Minimizing the risk of failure in motors/adjustable speed drives

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

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By Kirk Franz

PlantServices.com

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.