Monitor induction motors to drive power quality

Motors are essential pieces of electrical equipment. The most common of motors in use today is the poly-phase induction motor, with more than 90% of these being squirrel cage induction motors. The poly-phase induction motor is preferred for several reasons:

• It is relatively inexpensive
• It has a rudimentary design
• It is easily replaced
• It operates reliably
• It has a range of mounting styles and environmental enclosures.

Due to the significant capital and operational investments made by companies in induction motors — investments that impact the bottom line — knowing the condition of these motors is vital. Induction motors are generally robust, but they can fail prematurely. Causes of motor failures include poor maintenance practices, improper lubrication, harsh operating environment, inadequate source voltage or misapplication of the motor. All of these issues have one thing in common: excessive temperature rise.

Excessive heat is the nemesis of motors (Fig. 1). Temperature rise can originate in the bearings (lubrication, alignment, etc.) or the windings (design, voltage, etc.) or be imposed by external conditions (ambient temperature, atmosphere, etc.).

A permanent power monitoring device can provide a great deal of information about induction motors. By monitoring the voltage, current and temperature, today’s monitoring devices can provide data on many aspects of an induction motor, including:

• Quality of the motor’s terminal voltage
• Energy usage
• Loading concerns
• Excessive cycling
• Starting characteristics
• Environmental considerations.

Although each aspect is important, this article will focus on detecting problems with the terminal voltage to increase the life of an induction motor.

Induction motors have nameplate ratings to help ensure that the motor is used properly. The nameplate data should be observed as closely as possible, but there are times when external factors cause variations beyond the approved constraints of the motor. The quality of the terminal voltage depends on each phase’s magnitude, angle, frequency and duration of any deviation from the rated voltage. Deviations in one or more of these factors can reduce the operating life of induction motors. Combinations of these factors are grouped into eight categories of terminal voltage problems that can affect poly-phase induction motors:

• Undervoltage
• Overvoltage
• Unbalance
• Harmonics
• Transients
• Sags
• Swells
• Frequency deviation.

Induction motors are designed to operate within a limited range around their rated voltage (NEMA MG1 specifies ±10%). At full load current, a 10% overvoltage at the motor’s terminals can substantially increase the core losses of the motor resulting in overheating. An elevated ambient temperature exacerbates the problem. The overvoltage will always be less at the motor’s terminals than is measured by the monitoring device due to the voltage drop of the circuit.

Low voltage at the terminals of a fully loaded motor also results in additional heating, due to increased current flow. Adequately installed protective devices such as overload relays should limit this problem during operation, but starting during low voltage conditions is particularly taxing on the motor. When starting during severe undervoltage conditions, the developed torque may not be sufficient to allow the motor to come up to speed.

If the monitoring devices are located remotely from the motor, the voltage drop between the two should be accounted for when setting the alarm thresholds.

Voltage unbalance (including single phasing) is a major cause of motor failures (Fig. 2). Voltage unbalance in fully loaded poly-phase induction motors produces a disproportionately higher current unbalance (Fig. 3). The current unbalance causes additional losses in the motor resulting in a temperature rise (Fig. 4). Ultimately, increased heating can stress the motor’s insulation, shortening the life of the motor.

Many monitoring systems provide the ability to measure voltage unbalance using either the IEC method (based on symmetrical components) or the NEMA method (based on the maximum deviation from the average). Both methods are acceptable as long as the chosen method is used consistently. Measuring the voltage unbalance on an unloaded system is a more valid approach than measuring voltage unbalance on a system with loaded three-phase motors. Loaded three-phase motors have a tendency to “rebalance” an unbalanced system (to a degree), potentially masking the true severity of the problem.

Distorted voltage harmonics are additional frequencies that are integer multiples of the fundamental frequency. Voltage harmonic frequencies produce additional heat in the motor. Heating resulting from the I2R losses increases due to the additional harmonic currents. Eddy current and hysteresis losses are affected by the frequencies at the motor terminals. The higher frequency components associated with harmonics increase these losses.

Voltage harmonics include positive, negative and zero-sequence components depending on which harmonic frequencies are present (Table 1). Positive-sequence components develop torque in the same direction the motor is turning. Negative-sequence components develop torque in the opposite direction than the motor is turning. The zero-sequence components have no effect on the motor’s torque, but generate ancillary losses. The torque developed by the positive and negative sequential components opposes each other, resulting in higher currents and additional heating within the motor.

IEEE Standard 519-1995 provides a good guideline for the acceptable levels of voltage distortion to loads (including motors). A broad recommendation is to establish the voltage distortion monitoring limits at 5% total harmonic distortion (THD) and at 3% for any particular harmonic frequency. The types of connected loads and system impedances are major factors in the level of exposure a motor has to voltage distortion.

Transients are very fast (sub-cycle) discontinuities in the ac waveform on the plant’s electrical system. They are often caused by switching events or even by lightning. Voltage transients stress the insulation on the motor’s winding causing it to degrade over time or sometimes catastrophically fail. Factors that contribute to the effects of voltage transient include its magnitude, duration, rise-time, associated energy or even system impedances.

Damage due to voltage transients is generally either turn-to-turn or turn-to-ground depending on whether the electrical system is grounded or ungrounded. Due to their inductive nature, motors appear to be open circuits at high frequencies, often resulting in damage to the first turn or two of the windings. This condition is a classic indicator of damage due to voltage transients.

The ability to detect and alarm on high-speed voltage transient events is vital to ensuring longevity in the motor. Detecting transients requires that monitoring devices have a fast sample rate and a high dynamic range. Thresholds should be configured to detect at least two times the motor’s rated voltage. In order to minimize the filtering effects of the conductors, the voltage should be monitored in close proximity to the motor’s terminals.

Voltage sags and swells are momentary decreases and increases in the steady-state system voltage, respectively, as opposed to the long-term variations of undervoltage and overvoltage. Voltage sags can impact motors and their driven loads; voltage swells less so. Voltage sags reduce the available torque, stressing the motor and heating the windings.

The primary concern regarding voltage sags is their effect on a motor’s controls. During certain voltage sag conditions, the contactor’s coil loses its ability to effectively hold the contacts together causing the motor circuit to open. This problem becomes more complicated when the voltage sags to the threshold of the contactor’s ability to hold the contacts together. The contacts may then begin to bounce resulting in arcing, heat and damage. The variables that affect a motor’s susceptibility to particular voltage sags include the pre-event parameters, phase angle of occurrence, transitional phase shift and the contactor’s characteristics.

The coil should be expected to operate over a range of 85–110% of the rated voltage. The irony of motors is that their controls are sensitive to voltage sags, and yet the inrush currents associated with motor starts produce voltage sags.

The recommended power monitor settings for voltage sags and swells are 90% and 110% of nominal, respectively. This range ensures that all pertinent data is captured and available for analysis if a motor fails or operates erratically.

In most developed countries, governing bodies impose tight frequency constraints on utilities, so frequency deviations on the utility grid are rare. However, it is possible to experience significant frequency deviations when operating independent from the grid as is the case if you generate some or all of your own electricity. Three-phase induction motors are designed to operate most efficiently at their rated frequency. Significant frequency drift results in additional heating of the windings.

The recommended guideline for monitoring motors that may be vulnerable to frequency shifts is ±5% of the rated frequency. This range is further restricted as the terminal voltage deviates from the rated voltage of the motor.