Motor Drives
Fan power goes as the cube of speed, so a 20% reduction halves the energy. Then vector control made the better motor the easier one to command.
Skip to the animationMotors use about half the world's electricity and mostly run at fixed speed, throttling away the surplus — and since fan and pump power goes as the cube of speed, a variable-speed drive turns a 20% speed reduction into a 50% energy saving.
The cube law
For a centrifugal fan or pump, flow is proportional to speed and pressure to speed squared, so shaft power goes as the cube.
| Speed | Flow | Power |
|---|---|---|
| 100% | 100% | 100% |
| 80% | 80% | 51% |
| 60% | 60% | 22% |
| 50% | 50% | 12.5% |
Throttling with a valve or damper wastes the difference as pressure drop — like driving with the brakes on. A drive typically pays back in months, which makes this the largest single energy-saving opportunity in most industrial plants.
Why DC came first
A DC motor separates torque and flux onto different terminals: armature current sets torque, field current sets flux. A controlled rectifier on each gives near-independent control, and the control problem is almost trivial.
That simplicity is why DC dominated variable-speed drives until the 1980s — despite the motor being the worse machine, with brushes that wear, spark and rule out hazardous areas.
The induction motor's difficulty
An induction motor is rugged, cheap and sealed — but torque and flux come from the same stator current, so they cannot be commanded separately. Constant V/f control holds flux approximately and offers no direct torque control.
That is fine for a fan and unacceptable for a lift, a winder or a machine-tool spindle, where torque must be commanded precisely and immediately.
Vector control
Field-oriented control transforms the stator currents into a reference frame rotating with the rotor flux. In that frame one component controls flux and the other controls torque, independently — so the induction motor presents the same clean interface a DC motor always had.
The theory dates from 1971; it became practical when microprocessors could do the transformation thousands of times a second. That is what finally made the induction motor the default for demanding drives.
Braking
Drive a motor above synchronous speed and it becomes a generator, pushing energy back and raising the DC link voltage. That energy must go somewhere.
- Dynamic braking — dump it into a resistor. Simple, and wastes all of it.
- Regenerative — return it to the mains through an active front end. Pays for itself in lifts and cranes.
- Shared DC bus — one axis's braking energy accelerates another's, common in multi-axis machinery.
Problems the drive brings
| Problem | Cause | Countermeasure |
|---|---|---|
| Reflected wave | Fast edges on long cables double the terminal voltage | Output reactor, dv/dt filter, inverter-duty motor |
| Bearing currents | Common-mode voltage discharges through the bearing | Insulated bearing, shaft grounding ring |
| Upstream harmonics | Passive rectifier front end | Line reactor, 12-pulse, or an active front end |
| EMI | High dv/dt radiating from cables | Shielded cable, correct gland termination |
A fixed-speed motor needed none of these. They are real costs against a saving that is usually far larger — but they are why a drive retrofit is an engineering job rather than a swap.
The numbers you will be asked for
- Affinity laws
Q ∝ N · H ∝ N² · P ∝ N³
- Synchronous speed
N_s = 120f / p
- Slip
s = (N_s − N) / N_s
- Constant V/f
V/f = constant below base speed
- Field weakening
above base speed, flux ∝ 1/f, so torque falls
Watch it work
Check yourself
question 1 / 4
One question at a time. Pick an answer to see why it is right or wrong, then move on — there is no score to keep and nothing is saved.