Three Phase Asynchronous Motor Common Faults & Treatment Methods
What Is a Three-Phase Asynchronous Motor?
あ 三相非同期モーター — almost always called an 誘導電動機 in North America — is a rotating machine that converts three-phase AC electrical power into mechanical torque. “Asynchronous” means the rotor never reaches the speed of the rotating stator field: it always runs slightly slower, and that speed deficit (の スリップ) is exactly what induces current in the rotor and produces torque. It is the workhorse of industry — induction motors consume roughly 70% すべての産業用電力の (私たち. エネルギー省 / IEC estimates), which is why their failure modes and treatment methods matter to every maintenance program.
Key Construction Types
| Construction | Rotor type | 一般的な使用方法 | Service notes |
|---|---|---|---|
| Squirrel-cage (TEFC / ODP) | Cast aluminium or copper bars shorted by end rings | パンプス, ファン, コンベア, コンプレッサー | 最も一般的な; 頑丈な, no slip rings |
| Wound-rotor (スリップリング) | 3-phase winding + external resistors via slip rings | クレーン, mills, high-inertia starts | Adjustable start torque; brush/slip-ring wear |
| Line-start PM (synRM/PMSM) | Permanent-magnet or reluctance rotor | IE4/IE5 premium efficiency | より高い効率; needs drive or special design |
How a Three-Phase Asynchronous Motor Works
The principle is electromagnetic induction — no brushes, no external excitation on the rotor. The sequence is:
- Three-phase supply creates a rotating field. The 120°-spaced stator windings fed by 120°-spaced voltages produce a magnetic field that rotates at the 同期速度 Ns = 120・f / P (f = supply frequency, P = pole count).
- The rotor lags behind (スリップ). A stationary rotor would see a changing field and a large induced current, but as it speeds up the relative motion shrinks. At exactly Ns there would be zero relative motion, zero induced rotor current, and zero torque — so the rotor settles just below Ns.
- Rotor current is induced by slip. The slip frequency is fr = s・f (s = slip fraction). で 3% slip on 50 Hz, the rotor bars carry current at 1.5 Hz — enough to make torque, small enough to keep losses low.
- Torque balances the load. Torque rises with slip in the stable region; the motor finds the slip where electromagnetic torque equals the load torque. More load → more slip → more rotor current → more torque, up to the breakdown point.
- Losses become heat. 銅損 (I²R) in stator and rotor, iron loss in the core, and friction/windage all raise temperature. That is why thermal class and cooling are central to fault treatment.
同期速度 & Slip — Reference Table
| 極 (P) | Ns @ 50 Hz (回転数) | Ns @ 60 Hz (回転数) | Typical full-load slip |
|---|---|---|---|
| 2 | 3000 | 3600 | 3–6% |
| 4 | 1500 | 1800 | 2–5% |
| 6 | 1000 | 1200 | 1.5-4% |
| 8 | 750 | 900 | 1–3% |
作業例. A 4-pole, 50 Hz motor with a nameplate speed of 1455 回転数: Ns = 120 × 50 / 4 = 1500 回転数, slip s = (1500 − 1455) / 1500 = 3.0%, and the rotor current frequency is fr = 0.03 × 50 = 1.5 Hz. If slip climbs well above the nameplate value under the same load, suspect overload or broken rotor bars.
The Most Common Faults — and Where They Come From
Field data from industrial maintenance programs consistently shows a similar breakdown. Treat these percentages as a prioritization tool, not a precise census:
| Fault category | Share of failures* | Typical root cause | First symptom |
|---|---|---|---|
| 固定子巻線の絶縁 | ~32% | overheat, 電圧不平衡, 水分, 汚染 | 地絡, trip, burning smell |
| ベアリング / 機械的 | ~30% | 潤滑, 位置ずれ, 汚染 | ノイズ, 振動, hot bearing |
| ローター (bars / rings) | ~14% | 熱サイクル, start surge, casting defect | current swing, 低トルク, 熱 |
| 供給 / コントロール | ~14% | single-phasing, contactor, wrong connection | won’t start, ハム, reverse |
| 他の (environment, 負荷) | ~10% | blocked cooling, 過負荷, カップリング | overheat, 振動 |
*Aggregated from multiple motor-reliability surveys (例えば. IEEE/EPRI-style maintenance studies). Exact shares vary by industry and duty.
Diagnostic Decision Table — Symptom → Subsystem → First Test
| Observed symptom | Likely subsystem | First check (external → mech → elec) |
|---|---|---|
| Won’t start, no hum | 供給 / コントロール | 端子電圧, fuses, contactor, OLR setting |
| Won’t start, loud hum | 供給 (single-phase) / mechanical lock | Phase balance; manually rotate load (barring) |
| Starts but trips immediately | Electrical fault / 過負荷 | Megger to ground; verify OLR vs nameplate |
| Runs slow under load | 供給 / ローター | 電圧 & balance; broken-bar test (MCSA) |
| 過熱 | 熱 (all) | Load current vs FLA; cooling path; voltage balance |
| 振動 / ノイズ | 機械式 | ベアリング, alignment, balance; then electrical spectrum |
| Shell/case live | Insulation ground | Megger phase-to-frame; dry or rewind |
Engineering Data You Need for Diagnosis
Core Formulas
| 量 | 式 | 注意事項 |
|---|---|---|
| 同期速度 | Ns = 120・f / P | f in Hz, P = 極 |
| スリップ (分数) | s = (Ns − Nr) / Ns | Nr = 実際のローター速度 |
| すべり周波数 | fr = s・f | Frequency induced in rotor bars |
| Shaft torque | T = 9550·Pキロワット / n回転数 | Also T = 9.549·P / n |
| 電圧不平衡 (ありません) | %UB = max deviation from avg ÷ avg × 100 | ≤1% continuous; >5% not advised |
| Current unbalance (ルール) | ≈ 6–10× voltage unbalance | Negative-sequence heating |
| 定格電流 | I = P / (√3 · V · cosφ · η) | 3-phase apparent-power relation |
| 軸受の寿命 (L10h) | L10h = (C/P)p · 106 / (60·n) | p = 3 ボール, 10/3 ローラー |
Worked Example — Torque and Unbalance
あ 7.5 キロワット, 4-ポール, 50 Hz motor nameplated at 1455 回転数と 400 V:
- 定格トルク: T = 9550 × 7.5 / 1455 = 49.2 N・m.
- スリップ: s = (1500 − 1455)/1500 = 3.0% (matches the stable region).
- Voltage unbalance check: measured line voltages 400 V / 408 V / 392 V → average 400 V, max deviation 8 V → 2.0% アンバランス. Per NEMA MG 1 this already calls for ~5% load derating, and implies roughly 12–20% current unbalance — a real overheating risk worth correcting.
IEC 60034-30-1 効率クラス (IE)
| クラス | 相対効率 | Regulatory status (典型的な) |
|---|---|---|
| IE1 | 標準 (lowest) | Phasing out / banned for new in US & 欧州連合 |
| IE2 | 高い | Legacy minimum in some regions |
| IE3 | プレミアム | 現在のアメリカ. & EU minimum for most ratings |
| IE4 | スーパープレミアム | Growing adoption |
| IE5 | ウルトラプレミアム | Line-start PM / synRM emerging |
絶縁温度制限 (IEC 60034-1 / MGはありません 1)
| クラス | 最高巻取温度 (℃) | NEMA temp rise @ 40 °C amb | Practice |
|---|---|---|---|
| B | 130 | 80 °C rise (合計 120 ℃) | ベースライン |
| ふ | 155 | 105 °C rise (合計 145 ℃) | Used as insulation, run at Class B rise |
| H | 180 | 125 °C rise (合計 165 ℃) | High-duty / 高環境 |
の 10 °C rule: 毎 10 °C of sustained temperature above the design point roughly halves insulation life. A motor that runs hot is not just inefficient — it is on a timer. This is why “過熱” sits at the top of any treatment list.
Insulation Testing (IEEE 43-2013)
| メトリック | 基準 | Action |
|---|---|---|
| Minimum insulation resistance | ≥ 1 MΩ + 1 MΩ per kV rated (例えば. 400 V → ≥ 1.4 MΩ) | Below → dry / クリーン / 修理 |
| Polarization Index (PI = R10分/R1分) | > 2.0 良い; 1.5–2.0 questionable; < 1.0 dangerous | <1.0 → do not operate |
| Test voltage | 500 V DC for ≤1 kV windings (例えば. 400 V motor) | Use megger, lockout/tagout first |
A healthy 400 V winding reads well above 1.4 MΩ with PI > 2. A reading near or below the minimum, または以下のPI 1.0, 湿気を意味します, 汚染, or insulation breakdown — the leading cause of the “won’t start / trips / burns” cluster.
Best Applications & Where Faults Cluster
| 応用 | Why induction motors fit | Top fault to watch |
|---|---|---|
| パンプス & コンプレッサー | 継続勤務, 一定速度 | ベアリングの摩耗, single-phasing |
| ファン & ブロワー | High inertia, 単純なコントロール | Blocked cooling → overheat |
| コンベヤー & マテリアルハンドリング | Robust, 可逆 | 位置ずれ, 過負荷 |
| クラッシャー / ミキサー (high inertia) | High starting torque designs | Rotor-bar stress, 過熱 |
| Machine tools | 安定した速度 | Vibration from imbalance |
| 空調設備 & building systems | メンテナンスの手間がかからない | Contamination, 湿気の侵入 |
ステップバイステップ: Selecting & Maintaining for Reliability
ほとんど “common faults” are actually maintenance gaps. A short, repeatable program prevents the majority of failures:
- Size to the load, not the catalog. Match torque (T = 9550·P/n) とデューティサイクル; over-sizing wastes energy, under-sizing overheats.
- Verify the supply before commissioning. Confirm 3-phase voltage balance ≤1% (MGはありません 1), correct tap/connection (Δ vs Y), and proper phase rotation.
- Choose the thermal class for the environment. Use Class F insulation run at Class B rise for margin; specify Class H for high ambient or frequent starts.
- Baseline the insulation. Record IR and PI per IEEE 43 when new or after rewind — every future test is compared to this trend, not an absolute number.
- Baseline vibration. Capture the as-new spectrum; rising overall levels or new sidebands at 2×slip frequency flag broken rotor bars early.
- Grease on schedule. Fill to ~1/3–2/3 of the bearing cavity; 以上- or under-greasing is itself a leading cause of bearing failure.
- Keep it cool and clean. A 1-inch dust coat on TEFC fins can raise winding temperature 20 °C — clean the frame and check the fan.
よくあるエンジニアリングの間違い
| 間違い | Why it bites | Better practice |
|---|---|---|
| Ignoring 1–2% voltage unbalance | Creates 6–10× current unbalance and silent overheating | Correct supply; derate per NEMA MG 1 その上 1% |
| Replacing a motor without checking the load | Same fault recurs in weeks | Bar the load, check coupling/alignment first |
| Over-greasing bearings | Seal damage, churning heat | Fill 1/3–2/3 cavity on a schedule |
| Swapping any two leads “to reverse” on a running VFD | Can trip or damage drive | Reverse at the drive, not the terminal box |
| Judging health by frame temperature alone | Winding runs much hotter than the frame | Use RTD/thermistor or megger + trend |
| Skipping the PI test, reading only spot IR | Misses moisture/contamination trends | Run full 10-min IEEE 43 test |
トラブルシューティング表: 問題→原因→解決策
| 問題 | 考えられる原因 | 処理 |
|---|---|---|
| Motor does not start, no hum | 供給なし / open fuse / OLR tripped / control fault | Check voltage at terminals, fuses, contactor, OLR setting; restore supply |
| Motor hums but will not turn | 単相, mechanical lock, wrong Δ/Y connection | Measure 3-phase balance; bar the load; correct connection per nameplate |
| Trips immediately on start | Short circuit, 地絡, 過負荷, wrong OLR | Megger to ground; verify OLR vs nameplate; free the load |
| Runs slow under load | 低電圧, single-phase under load, broken rotor bars | Measure terminal voltage/balance; broken-bar (MCSA) test; rewind rotor |
| 過熱 / trips on thermal | 過負荷, blocked cooling, 電圧不平衡, winding fault | Check load current vs FLA; clean cooling path; correct unbalance; メガ |
| 過度の振動 / ノイズ | ベアリングの摩耗, 位置ずれ, imbalance, loose feet | Replace/regrease bearing; カップリングを調整する; バランスローター; tighten foundation |
| Bearing runs hot | Wrong/little/contaminated grease, fit too tight/loose | クリーン, refill 1/3–2/3; ベアリングを交換する; correct fit tolerance |
| Shell/case electrically live | Winding ground fault, 水分, damaged lead | Megger phase-to-frame; ドライ, 修理, or rewind; fix lead insulation |
| Current swings / 不安定な | Broken or loose rotor bars (かご) | Confirm with MCSA sidebands; reweld or replace rotor |
| Runs in wrong direction | Two phases swapped (phase rotation) | Swap any two supply leads at the disconnect/starter |
よくある質問
What are the most common three-phase asynchronous motor faults?
By share of failures: stator winding insulation breakdown (~32%), bearing/mechanical faults (~30%), and rotor-bar faults (~14%), followed by supply/control problems such as single-phasing. 過熱, 振動, failure to start, and live frame are the visible symptoms of these root causes.
Why is single-phasing so destructive?
When one phase opens, the two remaining windings carry the full three-phase load current. Within seconds the overloaded windings heat dramatically, and the motor may keep “走っている” on two phases while cooking itself. Modern overload relays often catch it, but older contactors or worn connections may not — and NEMA MG 1 limits continuous voltage unbalance to 1% to avoid it.
How do I tell an overheated motor from a warm one?
Frame temperature is not winding temperature — the winding runs far hotter. Use embedded RTDs/thermistors, or compare load current against nameplate FLA. IEC による 60034-1 / MGはありません 1, Class B/F/H limit total winding temperature to 130/155/180 ℃; sustained operation above the design point follows the 10 °C rule (each 10 °C halves insulation life).
What does a low insulation resistance or PI mean?
Per IEEE 43-2013, minimum insulation resistance is 1 MΩ + 1 MΩ per kV rated (ある 400 V motor → ≥1.4 MΩ), and the polarization index (R10分/R1分) should exceed 2. Low or falling values mean moisture, 汚染, or insulation breakdown — the leading cause of ground faults and burns. ドライ, クリーン, or rewind as needed.
Can a three-phase motor run missing one phase?
It should not. A motor already running may continue on two phases but will overheat quickly; a motor at rest usually just hums and will not accelerate. Treat any single-phase condition as a fault to clear before restarting.
How often should I test motor insulation?
Baseline at commissioning and after every rewind, then on a scheduled program (commonly annually for critical motors, or tied to predictive-maintenance rounds). Trend the PI over time — a dropping trend matters more than any single reading.
Why Choose Greensky for Three-Phase Motors?
When you need a replacement or a new design built to spec, Greensky supplies a full three-phase asynchronous (誘導) モーター range — plus BLDC, PMSM, and integrated gear-motor options — engineered to IEC 60034 およびNEMA MG 1 so the faults above are designed out, not discovered in the field:
- IE3 / IE4 efficiency: premium-class rotors and lamination stacks that stay cooler and last longer.
- Thermal margin: Class F insulation run at Class B rise, with optional Class H for harsh duty — aligned to the 10 °C life rule.
- Bearing reliability: selected SKF-grade bearings, correct grease fill, and shaft/end-cover fits that resist the #1 failure mode.
- フランジ & mounting compatibility: IEC B5/B14 and NEMA C-face — see our モーターフランジガイド.
- 低MOQ OEM/ODM: custom voltage, 囲い, 軸, and encoder for spares and new machines.
関連書籍
- 同期モーターと誘導モーター: 主な違い
- DCモーターとは? 種類, 原理 & 数式
- How to Troubleshoot a DC Motor (Fault Diagnosis)
- AC モーターと DC モーター: どれを選ぶか
- モーターフランジとは? IEC と NEMA の取り付け
- ギアボックスとギアモーター: 違い & 選択
- ロボットアームに減速機が必要な理由
- エンジニアが知っておくべき BLDC モーターの欠点
参照
- IEC 60034-1 — 回転電機: 評価と性能 (サーマルクラス, 温度制限). webstore.iec.ch/publication/67467
- IEC 60034-30-1 — Efficiency Classes (IE1～IE5) for Rotating Electrical Machines. webstore.iec.ch/publication/67784
- MGはありません 1 — モーターと発電機 (安全性, thermal rise, voltage-unbalance limits). nema.org/standards/view/mg-1-motors-and-generators
- IEEE 43-2013 — 回転機械の絶縁抵抗試験の推奨方法. standards.ieee.org/ieee/43/4385
- IEEE 112 — 多相誘導電動機の標準試験手順 (損失 & efficiency methods). standards.ieee.org/ieee/112/4213
- IEEE 841 — IEEE Standard for Petrochemical and Chemical Industry Motors (信頼性 / メンテナンス). standards.ieee.org/ieee/841/5393
- 私たち. DOE — Electric Motor Systems Efficiency & 信頼性 (maintenance guidance). energy.gov/eere/amo/articles/determination-electric-motors
- SKF — Bearing selection, 潤滑 & maintenance for electric motors. skf.com/us/products/maintenance-products/bearing-maintenance
- Siemens — Low-voltage motor systems & drive integration. siemens.com/global/en/products/drives.html
- Academic — MCSA / broken-rotor-bar fault diagnosis survey (induction motor condition monitoring). sciencedirect.com — Induction motor fault diagnosis review
