Short answer: A relay switches small control signals; a contactor switches motor or heavy loads; a motor starter is a contactor plus an overload relay. A PLC reads its inputs, runs its program and sets its outputs, over and over. A VFD (variable frequency drive) changes the frequency and voltage it feeds a motor, which changes the motor’s speed.
Controls sit on top of motor circuits, and motor circuits are heavily tested. The PSI outline gives Motors and Generators 5 items in Texas Journeyman NEC Knowledge and 2 in Calculations, plus Control Devices and Disconnecting Means on both parts. The Texas Journeyman Practice Pack has 1,000 practice questions on the 2026 NEC and step-by-step solutions for 299 calculation questions. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words. Drive and PLC details vary by manufacturer; follow the equipment manual and your supervisor. Drives can hold dangerous voltage after power is removed.
Relay, contactor or motor starter?
| Device | What it switches | Typical coil | Where you see it |
|---|---|---|---|
| Control relay | Small control signals (a few amps) | 24 V DC or 120 V AC | Inside control panels, interlocks |
| Contactor | Motor or heavy resistive loads | 24 V DC or 120 V AC | Lighting contactors, heaters, motors |
| Motor starter | Typically a contactor with overload protection for starting and protecting a motor | Depends on the starter | Many across-the-line motor installations |
| PLC output | Usually a relay or solid-state output that drives a coil, not the motor itself | Set by the output module | Automated machines |
| VFD | The motor’s power, at variable frequency and voltage | Controlled by keypad, wiring or network | Pumps, fans, conveyors |
The idea that ties them together: a small circuit (the control circuit) decides when a bigger circuit (the power circuit) turns on. In NEC 430 motor circuits you saw the hard-wired version: a three-wire start-stop station with a seal-in contact. A PLC can implement similar start-stop control logic in its program, while the motor power circuit, protective devices and any required safety functions remain separate design considerations.
How a PLC works
A programmable logic controller repeats one cycle, called a scan, many times a second:
- Read inputs. Pushbuttons, limit switches, sensors and overload contacts wired to input terminals.
- Solve the program. Most electricians work with ladder logic, which looks like the relay diagrams it replaced.
- Write outputs. Output terminals energize starter coils, solenoids, pilot lights or signal a drive.
Trace one input to one output
A conveyor’s start-stop logic, wired to a PLC:
| Field device | PLC address (example) | In the program |
|---|---|---|
| Start pushbutton, normally open | Input 0 | Normally open contact |
| Stop pushbutton, physically normally closed | Input 1 | Input is ON in normal operation; the run logic needs it to stay ON |
| Overload relay contact, normally closed | Input 2 | Input is ON in normal operation; the run logic needs it to stay ON |
| Conveyor motor starter coil | Output 0 | Coil |
In this simplified example, the program reads input 1 and input 2 as true while the physical stop and overload contacts are closed. The rung: start (input 0) in parallel with the seal-in contact from output 0, and that branch in series with the two permissive inputs, driving output 0. When you troubleshoot, check each piece in order: is the input light on at the PLC when the button is pressed, does the program show the rung true, is the output light on, and does the starter coil get voltage? That tells you whether the problem is the field device, the program or the output wiring.
A normally closed stop circuit can help a control system treat an open wire as a stop condition. However, an ordinary PLC input is not a substitute for a properly designed, risk-assessed safety function. Emergency-stop and personnel-protection circuits require the appropriate safety-rated design and equipment.
What a VFD is
A variable frequency drive takes fixed-frequency power, turns it into DC, then rebuilds AC at the frequency and voltage the motor needs. An induction motor’s speed follows the frequency, so the drive controls speed. Drives also give soft starts and can save energy on fans and pumps.
Worked example 1: motor speed and slip
The synchronous speed of an AC motor is 120 × frequency ÷ number of poles.
- A 4-pole motor at 60 Hz: 120 × 60 ÷ 4 = 1,800 rpm.
- The nameplate says 1,750 rpm at full load. Slip: (1,800 − 1,750) ÷ 1,800 = 0.0278, or 2.78%.
- On a VFD at 45 Hz: 120 × 45 ÷ 4 = 1,350 rpm synchronous.
Check: 1,350 ÷ 1,800 = 0.75, and 45 ÷ 60 = 0.75. Synchronous speed is proportional to frequency. Actual shaft speed is a little lower because of slip, which changes with load.
Worked example 2: volts per hertz
In a simple constant volts-per-hertz example, a drive keeps an approximately constant ratio of voltage to frequency below base speed. Actual drive behavior depends on its control mode, programmed settings and the motor.
- A 460 V, 60 Hz motor: 460 ÷ 60 = 7.67 V per Hz.
- At 45 Hz: 460 × 45 ÷ 60 = 345 V.
Check: 345 ÷ 45 = 7.67. Same ratio.
Worked example 3: conductors feeding a drive
For a single motor on an adjustable-speed drive, the conductors that supply the drive are sized from the drive’s rated input current, at 125% (430.122(A)). Drive systems are in Part X of Article 430, and control panels built for industrial machinery are in Article 409. Texas uses the 2026 NEC for installations from September 1, 2026 (TDLR rule adoption notice).
- Drive nameplate input current: 21.0 A.
- Minimum conductor ampacity: 21.0 × 1.25 = 26.25 A.
Check: 26.25 ÷ 1.25 = 21.0.
Safety you must know before touching a drive
- Drive DC bus capacitors can stay charged after the disconnect is off. The drive label states how long to wait; follow the manual’s procedure to verify the bus is discharged.
- Lockout applies to every source: the drive’s supply, any bypass and any control power from another panel.
- Only qualified persons take energized measurements inside drives and control panels.
Check yourself
Our own practice questions. Answers below each.
- A 6-pole motor at 60 Hz. Synchronous speed? Answer: 120 × 60 ÷ 6 = 1,200 rpm.
- Same motor on a drive at 50 Hz? Answer: 120 × 50 ÷ 6 = 1,000 rpm.
- The PLC input light for the start button turns on, but the output light never does. Where do you look? Answer: The program conditions in that rung, such as the stop and overload inputs, before the output wiring.
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A printed workbook for the two-part exam
Texas Journeyman Electrician Exam Prep 2026 NEC (Tsai Electro, paperback): 486 practice questions and 2 full two-part mock exams for the Texas Journeyman exam on the 2026 NEC. Use it alongside your own code book. Not affiliated with TDLR or PSI.
FAQ
What is a VFD?
A variable frequency drive. It converts incoming AC power to DC and back to AC at an adjustable frequency and voltage, which lets it control the speed of an AC induction motor and start it softly.
What is the difference between a relay and a contactor?
Both are switches operated by a coil. A relay switches small control currents; a contactor is built to switch motors and other heavy loads. A motor starter is a contactor with an overload relay added.
How does a PLC work?
It repeats a scan: it reads its inputs, solves its program, usually written in ladder logic, and then sets its outputs. The cycle repeats many times a second.
How do you calculate motor speed from frequency?
Synchronous speed equals 120 times the frequency divided by the number of poles. A 4-pole motor at 60 Hz runs at 1,800 rpm synchronous; actual speed is a little lower because of slip.
Sources checked October 2026
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- NFPA 70 (NEC), 2026 edition, Articles 409 and 430, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
