Here are some answers to frequently asked questions about motor control.
Motors provide the driving power in almost every stage of the manufacturing and production process. With this much productive capacity at stake, proper understanding of motor control is critical. Here are the answers to some frequently asked questions about discrete motor controls.
Can you describe what a push button is and what its components are?
We most often use push buttons to manually energize or de-energize a control circuit. Push buttons come in many sizes, colors, and configurations. However, they all consist of three basic components — operator, legend plate, and contact block.
The operator is the part you push or pull to open and close the contacts. Styles include the flush head, extended head, and mushroom head. Special-purpose operators are for specific applications — such as an emergency stop (E-stop) mushroom button that complies with global standards.
Legend plates are the labels that are installed around a push button and identify its purpose. They come in many sizes, colors, and languages. Examples of label text include RUN, START, and STOP.
The contact block may house many sets of contacts that open and close when you operate the switch. The normal contact configuration allows for one normally open and one normally closed set of contacts within a contact block. A switch may contain stacked contacts that change state with a single push of a button.
What are the two most common methods of motor starting?
Full-voltage (across-the-line) and reduced-voltage (popular for low-horsepower motors). Before choosing the full-voltage method, check that the driven load can handle the full starting torque without damage. Make sure the resulting current demand doesn't cause excessive voltage drop. Advantages of full-voltage starting are maximum efficiency and maximum torque at startup. Motors using full-voltage starting will usually be up to speed in seconds, but the current rush can hinder other equipment in the plant.
Reduced-voltage starting occurs when the windings receive less than full-line voltage or when the winding connections (such as part winding or wye-delta) reduce the starting current. It reduces the torque the motor develops and the current it draws. In some methods, such as autotransformer or wye-delta starting, the line current and torque reduce in direct proportion. In other methods, such as series impedance or solid-state reduced voltage starting, torque changes in a squared relationship to the current.
The main reason for using reduced voltage is to control either the inrush current or torque during motor starting. Current-draw restrictions may require larger motors to start with reduced voltage. You'll also see this, regardless of horsepower, in applications where reducing torque extends equipment life or improves the process.
What are the methods of connecting control voltage, and how do they differ?
The three methods are common control, separate control, and transformer control. With common control, you take the control voltage directly from the power line. Thus, the control voltage and line voltage share a common source. You often encounter this method in commercial, light industrial, or agricultural applications. You'll rarely see it in applications over 240V. The advantage of common control is you don't need additional components, transformers, or power supplies. The disadvantage is you must take extra measures to guard operators and maintenance people against shock hazards.
When using separate control, you take the control voltage from a source outside the starter enclosure and bring it to the starter. It's possible to wire several starters from one source, such as a transformer, and provide some cost-savings. Separate control is a good option when you need a separate power supply for other purposes — for example, when you need to power motor instrumentation. If you must maintain control power, then separate control is essential. You can best meet this last criterion by adding a UPS to the control power system.
A disadvantage of separate control is you must add supplemental circuitry, such as a disconnect auxiliary contact, to meet the rules of 430.74 of the NEC. Another disadvantage is increased wiring cost, because you're bringing the control voltage in from a second source rather than from the motor's power source.
Transformer control is the most popular method of deriving control voltage. The primary of the transformer connects to the power line inside the enclosure but after the disconnecting means. The secondary feeds the control circuit. The preferred secondary voltage today is 120V. Currently, 24V control — particularly the DC associated with programmable logic controllers (PLCs) — is gaining popularity. Because your PLC typically runs on 120V, you'll need to power it from a 120/208V transformer. If you design your distribution correctly, this transformer won't power convenience receptacles or lighting. As a result, you should consider buying a dedicated transformer.
What are the different types of control relays, and what should I consider when determining which type of relay to use?
Relays come in many different ratings and configurations and serve many different functions. For example, you can use relays as input/output (I/O) interfaces to devices that have solid-state I/O, like PLCs and drives.
Control relays actuate contactors that switch line power to motors and other high-current loads. Relays, on the other hand are low-power switching devices. General-purpose, industrial, and timing relays are available with IEC and NEMA ratings. These are ideal for applications that require a quick, low-cost, average life and easy-to-replace installation.
One manufacturer uses industrial relays (machine control relays) to switch solenoids and provide logic switching for control circuits. Industrial relays are generally repairable, can switch higher currents than general-purpose relays, and have as many as 12 contacts.
When looking at a control diagram with timers, I get confused by the terminology. What are the designations for timing relays, and what do they mean?
Timing relays provide sequential logic within a control circuit. You can set up a time delay to occur upon sensing when a signal comes or goes. Timing relays have both timed contacts and instantaneous contacts. The designations for the timed contacts are NOTO, NCTO, NOTC, and NCTC.
What do these mean? NO means normally open, and NC means normally closed. TO means timed open, and TC means timed closed.
TO contacts begin timing when you apply power to the timer — they change from their normal condition after power up. Conversely, TC contacts begin timing when you remove power. A NOTO contact changes from open to closed immediately when you apply power. After you remove power, the contacts remain closed until the end of the timing period — and revert back to open. The NCTO works in a similar manner, except the contacts are in the opposite position and take the opposite action.
What are the three types of limit switches, and what are the most common applications of these instruments?
Limit switches convert mechanical motion into electrical signals. Factory-sealed limit switches have either a prewired cable or factory-installed quick disconnect receptacle. They offer reduced installation time and can operate in a variety of harsh environments. Installers can quickly and easily replace these switches because the wiring will remain intact when you remove the switch, which contains the contacts. Some switches provide diagnostic capabilities and output options.
Two popular applications of limit switches are the indication of end-of-travel and misalignment. Use limit switches where you must know travel or position — especially on such things as transfer machines, conveyors, and valves.
How do the several overload class relay (OCR) designations relate to specific application needs? What considerations apply to choosing a motor protection device?
U.S. industry standards designate an OCR by a trip-class number (10, 20, or 30) that indicates maximum time in seconds at which the OCR will trip when carrying a current equal to 600% of its current rating. This rating is directly related to the thermal starting capacity of a motor. This is referred to as the safe stall time (SST). General applications use a Class 20 OCR, while specific applications may require a Class 10 or 30 OCR. You'll often find Class 10 with hermetic motors, submersible pumps, or motors with short locked rotor time capability. You'll typically find Class 30 overloads with high-inertia loads or hard-to-start loads with a long acceleration time. Motor manufacturers have traditionally designed NEMA motors to Class 20 overload protection criteria, while designing IEC motors to Class 10 overload protection criteria.
Motor control involves selecting from many options. To make the right choices, determine what you want the motor to do, draw out your control scheme, and choose the components that will do the job.
Lukitsch is commercial engineering manager, Rockwell Automation, Milwaukee.