What is 2× line frequency?

2× line frequency (100 Hz for 50 Hz systems, 120 Hz for 60 Hz systems) is the vibration frequency caused by the alternating magnetic field in an electric motor. It is always present at some level. Elevated 2× line frequency can indicate electrical problems such as stator eccentricity, shorted laminations, unbalanced phase voltages, or loose iron.

What indicates broken rotor bars?

Broken rotor bars are indicated by sidebands around 1× line frequency spaced at pole pass frequency (2 × slip × line frequency). In current spectrum analysis (MCSA), look for sidebands at f_line ± n × pole_pass. In vibration, look for rotor bar pass frequency with sidebands at 2× line frequency.

How to distinguish electrical from mechanical unbalance?

Key test: turn off power and observe coast-down. If the 1× vibration drops instantly at power cutoff, the source is electrical (magnetic unbalance). If it decreases gradually with speed, the source is mechanical. Electrical unbalance at 1× appears at exactly line frequency or 2× line, while mechanical unbalance appears at shaft running speed.

What is slip frequency?

Slip frequency is the difference between the synchronous speed and actual rotor speed, expressed as a frequency: f_slip = s × f_line, where s = (Ns - N)/Ns. Slip is necessary for torque production in induction motors. Typical full-load slip is 1-5% depending on motor size and design.

How to distinguish electrical from mechanical faults?

Use the power-off test: electrical vibration disappears instantly when power is removed, while mechanical vibration decreases gradually with coast-down. Electrical faults typically produce vibration at exact multiples of line frequency (100/120 Hz), while mechanical faults produce vibration at shaft speed and its multiples. Current signature analysis (MCSA) can further isolate rotor-specific faults.

免費工程工具 #039

馬達電氣故障頻率計算器

計算同步轉速、轉差率、2× 電源頻率、極通過頻率、轉子條通過頻率及診斷邊帶，用於電動機分析。

轉差率 2× 電源極通過轉子條

結果

同步轉速

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轉差率

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轉差頻率

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1× 電源頻率

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2× 電源頻率

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極通過頻率

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軸頻率 (1×)

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同步轉速

交流感應電動機的同步轉速取決於電源頻率和極數：

轉差率

轉差率是同步轉速與實際轉子轉速之間的差值，以分數或百分比表示：

標準感應電動機的典型滿載轉差率為 1–5%。

轉差頻率

2× 電源頻率

由於交變磁場，此頻率（50 Hz 電源為 100 Hz，60 Hz 電源為 120 Hz）始終存在於電動機振動中。2× 電源頻率升高表示電氣問題：相不平衡、氣隙偏心或定子繞組故障。

極通過頻率

極通過頻率是斷條分析的重要指標。電流頻譜（MCSA）中 1× 電源頻率周圍的極通過頻率邊帶是轉子條故障的典型特徵。

轉子條通過頻率

實務範例

範例 — 4 極電動機，50 Hz，1475 RPM

已知： f_電源 = 50 Hz，極數 = 4，N = 1475 RPM

N_s = 120 × 50 / 4 = 1500 RPM

s = (1500 − 1475) / 1500 = 1.67%

f_轉差率 = 0.0167 × 50 = 0.833 Hz

2× 電源 = 2 × 50 = 100 Hz

極通過頻率 = 2 × s × f_電源 = 2 × 0.0167 × 50 = 1.67 Hz

⚠️ 注意： 電動機轉速隨負載變化。請確保使用運轉條件下實際測量的轉速，而非銘牌轉速。使用轉速計或頻閃儀測量可為依賴轉差率的計算提供最準確的結果。

馬達電氣故障頻率計算器

結果

轉子條分析

轉子條邊帶 (f_電源 ± n × f_極通過)

同步轉速

轉差率

轉差頻率

2× 電源頻率

極通過頻率

轉子條通過頻率

實務範例