Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. They are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent. The actual transformer or generator supplying power to this circuit is omitted for simplicity. In reality, the circuit looks something like the picture below.

In the picture notice the number labelling the wire connecting the lamp and the switch. In the real world, that wire would be labelled with that number, using heat-shrink or adhesive tags, wherever it was convenient to identify. Wires leading to the switch would be labelled "L1" and "1," respectively. Wires leading to the lamp would be labelled "1" and "L2," respectively. These wire numbers make assembly and maintenance very easy.

OPERATING A 3-PHASE (3∅) MOTOR

1. When the Forward Normally Open (NO) push-button is actuated the seal-in NO contact (N1) is energized closing the circuit. Contactor M1 is energized which starts the motor, at the same time the Normally Closed (NC) timed-closed contact (TD1) is energized, temporarily preventing the Reverse push-button from accidentally shorting the motor.
2. When the Reverse Normally Open (NO) push-button is actuated the seal-in NO contact (N2) is energized closing the circuit. Contactor M2 is energized which starts the motor, at the same time the Normally Closed (NC) timed-closed contact (TD2) is energized, temporarily preventing the Forward push-button from accidentally shorting the motor.
3. When the Stop push-button is pressed the circuit energy is interrupted and the NO seal-in contact returns to the Open position which de-energize the motor.

The picture below shows a more detailed schematic of the Motor Starter devices: The contact elements in M1 & M2 are operated simultaneously by an electro-magnet which is energized when the Forward or Reverse buttons are pressed. The Overload Heater elements are low-resistance strips of metal intended to heat up as the motor draws current. Overload heaters are not fuses they are designed to thermally mimic the heating characteristic of the particular electric motor they protect. They open the circuit when a set temperature is reached and won't close it again until the motor has cooled down. A description of 4 basic Timed-Relays is also given in the picture.

WIRING E-STOPS

The placement of the emergency-stop button is system specific and must take into account the needs of the process, it should disable a system as quickly and reliable as possible with minimum to no reliance on the PLC.

WIRING WITH INTERFACE RELAYS

Interface Relays allow you to connect devices that operate at different voltages. For example, in the picture below the IR1 Coil is activated by a NC Button installed in a 120Vac circuit. The IR1 Coil closes IR1 Contacts in the 24Vdc circuit which sends a digital signal to Input I1. The PLC then sends a signal to output Q1 which then activate the 24Vdc Coil IR4. Coil IR4 then closes the IR4 Contacts in the 120Vac circuit, which then activate the F Coils which then close the contacts in the F relay. Finally, this completes the 3∅ voltage circuit to the motor.

Using Programable Logic Controllers (PLC) makes building and reconfiguring industrial machines easier and more cost efective.

The Following illustrate the logic of utilizing the MAKE instruction in PLC programming.

The Following illustrate the logic of utilizing the BREAK instruction in PLC programming.

The Following illustrate the logic of utilizing both, the MAKE and BREAK instructions in the same PLC programm.

• Solid State Relays (SSR): is a device that switches on or off when an external voltage is applied across its terminals and contain no mechanical parts. They consist of an input sensor, a solid state switching device, and a coupling mechanism with no moving parts and may be designed to switch either AC or DC to the load at extremely fast speeds. Solid state relays have very limited contact wear due to no moving parts, but are more susceptible to overload damage in comparison to electromechanical relays. Applications: Medical, HVAC, Theatrical Lighting, Industrial Food Equipment, Elevators/Lifts, Control Panels, etc.


• Electromechanical Relays: are electrically operated switches that typically are used to control high power electrical devices. They are used in many of today's electrical machines when it is vital to control a circuit, either with a low power signal or when multiple circuits must be controlled by one single signal. Applications: General aviation, aerospace, and wireless technology industries.


• Timing Relays: Timing relays are employed wherever simple time-controlled processes are required. Timing relays are also and particularly required in connection with a control. Applications: on-delay, off-delay, and interval/one-shot modes.


• Reed Relays: They are very small hermetically sealed relays that are actuated via permanent magnets or electro-magnets. Applications: Monitoring of Liquid levels, seat belt & Emergency door closures, vehicles, household appliances, medical devices, telecommunications, industrial applications.


• Safety Relays: use monitoring logic, as well as overvoltage and short-circuit protection, in combination with redundant relays, to provide a high level of fail-safe operation. Positive guided contacts assure reliability for safety applications. Applications: Available models include E-stop, two-hand, safety gate, safety mat, light curtain, and speed safety relays.

• Limit Switches
• Speed Switches
• Pressure Controls
• Temperature Controls
• Float Switches
• Encoders, Multiplexing
• Proximity Switches
• Photoelectric Switches
• RF Identification
• Bar Code Readers and Decoders

A typical circuit used to supply electrical energy to most DC equipment is shown below: When the circuit is pluged into the AC outlet [A] the Alternating signal (AC) is passed through a Transformer [B] in order to sted it down closer to the equipment's operating voltage. The signal is then passed through a Rectifying circuit [C] in order to convert to a Direct signal (DC), the waves are then smoothed out by the capacitor [D] and finally procesed through a regulating circuit [E] to ensure the signal is clean and stable.

• Power Supply modules can be purchased at: Automation Direct, Mouser, ABB, etc.

• AC Motors:
  • Induction, squirrel-cage rotor (Asynchronous):
    • Single-Phase, Shaded-pole: computer fans.
    • Single Split-Phase: washing machines, furnaces.
    • Single-Phase Capacitor: refrigerators, air conditioning.
    • 3-phase (3∅): Lathes, Mill machines and other industrial applications.
  • Synchronous, squirrel-cage plus permanent-magnet rotor:
    • Where applications involve high kW output and low speed.
    • AC Servo applications with controllers.
    • Fans, Blowers, DC generators, Centrifugal pumps, compressors, Reciprocating pumps.
    • Speed, Torque and Direction can be controlled with Variable Frequency Drives.
• DC Motors:
  • Brushed with Commutator assembly: Series Wound, Compound Wound, Shunt Wound, Permanent Magnet
  • Brush-less without Commutator assembly: Servo applications with controller.

Below is the information listed in an Induction Motor Plate in order of importance:

1. Voltage, Frequency (Hz), Amperage: All good to know for VFD programming and Machine Design.
2. Diagrams: High Volts (460 VAC), Low Volts (230 VAC) lead connections diagram.
3. Premium Severe Motor, Inverter Duty: Indicates if the motor can be controlled with VFD computer.
4. Horse-Power (HP), Phases (1∅ or 3∅), RPM (at 60Hz), Operating Temperature, Insulation Class: All good to know for Machine Design.
5. Efficiency: percent of electric energy that is converted to mechanical energy.
6. Power Factor (PF): Indicates the percentage of useful energy from the total energy — and is best when it's as close to unity as possible.
7. Service Factor (SF): Is a measure of periodically overload capacity at which a motor can operate without damage. For example, a 10-hp motor with 1.15-SF could put out 11.5-hp for a few seconds.
8. Enclosure Type (NEMA for Motors only): Open Drip-Proof (ODP), Weather Protected (WPI/WPII); Totally Enclosed Fan Cooled & Blower Clooled (TEFC & TEBC), Pipe-Ventilated (TEPV), Air Over (TEAO), Non-Ventilated (TENV), Air to Air Cooled (TEAAC), Water to Air Cooled (TEWAC), and Explosion-proof (TEXP); and those with encapsulated or sealed windings. See also the IEC Ingress Protection (IP) and European EN standards. Also the U.S. National Electrical Code (NEC), National Fire Protection Association (NFPA).
9. Design A, B, C, or D: NEMA application designation, A & B power fans, pumps, HVAC, blowers and can be controlled by VFDs. C & D operate Cranes, Hoists, PD pumps, etc.
10. Frame Type: Used to locate the frame dimensions in a NEMA or IEC chart. Click the link for flange and face mounted motors bolt patterns. This link is for foot mounted motors.
11. Lubrication & Bearing: Lubrication specification and Bearing type. Important for maintenance.
12. Code A-V: Indicate the amount of Locked-Rotor Amperage (LRA) in kVA per Hp a motor will draw when started. However, if the motor is controlled by a VFD this code is of no importance.
13. Model Number: specific to the manufacturer.
14. CSA international: (Canadian Standards Association) sticker, is a group that provides testing and certification of many industrial products.
15. Name of the Manufacturer: useful if you plan to buy from them again.

First: You must determine the througput required of the equipment. This will yield the velocities (rpm), revolutions (rev) and accelerations (α) the motor must provide. A velocity-time chart is the ideal tool for these calculations. Use the chart to calculate the motor speed (ω) and peak acceleration (α_peak) by obtainig the area under the curve of the chart. Make sure to account for a gear reduction if one is used.

1 rev = A1 + A2 + A3

1 rev = (1/2)t₁ω + t₂ω + (1/2)t₃ω

α_peak = Δω / t₁

Second: Calculate the Load Inertia (J_load), This is about the most difficult part of the sizing process. Many tools are available to calculate Inertia. Once the inertia number is available now the preliminary peak torque (M_{peak pre}) and the RMS torque (M_rms) can be obtained.

M_{1} = M_{peak} = J_{load} \alpha_{peak}

M_{rms} = \sqrt{\frac{M_{1}^2 t_{1} + M_{2}^2 t_{2} + M_{3}^2 t_{3}}{t_{1}+t_{2}+t_{3}}}

Third: Pick a motor with a Torque-Speed graph that might accomodate the calculated peak torque, rms torque and rpm speed. Add the Motor Inertia (J_motor) to the peak torque calculated. Common in industry are Load/Motor inertia between 5/1 and 10/1. The new peak torque should fall within the instantaneous operation region of the chosen motor.

Fourth: If the new peak and rms torque fall within the two regions of the chosen motor, then you probably have the right motor. Next, pick a driver that will handle the current/voltage the motor needs.

Gear Boxes (GB): you can add GB if speed is not important but increased torque in the system is. However, consider that GB can be expensive, require maintenance, adds backlash and inertia.

Regeneration (Regen): occurs when the motor produces energy that needs to be dumped out as heat. Factors that increase Regen include high speed, high inertia, low friction, high desceleration rate, vertical applications. Adding counter-balances to a system help reduce Regen.

The following calculators can aid in motor sizing: Conveyor Calculator, Screw Drive (1), Screw Drive (2), Motion Analysis notes (SolidWorks).

Using Contactors and Switches: High Power motors that don't required complicated operation schemes use simple starting devices. The link in video 1 shows the Star-Delta method.

Frequency Inverters: a.k.a. Adjustable Speed Drives, Adjustable Frequency Drive (AFD), Variable Frequency Drive (VFD) and AC Drives are devices used to control the speed (rpm), torque & direction of AC motors by means of changing the voltage frequency. This is achieve through a technique called pulse-width modulation (PWM): (1) First, the VFD takes incoming 3-phase or 1-phase AC signal at a fixed frequency (2) the signal is turned into DC (3) which then can be used in the computer to manipulate and simulate different AC frequencies.

Vendors such as Kollmorgen, Sew-Eurodrive, Lenze, ABB, Dayton, Fuji Electric, etc. carry different Motors & VFDs to choose from.

GAM, Apex Dynamics, Misumiusa, THK are suppliers of couplings, gear reducers, and other robotics items.

The picture below shows a typical way to wire 1-phase and 3-phase VFD with 3-phase motors. Make sure the VFD has adequate cooling, as the components get very hot.

Pros of Using Variable Frequency Drives

• They provide substantial energy savings.
• Speed control is used to replace a valve or damper-type flow control.
• Smooth starting and stopping mechanisms reduce mechanical wear on equipment.
• Integrated features allow for easy implementation of future modifications.
• They allow for increased power factors.
• They have regenerative braking.
• They control speeds up to 100 Hz.

Cons of Using Variable Frequency Drives

• Higher initial capital costs
• Inverter duty motors should be used with VFDs to optimize motor life
• Harmonics may occur if VFDs aren’t installed per manufacturer specifications
• You will require additional heat dissipation

For video instructions on how to wire a VFD and how to interpret a motor name plate just follow the links.

Note: we cannot take the same address for Counters & Timers. In other words, if we are using T000 we cannot use C000 in the same program.

PLC Static Input/Output Check

• Start the CX Programmer from Omron > click New > in the Change PLC window enter a Device Name, Select Device Type.
• For the selected Device Type > click Settings... > In the Device Type Settings window select CPU type > click OK.
• In the Change PLC window select Network Type or the type of cable communication you're using with the PLC > click OK.
• Click the Work Online button to connect to the PLC > after the warning appears, click Yes
• Go to tools > SwitchBox Utilities > in the SwitchBox Utility window find the Bit Monitor section > under Address enter the address number of input 0.00 and output 100.00> notice how the status of the I/O is shown under the Value cell (on/off).

Run Mode vs. Programming Mode

• Run Mode starts to execute the program
• Programming Mode stop the execution of any program

Downloading & Uploading Programms

• PLC > Transfer > To PLC > click OK
• PLC > Transfer > From PLC > click OK

Changing the Programm

• click Work Offline > click on the output symbol in the ladder logic graph > change the address to 100.01 > click OK
• click Work Online > PLC > Transfer > To PLC > click OK

The image below shows how to setup a two-button operation to turn ON and OFF a motor or lamp. The buttons can be NO or NC and when combined with the simple PLC ladder logic this setup can save a lot of wiring time and material. R-click the image > "Open image in new tab" to see a slideshow sequence of the running program.