Professional-grade calculations for HP, RPM, Torque, Motor Selection, and Engineering Analysis
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| Poles | Synchronous Speed (RPM) | Typical Full Load Speed | Slip (%) | Common Applications |
|---|---|---|---|---|
| 2 | 3600 | 3450-3500 | 2.8-4.2 | High-speed fans, centrifugal pumps, grinders |
| 4 | 1800 | 1725-1750 | 2.8-4.2 | General purpose, most common industrial applications |
| 6 | 1200 | 1140-1180 | 3.3-5.0 | Heavy duty, high torque applications |
| 8 | 900 | 850-875 | 2.8-5.6 | High torque, low speed applications |
| 10 | 720 | 690-710 | 1.4-4.2 | Very high torque, slow speed |
| 12 | 600 | 575-590 | 1.7-4.2 | Extremely high torque applications |
| Frame Size | HP Range | Typical Voltage | Mounting Type | Application |
|---|---|---|---|---|
| 42 | 1/20 - 1/6 | 115V | Face Mount | Small appliances, fans |
| 48 | 1/8 - 1/2 | 115/230V | Face/Foot Mount | Small pumps, blowers |
| 56 | 1/4 - 3/4 | 115/230V | Foot Mount | General purpose, light duty |
| 143T-145T | 1 - 3 | 230/460V | Foot Mount | General industrial |
| 182T-184T | 3 - 7.5 | 230/460V | Foot Mount | Industrial machinery |
| 213T-215T | 7.5 - 15 | 230/460V | Foot Mount | Heavy industrial |
| 254T-256T | 15 - 25 | 460V | Foot Mount | Large industrial equipment |
| 284T-286T | 25 - 40 | 460V | Foot Mount | Heavy duty industrial |
| HP Rating | 2 Pole | 4 Pole | 6 Pole | 8 Pole |
|---|---|---|---|---|
| 1 | 77.0% | 85.5% | 82.5% | 77.0% |
| 1.5 | 84.0% | 86.5% | 87.5% | 85.5% |
| 2 | 85.5% | 86.5% | 88.5% | 86.5% |
| 3 | 85.5% | 89.5% | 89.5% | 88.5% |
| 5 | 87.5% | 89.5% | 89.5% | 88.5% |
| 7.5 | 88.5% | 91.0% | 90.2% | 89.5% |
| 10 | 89.5% | 91.7% | 91.0% | 90.2% |
| 15 | 90.2% | 92.4% | 91.7% | 91.0% |
| 20 | 91.0% | 93.0% | 92.4% | 91.7% |
| 25 | 91.7% | 93.6% | 93.0% | 92.4% |
| HP | 115V (1φ) | 230V (1φ) | 230V (3φ) | 460V (3φ) | 575V (3φ) |
|---|---|---|---|---|---|
| 1/4 | 5.8 | 2.9 | - | - | - |
| 1/3 | 7.2 | 3.6 | - | - | - |
| 1/2 | 9.8 | 4.9 | - | - | - |
| 3/4 | 13.8 | 6.9 | - | - | - |
| 1 | 16 | 8 | 2.5 | 1.25 | 1.0 |
| 1.5 | 20 | 10 | 3.7 | 1.85 | 1.48 |
| 2 | 24 | 12 | 4.8 | 2.4 | 1.92 |
| 3 | 34 | 17 | 6.6 | 3.3 | 2.64 |
| 5 | 56 | 28 | 10.6 | 5.3 | 4.24 |
| 7.5 | - | 40 | 15.2 | 7.6 | 6.08 |
| 10 | - | 50 | 19.6 | 9.8 | 7.84 |
An induction motor cannot run at synchronous speed. If it did, the rotor would see no relative motion of the stator field, no EMF would be induced, no rotor current would flow and no torque would be produced. It must therefore slip:
s = (ns − n) / ns with ns = 120f/P
A four-pole motor on 50 Hz has a synchronous speed of 1,500 rpm. A nameplate reading 1,440 rpm therefore means 4% slip at full load. Slip is near zero at no load and rises with torque, which is why shaft speed falls slightly as a motor is loaded.
Motor efficiency peaks somewhere around 75–100% of rated load and drops sharply below about half load, because the fixed losses — core loss, friction and windage — stay constant while useful output shrinks. An oversized motor therefore runs both less efficiently and at worse power factor than a correctly sized one, which is the usual argument against specifying generously "to be safe".
At standstill slip is 1 and the rotor behaves like a shorted transformer secondary, so only leakage impedance limits current — hence six to eight times full-load current on direct-on-line starting. As the rotor accelerates, slip falls, rotor impedance rises and current drops to its running value.
The difference between synchronous and actual speed, as a fraction of synchronous speed. An induction motor must slip to induce rotor current and produce torque.
ns = 120f/P, where f is supply frequency and P the pole count. A four-pole motor on 50 Hz has a synchronous speed of 1,500 rpm.
It is a four-pole machine with 4% slip at full load, since synchronous speed is 1,500 rpm. Slip approaches zero at no load and rises as the motor is loaded.
Mechanical shaft output. Divide by the efficiency to find electrical input, which is what the supply, cable and protection must be sized for.
Efficiency and power factor both fall away below about half load, because fixed losses stay constant while useful output shrinks. A correctly sized motor runs more efficiently and draws less current for the work done.
A multiplier indicating permissible continuous overload. A 1.15 service factor allows 15% above rated load, at the cost of higher temperature rise and reduced insulation life.