How Electric Motors Work (and Run Everything)

Part 15 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Around half of all the electricity used on Earth ends up doing one thing: spinning an electric motor. The IEA's 4E Electric Motor Systems Annex puts the figure at 53% of global electricity consumption, swallowed by the motors driving pumps, fans, compressors and conveyors. The older textbooks say "more than 40%," a figure that traces to a 2011 IEA report; the number has only climbed since. Walk into a Malaysian chiller plant or a moulding factory and the local share is higher still, because almost everything that moves in there is a motor. So understanding how electric motors work is understanding where most of your power bill actually goes. And the machine doing nearly all of that work is one specific, unglamorous design: the squirrel-cage induction motor.

How electric motors work: what's inside an induction motor

Two parts, and only one of them is wired to anything.

The outer part is the stator, a ring of iron carrying three sets of windings. Feed those windings three-phase AC and they conjure a magnetic field that sweeps around the inside of the ring like the lit bulb chasing around a marquee sign. That rotating magnetic field is the whole trick of three-phase power, and it needs no moving parts to produce: the three currents peak in turn, 120 degrees apart, so the magnetic "peak" appears to rotate on its own.

The inner part is the rotor, and here is the surprise. It is not a coil, not an electromagnet, not connected to the supply at all. It is a cage of aluminium or copper bars, shorted together by a ring at each end, buried in an iron core. It looks like a hamster wheel, which is where the name comes from. No brushes touch it. No slip rings feed it. No wire runs to it. The spinning part of the machine that runs your whole plant is connected, electrically, to nothing.

How does an induction motor actually turn?

The rotating field reaches into the cage and creates the very current that drags the cage around.

This is Faraday's law of induction wearing its third costume in this course. In a generator a moving magnet induces current in a coil. In a transformer a changing field induces voltage in a second winding. In an induction motor the rotating stator field sweeps past the stationary cage bars and induces currents in them, exactly like a transformer whose secondary happens to be free to spin. Those induced currents turn the rotor into its own magnet, and the two magnetic fields, the stator's and the rotor's, try to line up. But the stator field keeps moving, so the rotor is left perpetually chasing it. That chase is torque.

Picture dragging a strong magnet across a sheet of copper. You never touch the copper, yet it shuffles along after the magnet, tugged by the currents your moving field stirs up in it. The rotating stator field is the magnet; the cage is the copper. That is the entire principle, and it is why the machine "works by induction."

Why does a "1,500 rpm" motor run at 1,440?

Because the rotor can never quite catch the field it is chasing, and that gap has a name: slip.

Think it through. If the rotor ever matched the field exactly, there would be no relative motion between them, nothing sweeping past the bars, no induced current, and therefore no torque. The moment it caught up it would stop pulling and fall behind again. So it must always lag by a little to keep inducing current in itself. On a 50 Hz supply a four-pole motor has a synchronous field speed of 1,500 rpm, but under full load the rotor turns nearer 1,440. That few percent shortfall is the slip, typically 1 to 5%, and it is not a defect. It is the price of the whole induction principle. Load the motor harder and it slips a touch more; ease off and it creeps back toward 1,500. An induction motor is almost, but never quite, a constant-speed machine.

Why did this design take over industry?

Because there is nothing to wear out and nothing to spark.

A brushed motor drags carbon brushes against a spinning commutator; they wear down, throw sparks, and need replacing, and the sparks make them dangerous around dust or fumes. The squirrel-cage rotor has none of that. Nothing rubs, nothing arcs, nothing is consumed but the two bearings it turns on. It is cheap to build, rugged, and close to maintenance-free, which is why more than nine in ten industrial motors are this one design. The catch, and the reason the rest of this article exists, is that a machine this simple pushes all its awkward behaviour out onto the electrical supply feeding it.

Why does starting a big motor dim the lights?

Because at the instant of switch-on the motor looks almost like a short circuit.

At standstill the rotor is not yet turning, so it generates no opposing voltage to limit the current. Throw a large motor straight across the line, called direct-on-line starting, and it gulps 6 to 8 times its normal running current, occasionally up to ten, for the couple of seconds it takes to spin up. That surge drags the voltage down across the whole switchboard, which is how your own plant manufactures its own voltage sags: the lights dip and a sensitive line nearby can trip every time the big compressor kicks in.

There is a ladder of fixes, cheapest to most capable. A star-delta starter wires the motor one way to start and switches it over near full speed, cutting the starting current to about a third, crudely but cheaply. A soft starter uses electronics to ramp the voltage up smoothly, holding inrush to two or three times rated and sparing the belts and couplings a mechanical jolt. At the top sits the variable frequency drive (VFD), which ramps both voltage and frequency so the motor accelerates from a standstill and, as a bonus, lets you run a fan or pump at part speed and save real energy. The VFD's trade-off is that its electronic front end draws current in sharp gulps rather than a smooth sine, injecting harmonics back onto your supply. Every rung of the ladder buys you something and costs you something.

Why does an idling motor have terrible power factor?

Because an induction motor draws a fixed lump of magnetising current whether it is working hard or barely working at all.

To build its magnetic field the motor pulls a steady magnetising current, roughly 20 to 60% of full-load current, and that current does no useful work; it is purely reactive power, stored in the field and handed back every cycle. At full load the large working current dominates and the power factor (how much of the current the motor pulls actually does useful work) sits at a healthy 0.85 or so. Run the same motor lightly loaded and the working current shrinks while the magnetising current stays put, so power factor collapses toward 0.2 or 0.3. An oversized motor loafing along at a fraction of its rating is therefore permanently dragging your site's power factor down, which in Malaysia means a TNB penalty for falling below 0.85. The fix is unglamorous: size the motor to the load, do not buy big "just in case."

How to read a motor nameplate

The metal plate on the side tells you everything you just read, if you know what you are looking at.

Four fields matter most. kW is the rated shaft output, the useful mechanical power. rpm is the full-load speed; if it reads 1,440 you are looking at a four-pole motor and the 60 rpm shortfall from 1,500 is the slip. PF or cos-phi is the full-load power factor. And the IE efficiency class, set by IEC 60034-30-1, runs IE1 (standard) up through IE3 (premium) to IE5 (ultra premium), each rung shaving off losses.

That last field is where the money hides. Over a fifteen-to-twenty-year life, the purchase price of a motor is only 1 to 5% of what it costs to own; the electricity it drinks is the other 95% and more. A one or two percentage-point gain in efficiency pays back the price premium of a higher IE class in a year or two, then keeps paying for the rest of the motor's life. Buying a motor on its sticker price is like choosing a car on the showroom tag and ignoring two decades of fuel, except here the fuel is almost the entire cost. You do not buy a motor. You buy the electricity it will spend the next twenty years spinning into work.

This is Part 15 of 23 in Cobler's Electricity Fundamentals series. Previous: Three-Phase Power Explained: Why Factories Get 400 V. Next: How Electricity Meters Work: Disc to Smart Meter.

Want to see which motors on your site are oversized, slipping or dragging your power factor down? Book a demo and CobiNeural will show you the live load, efficiency and power factor on every motor you run.