AC Motors and Variable-Frequency Drives
Induction motor speed and slip, V/Hz control, drive setup and parameters, motor cables, derating, harmonics and braking.
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How an induction motor sets its speed
The stator of a three-phase motor makes a magnetic field that rotates at the synchronous speed ns = 120 × f ÷ p (rpm, with f the supply frequency in Hz and p the number of poles). The rotor runs slightly slower, because torque needs the rotor to cut field lines: the difference is the slip, s = (ns − n) ÷ ns, typically 2 to 6 percent at rated load. Changing the frequency therefore changes the speed. A drive does this: it makes the three-phase supply at whatever frequency you ask for.
Worked example: speed at different frequencies
- A 4-pole motor on 60 Hz has ns = 120 × 60 ÷ 4 = 1 800 rpm, and runs at about 1 746 rpm at full load.
- On a drive at 30 Hz: ns = 120 × 30 ÷ 4 = 900 rpm, and with 4 percent slip the shaft turns at 864 rpm. Speed control is about 1:10 to 1:20 with plain V/Hz, more with vector control.
- More poles means lower speed: 2-pole 3 600 rpm, 4-pole 1 800, 6-pole 1 200, 8-pole 900 rpm at 60 Hz (3 000, 1 500, 1 000, 750 at 50 Hz). More poles also mean a larger frame for the same power, because torque is what sets the size.
Inside a VFD
The drive rectifies the mains to a DC bus (566 V for a 400 V supply, from 400 × √2), then an inverter of power transistors (IGBTs) switches the bus at kilohertz rates, in pulse-width modulation, to build a sine-like current at the chosen frequency and voltage. The DC bus capacitors store energy and, for braking, must absorb it (below).
Control methods: V/Hz, sensorless vector, closed-loop vector
| Method | How it works | Speed regulation | Torque at low speed | Use for |
|---|---|---|---|---|
| Scalar V/Hz | Voltage is set in proportion to frequency (with a boost at low speed) | About 2 to 3 percent of base speed, with slip | Limited; poor below 3 to 5 Hz | Fans, pumps, conveyors, simple loads |
| Sensorless vector (SVC) | The drive estimates rotor position from the motor model and currents, and controls torque and flux | About 0.5 to 1 percent | Good down to about 1 Hz, 150 percent torque | Cranes, mixers, extruders, general machines |
| Closed-loop vector (FOC) | Encoder feedback; field-oriented control of flux and torque | Under 0.1 percent; zero-speed torque | Full torque at zero speed | Hoists, winders, servo-like accuracy |
V/Hz. The flux in the motor is proportional to V ÷ f. For a 400 V, 50 Hz motor the ratio is 8.0 V per Hz, so at 20 Hz the drive outputs about 160 V. At low speed the winding resistance matters, and a voltage boost is added. Above the base frequency the voltage cannot rise further, so the flux weakens: torque falls as 1/f and power stays constant (the field-weakening region).
Setting up a drive
- Enter the nameplate: rated voltage, current, frequency, speed (rpm), power and power factor. The drive builds its protection and control from these. Wrong data gives poor torque and false overload trips.
- Run the auto-tune (stationary or rotating) for vector control: it measures the stator resistance and inductance.
- Set the limits: minimum and maximum frequency, acceleration and deceleration times, current limit, motor overload (the drive’s I²t protection).
- Choose the control source: keypad, digital inputs (start, stop, forward, reverse), analog speed reference (0 to 10 V or 4 to 20 mA, see the Analog Scaling Tool), or a fieldbus (Modbus, EtherNet/IP, PROFINET: see Connecting PLCs, Drives and HMIs).
- Wire the safety: an emergency stop must remove power or use the drive’s safe torque off input, not just a program stop (see Braking, Safe Stopping and Troubleshooting).
- Check direction and test at low speed with the load uncoupled where possible.
- Save the parameters and record them: a replaced drive must be loaded with the same set.
Ramps, torque and acceleration time
The acceleration time must be long enough that the torque the drive can produce is not exceeded. A drive can deliver around 150 percent of rated current for a minute (check the data). The time to accelerate an inertia J through a speed change Δω with an available torque T is t = J × Δω ÷ T.
Worked example: acceleration time of a fan
- The 1.5 kW, 1 440 rpm motor of Choosing a Motor Technology has a rated torque of 9.95 N·m and a rotor inertia of 0.0045 kg·m²; the fan adds 0.05 kg·m². Total J = 0.05 kg·m².
- Speed change: 1 440 rpm = 150.8 rad/s. With 150 percent of rated torque (14.9 N·m) and ignoring the load torque: t = 0.05 × 150.8 ÷ 14.9 = 0.55 s at the very least.
- In practice the fan load torque takes part of the motor torque, so the real time is longer: set the ramp to several seconds. A ramp set shorter than the physical limit trips the drive on overcurrent.
Braking and regeneration
When the drive slows a load faster than friction would, or lowers a load, the motor becomes a generator and pushes energy back into the DC bus, whose voltage rises. If it exceeds the limit the drive trips on overvoltage. The options are to lengthen the deceleration time, to burn the energy in a braking resistor (a chopper switches it across the bus), or to return it to the supply with a regenerative front end; drives on a shared DC bus can use each other’s energy.
Worked example: braking energy
The fan above stored E = ½ J ω² = ½ × 0.05 × 150.8² = 620 J. Stopping it in 2 s means an average braking power of 620 ÷ 2 = 310 W (the peak is higher). Part of the energy is lost in friction, but a braking resistor must be sized for the peak power and for the heating of repeated stops. A fan with its own drag may need no resistor at all. Check the drive’s data for the minimum resistor value.
Pumps and fans: the affinity laws
For a centrifugal pump or fan, flow is proportional to speed, pressure to the square of speed, and power to the cube. Slowing the machine saves energy far faster than the speed drops.
Worked example: running at 80 percent speed
Flow = 80 percent; pressure = 0.8² = 64 percent; power = 0.8³ = 51 percent. A 20 percent reduction in flow saves nearly half the power, which is why a drive is better than throttling a valve or damper. For 8 000 hours a year the saving on a 7.5 kW fan runs to thousands of kWh.
Installation, cables and harmonics
- Motor cable: shielded, with the shield bonded 360° at both ends, as short as practical. Long cables cause reflected voltage spikes that damage the motor insulation: use an output reactor or dv/dt filter above the manufacturer’s limit (commonly 50 to 100 m).
- Separate the motor cable from control and signal cables (see Cabling, Grounding and Noise).
- EMC and harmonics: the rectifier draws current in pulses. A line reactor or DC choke reduces the harmonics and protects the drive from supply disturbances; an EMC filter limits conducted noise.
- Cooling: the drive dissipates around 2 to 3 percent of its rating as heat. Keep the airflow clear and the enclosure temperature within the rating (derate above 40 °C or at altitude).
- Bearing currents: fast switching can drive current through motor bearings: use insulated bearings or shaft grounding rings on larger or long-cable installations.
- Do not switch the output with a contactor while running unless the drive is stopped.
- Motor cooling at low speed: a shaft fan cools less at low speed, so a constant-torque load at low speed needs an independent fan (forced ventilation) or an inverter-duty motor.
Faults and troubleshooting
| Fault or symptom | Likely cause | Check |
|---|---|---|
| Overcurrent on starting | Ramp too short, locked rotor, a short in the motor or cable | Lengthen the ramp, check the mechanics, megger the motor and cable |
| Overvoltage while slowing | Regeneration into the DC bus | Longer deceleration, a braking resistor, or a regenerative unit |
| Undervoltage or phase loss | Supply sag, a blown fuse or a loose connection | Measure all three phases on the input |
| Motor overload trip | Wrong nameplate data, a stuck load, poor cooling | Check the set current and the real current with a clamp meter |
| Drive overheating | Blocked airflow, high ambient temperature, a fan failure | Clean and check the cooling fans and the enclosure |
| Earth (ground) fault | Cable or motor insulation failure, moisture | Insulation test with the cable disconnected |
| Motor will not reach set speed | Current limit reached, an analog reference scaling error | Check the limits and the reference signal with a meter |
| Noisy or rough at low speed | Scalar control with a light load, carrier frequency | Try vector control, raise the carrier frequency, check the boost |
| Unexplained nuisance trips | Electrical noise, a poor ground, long motor cables | Shielding and grounding, an output filter, separate the cabling |
Related
- Motion Control and Actuators: the overview of this guide.
- Choosing a Motor Technology and Braking, Safe Stopping and Troubleshooting: motor choice, and safe stopping and braking.
- Servo Systems and Tuning: the closed-loop alternative.
- Motors, Drives and PID Control: wiring a drive to a PLC.
- Power Electronics for the converter electronics.
FAQ
Can I run any motor on a drive?
Most standard three-phase induction motors work, but check the insulation rating and the cooling at low speed. Inverter-duty motors are built for it. A motor not rated for inverter use may fail early on long cables or at high bus voltages.
Why is the motor current higher than the nameplate at low speed?
At low speed there is less cooling and the drive supplies a boost. If the load needs full torque at low speed, the current is the full rated value, so the motor is thermally limited; it is the cooling, not the current, that must be addressed.
Can a single-phase supply run a three-phase motor?
Yes, with a drive rated for single-phase input (and usually derated for the higher input current). The motor sees a proper three-phase supply at its terminals.
How do I stop a drive safely?
Use the drive’s safe torque off (STO) input in the emergency stop circuit, with a category-appropriate stop function. A program stop or a keypad stop is not a safety function (see Braking, Safe Stopping and Troubleshooting).
References
- Hughes, A. and Drury, B., Electric Motors and Drives, 5th ed., Newnes.
- Siemens, Rockwell Automation (Allen-Bradley PowerFlex), ABB, Danfoss and Yaskawa drive manuals: parameter and installation guides.
- IEC 61800-3 (EMC for adjustable speed drives) and IEC 61800-5-1 (electrical safety); NEMA MG-1, Part 31 (inverter-fed motors).
Last reviewed: October 2026. Values are typical examples; follow the drive and motor manufacturer’s instructions and local electrical codes.