Standard Control Components
Not every motor-control application requires expensive drives or electronic controllers. In many cases, single-speed starters are all that’s needed, and they have been the most widely used motor controller for years. Now, newer ones offer engineers simpler, less expensive, and easier to apply controllers for their systems. These advanced controllers are also more intelligent, and come in many more types and sizes. The family includes manual controllers and starters, as well as magnetic, full-voltage reversing, autotransformer, solid-state, and reduced-voltage starters.
Easy does it
Manual controllers are still the simplest. They operate with toggle or rotary switches that open and close a set of contacts in series with the motor. They work best with low-horsepower, single-phase loads that don’t need remote control or aren’t switched frequently. Manual controllers are also suited for applications where overload protection isn’t required such as in impedance-protected motors, or where protection is integral to the motor such as in small fans with imbedded thermal cut-out switches.
Another device, manual starters, are similar to manual controllers, but they also protect motors from overloads. The amount of protection is usually adjusted through different values of heater elements or dial settings. Excessive current automatically opens the contacts to protect the motor windings from overcurrent, overheating, and failure. An integral overload relay reacts to an inverse-time curve that progressively opens faster as the current increases. Typical applications for manual starters include low-horsepower, single-phase motor loads where overload protection is not initially provided and frequent operation or remote control is not required. Common applications include small air compressors and exhaust fans.
A self-protected starter, a variation of the manual starter, is often used in multiple-motor control panels. They usually include low-level, instantaneous overcurrent protection which lets a single upstream short-circuit-protection device protect several starters. Thus, the motors don’t need individual short-circuit protection. (Combination ratings are often called group motor ratings.) The manual starters can be used on single and three-phase motor loads up to 600 V at less than 80 A. Typical applications include machine tools where multiple motors are controlled from a single operators panel, and individual disconnects for each motor aren’t required.
Magnetic starters, sometimes called full-voltage, nonreversing (FVNR) starters, are also quite common. They contain a magnetic contactor and an overload relay. The contactors comprise a set of stationary and moving contacts with a spring and magnetic coil. Without a secondary control voltage applied to the magnetic coil, the spring forces the moving contacts away from the stationary contacts, opening the circuit to the motor. But with power applied to the coil, a magnetic field overcomes the spring force and pulls the moving contacts to the stationary contacts, closing the circuit and energizing the motor. The overload relay is similar to the manual starter, but it includes a trip contact. The trip contact opens the circuit to the contactor coil under an overload condition, opening the contactor and deenergizing the motor.
Magnetic starters have longer electrical and mechanical lives and can be operated remotely. They automatically control motor loads from thermostat switches, float switches, and PLCs, and are used for a wide variety of applications including pumps, fans, compressors, and blowers. Often, magnetic starters control three-phase motors with small-to-medium power requirements. The basic contactor and overload relay are building blocks for other single-speed starters including part winding, autotransformer reduced voltage, full-voltage reversing, and wye-delta reduced-voltage starters.
Full-voltage reversing starters, similar to nonreversing starters, engage two contactors to spin the motor in either direction by reversing the phase relationship. Phases are reversed in the line-side connections to each contactor, while load-side connections of each contactor are tied together and brought to the motor. The two contactors are electrically and mechanically interlocked to prevent accidental closure of both at the same time. An identical overload relay is used with reversing starters. Typical applications include overhead doors, cranes, and hoists.
Part winding and wye-delta motor starters are special configurations of the basic magnetic starter that use reduced voltage. They handle special motors equipped with individual windings and configurations for smoother motor starting. The starters are typically intended for high-power motors where electrical and mechanical stresses are highest. A typical application is in a large air conditioning compressor for an industrial or commercial facility. If started under full line voltage, voltage in the rest of the facility could severely dip. Quite often, the utility company requires such motors be started under reduced voltage.
An autotransformer starter, another type of reduced- voltage starter, switches the windings of a transformer. It applies different voltages to a standard motor during start-up. Separate contactors control the connection of the motor to transformer secondary windings. Like part-winding and wye-delta starters, autotransformer starters are typically used on high-horsepower motors, but don’t require special motors. Autotransformer starters also work well for applications that require high starting torques such as mixers and grinders.
The last type of single-speed starter is the solid-state, reduced-voltage starter (SSRV). It differs greatly from the previously discussed devices in that it has no set of mechanical contacts to switch power to the motor. Instead, SSRVs replace discrete contacts with power semiconductors. This type of reduced-voltage starting provides smooth starting and stopping which minimizes both electrical and mechanical stress to the system. SSRV starters can be used in most applications where conventional part-winding, wye-delta, and autotransformer starters are employed, but must provide the required starting torque. They can also fit where such devices don’t provide smooth enough starting or stopping characteristics, such as on conveyor bottling lines.
Unfortunately, PLCs with semiconductor output stages have two potential problems. They require a sufficient load current to latch, and they produce a small leakage current in the “off state.” In some applications the leakage may be so high that the controller will fail to drop out when given a stop signal.
For example, the high impedance of the starter input terminals alone may not support the minimum current needed to maintain the triac’s on state. A resistor placed across the terminals may be required to develop the load current needed to latch the device. Typically, a 50 mA load will latch it, but confirm this value against the PLC manufacturer’s specifications. Then calculate the value of the resistor: Rn = 120/Il, where Rn is the load resistor needed, and Il is the latch-in current from the PLC specifications. The wattage is Wm = Il 2Rn. In addition, a 2.4 k½ resistor (7 W) may be needed in parallel with the starter input.
In another example, a triac-output starter with an input impedance of about 18 k½ and a leakage current higher than 1.3 mA, has contacts that can remain closed when sensing a stop or open signal. To correct such problems in the field, install a voltage divider. The resistance values in the table provide 24 V to the P and 3 terminals when 100 V is supplied by the triac output. The 24 V is sufficient to close the controller and with the leakage current shown, the voltage at the P terminal will be 3 V or less, ensuring dropout.
By comparison, transistor output module leakage current ranges from 0.1 to 3.5 mA. This may be sensed as a true signal and can be remedied with a 1-k and 200-½ resistor divider as illustrated.