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4-pole motor: asynchronous speed, slip, rotor frequency

Short answer

Synchronous speed = 120×60/4 = 1800 r pm. Slip = (1800-1755)/1800 = 0.025 (2.5 %). Rot or fre quency = 0.025×60 = 1.5 Hz.

A 208 V, 60 Hz, 4 pole, 3-phase induction motor has a full-speed of 1755 rpm. Calculate: (i) asynchronous speed, (ii) the slip and (iii) rotor frequency.

Given: 208 V, 60 Hz, 4-pole, full-load speed 1755 rpm.

(i) Asynchronous (synchronous) speed

Ns=120fP=120×604=1800 rpmN_s = \frac{120 f}{P} = \frac{120\times 60}{4} = 1800\ \text{rpm}

(ii) Slip

s=Ns−NrNs=1800−17551800=0.025=2.5%s = \frac{N_s - N_r}{N_s} = \frac{1800-1755}{1800} = 0.025 = 2.5\%

(iii) Rotor frequency

fr=sf=0.025×60=1.5 Hzf_r = sf = 0.025\times 60 = 1.5\ \text{Hz}

Answer: (i) 1800 rpm; (ii) 0.025 (2.5 %); (iii) 1.5 Hz.

✓ Answer checked by Claude Opus 5.5 (high effort)

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