Electric Motor Basics: How Motors Work, the Main Types, and How to Size One
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What Is an Electric Motor?
An electric motor is an energy-conversion machine: it takes electrical power at its terminals and delivers mechanical power at its shaft. Every motor, regardless of type, obeys the same physical chain — electrical energy creates a magnetic field, that field acts on current-carrying conductors, and the resulting force becomes shaft torque.
The core physics in one line
A conductor carrying current I in a magnetic flux density B experiences a force F = B·I·L (Lorentz force). Arrange many conductors on a rotor inside a stator field and the net tangential force becomes torque. That is the whole trick; everything else is engineering around how to keep the force pointing the same way as the rotor turns.
Anatomy of a motor
- Stator — the stationary part. In AC motors it carries the windings that generate the rotating field; in PM motors the field can come from permanent magnets fixed to the stator.
- Rotor — the rotating core on the shaft. It may carry windings (induction squirrel-cage or wound rotor), permanent magnets (PM/BLDC/PMSM), or a commutator (brushed DC).
- Air gap — the small clearance between stator and rotor. It must be tiny to concentrate flux but large enough to let the rotor spin; it strongly affects efficiency and torque.
- Commutator & brushes (brushed DC only) — a mechanical switch that flips current direction in the rotor as it turns. They wear out; this is the reason brushless designs exist.
- Bearings & housing — support the shaft and remove heat. Bearing life, not winding life, is the usual limit on a brushless motor.
How an Electric Motor Works (Step by Step)
- Energise the field. Current in the stator windings (or fixed magnets) produces a magnetic field across the air gap.
- Create rotation. In an AC motor the alternating supply naturally produces a rotating magnetic field. In a DC motor a commutator (brushed) or electronic controller (brushless) switches the field in step with rotor position.
- Produce torque. The field acts on rotor current or rotor magnets, generating a tangential electromagnetic force — torque — via the Lorentz law.
- Deliver mechanical power. Torque spins the rotor; shaft power is the product of torque and angular speed:
P = T · ω. - Sustain commutation. The switching must track rotor angle. Brushed DC does it mechanically; BLDC/PMSM do it electronically (sensors or back-EMF estimation), which is why brushless motors need a controller.
AC vs DC: where the difference really lives
The supply is almost a side issue. The decisive difference is how commutation happens. An AC induction motor needs no commutation at all — rotor current is induced. A brushed DC motor commutes mechanically (wearing parts). A BLDC motor commutes electronically and is, electrically, a synchronous machine fed from DC through an inverter. See our BLDC basics guide for the electronic-commutation detail, and brushed vs brushless for the trade-offs.
Motor Type Comparison
The single most useful view for a specifier is a side-by-side of the families. The numbers below are typical ranges, not hard limits — exceptions exist at every boundary.
| Family | Commutation | Typical efficiency | Speed control | Maintenance | Relative cost | Where it wins |
|---|---|---|---|---|---|---|
| AC induction (IM) | None (induced rotor current) | IE2–IE4 (85–96%) | Needs VFD for good control | Very low | Low | Pumps, fans, conveyors, compressors |
| AC synchronous / PMSM | Synchronous lock to supply | IE4–IE5 (95–98%) | VFD, precise | Low | Medium–high | HVAC, high-efficiency drives, EVs |
| Brushless DC (BLDC) | Electronic (ESC/inverter) | 85–95% | Excellent | Very low | Medium | Fans, appliances, robotics, EVs |
| Brushed DC | Mechanical commutator | 75–85% | Excellent (voltage) | High (brushes) | Low | Low-cost, simple drives, starters |
| Stepper | Electronic step sequencing | 40–70% | Position (open-loop ok) | Low | Medium | CNC, 3D printers, positioning |
| Universal (series) | Mechanical, AC or DC | 60–80% | Excellent | Medium | Low | Hand tools, vacuum cleaners |
Synchronous vs asynchronous — the slip point
In an induction motor the rotor always turns slightly slower than the rotating field; that slip is what induces rotor current, so torque costs a few percent of speed. A synchronous motor locks exactly to the field frequency — zero slip, higher efficiency, but it needs assistance to start. Permanent-magnet versions (PMSM, and BLDC as its DC-fed cousin) remove rotor losses entirely and reach the highest IE classes.
Motor Efficiency Classes and the IE System
Efficiency is mechanical output ÷ electrical input. Losses split into copper (I²R winding heat), iron (core magnetisation), friction/windage, and stray load losses. The IEC 60034-30 series standardises minimum efficiency by the IE code:
| Class | Typical full-load efficiency (4-pole, ~5–50 kW) | Relative loss vs IE1 | Typical use |
|---|---|---|---|
| IE1 (Standard) | 85–91% | Baseline | Legacy / non-regulated markets |
| IE2 (High) | 87–93% | ~15% lower loss | General industrial |
| IE3 (Premium) | 89–95% | ~30% lower loss | Regulated minimum in US/EU/China |
| IE4 (Super Premium) | 91–96% | ~45% lower loss | High-run-hour applications |
| IE5 (Ultra Premium) | 93–98% | ~60% lower loss | PM synchronous, best-in-class |
Each step up roughly halves the remaining loss. On a motor that runs thousands of hours a year, the IE class — not the purchase price — dominates lifetime cost. Our efficiency-class explainer goes deeper, and our BLDC range reaches IE4–IE5 through permanent-magnet design.
Engineering Data: Formulas, Temperature, Duty
The formulas you actually use
| Quantity | Formula | Notes |
|---|---|---|
| Shaft power from torque & speed | P(kW) = T(N·m) × n(rpm) / 9550 | So T = 9550 · P / n |
| Synchronous speed (AC) | ns(rpm) = 120 × f(Hz) / p | p = number of poles |
| Slip | s = (ns − n) / ns | Induction only; 2–5% typical |
| Efficiency | η = Pout / Pin | Always < 1 |
| Mechanical power | P = T · ω ; ω = 2πn/60 | ω in rad/s |
Temperature limits — what actually kills a motor
Winding life halves for every ~10°C of sustained overload. IEC 60034-1 classes the insulation by the maximum allowable winding temperature (ambient 40°C assumed):
| Insulation class | A | E | B | F | H |
|---|---|---|---|---|---|
| Max winding temp (°C) | 105 | 120 | 130 | 155 | 180 |
For rolling bearings, SKF’s failure-mode data puts the practical shell-temperature ceiling near 95°C with a rise of no more than 55°C above ambient — well before the winding limit is reached. Bearing temperature, not insulation class, is usually the first thing to trip in the field. If a motor runs hot, work through common torque and thermal faults before replacing it.
Duty cycles (IEC 60034-1)
- S1 — continuous, constant load (the default for pumps and fans).
- S2 — short-time, then cool-down.
- S3–S9 — intermittent, periodic, or variable load with defined ratios. A motor rated S3 will overheat on continuous duty at the same power.
Best Applications by Motor Type
| Application | Best-fit family | Why |
|---|---|---|
| Centrifugal pumps, HVAC fans, conveyors | AC induction (IE3+) | Cheap, rugged, runs for years unattended |
| EV traction, robotics, servo | BLDC / PMSM | High efficiency, precise torque, regeneration |
| Precision positioning, CNC, 3D print | Stepper / servo | Open-loop position or tight closed-loop control |
| Low-cost appliances, cordless tools | Brushed DC / universal | Simple drive, low bill-of-materials cost |
| Long-life, inaccessible ducts | BLDC (sealed, sensorless) | No brushes to replace; see appliance applications |
| Conveyors needing high torque at low speed | Gear motor (any family + reducer) | Gears trade speed for torque; compare reducer types |
Where axial length is tight, a pancake BLDC with an integrated reducer is often the only fit — see flat BLDC gear motors and micro gearboxes.
How to Select a Motor: Step by Step
- Define the load. Continuous torque and speed at the output shaft, plus peak/inrush needs (starting torque, acceleration).
- Pick the family. Constant-speed pump → induction; precise variable-speed → BLDC/PMSM; positioning → stepper/servo.
- Compute shaft power. Use
P = T·n/9550, then divide by transmission efficiency (gears, belts, couplings). - Apply duty & service factor. Intermittent or shock-loaded loads need S-rating and a service factor.
- Choose the efficiency class. Higher IE pays back on high run-hours; see the IE table above.
- Check the environment. IP rating, insulation class, ambient/altitude derating, flange and shaft dimensions (flange guide).
- Confirm the drive. A VFD-rated motor is mandatory if you vary speed; a standard motor on a drive overheats (see mistakes below).
For the load-side torque arithmetic that feeds step 3, see how to calculate motor torque, and for a full reducer-inclusive method, speed-reducer motor selection.
Worked example — sizing an induction motor for a centrifugal water pump
This is the kind of calculation competitors omit. Follow the chain and you can reproduce it for any pump.
| Step | Calculation | Result |
|---|---|---|
| Given | Flow Q = 60 m³/h, Head H = 18 m, water (ρ=1000), pump ηp = 0.72, 16 h/day S1, ambient 40°C | — |
| 1. Hydraulic power | Phyd = ρ·g·Q·H / 3.6×106 = 1000·9.81·60·18 / 3.6e6 | 2.94 kW |
| 2. Pump shaft power | Pshaft = 2.94 / 0.72 | 4.09 kW |
| 3. Motor rating | Allow coupling 0.98, round to next IEC frame | Select 5.5 kW |
| 4. Input power (IE3) | Pin = 4.09 / 0.90 | 4.54 kW |
| 5. Speed | 2-pole, 50 Hz: ns = 120×50/2 = 3000, slip ~3% | ~2900 rpm |
Common Engineering Mistakes
- Running a standard motor on a VFD. Inverter voltage spikes and extra iron loss push winding temperature up 10–20% and break down turn-to-turn insulation. Use a properly rated (insulation class F/H, reinforced) inverter-duty motor.
- Sizing from power alone. Two loads with identical kW can need very different starting torque and duty. Always start from torque × speed, then duty.
- Ignoring duty cycle. An S3-intermittent motor run continuously at nameplate power will overheat within hours.
- Buying on purchase price. On high run-hours the IE class outweighs capital cost within two years — see the pump example.
- Wrong IP / cooling for the site. A fan-cooled motor in a dusty or wash-down area fails early; match IP54/IP55 or higher and forced ventilation where needed.
- Skipping ambient/altitude derating. Above ~1000 m or 40°C, convection drops and you must derate or upsize.
- Confusing Kv with efficiency in BLDC. Kv (rpm/V) does not determine efficiency; the motor constant Km = Kt/√R does. Covered in our BLDC guide.
Troubleshooting: Problem → Cause → Solution
Salvaged from field practice (the “see, hear, smell, touch” method), upgraded to a table. The original page mixed in off-topic VFD board-repair text here; that has been removed.
| Problem | Likely cause | Solution |
|---|---|---|
| Overheating / thermal trip | Overload, poor ventilation, high ambient, single-phasing | Check current vs nameplate; clear vents; verify all phases; overheating deep-dive |
| Loud humming, no start | Single-phasing or locked rotor | Test supply phases and contacts; free the load |
| Excessive bearing noise | Lubrication loss, contamination, bearing damage | Re-grease or replace bearing; keep out dust/moisture |
| Vibration | Misalignment, unbalanced load, loose footing | Re-align coupling; tighten mounts; balance rotor |
| Burnt smell / smoke | Winding short, insulation failure | Megger the windings; replace or rewind; find root cause |
| Low speed / weak torque | Slip too high, voltage sag, wrong poles | Confirm supply; check rotor bars; verify pole count |
| Shaft current damage (VFD) | Common-mode voltage, no bearing insulation | Use insulated bearing or shaft grounding on drives > ~160 kW |
Frequently Asked Questions
What is the difference between an AC and a DC motor?
The supply type is secondary; the real difference is commutation. DC motors switch current mechanically with brushes (or electronically in BLDC). AC induction motors induce rotor current with no commutator at all, while synchronous and BLDC motors lock the rotor to a field electronically. For the BLDC specifics, see our BLDC basics.
What does an IE efficiency class mean?
IE1 through IE5 (IEC 60034-30) set minimum full-load efficiency for AC motors. Each step roughly halves the remaining loss: IE3 is the common regulated minimum, IE4–IE5 are premium/super-premium, typically permanent-magnet designs. Higher class costs more upfront but saves energy every running hour.
Can I run a standard motor on a variable-frequency drive?
Not safely for long. Inverter switching adds voltage spikes and harmonic iron loss that raise winding temperature 10–20% and stress insulation. Use an inverter-duty motor with reinforced insulation (class F/H) and forced cooling at low speed.
How do I size a motor for my load?
Start from output torque × speed to get shaft power (P = T·n/9550), divide by transmission efficiency, then apply duty cycle and a service factor. The worked pump example above shows the full arithmetic; the reducer selection guide adds the gear stage.
Why does my motor overheat?
Most overheating is overload, poor ventilation, high ambient, or single-phasing — not a bad motor. Measure current against nameplate, clear the cooling path, and confirm all three phases are present before replacing anything.
What is slip in an induction motor?
Slip is the small speed difference between the rotating stator field and the rotor: s = (ns − n) / ns. It is what induces rotor current, so a few percent of slip is normal and necessary; zero slip would mean zero torque in an induction machine.
Brushless or brushed — which should I choose?
Choose brushed for the lowest bill-of-materials cost and simple voltage control where maintenance access is easy. Choose brushless (BLDC/PMSM) when you need efficiency, life, low noise, or precise speed/torque — the trade-off is the controller. Compare them in brushed vs brushless.
Why Choose Greensky?
- Full motor family under one roof. BLDC, brushed DC, AC induction, and geared versions — so the recommendation follows the calculation, not the catalogue.
- Motor + gearbox + controller as a system. Supplied and validated together, which removes the interface mismatch you get when three vendors each meet their own spec. Pair with our controllers and 48 V BLDC drives.
- Efficiency to IE4/IE5. Permanent-magnet designs for high run-hour duty where the energy saving pays back fast.
- Custom shafts, flanges, connectors and ratios. Through custom motor development and OEM/ODM programmes — usually faster than forcing a standard part to fit.
- Engineering support in English. We spec from your torque, speed and duty data, not from a keyword.
Related Technical Resources
- Brushless DC motor range and specifications
- Brushed DC motors — when they are still the right choice
- Motor controllers and drives
- How to calculate motor torque
- How to select a motor for an industrial application
- Motor efficiency class explained
- What is the use of a DC geared motor
- Different types of speed reducers compared
- Direct drive motor vs gear motor
- BLDC basics: commutation, Kv, and sizing
- All DC motor technical articles
References and Standards
The efficiency figures, insulation classes, duty definitions, and failure modes cited above are drawn from published international standards and manufacturer technical documentation, reviewed against current editions by the Greensky Power engineering team.
- International Electrotechnical Commission. IEC 60034-1:2022 — Rotating electrical machines, Part 1: Rating and performance. https://webstore.iec.ch/en/publication/65446
- International Electrotechnical Commission. IEC 60034-30-1 — Efficiency classes of line-operated AC motors (IE-code). https://webstore.iec.ch/publication/91195
- International Electrotechnical Commission. IEC TS 60034-30-2 — Efficiency classes of variable-speed AC motors. https://webstore.iec.ch/publication/30830
- American National Standards Institute / NEMA. ANSI/NEMA MG 1 — Motors and Generators. https://webstore.ansi.org/standards/nema/ansinemamg2021
- National Electrical Manufacturers Association. Motors and Generators product section. https://www.nema.org/products/pages/motor-and-generator.aspx
- U.S. Department of Energy. Determining Electric Motor Load and Efficiency. https://www.energy.gov/sites/prod/files/2014/04/f15/10097517.pdf
- International Energy Agency. Electric Motors — Energy System, Industry. https://www.iea.org/energy-system/industry/electric-motors
- SKF. Bearing Failures and Their Causes. https://www.skf.com/group/support/bearing-failures-and-their-causes
- Siemens AG. SIMOTICS Electric Motors — Technical Documentation. https://www.siemens.com/global/en/products/drives/electric-motors.html
- IEEE Xplore. Permanent magnet synchronous and brushless DC motor drives — technical literature. https://ieeexplore.ieee.org/document/6342334
- maxon motor ag. EC Motor Technology — Brushless DC Drive Fundamentals. https://www.maxongroup.com/maxon/view/content/ec-technology
- FAULHABER. Drive Technology Know-How Library. https://www.faulhaber.com/en/know-how/
- Yaskawa. Technical Downloads and Application Notes. https://www.yaskawa.com/downloads/search-index
