Electric Motors

§1The prime mover as a component

This library’s lens is mechanical, and through it a motor is four things: a torque–speed curve, a thermal budget, a standardised lump of iron, and a shaft end that the rest of this section connects to.

The electrical interior — windings, fields, power factor — belongs to the electrical shelves; what the machine designer contracts for is the curve (§3), which says what torque arrives at every speed and therefore whether the fan, conveyor or screw ever reaches its duty point; the thermal budget, because a motor is ultimately a heater with a by-product of torque, rated by its insulation temperature and its duty cycle, and killed almost exclusively by heat; the standard frame (§6), which makes motors interchangeable commodities the way the fastener pages made bolts; and the shaft end, a keyed cylinder to which everything earlier in this section — coupling, sheave, sprocket, pinion — bolts, keys and aligns. The star of the page is the three-phase squirrel-cage induction motor, the machine that runs most of industry: no brushes, no contacts to the rotor, nothing to wear but two bearings from the bearing page — and one defining behaviour, which §2 states as a supply-frequency fact before §3 draws it.

§2Speed is the supply

An induction motor’s speed is not a design variable of the load or the operator — it is set by the mains frequency and the winding’s pole count, minus a small, load-dependent slip.

The stator’s field rotates at synchronous speed; the rotor must lag it slightly — the lag, slip, is what induces the rotor currents that make torque — so a loaded motor runs a few per cent under synchronous, and the ladder above is the entire speed catalogue of fixed-frequency machinery. The everyday example carries the arithmetic: the ubiquitous “1440 rpm” 4-pole motor is a 1500-rpm-synchronous machine at full load, running 4.0% slip — and since slip shrinks toward nothing as load lightens, the motor’s speed barely moves across its whole working range, the near-vertical right-hand wall of the hero’s curve. That constancy is the machine’s promise and its constraint in one. Promise: a fan, pump or line shaft gets a metronome, free. Constraint: process speed cannot be chosen — for a century the mechanical answer was everything this section has built (sheave ratios, chain sprockets, gearboxes), turning the motor’s one speed into the machine’s many; §5 is about the electronic answer that changed the trade. Either way, every drive calculation in this section starts from this ladder’s number.

§3Torque, power and the curve

Power is torque times speed — so a motor’s nameplate kilowatts convert to shaft newton-metres only at a stated rpm, and the curve says what happens everywhere else.

That 19.9 N·m is the hero’s marked point, and the curve around it is the machine’s character sheet. At the far left, locked-rotor (starting) torque — comfortably above full-load for the standard designs, so the motor starts loaded — bought at the price §4 counts. Rising toward breakdown torque, the summit around 2–2.5× full-load near 80% speed: the absolute most the motor can give, and the overload headroom that lets it shrug momentary jams. Then the plunge down the right-hand wall through the operating region, where §2’s few-per-cent slip lives: here the curve is so steep that torque demand barely moves speed — the motor behaves as the constant-speed source the section’s ratios all assumed — and here, only here, is the motor happy, because everywhere left of the knee the large slip is dumping proportionate heat into the rotor. Two reading rules complete the literacy: match the load’s torque curve (a fan’s square law, a conveyor’s flat line) against the motor’s and the intersection is the running point, stable only on the steep wall; and remember the summit is thermal fiction — breakdown torque is available for seconds, not service.

§4Starting

A direct-on-line induction motor announces itself with an inrush of six to eight times full-load current — and every starting method is a different way of paying, or deferring, that bill.

Direct-on-line (DOL) is the honest default: full voltage, full starting torque, and the famous 6–8× current surge for the seconds the rotor spends climbing the curve’s left half — fine for the supply and the machine up to the size where lights flicker and couplings snap, and the mechanical half of that sentence matters here: DOL delivers its torque as a step, and shafts, keys and gear teeth feel it as the impact the earlier pages armoured against. Star–delta is the classical economy: start with the windings in star so each sees line voltage ÷√3, and since both current and torque follow voltage squared, both drop to exactly 1/3 — a gentle, cheap start for loads that can accelerate lightly (fans, unloaded machines), followed by the changeover thump to delta. Soft starters ramp the voltage electronically, trading torque for grace continuously and retiring the thump. And the modern default, §5’s drive, starts by ramping frequency itself — full torque available from zero speed at modest current, the first-ever free lunch in this paragraph. One rating survives them all: starting is rotor-heating duty, so every motor carries a starts-per-hour budget, and frequent-start service is specified, not improvised.

§5The drive changes everything

The variable-frequency drive hands §2’s ladder to software: synthesise any frequency, hold volts proportional to hertz, and the one-speed workhorse becomes a speed-controlled servo of a sort.

The governing recipe is constant V/f: winding flux follows voltage-over-frequency, so the drive raises them together and the motor carries its full rated torque at any speed from a crawl to its base — the whole torque–speed curve of the hero sliding left and right on demand. Above base speed the voltage ceiling arrives and the regime flips to constant power: frequency keeps rising, flux weakens, and torque falls inversely — run 2× base speed and the shaft offers 0.5× the torque, the exact trade a gearbox would have made, now made in silicon. Two mechanical footnotes keep drive retrofits honest. Cooling: the standard motor’s fan is on its own shaft, so a motor loafing at 20% speed under full torque has 20% of its wind — continuous slow duty wants a separately powered blower or a derate, or the thermal budget of §1 quietly forecloses. And bearings: fast-switched drives can drive small currents through the motor’s bearings, micro-pitting raceways in exactly the pattern the bearing page called death-by-electricity’s cousin — the cure (insulated bearings or a shaft-grounding ring) is a checkbox at ordering time and a mystery failure eighteen months after forgetting it.

§6Frames, mounts and the shaft end

The motor’s mechanical interface is as standardised as a bolt’s — one number fixes the geometry, three letters fix the mounting, and the shaft end is a keyed handshake with the rest of this section.

The IEC frame number is the shaft-centre height in millimetres: a frame 112 motor’s shaft axis stands 112 mm above its feet, and with the frame number come standardised foot spacings, flange dimensions and shaft-end diameter — the interchangeability contract that lets any maker’s 112-frame drop onto the last one’s bolt holes and coupling. Mounting designations read as a small alphabet: B3 foot-mounted on a bed, B5 the large through-bolted flange hanging on a gearbox face, B14 the smaller tapped-hole face flange, with combined and vertical variants built from the same letters. The shaft end arrives as a ground cylinder in a standard tolerance with a keyway cut to the keys-and-keyseats page’s sections — every sizing, fitting and stress-raiser lesson there applies verbatim at this, its most-visited address — and what hangs on it is this section in miniature: a coupling aligned to the closing page’s procedure, or a sheave or sprocket whose overhung pull the motor’s bearings must carry, which is why sheave diameters and belt tensions appear in motor manuals at all. The prime mover, in the end, joins the machine on purely mechanical terms — which is exactly why it belongs in this section.

§7Quick reference

The working core of the page on one card rack.