Induction Motors and Drives
Slip, torque, the nameplate, efficiency classes, starting methods and variable-speed drives.
How an induction motor works
The stator carries three windings that, fed by three-phase current, create a rotating magnetic field at the synchronous speed ns = 120 f ÷ p (see Three-Phase Systems). The rotor is a squirrel cage of bars short-circuited at the ends. The field cuts the bars and induces currents in them, and those currents make torque. The rotor can never quite reach synchronous speed, because at synchronous speed nothing would cut the bars and the torque would be zero: it runs a little slower, by the slip s = (ns − n) ÷ ns.
It is the most common machine in industry: robust, cheap, with no brushes or commutator, and starts itself. About two thirds of industrial electricity goes through induction motors.
Reading the nameplate
| Plate entry | Example | Meaning |
|---|---|---|
| Power (kW, hp) | 11 kW | mechanical output at the shaft at rated load |
| Voltage and connection | 400 V Δ / 690 V Y | delta on a 400 V supply, star on a 690 V supply |
| Current (FLA) | ≈ 20.5 A | full-load current at rated voltage: use it to size the overload relay and cable |
| Speed | 1 470 rpm | rated shaft speed; synchronous 1 500 rpm (4 poles at 50 Hz) |
| Power factor and efficiency | cos φ 0.85, η 91 % | at rated load: both fall at light load |
| Frequency | 50 Hz | a 50 Hz motor on 60 Hz runs 20 percent faster and needs the voltage to match (volts per hertz) |
| Service factor, duty, class | S1, insulation class F, IP55, IE3 | continuous duty; temperature rise 105 K; dust and water protection; efficiency class |
The motor’s slip at rated load is (1 500 − 1 470) ÷ 1 500 = 2.0 percent; the rotor currents then have a frequency of s f = 1.0 Hz. The rated torque is T = P ÷ ω = 11 000 ÷ (2π × 1 470 ÷ 60) = 71.5 N·m.
Torque and speed
Torque rises with slip up to a maximum (pull-out or breakdown torque, 2 to 3 times rated) and falls beyond it. In the normal running range the curve is nearly a straight line: a motor loaded to half torque runs at about 1 485 rpm, only 0.5 percent above its rated speed. The stable operating point is where the motor torque equals the load torque, with the motor curve steeper than the load curve.
Starting methods
At standstill the motor looks like a short-circuited transformer, so it draws a locked-rotor current of 5 to 8 times its full-load current, with a torque of 1 to 2.5 times rated. The supply cable, the transformer and other loads must tolerate the voltage dip, and the starting method is chosen to limit it:
| Method | Start current | Start torque | Notes |
|---|---|---|---|
| Direct on line (DOL) | 5 to 8 × In | 100 % | simple, cheap; below about 5 to 7.5 kW or where the supply is strong |
| Star-delta | ⅓ of DOL | ⅓ of DOL | for light starts (pumps, fans); needs a motor wound for delta at the line voltage; a transient at the changeover |
| Soft starter | adjustable (2 to 4 × In) | adjustable; torque falls as V² | smooth ramp, bypass contactor after start; also soft stop for pumps |
| Variable-speed drive | about 1 to 1.5 × In | full torque from standstill | best control; also saves energy and gives speed control |
| Autotransformer, rotor resistance | adjustable | adjustable | older methods; wound-rotor and large HV motors |
Worked example: starting the press motor
Worked example: DOL, star-delta and soft start for the 11 kW motor
- DOL: Istart = 7 × 20.5 = 144 A, with a torque of about 0.51 times rated on the simple curve (real motors have more). On the 100 kVA transformer of Transformers and Distribution (rated 144 A) that is 1.0 times the transformer rating: a visible dip.
- Star-delta: the winding sees 400 ÷ √3 = 231 V, so both the current and the torque fall to a third: 48 A and 0.17 times rated torque.
- The press pump starts unloaded (the unloading valve of hydraulic design passes the oil to the tank at 5 bar), so the starting torque needed is only about 5 percent of rated: star-delta works. A press that started against full pressure (a constant-torque load of 1.0 times rated) could not start in star, because the star torque of 0.17 is below the load.
- Soft starter at 60 percent initial voltage: current 86 A (current ∝ V), torque 36 percent of DOL (torque ∝ V²).
- Acceleration time with a total inertia J = 0.35 kg·m² (motor and pump) and an average accelerating torque of 64 N·m: t = J ω ÷ T = 0.35 × 154 ÷ 64 = 0.84 s.
Variable-speed drives
A variable-frequency drive (VFD or inverter) rectifies the AC to DC and chops it into three-phase AC of adjustable frequency and voltage. The synchronous speed follows the frequency; the voltage is raised in proportion (V ÷ f constant) so the flux, and so the torque, stay constant: at 25 Hz the drive supplies 200 V for 400 V at 50 Hz (8 V per Hz). Above base speed the voltage cannot rise further, so the torque falls (field weakening). Benefits: a soft start with low current, precise speed control, energy savings on pumps and fans, braking and reversing. The cautions are harmonics (Power Factor, Harmonics and Energy), motor cooling at low speed (a fan on the shaft slows with the motor), long cable limits (dV/dt and reflected voltage; use output filters), and bearing currents.
Worked example: a fan on a drive
- A fan taking 8.8 kW at full speed runs at 80 percent of that flow most of the time. By the fan laws (see Ducts, Fans and Air Distribution) the power falls with the cube of the speed: 8.8 × 0.8³ = 4.5 kW.
- The saving is 4.3 kW over 4 000 hours: 17 178 kWh, about 2 061 a year, against a throttling damper which would save far less. A drive pays for itself in one or two years on a fan or pump that runs most of the day.
Efficiency classes and energy
IEC 60034-30-1 defines efficiency classes: IE1 (standard), IE2 (high), IE3 (premium), IE4 (super premium), with IE3 now compulsory for most new motors from 0.75 kW in the EU, and equivalent NEMA Premium in the US. Typical figures for a four-pole 11 kW motor: IE2 89.8 percent, IE3 91.4 percent, IE4 92.7 percent.
Worked example: what is a better motor worth?
- Electrical input for 11 kW out: IE2 12.25 kW, IE3 12.04 kW, IE4 11.87 kW.
- Over 4 000 hours a year the IE3 saves 858 kWh (103 a year at 0.12) and the IE4 1 533 kWh (184). A motor’s running cost over its life is about 50 to 100 times its purchase price, so efficiency matters more than price.
- Right-sizing matters too: a motor loaded under 40 percent has a poor power factor and efficiency. The load of 8.5 kW plus a 15 percent margin needs 9.8 kW: choose 11 kW (the next standard size), not 15 kW.
Heating, ratings and protection
A motor is limited by temperature, not torque. Insulation class F allows a winding temperature rise of 105 K over a 40 °C ambient; class H more. Every 10 K above the rated temperature roughly halves the insulation life (50 percent of it left). High ambient or altitude reduce the rating: at 55 °C ambient the 11 kW motor should be derated to about 9.7 kW (88 percent), and above 1 000 m again. Protect with an overload relay or motor protection circuit breaker set at about the nameplate current (here 21.6 A, 1.05 × 20.5), a trip class 10 (trips in 10 s at 7.2 times) or 20 for hard starts, thermistors (PTC) in the windings, and phase-loss protection. Short circuits are covered by fuses or a magnetic breaker (Protection, Earthing and RCDs).
Related
- Previous: Transformers and Distribution. Next: Cable Sizing and Voltage Drop.
- Variable-Frequency Drives and Motion Control and Actuators.
- Motor Torque and Inertia Matching Calculator.
- Electrical Power: the whole guide.
FAQ
What does slip mean?
The shortfall of the rotor speed below the synchronous speed, as a fraction. It rises with load: about 2 to 5 percent at full load for medium motors.
Why does my motor draw so much current at start?
At standstill there is no back-emf to oppose the supply, so the current is limited only by the leakage impedance: 5 to 8 times rated.
Can I reverse a motor?
Yes: swap any two of the three phases (a reversing contactor does it). Wait for the motor to stop, or use a drive.
Can any motor run on a drive?
Most can, but check the insulation (inverter-duty windings), cooling at low speed and the cable length; a drive-rated motor with an encoder is better for precision.
References
- Wildi, T., Electrical Machines, Drives and Power Systems, Pearson; Fitzgerald, A. E., et al., Electric Machinery, McGraw-Hill.
- IEC 60034-1 (rating and performance), IEC 60034-30-1 (efficiency classes), IEC 60034-12 (starting performance), IEC TS 60034-25 (motors on converters).
- US DOE, Improving Motor and Drive System Performance: A Sourcebook for Industry; NEMA MG-1.
Last reviewed: October 2026. Motor ratings and efficiencies are typical for a four-pole 11 kW machine.