Interest
Motor controls
Split out from the industrial interest page so the control circuits can be read on their own. This is study material, not a list of machines Jeffrey has started.
Ladder diagrams
A ladder is read like a railing. The two uprights are the control-power lines, often from a control transformer at 120 volts or 24 volts. Each rung is a circuit. Contacts in series are AND. Contacts in parallel are OR. A coil on the right is the load of that rung. The coil’s own auxiliary contacts may show up on other rungs. Wire numbers on the drawing match labels on the wires.
Three-wire start/stop
A two-wire control (a switch that stays closed) will restart a machine when power returns. A three-wire circuit will not. Stop is a normally closed pushbutton in series. Start is a normally open pushbutton in parallel with a normally open auxiliary contact on the starter. Push start, the coil picks up, the auxiliary contact seals in around the button, and the motor runs after the finger leaves the button. Stop opens the rung and the seal drops out. A power loss does the same thing. That is why a maintained switch is the wrong device on a machine that should not restart by itself.
Contactors, starters, overloads
A contactor is a relay big enough for the motor current. A starter is a contactor plus an overload relay. The overload watches heat, or current, and opens the control circuit before the motor cooks. Short-circuit protection is still the breaker or the fuses. The overload is not a substitute for that.
Sizing starts at the nameplate full-load amperes, not at a guess from horsepower. NEC 430.32 is where the adopted code sets the percentage, often 115 percent or 125 percent of nameplate FLA depending on the service factor and the temperature rise. The nameplate and the code year on the job pick the number. A separate branch-circuit breaker is sized from a different article (430.52) and is allowed to be larger so the motor can start.
Forward and reverse
Reversing swaps two of the three phase leads. Two contactors do that. Both must never be closed together, or the result is a phase-to-phase fault. An electrical interlock puts a normally closed auxiliary of each contactor in series with the other coil. A mechanical interlock is the second layer. The stop button is common to both directions.
Control transformers, pilots, timers
A control transformer makes the ladder voltage from the line. One side of the secondary is often grounded, and the overcurrent protection for the control circuit follows the transformer article and the listing. Pilot devices are the buttons, selector switches, and limit switches on the machine. On-delay and off-delay timers are coils with contacts that change state after a set time. The drawing says which kind. Guessing is how a conveyor starts in the wrong order.
Wye-delta, soft starters, VFDs
A wye-delta starter connects the motor windings in wye for the start, so each winding sees line voltage over √3, and current and torque are about one third of a delta start. It then transitions to delta. The motor has to be built for that. A soft starter ramps the voltage with SCRs instead. A VFD ramps frequency and voltage together, so the motor can run at the speed the process wants, not only at full speed.
On a centrifugal fan or pump, power falls off roughly with the cube of speed. The Department of Energy’s motor tip sheet walks an example where half speed cuts the input to a small fraction of full-load power, after drive and motor losses. Soft start also avoids the five-to-seven-times current of an across-the-line start. The cost is harmonics, bearing currents if the installation ignores them, and programming. Parameters that matter in the field are motor nameplate data, accel and decel, minimum speed, and whether a bypass contactor is provided.
MCCs
A motor control center is a lineup of buckets on a vertical bus, fed by a horizontal bus. Each bucket is a starter, a feeder, or a drive. Units are withdrawn only in the manner the manufacturer describes, and only after the disconnect for that unit is open and locked. The bus on the back is not something a bucket pull exposes a person to on purpose.
Troubleshooting with a meter
The safe order is the lockout page, not a meter on a live starter because it is faster. If the employer’s program allows troubleshooting energized, it is energized work: justified, permitted if the program says so, and done in the PPE the arc-flash label or the study requires. A continuity check is done dead. Voltage checks use a meter rated for the circuit, proved live-dead-live. A ladder is then walked: is control power present, does stop pass voltage, does start, does the overload contact, does the coil have voltage and is the mechanical side free. Lockout/tagout is the default, not the exception.
What's happening now
- Adjustable Speed Drive Part-Load Efficiency U.S. Department of Energy, Motor Systems Tip Sheet #11
Shows why a VFD on a variable-torque fan or pump can cut input power sharply when speed comes down, and why part-load efficiency still has to be counted.
- Optimizing Your Motor-Driven System U.S. Department of Energy
A short plant guide that includes adjustable-speed drives among the practical ways to stop wasting motor energy.
- Motor Systems U.S. Department of Energy
The DOE index of motor tip sheets, including when an inverter-duty motor is specified and how drives and motors misbehave together.
Code section numbers, energy-code rules, and utility practices change by edition and by city. The authority having jurisdiction, and the employer’s written program, decide what applies on a given job.