The Mysteries of Motor Control
Unlocking the truth about motor controls requires diligent planning, extensive research and careful specification
Motor-control design is one of the more mundane engineering tasks to perform, and most engineers treat it as one of those 'necessary evils.' However, there are several aspects, not always considered, that can have a significant impact on the functionality, protection and safety of a motor-control installation.
One of the most important issues-and one that is often overlooked-is single-phase protection for three-phase motors. It is common for electrical utilities to experience local or limited-area single-phase conditions, and whenever there is a single conductor breakage or a single fuse interruption, there will be a single-phase condition for the areas fed by that circuit. Induction motors are adversely affected by these single-phase conditions, suffering from reduced torque, increased current in the remaining energized phases and increased internal temperatures. Personnel at electrical utilities and motor-rewinding companies report that single-phasing is the most common electrical failure mode for three-phase motors.
While some are of the opinion that single-phase protection is too expensive or causes unnecessary outages, this is not the case! Unless the motor-circuit protection removes power to the motor during single-phase conditions, a typical motor with a load of greater than 50 percent will fail due to overheating within two to five minutes. Not only is the process interrupted, but the outage time is also increased from the time it takes to restore all three phases to the time it takes to order, receive and install a new motor.
Most fuses, circuit breakers and overloads do not take a motor off-line due to a single-phase condition until after the motor is severely damaged. However, single-phase protection that is usually a part of a solid-state overload can take a motor off-line within two to three seconds of a phase loss, saving the motor from any serious damage.
Melting-alloy and bimetallic overloads have been in use for many years, each having its individual pros and cons. The melting-alloy type employs a eutectic metal alloy that melts upon application of a current higher than its rated value. Upon tripping, the metal alloy must experience a time delay to permit the eutectic to solidify before the overload can be reset. While this delay can be a nuisance, it can also be beneficial because it allows the motor to cool down before permitting it to restart. This overload is not ambient-temperature-compensated, however, so it may trip prematurely if the ambient temperature of the starter is higher than the motor ambient temperature.
The bimetallic overload has two advantages over the melting alloy type: it can be ambient-compensated and it can offer a reduced reset time. The ambient compensation permits the motor and the starter to be physically separated into locations with differing ambient temperatures. The bimetallic overload cools more rapidly and can be reset more quickly.
Solid-state overload devices offer all of the advantages of ambient-compensated overloads-requiring no compensation-along with many more benefits. The operating speed may be selected on most solid-state overloads, with choices ranging from class 10 to the slower class 20 or 30. The class of the device indicates the tripping time, e.g., a class-10 relay trips in ten seconds at six times its rated current. In addition, most of the solid-state overloads have single-phase, phase-reversal and low-voltage protection. The use of the solid-state overload type reduces installation costs for each starter from 20 to 30 percent, as there is no heater selection or installation time required. With these protective functions, the solid-state overload is a very cost-effective device for most applications.
Also, solid-state overload devices can reduce the stocking inventory needed for electrical equipment. In one manufacturer's catalog, there are over 3,800 cataloged overloads in 153 tables. While several of these are duplicated items, the devices may not trip at the same current, requiring the person selecting the heater to exercise some degree of skill. So, when the number of overloads or the type of starter is changed, the full-load current must be checked to assure that the proper overload has been selected (the Table above offers an example taken randomly from a manufacturer). In comparison, solid-state overloads require only nine different sizes to cover motors with full-load currents from 3 to 810 amperes (A).
Thermal-magnetic breakers, instantaneous breakers, motor-circuit protectors and fuses are the choices for short-circuit protection for motors and motor starters.
Many engineers and owners prefer the thermal-magnetic breaker because of its convenience: Whenever the motor breaker trips, the breaker may be easily reset and the process restarted without having to change fuses or maintain replacements. For overload or low-level fault conditions, the breaker works just fine, but when the fault escalates into a bolted fault, that is not the case. Under maximum fault conditions, the breaker opens and protects itself, but fault current flowing through the impedance of the motor overload and the conductors can cause them to heat to incandescence, resulting in their destruction. This may necessitate the replacement of the overload and branch-circuit conductors or, in the case of an exploded overload element, the entire starter may have to be replaced due to all enclosure surfaces being covered with products of ionization.
The instantaneous breaker is allowed by the National Electrical Code (NEC) if certain criteria are met:
The breaker must be fully adjustable over the range of anticipated inrush currents.
The breaker must be part of a listed combination motor controller having coordinated motor-overload, short-circuit and ground-fault protection. The instantaneous breaker is the general class of breaker in which the MCP breaker is a special case for lower current applications below 150 full-load amperes (FLA).
The motor-circuit protector (MCP) improves on the performance of the thermal-magnetic circuit breaker for motor short-circuit protection. Even though with bolted, three-phase faults there is really no change in performance between the two breaker types, the low-level fault performance of the MCP is significantly better.
With these more common medium-level faults, operating characteristics allow MCPs to open faster; variable magnetic features also let MCPs operate faster than thermal-magnetic breakers or fuses.
Fuses are the protection of choice where the potential of high-level faults is present. With fault duties above a few thousand amperes, the current-limiting, dual-element fuse can limit the fault let-through current to a level that leaves both the overload element and the branch-circuit conductor unscathed. However, there may be some incidental welding within the starter contacts. This type of starter protection is referred to as 'Type 2 Protection.'
Section 110-10 of the NEC addresses this situation, requiring that '... the circuit-protective devices used to clear a fault do so without extensive damage to the electrical components of the circuit.' Individually, any one of the protection schemes can potentially result in some damage to portions of the electrical system.
For optimum motor protection, MCP breakers should be used in concert with current-limiting fuses in starters that have solid-state overloads. This combination should result in the best protection over the entire range of overcurrents, from the lowest level overload up to the maximum available fault.
Today, converting motor controls to variable-frequency drives (VFDs) is a widespread practice that permits increased flexibility in operation and energy conservation. VFDs are being installed in both industrial processes and heating, ventilation and air-conditioning applications where either constant-speed drives or mechanical speed-adjustment mechanisms were previously the norm.
The proliferation of VFDs has produced an entirely new set of challenges and potential problems. The design of the motor/drive combination must assure that the motor is 'VFD-rated' so that it can withstand the increased internal forces generated by the distorted waveform and the varying frequencies of the VFD. Also, some measures must be taken to mitigate the harmonics generated by the VFD at the source. As has been discussed ('Waving Goodbye to Harmonics,' February 1996), the introduction of resonant, harmonic filters or the utilization of 12- or 18-pulse VFDs typically reduces harmonics to a level below the requirements of the Institute of Electrical and Electronics Engineers (IEEE) standard 519.
Designing conductors between the motor and the VFD requires more than merely checking the full-load current. Due to the unique waveform and VFD impedance characteristics, the peak voltage on the motor-to-drive conductors can easily exceed the breakdown voltage of the insulation. Limiting the distance between motor and drive, inserting series inductors or utilizing specially designed VFD conductors can reduce the chance of insulation failure. There are several software packages available that ascertain if there is a potential problem with the conductor length in a particular installation.
One of the benefits of VFDs is that they have integral overload, single-phase and ground-fault protection. These extra features provide a level of protection equal to-or better than-that provided by the solid-state overload relays previously discussed.
Some suppliers and manufacturers have marketed soft-start accessories as a means of saving energy and, thus, cutting costs. Starting a motor directly across the line produces a current on the order of six times the motor full-load current that lasts 3 seconds or so. This current rapidly declines to the motor full-load current once the motor achieves between 75 and 80 percent of its full-load speed.
Claims of energy conservation are based on the fact that the soft-start accessory reduces current surge to a level below the motor full-load current during the starting phase. This current reduction is supposed to result in fewer total kilowatt-hours consumed by the motor during starting, which reduces energy costs.
The problem with this claim is that the math does not add up (see 'The New Math of Soft-Start Savings,' page 32). Annual savings may be as little as $20 per device-a tiny amount as compared to the cost of installing a soft-start module, which can run about $2,500.
This is not to say that there is no place for soft-start accessories. They significantly reduce the inrush current on induction motors. Where periodic voltage dips are objectionable to the end-user, the soft start is far superior to other methods of reduced-voltage starting.
Engineers should also be aware of some of the NEC issues that are commonly overlooked in the design process. Some common oversights include:
For small motors with a current rating of 8 amperes (A) or less, the setting of the thermal magnetic breaker may not exceed a 15-A trip rating. This means that the breaker feeding one or more 120-volt (V) fan-coil motors rated at, for example, 150 watts each must still be sized at 15 A, not 20 A (NEC 430-52). In addition, the circuit is limited to no more than 10 motors, as increasing the breaker to a 20-A trip violates the above provision of the code.
Every motor must be grounded, either through the motor branch circuit conductor within the terminal compartment or to a grounding lug on the outside of the terminal compartment (NEC 430-12e). This assures rapid operation of the overcurrent device in the event of a phase-to-ground fault, limiting damage to the motor.
For motors having starters with a full-load rating of less than 100 A, the conductors must be sized as though they were rated 60°C in accordance with [NEC 110-14 (c)(1)]. The only instance where conductors may be sized based on the 75°column is when the starter and motor lugs are rated and labeled for 75°C. For starters with full-load ratings of greater than 100 A, the conductors must be sized as though they were rated 75°C in accordance with NEC 110-14 (c)(2), even if they are XHHW-rated at 90°C.
High-efficiency motors may have starting inrush currents that are greater than normal. In such cases, dual-element fuse ratings may be increased up to 225 percent of the FLA; breaker ratings up to 100 A may be increased up to 400 percent of FLA; and breaker ratings over 100 A may be increased up to 300 percent of FLA (NEC 430-52[c], Exception No. 2).
Ignorance isn't bliss
There are many hidden issues in the design of motor-control systems in modern facilities. For installations that do not have any faults or other malfunctions, and where NEC is not followed, virtually all of these design elements may be ignored.
However, it only takes one problem to demonstrate why these issues are important for complete system protection. Burned motor windings, NEC violations, motors vibrated apart and conductor-insulation breakdown are just some of the potential problems awaiting the unwary designer. Suitable application of the above principles can help result in superior protection of any electrical installation.
While soft-start devices are very useful in environments where engineers hope to reduce voltage dips, can soft-start accessories really be a way to cut energy costs significantly, as some suppliers and manufacturers often contend?
The theory behind some claims of 'amazing energy-conservation' is that the soft-start accessory reduces current surge to a level below the motor full-load current during the starting phase, resulting in fewer total kilowatt-hours consumed by the motor during starting, reducing energy costs.
To check the math, consider an example: Assume that a 50-horsepower motor is to be started both with and without a soft-start device. By comparing the energy saved by the soft-start device versus an across-the-line start during the starting phase, a value may be determined for the energy savings for one start. If this motor starts four times a day, the payback period for the procurement and installation costs of this device may be determined.
Assuming that the surge lasts for 2 seconds at six times full-load current (390 A), the expended energy may be calculated as:
390 A x 460 V x 1.732 x 2 seconds÷ 3,600 seconds per hour = 0.173 kWh.
So, the power content of the starting surge is 0.173 kWh. If the soft-start device actually saves all of the energy consumed during this starting phase, the total power saved during the year would be 252 kWh per year. At 0.08 cents per kWh, the end-user can expect an annual savings of a whopping $20.
Considering that the installed cost of this soft-start module is about $2,500, the simple payback works out to be about 125 years-outside of the most owners' ranges.
Sizing Motor Circuits: Back to the Basics
Sizing the elements of a typical motor circuit is an elementary task, but a review of the basics always helps keep everything in perspective.
The motor short-circuit protective device should normally be sized at the next breaker smaller than 250 percent of the motor full-load amps with a minimum 15-ampere breaker. If fuses are used, they should be sized at the next fuse smaller than 175 percent of the motor full-load amps. Manufacturers' published breaker selection charts may reduce these sizes even further, in some cases.
The branch-circuit conductor should be sized at the next conductor size larger than 125 percent of the motor full-load amps. The disconnects for the motor and the motor controller are both horsepower-rated and should be sized based on the rating of the motor.
For multiple motors , the National Electrical Code (NEC) modifies these rules for the additional motors. For a circuit breaker, add the full-load amps of all of the motors on a feeder and then add 125 percent of the largest motor full-load amps. The breaker for this feeder should then be sized at the next smaller standard breaker size. The same process should be used for fuses, except for adding 75 percent of the largest motor full-load amps. The feeder conductor should be sized by taking the sum of all of the motor full-load amps, adding 25 percent of the full-load amps of the largest motor and then selecting the next larger standard conductor size.
For the branch-circuit and overcurrent-protection sizing, the full-load current must be based on either NEC 430-150 or the motor nameplate values for general duty, three-phase motors. For torque motors , the NEC only permits the use of nameplate ratings to be used for overcurrent sizing.
The sizing for variable-frequency-drive (VFD) motors must use the nameplate rating of the power-conversion equipment where the nameplate rating is available. Where it is not available, 150 percent of the table values in article 430-149 and -150 should be utilized.