Single Phase Motors

What are the electrical safety limits for a gasfitter and typical skin resistance values?

Max: 100VA AC, 150VA DC

Dry skin resistance: 100k–600k ohms

Wet skin resistance: ~1k ohm

Shock thresholds:

15mA: painful

15–75mA: can’t let go, breathing may stop

100–200mA: heart skips, likely fatal

200mA: severe burns, heart may stop

What are the general characteristics of single phase motors?

Common where 3-phase not available (resi/commercial)

Efficiency: 60–68%

Sizes: up to 15HP, often fractional HP

Less efficient, heavier than 3-phase

How does a resistance split phase induction motor operate and what are its features?

Two stator windings (main and aux) with different R and L

Run winding: large conductors, more turns, bottom of stator

Aux (start): small conductors, fewer turns, 90° to run winding

Start winding disconnects at ~75% full speed

Start torque: 125–150% of running torque

Start current: 5–6× running

Phase angle: ~30°

Constant speed (typically 1750 RPM, 4-pole)

Reversible (at standstill), Reversing (while running)

Used for: washers, sump pumps, fans (low inertia loads)

What is a capacitor start induction motor and how does it compare to a resistance start motor?

Adds capacitor to aux winding for phase shift

Nearly 90° phase angle

Same number of turns in run and aux windings → more startup flux

Lower current draw (~30% less)

Higher starting torque

Canister-mounted capacitor

After start, operates like resistance split phase motor

What is a permanent split capacitor (PSC) motor and its pros/cons?

Aux winding and capacitor stay in circuit continuously

Advantages:

Quiet, 2-phase-like operation

Good power factor, efficiency

No centrifugal switch

Can reverse while running

Disadvantage:

Uses large, costly oil-filled capacitor

Types:

Single Voltage Non-Reversible

Dual Voltage Reversible

Single Voltage Reversible

2-Speed Single Voltage

What is a two value capacitor motor and what makes it unique?

Combines PSC and capacitor start motor features

Uses 2 capacitors:

Start capacitor: high capacitance, electrolytic

Run capacitor: continuous rated, oil-filled

Uses cutout switch

Provides both strong starting and running performance

Reversible by swapping start winding leads

What is a shaded pole motor and what are its limitations?

Aux winding: 1-turn copper ring around each pole

No centrifugal switch

Phase shift from shading creates rotation

Very inefficient (5–35%)

Low starting torque

Reversing requires mechanical changes or shaded ring circuiting

Variable speed by adjusting applied voltage

Used in low HP applications

How are single and dual voltage motor leads typically configured?

May have 2 to 9 leads (for reversing, thermal, capacitor, etc.)

"T" labels for leads common

NEMA rotation: CCW viewed opposite shaft end

IEC rotation: CW viewed from shaft end

Both result in same actual direction

What is important about thermal protection in single phase motors?

Leads labeled "P"

One lead is current sensing (has resistance), one is breaking (0 ohms)

In dual voltage motors:

2 main coils in parallel (low voltage), in series (high voltage)

Start coil remains in parallel with one main coil

What are basic maintenance steps for a single phase motor?

Lubricate

Check stator and bearing temps

Check shaft for bearing wear

Check insulation with megger

Replace rather than repair in most cases

What are the key nameplate specs to interpret on a single phase motor?

Service Factor: how much overloading allowed without overheating

Insulation Classes:

A: 105°C

B: 130°C

C: 155°C

D: 180°C

Enclosure: TEFC (Totally Enclosed Fan Cooled = most common)

Frame: standard for motor size and mounting pattern

How do you calculate motor current draw and power loss?

Motor HP = mechanical power

current draw = electrical input

1 HP = 746 Watts

Efficiency losses:

Electrical: iron loss (core), copper loss (resistance)

Mechanical: friction in bearings, windage (air resistance)

How do you calculate current draw from nameplate values?

Given: 1.5 HP, 115V, 75.5% eff, 73% power factor

1.5 × 746 = 1119W

1119 / 0.755 = 1482W (real power)

1482 / 0.73 = 2030W (apparent power)

2030W / 115V = 17.7 Amps

What are the four main components of a resistance split phase motor?

Frame

Centrifugal switch

Stator

Rotor

What is the typical HP range for resistance split phase motors?

1/6 to 3/4 HP

Is a resistance split phase motor reversible?

Yes, if start winding leads are brought out to the terminal box

What distinguishes aux windings in PSC vs resistance start motors?

PSC: aux coil rated for continuous duty

Resistance split phase: aux coil only used for start

What component does a capacitor start motor have that a PSC motor doesn’t?

Cutoff switch (to remove start cap after startup)

How does capacitance differ between capacitor start and PSC motors?

Start motor: higher capacitance, oil-filled, continuous duty rated

What extra components does a two value capacitor motor have?

Cutout switch

Start capacitor (higher capacitance than run cap)

Is a two value capacitor motor reversible?

Yes

How do you reverse a shaded pole motor?

Reverse armature or change field winding connection

Motor Types Chart