Electric Motor Basics for Electricians: Single-Phase and Three-Phase Essentials

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Single-Phase Motor Essentials for Electricians: Types, Wiring, and Applications

Capítulo 2

Estimated reading time: 10 minutes

+ Exercise

1) Typical Use Cases in Residential and Light Commercial Work

Single-phase motors show up wherever you have 120 V or 240 V supply and modest horsepower needs. In the field, the fastest way to narrow the motor type is to start with the application, the starting load (easy vs hard to start), and whether speed control is expected.

Split-phase (resistance-start, induction-run): General-purpose loads with moderate starting torque. Common in older belt-driven blowers, small shop tools, older pumps, and some older compressors (less common today).
Capacitor-start (CSIR): Higher starting torque for hard-start loads. Common in small air compressors, sump/utility pumps, pressure washers, and some refrigeration/AC auxiliary equipment.
PSC (permanent split capacitor): Very common in HVAC blowers, air handlers, furnace draft/inducer motors, condenser fan motors, and bath fan motors. Good efficiency and smooth operation; typically lower starting torque than capacitor-start.
Capacitor-start/capacitor-run (CSCR): High starting torque plus good running efficiency/power factor. Used where both strong start and good run performance matter: some pumps, larger blowers, and specialty HVAC applications.
Shaded pole: Very small fractional-horsepower, low starting torque. Found in small desk/room fans, small appliance fans, older bathroom fans, small refrigerator evaporator fans (older designs).
Universal (series-wound): Runs on AC or DC; high speed and high starting torque. Found in portable tools (drills, saws), vacuums, blenders, mixers, and some small appliances. Often paired with electronic speed control.

2) Identification Cues in the Field (What You Can See Quickly)

Start with the nameplate and the “hardware clues”

Voltage: Many single-phase motors are 115/230 V dual-voltage (especially induction motors). Universal motors are commonly 120 V only in portable tools.
Capacitor can present? A metal or plastic can (often oval) mounted on the motor or in the equipment cabinet is a strong clue for capacitor-start, PSC, or CSCR.
Centrifugal switch or start relay? Many capacitor-start and split-phase motors use a centrifugal switch inside the endbell (you may hear a click as it comes up to speed). Some use a current relay or PTC device instead (common in hermetic compressor circuits, but you may see relays in packaged equipment).
Number of leads and lead colors: More leads often means dual-voltage or multiple speed taps (common on PSC blower motors). Fewer leads is common on shaded pole and many universal motors.

Type-by-type “spotter’s guide”

Motor type	Common visual cues	Typical lead situation
Split-phase	No capacitor can; may have centrifugal switch	Often 4–6 leads (dual-voltage variants common)
Capacitor-start (CSIR)	Large start capacitor; often centrifugal switch	Often 4–6 leads; start circuit switched out after start
PSC	Run capacitor always in circuit; no centrifugal switch	Often multiple speed taps (3–6+ leads) plus capacitor leads
CSCR	Two capacitors (start + run) or a dual-section capacitor; switching device present	More complex lead set; start cap switched out, run cap stays
Shaded pole	No capacitor; very small; often open frame; simple construction	Usually 2 leads only
Universal	Brushes/commutator; often loud/high speed; may have speed control board	Often 2 leads to supply plus internal brush/field wiring

Practical step-by-step: quick field identification workflow

Look for a capacitor can on or near the motor. If none, suspect shaded pole, split-phase, or universal.
If capacitor present: check if it’s labeled “RUN” (often 370/440 VAC, lower µF) or “START” (often 125/250 VAC, higher µF). A run capacitor strongly suggests PSC or CSCR; a start capacitor suggests CSIR or CSCR.
Listen/observe startup: a distinct click shortly after startup suggests a centrifugal switch (split-phase, CSIR, CSCR).
Count and label leads before disconnecting (photo + tags). Multiple speed taps strongly suggests PSC blower motor.
Check for brushes (service caps/brush holders). Brushes indicate universal motor.

3) Simplified Wiring Diagrams and Lead Labeling Conventions

Field wiring varies by manufacturer. Always follow the nameplate diagram when available. The simplified diagrams below help you understand what you’re looking at so you can troubleshoot safely and avoid miswiring.

Common lead labels you may encounter

T-leads (T1, T2, T3…): Common on NEMA-style induction motors, especially dual-voltage.
U/V/W: More common on three-phase, but sometimes appears in documentation.
PSC blower motor taps: Often labeled COM (common), HI, MED, LO, plus capacitor leads (sometimes CAP or two brown wires).
Capacitor leads: Many PSC motors use BRN and BRN/WHT to the run capacitor; some use two brown leads with no polarity.

Split-phase (resistance-start) simplified diagram

Start winding is used only for starting and is disconnected by a centrifugal switch once up to speed.

   L1 --------------------+----------------- Run winding ---------+---- L2/N (or L2 on 240V)  |                    |                                         |                    +-- Start switch -- Start winding ----------+

What to expect: No capacitor. If dual-voltage, you may see multiple T-leads with a nameplate connection chart.

Capacitor-start (CSIR) simplified diagram

A start capacitor boosts starting torque; the start circuit is switched out after startup.

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   L1 --------------------+----------------- Run winding ---------+---- L2/N  |                    |                                         |                    +-- Start switch -- Start cap -- Start winding --+

Field cue: Start capacitor often has higher µF and lower VAC rating than run capacitors. The start capacitor is not intended for continuous duty.

PSC (permanent split capacitor) simplified diagram

Run capacitor stays in circuit at all times; typically no centrifugal switch. Many PSC motors have speed taps: you energize one speed lead at a time.

               +---------------- Run winding ---------------+   L1 (hot) ----+                                         +---- L2/N (neutral)               +-- Run capacitor -- Aux winding ---+   (Speed tap selection often on the L1 side: HI/MED/LO)

Typical blower wiring concept: COM goes to neutral (or L2 on 240 V). One speed lead goes to hot through a relay or switch. Capacitor connects between the two designated capacitor leads (often brown wires).

Capacitor-start/capacitor-run (CSCR) simplified diagram

Uses a run capacitor continuously and a start capacitor only during starting (via switch/relay). Sometimes implemented with a potential relay or centrifugal switch.

   L1 --------------------+----------------- Run winding ------------------+---- L2/N  |                    |                                                  |                    +-- Run cap -- Aux winding -----------------------+  |                    +-- Start switch/relay -- Start cap --(in parallel with run cap path)--+

Field cue: Two capacitors (or a dual capacitor) and a switching device. If the start switch fails, the motor may not start or may overheat quickly.

Shaded pole simplified diagram

Electrically simple: a single main winding; shading coils are part of the stator construction and not separately wired.

   L1 (hot) ---- Main winding ---- L2/N (neutral)

Field cue: Usually only two leads. If it hums and won’t start, replacement is common because internal parts are not typically serviceable.

Universal motor simplified diagram (series-wound with brushes)

Field winding and armature are in series. Speed control (if present) is often an electronic controller in series with the motor.

   L1 (hot) ---- Field winding ---- Brush/Armature ---- Brush ---- L2/N

Field cue: Brushes, commutator, and often a strong “tool-like” sound. If it runs only when you tap it or it sparks excessively, suspect brushes/commutator issues.

Practical step-by-step: safe lead handling and basic verification

De-energize and verify with a meter (absence of voltage) before touching leads.
Photograph and tag every lead before disconnecting (especially PSC multi-speed motors).
Discharge capacitors using an appropriate resistor/discharge tool; verify with a meter before handling.
Identify common vs speed leads (PSC): common often measures continuity to all speed leads; speed leads may show different resistances to common (higher resistance often corresponds to lower speed tap).
Check for a start switch/relay (CSIR/CSCR/split-phase): inspect mechanical freedom (centrifugal mechanism) if accessible, or relay contacts if external.

4) Common Symptoms by Type and Likely Causes (Troubleshooting Map)

Many single-phase motor complaints sound similar (humming, no start, overheating). The motor type determines the most likely failure points. Use the symptom to decide what to test first.

Split-phase: common symptoms and causes

Hums, won’t start; starts if spun by hand: start winding open, centrifugal switch stuck open, or start circuit connection issue.
Slow start, low torque: high resistance connection in start circuit, weak supply voltage, or failing centrifugal switch not fully closing at rest.
Overheats/trips overload: centrifugal switch stuck closed (start winding stays in circuit), overloaded driven equipment, or low voltage.

Capacitor-start (CSIR): common symptoms and causes

Hums, won’t start; may start unloaded but not under load: failed start capacitor (open/low capacitance), start switch/relay not closing, or open start winding.
Starts then quickly overheats: start switch stuck closed (start capacitor and start winding remain energized), incorrect replacement capacitor value, or mechanical overload.
Bulged/leaking capacitor: capacitor failure; often accompanied by hard starting and humming.

PSC: common symptoms and causes

Hums, won’t start; starts if spun: failed run capacitor (most common), seized bearings, or incorrect speed tap energized (wrong wiring).
Slow to start, low airflow/torque: weak run capacitor (capacitance drift), low voltage, or blower wheel drag.
Overheating: wrong capacitor µF rating, blocked airflow over motor, incorrect wiring causing wrong winding relationship, or excessive static pressure/load.
Runs only on one speed / wrong speed: open speed tap lead, failed speed relay/control board, or miswired speed selection (multiple taps energized at once can damage the motor/control).

CSCR: common symptoms and causes

Hard start or no start: failed start capacitor, failed start relay/switch, or open start winding.
Runs but with poor efficiency/overheats: failed run capacitor or incorrect run capacitor value.
Repeated capacitor failures: start circuit not dropping out (relay/switch issue), wrong capacitor voltage rating, or excessive starts per hour.

Shaded pole: common symptoms and causes

Hums, won’t start: seized bearings, debris in fan, or shorted turns (often not economical to repair).
Overheats: blocked airflow, wrong voltage applied, or mechanical drag.
Noisy operation: worn sleeve bearings or fan imbalance.

Universal motor: common symptoms and causes

Runs intermittently / needs tapping: worn brushes, weak brush springs, dirty commutator, or broken brush lead.
Excessive sparking: worn brushes, rough/dirty commutator, armature issues, or overloaded tool.
High speed but low power: poor brush contact, partially open field winding, or voltage drop in cord/switch.
Overheating/burning smell: blocked cooling air, overload, stalled rotor, or failing armature/field insulation.

Practical step-by-step: symptom-driven checks (quick and targeted)

If the motor hums and won’t start: (a) verify supply voltage at the motor under attempted start, (b) check for mechanical binding (spin by hand with power off), (c) if capacitor-equipped, test/replace capacitor with correct µF and VAC rating, (d) if switch/relay-equipped, inspect operation (stuck open/closed).
If the motor starts slowly: (a) measure voltage drop during start, (b) check capacitor value (PSC/CSIR/CSCR), (c) check load (pump impeller, blower wheel, belt tension), (d) confirm correct wiring/speed tap selection.
If the motor overheats or trips overload: (a) confirm the start circuit drops out (split-phase/CSIR/CSCR), (b) verify correct capacitor type/value (run vs start), (c) check airflow and ambient temperature, (d) check driven load for binding or overloading.
If troubleshooting points to a start switch/centrifugal mechanism: (a) de-energize, (b) access endbell if serviceable, (c) inspect for dirt, broken springs, stuck weights, or burned contacts, (d) verify continuity of the start circuit at rest and open circuit at speed (where testable per manufacturer guidance).

Now answer the exercise about the content:

A blower motor in an air handler has multiple speed tap leads and a run capacitor that stays in the circuit during operation, with no centrifugal switch. Which single-phase motor type best matches these field clues?

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PSC motors keep a run capacitor in the circuit continuously and commonly have multiple speed taps, especially in HVAC blower applications. They typically do not use a centrifugal switch.