Minimizing the risk of failure in motors/adjustable speed drives

June 9, 2006
Only the end user of a product is in the position to evaluate the risk/reward relationship for a particular application.

If the probability of a failure occurring within one year is two percent and the cost of the failure is $200,000, then the expected loss in that year is $4,000. You can minimize or control the risk of combining the motor and the drive on a given application by knowing some of the most important failure factors in either purchasing the motor and drive separately or by using the package concept. The factors that affect the purchase are plant standardization, the ability of the adjustable speed drive to perform in the application, communication requirements, and the ease
of installation.

The most widely used adjustable speed drives are pulse width modulated inverters with either V/Hz control or open loop vector control. Many drives can be operated in either mode to provide additional flexibility without additional costs. The open loop vector control gives improved speed and torque control and higher starting toque per amp than does the V/Hz drive. The downside of the open loop vector control is a longer set-up time to optimize performance.

There are several considerations in buying pulse width modulated adjustable speed drives. Inverters come in standard voltage ratings and their rating must match your line voltage. From a reliability point of view, lower voltages are easier on the motor.

The current ratings of the inverter must match the motor current requirements both at full load and during acceleration. Confirm the inverter current rating with the motor manufacturer, especially on motors operating below 1,200 rpm and whenever acceleration torque is critical. The linear relationship of motor current and torque deteriorates as torque output increases above the 140 percent level.

It may be useful for the drive to have an adjustable carrier frequency to optimize system reliability. Modern inverters typically use carrier frequencies of two to 20 kilohertz. Lower carrier frequencies generally impose less long-term stress on the motor insulation system and reduce the incidence of bearing shaft current damage. Higher carrier frequencies have a positive effect on motor efficiency and on motor noise levels.

The high switching rates of inverter power devices can place high switching voltages across the motor terminals. These high voltage surges are primarily a function of the rate of change of the voltage output (dV/dt) for the inverter and the length of the cable connecting the inverter to the motor. Any generalizations of acceptable cable lengths are misleading because there are many switching topologies and devices used on inverters. You can expect the manufacturer of the inverter to provide adequate guidance on expected surges for cable lengths up to 400 or 500 feet. This is important knowledge to have when selecting the motor. Special high frequency filters might be required to prevent ringing within the cable itself if its length exceeds recommended values.

Most pulse width modulated drives are either dual-rated or available as different drives suitable for variable torque or constant torque loads. The variable torque pulse width modulated drives typically have less overload capability and cost less.

The three-step approach

Unfortunately, there is no cost effective "guaranteed" failure-free motor that fits every application. The best way to evaluate the risk of failure for the motor selection consists of a three-step approach. The first step is to fully understand the required operating and environmental considerations of your application. The second step is to educate yourself about the potential problems in connecting AC motors to pulse width modulated drives. The third step is to enlist the assistance of motor and drive manufacturers knowledgeable about the latest offerings in motor and drive technology. Their experience with motors and drives compatibility goes a long way toward system reliability.

The application and operation parameters must be defined to properly specify the motor. The risk potential greatly increases if the parameters are not considered. As a minimum, know the following before attempting to select a successful motor/drive package.

Know the speed versus torque profile of the load. The two most common profiles are variable torque used for centrifugal fans and pumps and constant torque used for conveyors, extruders, positive displacement pumps, and similar loads. Variable torque loads are the easiest applications for motors/drives because load horsepower varies as the cube of the shaft rpm for centrifugal loads.

For example, a variable torque load requiring 10 hp at 1,800 rpm only requires about 4.2 hp at 75 percent of base speed (1,350 rpm). The self-cooling capability of the motor is greatly reduced at low speeds, but the motor easily dissipates the lower internal heat losses. Applications in which a pump or fan has a damper or a valve on the output changes this relationship and must be studied.

Constant torque loads present a different problem. To maintain constant torque at low speeds, the motor requires relatively constant current throughout its speed range. This results in constant heat generation that must be dissipated at low speeds.

The second item is the maximum and minimum speed required from the motor. Speeds greater than the designed nominal speed may require precision balancing of the rotor. The maximum safe continuous overspeed for NEMA Design B motors is defined in NEMA MG1 Section 30. If you need even higher speeds, consult the motor manufacturer.

The next factor is the motor horsepower required at "base" frequency. The most common base frequency is 60 Hz.

You should know which motor enclosure is required for environmental conditions. The most common enclosures are open drip-proof and totally enclosed-fan cooled. The environmental conditions dictate which motor to use. Use a TEFC motor outdoors if the risk of moisture or dust contamination exists. Use open drip-proof motors indoors in clean and dry areas. Don't overlook hazardous Division 1 and Division 2 areas that require special consideration not covered here.

The fifth consideration is the voltage and frequency of available commercial power.

Finally, specify the duty cycle of the motor. This is the length of time the motor runs at various speeds within the required speed range. If a continuous duty cycle is assumed for all speeds, the motor must be oversized or designed to cool itself during the worst case condition--the minimum continuous operating speed for constant torque loads.

Possible problems

There are potential motor problems that may occur with pulse width modulated drives. The first issue is motor winding heating. The motor operating temperature at rated full load is always greater under adjustable speed drive power than it is under sine wave power. The reason is the increased harmonic content of the adjustable speed drive output voltage and frequency. Tests show that the operating temperature can increase five to 25 percent over the temperatures resulting from sine wave power. The magnitude of the increase is dependent on the motor design and the carrier frequency--transistor switching speed--of the pulse width modulated output.

A higher carrier frequency reduces extra temperature rise, but causes other problems. Most motors with a 1.15 Service Factor and Class F insulation provide sufficient thermal margin to compensate for the extra temperature when operated at rated speed and load. However, the extra heating attributed to the drive harmonics is only one portion of the heating picture.

Constant torque loads require constant motor current at low speeds, which results in constant heating. Dissipating heat at low speeds may require auxiliary motor cooling from a constant speed blower package mounted on the motor, an oversized motor, or a specially designed motor with a large thermal window. Most motor manufacturers have standard catalog motors that address the thermal issues of constant torque applications.

Another issue seldom mentioned is the effect that motor bearing temperature has on lubrication life. A rough guideline is that raising the temperature of the lubricant by 15 degrees C halves lube life. Motors operated at extremely low or high speeds continuously may require special greases or lubrication frequency. Only the motor manufacturer can offer advice for these situations.

A common motor issue that must be considered is the voltage stress the adjustable speed drive imposes on the motor windings. Pulse width modulated drive output is continually switched on and off rapidly. The relative duration of the on-and-off times determine the effective voltage output waveform. The motor sees this on-and-off switching as a transient voltage surge. A higher carrier frequency means more frequent pulses.

The magnitude of the voltage spikes at the motor terminals can be as high as three times the DC bus level of the drive. The actual level of the voltage spikes or overvoltage at the motor terminals is primarily a function of the voltage rise time and the cable characteristics.

These high voltage spikes cause early insulation failure in one of two ways. The repeated spike-induced dielectric stress eventually fatigues the insulation. This is very much like the repeated bending of a piece of metal. The second way is by high voltage levels ionizing the air trapped in voids in the insulation. This process, known as corona discharge, erodes insulation by bombarding it with ions.

Most standard motors wound for 200 to 600 volts input use the same insulation materials. These materials have a nominal voltage rating of 600 volts. Therefore, few if any voltage stress failures have been documented in 230-volt applications. However as one increases the line voltage, the potential for winding failures increases. NEMA Standard MG1, Section 31 for inverter duty motors recognizes the problem. This standard requires the stator insulation for low voltage motors to withstand 1,600 peak volts. This effectively addresses most 460-volt applications but only some of the 575-volt applications. A new standard to be published soon better addresses the 575-volt applications.

Magnet wire manufacturers contributed to the cause by developing wire insulation reported to have better resistance to voltage spikes. The problem is that there are no industry standard test procedures to prove or disprove the claims that the wire is spike resistant when used as a component of the complete motor insulation system. Despite this shortcoming, many motor manufacturers have seen spike resistance improvement using their own test procedures. Most now use the wire in their inverter duty motor product lines and some in their premium efficient motors. The inverter manufacturer is in the best position to predict the surge voltage. You should review that information with the motor manufacturer to properly select the motor insulation system.

NEMA Section 30 limits the use of standard motor insulation systems on inverter applications to 1,000 volts peak whereas the limit under NEMA Section 31 is 1,600 volts. For years people used the standard insulation system on line voltages of 460 volts and less with a reasonable degree of success. Our experience indicates that the failure rate increases on standard insulation system by a factor of three to four when used on inverters compared to sine wave power.

When reviewing these failures, two points stand out--machine wound motors are more susceptible to failure than hand-wound motors and over half the motors that failed had contaminants on the motor windings. The overall failure rate was less than one percent and it should be noted that the peak voltages at the motors were less than 1,250 volts. Be conservative when choosing motor enclosures to maximize your probability of success.

Final consideration

The potential for bearing currents exists on variable speed induction motor drive systems. Although the phenomenon of bearing currents seems to be understood in theory, much work needs to be done before it can be accurately predicted. The National Electrical Manufacturers Association has a newly formed committee to develop application guidelines and standards for this potential problem.

The semiconductor industry brought the problem of bearing currents to the forefront several years ago when they noticed that motors developed unacceptable vibration levels after only 18 months in operation. Upon inspection, they found classic bearing current fluting of the inner race of the bearing. Several solutions were found: shaft grounding, bearing insulation on both bearings, and special electrical filters.

Explanation of the phenomenon

First, we can say that every power electronic converter generates a common mode voltage. This voltage is a function of the switching speeds of the devices, the switching patterns, and the bus voltage of the inverter. The motor bearing current is a common mode component of the current resulting from the capacitive coupling of the motor windings, the rotor assembly, the bearing, and the motor frame ground.

Current guidelines are based on experience and some modeling. We did not find any real field problems when inverters first became popular back in the eighties. The carrier frequencies were generally below 2.5 kHz and the turn-on time of transistors was slower--a lower value for dV/dt. The general assumption is that if there was damage being done, it was inconsequential compared to other wear factors on the bearing. We still believe that to be true on switching frequencies below four kHz. There are many successful applications using higher carrier frequencies but there is also insufficient data to make a definitive statement.

Most standard applications do not require a motor drive package from a single supplier if you ask the right questions of your suppliers. For applications that are critical or for applications that require high response, selecting a packaged motor/drive set gives you the advantage of single point of responsibility.

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