion-proof motors are not required in Division 2 locations.
Figure 1. Typical of high-amplitude EDM discharges, this waveform shows the buildup of voltage on an ungrounded motor shaft, followed by a discharge. The vertical line indicates a sharp drop in voltage. (Graphics courtesy of Marathon Electric Motors)
Shaft grounding devices are not permitted on non-explosion-proof motors that will be used in Division 2 locations, even if they are installed inside the motors. This is because non-explosion-proof motors are not designed to fully contain an arc-induced explosion within the enclosure. End users seeking bearing-current protection for a Division 2 location should consider purchasing a Division 1 (explosion-proof) motor with suitable protection from bearing currents.
How Inverters Cause Bearing Failure
Because inverters can save 30 percent or more in energy costs, they have been cited as a key technology for those wishing to make their processes more energy efficient. However, inverters cause voltages and currents that can damage bearings. In fact, the costly repair or replacement of failed motor bearings can offset much of the savings an inverter yields and severely diminish the reliability of an entire system.
Although it is now common knowledge that two-level inverters cause unwanted motor shaft voltages, many end users who purchase three-phase alternating current (AC) induction motors do not realize that models labeled "inverter-duty" or "inverter-ready" might not be equipped to prevent bearing damage. While many such motors have inverter-rated insulation systems to protect their windings, their bearings are often ignored. If they are to be used with inverters, they need bearing protection.
Bearing damage is often overlooked until it is too late to save the motor. In fact, electrical damage has become the most common cause of bearing failure in inverter-controlled AC motors.
In 1993, inverter manufacturers introduced the first 1,200-volt insulated gate bipolar transistors. Transistor turn-on time dropped from 2 to 0.5 microseconds or less. The old 230-volt inverters were replaced by 460-volt and 575-volt models. These faster rise times and higher voltages increased motor shaft voltages dramatically, resulting in a sudden global increase of bearing failures during the 1990s.