fractional horsepower motors
1. What Fractional HP Means
Fractional horsepower (FHP) motor = small motor rated <1 HP (1 HP =746W), typically to HP (35W–746W). Dominant in single-phase domestic applications where small, cheap, single-phase supply suffices. The name comes from historical HP rating: integral HP ≥1 HP, fractional <1.
2. Types of FHP Motors (exam list)
| Type | Principle | Starting | Applications (fractional) |
|---|---|---|---|
| Single-phase induction (capacitor-start, capacitor-run, shaded pole) | Induction (RMF via auxiliary) | Low-moderate | Fans, pumps, refrigerators, washing machines, mixers, toys — most common FHP |
| Universal motor (AC/DC series) | Series DC motor with laminated field, works on AC or DC (universal) | High torque, high speed (10k rpm) | Mixers, vacuum cleaners, drills, sewing machines — high speed, small |
| Shaded pole | Single-phase induction with copper shading ring | Very low torque | Small fans, record players, hair dryers |
| Stepper / BLDC (small) | Permanent magnet, electronic | Precise position | Printers, robotics (increasingly) |
FHP induction motors are capacitor-start (start winding + capacitor, centrifugal switch) or capacitor-run (permanent capacitor for quiet fan).
3. Why FHP Important
- Quantity: billions produced, most common motors in homes.
- Single-phase supply: homes have 1-phase 230V, so FHP single-phase is default; industry uses integral HP 3-phase.
4. Worked Checks
Ceiling fan 70W =0.094 HP → FHP, single-phase capacitor-run (permanent capacitor 2–3µF) for low noise, efficient.
Refrigerator compressor 150W =0.2 HP → FHP hermetic single-phase induction.
Industrial pump 5 HP = integral HP → 3-phase.
5. Exam Points
- Define fractional HP as <1 HP (746W) — 3 marks, very frequent.
- List types: single-phase induction (capacitor), universal, shaded pole with one application each — 5-7 marks, very frequent (the most common FHP question).
- Distinguish fractional (<1 HP, home, single-phase) vs integral (≥1 HP, industrial, 3-phase) — 5 marks, frequent.
- Give examples: fans, pumps, mixers, refrigerators as FHP — 3-5 marks, frequent.