The squirrel cage ac induction motor is widely used and delivers more power to industry than any other electric device. There is a catch however. These motors are sometimes hard to start because of the large inrush current (locked-rotor current), which ranges from about seven times full-load current for a standard motor to twenty times full-load current for some high-efficiency motors.
Inrush current causes dynamic stresses on electrical distribution systems. Starting large motors may cause a transient-line undervoltage condition. A large starting current causes a considerable voltage drop on the line due to line impedance. This is true especially on older power distribution systems that often have over-loaded conductors and undersized substation transformers with poor voltage regulations.
Line undervoltage, although temporary, may cause nuisance conditions, such as lights dimming, to more serious ones such as contactors or relays dropping out. There are known cases of generator start or automatic transfer to an alternate power source as a consequence of an undervoltage condition due to starting large motors across the line.
Locked-rotor current in mechanical terms translates as locked rotor torque. This high starting torque causes mechanical stresses on the drive components or equipment, leading to excessive wear. It may also result in fatigue failures of couplings, bearings and gearboxes.
Reduced-voltage starters are the most popular method used to prevent locked rotor current. These devices control voltage from the instant it is applied to the instant it reaches the nominal line level and the motor is energized.
When selecting a reduced-voltage starter, engineers should consider several factors:
• Initial voltage needed to break loose the load.
• Necessary or allowed acceleration time.
• The shape of the voltage ramp curve, which is how the voltage changes and how it increases during the acceleration time.
• Motor demand for current and driven load demand for torque.
• Limits on the inrush current and starting torque due to driven loads.
• Starter efficiency and its effect on power demand.
• Electrical codes for installation and operation.
• Environmental conditions of the installation site.
• Utility penalties for exceeding peak demand and undervoltage conditions for transmission lines.
The various types of reduced-voltage devices are:
The autotransformer starter uses three contactors and an autotransformer with selectable taps. It enables:
• Two-step voltage starting with smooth acceleration and current-limiting of the transformer inductance.
• Flexibility of application choices.
• Acceleration times up to 30 sec.
The initial voltage is usually 65% of the line voltage. It can be set as low as 50% to reduce the starting torque. Or it can be set as high as 80% for higher starting torque when necessary, however, the resultant transformer losses may reduce efficiency.
Constraints are its high price and its large size, when compared to other types.
A primary resistor starter uses contactors and resistors. The number of contactors determines the voltage steps.
Low circuit inductance makes voltage steps abrupt. The large voltage drop on the resistor bank and heat losses lower this starter’s efficiency. Heating usually limits the acceleration to 5 sec.
This starter offers good starting torque and relatively low cost. But it requires space for the resistor bank and contactors.
A primary reactor starter is similar to the primary resistor type and offers smooth acceleration to 15 sec. Efficiency is poor due to additional inductance. Its poor power factor negatively affects motor flux and torque-producing current components. Applications are limited to medium and high voltage large horsepower motors.
Continue on page 2
A part winding is commonly used for reduced-voltage starting. It is an acrossthe- line starter that uses contactors to apply line voltage to a fraction of the motor winding during acceleration. It’s limited to motors with nine or twelve leads. Starting times are limited to 15 sec. The impedance it adds in the circuit does not affect starting torque. Starter efficiency is good. However, the need for a special motor and its rough acceleration make it the least attractive choice.
The wye-delta is a full-voltage starter that requires a 6-lead motor and contactors that allow the motor winding to be connected in wye configuration during acceleration. Wye-connected motor windings experience phase voltage rather than full-line voltage. Efficiency is good; inrush current and initial torque is reduced; but switching to full line is not a smooth transition.
The solid-state starter uses silicon-controlled rectifiers (SCRs) to apply regulated voltage for smooth motor starting. Such starters offer extended acceleration times, adjustable-voltage ramp curves, good efficiencies, adjustable current or torque limits, and a variety of protective features. They offer a soft start and smooth stop, electronic overload protection, adjustable torque boost to break loose loads with friction, and reduced initial torque to avoid mechanical stresses. Other reduced-voltage starters require additional separate devices to offer these features.
Engineers should be cautious sizing these starters. The SCR has a definite maximum current rating and can not handle high current levels for more then a few milliseconds or cycles. Thus, it is important to evaluate the necessary starter full-load current rating for each given application.
Solid-state starters use semiconductor switching technology and are similar to solid-state adjustable-speed drives or solid-state power supplies. They draw non-sinusoidal current waveforms. However, harmonic distortions because of voltage regulation take place for only a brief acceleration time. The IEEE standard 519, which defines harmonic distortion, prescribes the distortion limits in the steady-state operation.
Selecting solid-state starters
In a typical motor circuit, the major elements are the motor that moves the load, controls (including the starter) that govern how and when to start and stop the motor, and the line that delivers the power. Each of these elements affect your choice of solid-state starter. In addition, engineers must consider:
Ratings and horsepower. An SCR solid-state starter is selected based on voltage rating, continuous-current rating, and the product of both — the horsepower rating. Its critical job occurs during the starting, which is a dynamic process. For low-inertia applications, such as directly coupled pumps or fans, blowers, and compressors, choose a starter based on the full-load current, voltage, and horsepower rating of the selected motor. Remember these starters use semiconductor power-switching devices with limited peak-current rating. This limit, usually between 300% and 600% of the continuous current rating, limits the starting torque.
Heat. Most SCRs are installed on heat sinks and they operate continuously at the rated ambient temperature under the full load. However, operation at the peak current rating is limited to a minute or less, which limits acceleration time.
Inertia. When selecting starters for high inertia loads, use caution. The starter must be selected based on its ability to accelerate the load. Typical highinertia loads, such as centrifuges, crushers, loaded conveyors, chippers, grinders, mixers, and stirring machines, require maximum torque for extended periods of time to accelerate.
Calculate the inertia of the load, gearboxes, and couplings at the point of the motor shaft, and determine the torque required to drive the system.
Current. The next step is to obtain the motor full-load current and full-load torque from the motor nameplate. Starter-specification data, from manufacturers catalogs, lists continuous current rating, peak current or current limit, and maximum acceleration time.
Continue on page 3
To calculate maximum available starting current :
Compare the calculated required acceleration time with the starter’s maximum acceleration time setting. If required starting time is less then maximum starting time, a solid-state starter will suffice. If not, select a larger starter, obtain new data, and repeat the calculations.
Static friction. A starter must be able to overcome initial static friction and get the load moving. Static friction may be significant in applications involving traction loads like carts or conveyors. Solidstate starters offer a short duration voltage boost to provide a torque pulse to break frozen loads. However this pulse torque can not be higher than the previously calculated maximum torque available from the starter. For high static friction loads, select a starter based on its capability to deliver pulse torque.
Braking. When selecting a starter with an integral dc dynamic brake option, evaluate braking needs using the equations above. Here, acceleration time becomes stopping time. Refer to the motor and starter’s specification data and verify that excessive braking torque for fast deceleration will not exceed current rating limits. The thermal effect of rapid cycling can be cumulative, so consider that too. For rapid breaking and frequent starting or breaking duty cycles, consult the manufacturer.
Line voltage. Starters are available to meet the demands of any application once the horsepower rating matches the given voltage. However solid-state starters are current and voltage rated and the horsepower rating is more of a reference rating. The designer must consider the specific line voltage in a given power system.
Voltage varies due to changing loads, decreasing as the system load increases. Use the nominal line voltage only when designing a new system.
There are many power systems with permanent low-voltage conditions. For example, consider a 20-year old system designed with a 208/120 V wye-line power. Today this system supports many new loads and the line voltage barely exceeds 180 V. A motor installed in such a location will draw 15% more current. A solid-state starter chosen by its horsepower rating for a nominal voltage of 208 V may not have a sufficient current rating. Therefore, select a starter based on its continuous and short-time current rating rather than horsepower.
Solid-state starters offer a means of adjusting ramp-up time and shape of the voltage curve, while controlling the current within the selected limit. This makes them a good choice on older, regressed, and overworked power systems. They are a good choice where the substation transformer taps were moved to overcome a line voltage drop.
Short-circuit protection. Each installation has a specific level of available short-circuit fault current. It is usually specified in thousands of symmetrical, steady flow, amperes (kA sym).
A starter as a combination controller must protect from overload, locked rotor current, and short-circuit current levels. Starters are offered with and without short-circuit protective devices. Those without extra devices have a limited shortcircuit rating. A starter itself is designed only to interrupt currents of the magnitude of its continuous ampere rating. Circuit interrupting devices such as fuses and circuit breakers safely open the circuit under a fault-level current condition. The interrupting rating is given in thousands of symmetrical amperes (kAIC sym).
Manufacturers of starters provide a short-circuit rating for the starters or the combination controller. The rating depends on the type and rating of the interrupting device. Fast-acting fuses, for example, provide more protection and enable a higher short-circuit rating than thermal magnetic molded-case circuit breakers. Once the available fault current level is known, select a starter with a short-circuit rating higher or equal to the available fault level.
Continue on page 4
Utility rates change with peak current demand. Eliminating the peaks that full voltage and conventional reduced-voltage starters draw from the line can save money. This is not always the case, but a solid-state starter offers more adjustability than reduced-voltage starters and the highest degree of starting current regulation. (The other starters regulate voltage.)
What are your control needs?
Solid-state starters offer more then just on/off control. Many offer protection from overloads, jams, and locked rotors due to abnormal load conditions. Features such as ground leakage, ground fault, single phase, phase unbalance, and shorted phase protection are available on many models without the need for a separate motor-protective relay. Starters protect from abnormal line conditions like phase loss, phase unbalance, phase reversal, and over and undervoltage. Many models do this without extra voltagemonitoring relays. The most advanced designs offer communication capabilities with networks and can report power data such as kilowatts, kilowatt-hour demand, and power factor.
Aside from this, you may have to conform to various electrical and local codes. Your starter may require a disconnect, a fully rated bypass contactor for acrossthe- line starting, a thermal overload relay, or an emergency-stop push button. The control system may require a factory pre-wired circuit for 3-wire or 2-wire operation. Interfacing with most starters will require isolated contact closures.
Manufacturers publish data on these factors as well as environment, ambient temperature, elevation, and indoor or outdoor enclosure classifications that will help you ensure the starter meets your design criteria.
Stan Komander is product mangager, Baldor Electric Co., Ft. Smith, Ark.