A deceptive cause of motor failure is the hard-to-start load. You must take special care when choosing premium-efficiency motors to drive such loads.
Motor burnouts, nuisance tripping, and the resultant downtime are serious aggravations, particularly when there seems to be no obvious clue to the cause. After checking for the usual causes of motor failures, such as a bad bearing permitting the rotor to rub on the stator, overvoltages, overcurrent, improper electrical protection, drive misalignment, etc., suspect a fairly common but often overlooked cause: a hard-to-start load.
What is a hard-to-start load? A hard-to-start load is one that has high inertia. This kind of load takes excessive time to accelerate to full speed, ultimately causing motor overheating. Typical high-inertia loads are certain fans, blowers, pumps, and some kinds of machine tools. There's no precise definition of high inertia. Therefore, how can you pinpoint high inertia as the problem? Motor application engineers sometimes use this rule-of-thumb: "If the load inertia is more than twice the motor's own inertia, make a thorough analysis of the motor, load, and overcurrent devices." This approach usually requires more data than the typical user may have at hand, so you must use a more practical method.
For the plant/facility electrical person, an effective technique simply requires observation and recording of the acceleration time needed for the motor and load to reach full-speed during an across-the-line start. If this time is more than a few seconds, and if the application requires frequent starting, there's a good chance inertia is the problem.
Understanding high inertia and the motor To solve the motor problems caused by high-inertia loads, it is important to understand the capabilities and effect of the driving motor. For example, a NEMA Design B squirrel-cage induction motor is probably the most commonly used 3-phase motor. (Keep in mind torque is the motor's ability to start and turn the load.) The torque (developed by the motor as it goes from zero to full speed) remains low until it reaches about 80% of full speed.
During this period, the current needed to produce this torque remains close to the starting value of five to six times the nameplate full-load amperes. This produces high heating until the motor stops accelerating. If the load demands substantial driving torque, as most high-inertia loads do, the net torque available for acceleration is less, prolonging the acceleration time to reach full speed. Therefore, the high heating losses persist much longer with higher risk of overtemperature damage to the motor.
If starting is infrequent, this extra heating will have time to dissipate with only moderate overtemperature stress. But if starting and stopping is frequent, then the heat will be produced faster than it can be dissipated, leading to nuisance "trip-outs" and an ultimate "roast-out" of the motor.
Premium-efficiency motors are most vulnerable Since passage of EPACT (Energy Policy Act of 1992), a great many motors for common applications, as of October 1997, must be of energy-efficient design. Most have a much higher starting current and somewhat lower starting torque than the standard motors of old. This means when you must start a high-inertia load, the motor starting torque may not be sufficient for the job. Starting current can also be excessively high, causing trip-out delays, and possible damage to personnel/equipment.
Of particular concern is the Design E motor. This NEMA design motor is quite new, with only a few models available. It is very efficient, as given in NEMA standards, but it has a significantly higher starting current that can be troublesome.
Be especially watchful when replacing a standard NEMA Design B motor driving a load that may be somewhat hard-to-start, such as a conveyor belt or certain machine tools. Design B motors have maximum locked-rotor currents that are below NEMA maximum values (usually about six times full-load current). However, a Design E motor could draw a locked-rotor current as much as 55% higher than that of a comparable Design B motor. As a result, the existing motor starter and overcurrent protection would be inadequate.
If you're using the motor in motor-reversing applications, be extra suspicious. A full-voltage reversal from 100% forward to 100% reverse ("plug-reversing") will result in four times the normal acceleration losses. In addition, certain types of electrical braking can impose substantial losses. Typically, such special applications are usually carefully designed at the outset.
Effective solutions There are several solutions to the motor problems created by high-inertia loads, but some answers are more practical than others.
Internal temperature protection. First, consider the motor itself. If there has already been a motor burnout, a simple rewinding to the original design specification will result only in another such failure eventually. Instead, check with the rewinding motor shop about embedding internal-temperature protectors within the new winding. Such devices sense the actual winding temperature directly and will trip the motor starter when it reaches an unsafe temperature. This eliminates the nuisance tripping resulting from the inherently less-sensitive indirect relay-sensing methods. Furthermore, the direct temperature-sensing method takes into account the motor's thermal storage and cooling capacities.
Better insulation. When considering the motor rewind option, another solution to consider is the use of a higher temperature-class insulation system. This may not always be possible if the motor was already using the maximum Class H system.
NEMA Design D motor. For cases requiring frequent starting or reversing, such as some machine tool or automation applications, replacement of a failed motor with a NEMA Design D motor is a possible solution. This type of motor produces much higher average torque from zero to full-speed, and thus will accelerate and decelerate the load much more rapidly. Its higher resistance rotor design achieves this while drawing lower current, further reducing total motor heating. The drawback is its higher price and somewhat lower efficiency; but it can be a good trade-off for some high-inertia or frequent acceleration applications.
Solid-state protection. Modern solid-state protective relays offer very precise protection. The motor's losses are actually computed from current measurements using sophisticated "symmetrical component" techniques. Then, you match the motor temperature profile to the cooling profile to determine an approaching unsafe temperature. This provides a much closer degree of protection than indirect methods under more difficult conditions of operation. Until recently, the economic payoff has been valid only for more expensive motors or those critical to the process or operation. Recent advances in solid-state protective devices have made this kind of protection available in less costly designs.
It's important to remember traditional electromagnetic protective relays try to simulate the motor's heating and cooling characteristics through indirect methods (most often done by thermal-sensitive heater elements). However, the heater must be sized to permit the 600% normal starting current without tripping (for standard NEMA Design B motors), which means somewhat less protection at normal running current. For energy-efficient designs, such overcurrent protection is difficult to obtain. Thus, when high load inertia results in longer-than-normal starting times, the heater element may have to be oversized to permit starting. This is undesirable because it results in an even lower degree of protection as well as the possibility of eventual motor burnout.
Consult the motor and control manufacturer to achieve the best results. Always evaluate the motor's specifications to assure best results.
