Motor Drives

A drive combines power electronics and control to run a motor at any speed and torque. Learn VFD architecture, V/f and vector control, braking and regeneration, the affinity laws, drive sizing and installation issues.

A variable-frequency drive (VFD) rectifies the mains to a DC link, then inverts it to a variable-voltage, variable-frequency supply for the motor. Together with control software it can hold speed or torque precisely, start motors gently and save large amounts of energy.

Drives run around a third of the world's industrial motors and all EVs, lifts, CNC spindles and modern trains.

An analogy

A drive is like an automatic gearbox and cruise control for an electric motor: it gives exactly the speed and torque needed at every moment, instead of running flat-out and braking.

VFD building blocks

A VFD has three blocks: a rectifier (usually a six-pulse diode bridge), a DC link (capacitors, sometimes a choke) and an IGBT inverter producing PWM. A control board reads commands (speed reference, start/stop) and feedback, and protects against overcurrent, overvoltage and overtemperature.

V/f control

Keeping V/f constant keeps motor flux constant, so rated torque is available across the speed range below base speed. Low-speed boost compensates for stator resistance. It is simple, open-loop and fine for pumps and fans.

Vector (field-oriented) control

FOC transforms the three motor currents into a rotating frame aligned with the rotor flux (d-q axes). The d-current controls flux and the q-current controls torque, exactly like a DC motor's field and armature. With an encoder (or a sensorless observer) it delivers full torque at zero speed and fast dynamic response.

Braking and regeneration

Decelerating a load turns the motor into a generator; energy flows back into the DC link and raises its voltage. Options: dissipate it in a braking resistor via a brake chopper, share it on a common DC bus with other drives, or return it to the grid with an active-front-end (regenerative) rectifier — important for cranes, lifts and centrifuges.

Installation effects

Fast PWM edges on long motor cables cause reflected-wave overvoltage (up to 2× V_dc) that stresses motor insulation; dv/dt or sine filters fix it. Common-mode voltage drives bearing currents that pit bearings — insulated bearings and shaft grounding rings help. Six-pulse rectifiers inject harmonics; line reactors, DC chokes or AFEs reduce them.

Formulas

• Affinity laws (pumps/fans): Q ∝ n, H ∝ n², P ∝ n³
• Constant flux: V / f = V_rated / f_rated
• Kinetic energy to brake: E = ½ J ω²
• Braking resistor: R_br = V_chopper² / P_brake,peak

Worked example: Fan energy at reduced speed

Given: Fan runs at 80 % speed instead of 100 %

Find: Power relative to full speed.

• P ∝ n³ → 0.8³ = 0.512
• Savings ≈ 49 % of fan power

Answer: ≈ 51 % power — nearly half saved

Worked example: Braking resistor for a flywheel stop

Given: J = 10 kg·m²; Stop from 1500 rpm in 5 s; Brake chopper at 750 V

Find: Energy, peak braking power and resistor value.

• ω = 2π × 1500/60 = 157.1 rad/s → E = ½ × 10 × 157.1² = 123.4 kJ
• Average power = 123.4/5 = 24.7 kW; linear ramp → peak ≈ 2 × 24.7 = 49.3 kW
• R_br = 750²/49 300 = 11.4 Ω (energy rating ≥ 123 kJ per stop)

Answer: ≈ 11 Ω, 49 kW peak, 123 kJ per stop

Common mistakes

• Wrong: Assuming a standard motor can run at low speed and full torque indefinitely on a VFD. — Right: Self-cooled motors lose fan cooling at low speed; derate or add forced cooling.
• Wrong: Running 100 m of unshielded cable to a motor with no filter. — Right: Use shielded VFD cable, proper grounding, and dv/dt or sine filters for long runs.

Applications

• HVAC & water: VFDs on fans and pumps are the single biggest industrial energy-saving measure, often paying back in under two years.
• Lifts & cranes: Regenerative drives recover braking energy and give smooth, precise positioning.
• Electric vehicles: The traction inverter is a high-performance motor drive with FOC, field weakening and regenerative braking.

Key takeaways

• VFD = rectifier + DC link + inverter + control.
• V/f for simple loads; FOC for high performance.
• Pumps/fans: P ∝ n³ — huge savings from speed control.
• Handle regeneration (brake resistor or AFE) and cable/bearing effects.

Going further

• Direct torque control: DTC selects inverter voltage vectors from a switching table to keep flux and torque within hysteresis bands, giving very fast torque response without a PWM modulator.
• Model predictive control: MPC evaluates candidate switching states against a motor model each cycle and picks the one minimising a cost function — increasingly practical with fast processors.
• Drive efficiency classes: IEC 61800-9 defines IE/IES classes for drives and motor systems, encouraging evaluation of the complete power drive system rather than components alone.