How are electric motors classified by torque capacity?
There is no single set of low, medium and high torque bands, with fixed N·m limits, that covers every electric motor. Those labels need a product family or application for context. A useful specification states the shaft, speed, duration and cooling conditions associated with the rating. Start with what torque means and the units of torque before comparing catalogue numbers.
What makes a motor “high torque”?
A motor that comfortably drives a small lid mechanism could be inadequate for an industrial mixer. High torque is a relative description, not a complete sizing specification. Even matching numbers can describe different capabilities: 10 N·m continuously is not equivalent to 10 N·m for a brief peak. The benefits of higher torque and the consequences of insufficient torque depend on the load's operating requirements.
Six torque ratings worth distinguishing
| Rating | What it describes | How to use it |
|---|---|---|
| Rated torque | Torque specified at the rated speed and operating conditions. | Read alongside rated output, speed and duty type. |
| Continuous torque | Torque sustainable at the stated speed and thermal conditions. | Check sustained loads against this limit; it can vary with speed. |
| Peak torque | A permitted temporary torque level. | Check acceleration and transient loads against duration, repetition and drive-current limits. |
| Starting torque | Torque produced when starting from rest with the specified supply and starting method. | Must overcome the initial load; check the rest of the acceleration curve too. |
| Holding torque | Stationary position-holding capability, particularly for an energised stepper. | Do not substitute it for running torque. |
| Breakdown torque | The maximum of an induction motor's torque–speed curve under specified conditions. | Not a continuous rating; exceeding it can cause the motor to lose speed and stall. |
These torque ratings describe different conditions. Starting torque, holding torque and continuous torque are not interchangeable. The torque–speed curve shows the available operating envelope.
Motor technology is not a torque ranking
Induction, synchronous, DC, stepper and servo do not form a low-to-high torque scale. Servo describes a feedback-controlled motion system as well; it is not a motor construction category on the same footing as induction or DC. Use the relevant performance data for each system:
Induction motors: Check rated, starting and breakdown torque. Operation at low speed with a drive also requires a cooling and continuous-load check.
DC motors and brushless DC motors: Current, magnetic design, drive capability and thermal limits determine available torque. DC construction alone does not guarantee a high starting-torque rating.
Stepper motors: Use running torque at the target speed as well as holding torque. Servo motors: Check both continuous and peak torque envelopes at that speed. A sound stepper–servo comparison keeps these ratings distinct.
Torque motors: Direct-drive designs often target high torque at relatively low speed. The actual continuous and peak ratings still depend on the model and cooling arrangement. When comparing motors capable of high torque, include size and operating requirements.
Same kilowatts, different shaft torque
For mechanical shaft output at a given operating point, T (N·m) ≈ 9550 × P (kW) / n (rpm). An illustrative 2.2 kW output at 1400 rpm corresponds to approximately 15 N·m. At 2800 rpm, the same output corresponds to approximately 7.5 N·m. Equal power does not mean equal torque when the speeds differ. This is the practical link between torque and power and rotational speed.
Slowing a particular motor to half speed does not automatically double its torque. That conclusion would require it to maintain the same shaft power at the new speed. Check the motor and drive curve when establishing the balance between torque and speed.
Specify which side of the gearbox you mean
A gearbox can trade speed for output torque. For a simplified constant-speed calculation, Toutput ≈ Tmotor × i × η. A 2 N·m motor with a 10:1 reduction and 90% efficiency produces approximately 18 N·m at the gearbox output, at one tenth of the motor speed. The motor itself has not become an 18 N·m motor. When selecting a gearbox, also check its permitted output torque, mechanical strength and thermal capacity.
Do S1, S2, S3 or IE3 indicate torque capacity?
S1 means continuous duty; S2 means short-time duty followed by adequate cooling; S3 means intermittent periodic duty. S3 alternates loaded operation with de-energised rest, with starting having no significant thermal effect. IE3 and IE4 are efficiency classes. None of these codes specifies a fixed torque in N·m.
NEMA design letters A, B, C and D distinguish torque–speed, slip and starting characteristics for the three-phase induction motors within their scope. Design C has high starting torque; design D has very high starting torque. These letters are not universal torque bands for every electric motor.
Sources: WEG — Motor specifications and duty types · Kollmorgen — Torque terminology · Oriental Motor — Stepper torque characteristics · ABB — Nameplates and NEMA design letters · UC San Diego — Power and gear ratios