A reversible gear motor and a continuous-duty gear motor can look identical, mount identically and cost almost the same. The difference is a single character on the nameplate — and it decides whether your machine can run for eight hours or has to stop every half hour.
Almost every application article on this site mentions that a motor is “reversible, 30-minute rated”. This one explains what that rating is actually protecting, how to work out whether your duty cycle fits inside it, and what to do when it does not. It is the least glamorous specification on a gear motor and the one most often ignored until a winding fails in the third month.
In the compact AC gear motor naming convention, the letters after the frame and power tell you the duty and the shaft type. The ones that matter here are the duty letters.
| Suffix | Motor character | Duty rating | Typical use |
|---|---|---|---|
| GN | Reversible, with damping brake; pinion (gearhead-mating) shaft | 30 minutes | Doors, gates, indexing moves, anything that must reverse |
| GU | Reversible, with damping brake; round output shaft (right-angle families) | 30 minutes | Right-angle drives that must reverse |
| A (or induction equivalent) | Standard induction motor, no damping brake | Continuous | Conveyors, pumps, fans, mixers running one direction |
| S | Speed-control motor with tachogenerator | Continuous within the safe-operation line | Machines needing adjustable speed |
So a model ending GN or GU is a reversible motor, and a reversible motor in this family is rated 30 minutes. A model without the reversible lettering is an ordinary induction motor and is continuously rated. That is the entire distinction, and it is decided at order — you cannot convert one into the other afterwards.
It is not a timer and it is not an electronic limit. It is a thermal statement: at rated load, in a 40 °C ambient, the winding reaches its permitted temperature rise in about 30 minutes of continuous running. After that, the insulation starts to age faster than its design life allows.
Three consequences follow, and all three are useful in the field.
It is about temperature, not minutes. If the motor’s case surface stays below roughly 90 °C, it can run considerably longer than 30 minutes. A motor that is hot to the touch but not painful, mounted in moving air, at part load, will outlast the nominal rating comfortably. The rating assumes full load, still air and 40 °C ambient — conditions that are frequently not the real ones.
Interruption resets it partially, not fully. Because the winding cools between runs, a motor that runs 20 minutes and rests 10 can accumulate far more than 30 minutes of running per hour. The relationship is not linear: the shorter the pause, the less cooling, and there is a curve for every frame where a short run with a very short pause is no better than continuous operation.
The gearhead helps. A reversible motor bolted to a substantial gearbox has extra thermal mass and surface area attached to it. Manufacturers’ data for these families show that a reversible motor running with its gearhead reaches thermal equilibrium later than the same motor alone — in some published cases three to four times later. The metal around the motor is, in effect, a heat sink. This is one reason the assembled gear motor is rated as a unit rather than as a bare motor.
Two mechanisms, and both are the price of being able to reverse on command.
The first is the damping brake. A reversible motor carries a light friction brake that stops the shaft quickly when power is removed, so that the load does not coast and the next direction command starts from a known position. That brake does work on every stop, and the energy it absorbs becomes heat inside the motor housing.
The second is starting. A reversible motor has a higher starting current than an equivalent induction motor, because it is designed to accelerate a load and change direction rather than to run steadily. Every reversal is a start, so a machine that cycles twenty times an hour asks its motor to absorb twenty starts per hour — each one a burst of current, each one a slug of heat in the winding.
Put together: a reversible motor in a machine that reverses frequently is doing more thermal work per operating hour than its continuous-duty cousin doing the same job in one direction. The 30-minute rating is not conservative bookkeeping; it is a fair description of what the design can dissipate.
Before deciding whether 30 minutes is enough, write down the real cycle. Three numbers do it.
| Duty pattern | Run / rest per hour | Duty % | Verdict on a 30-minute reversible motor |
|---|---|---|---|
| Door or gate, occasional use | 3 × 10 s with long pauses | Under 5% | Comfortable; thermal margin is large |
| Indexing table, 10 moves/min | Short moves, short dwell | 20–40% | Acceptable if the case stays below about 90 °C; monitor it |
| Reversing conveyor in a wash station | 5 min run, 2 min rest | 70% | Marginal at full load; derate the load or step to a continuous motor |
| Continuous forward running | Uninterrupted | 100% | Wrong motor — use the induction (continuous) version |
The last row is the one to act on. Plenty of machines are built around a reversible gear motor because it was already in the design’s parts list, and then run it one direction for eight hours. That is buying a brake you never use and a duty limit you will eventually exceed. The continuous-duty version of the same frame is usually cheaper and has no 30-minute constraint.
If the duty cycle genuinely does not fit, there are three real options and one that is not.
What does not work is fitting an external fan to a totally enclosed motor and calling it continuous, or simply ignoring the rating because “it has been fine so far”. Winding insulation fails by cumulative thermal ageing, so a motor run past its rating does not fail on day one — it fails in the second or third year, usually out of warranty and usually during a busy period.
This is the second misunderstanding attached to the same suffix. “Reversible” describes a capability, not a speed.
When you reverse a running motor, the rotating field reverses and the motor acts as a brake until the shaft stops and accelerates the other way. That interval is called plugging, and it produces several times rated current plus a torque shock transmitted straight through the gearbox. The motor will survive a certain amount of this; the gear teeth and the coupling will not survive very much.
Two rules keep reversing machines reliable. Allow the shaft to reach standstill before the next direction command — a dwell of a few tenths of a second is usually enough at small inertia, longer for a high-inertia load. And check that the mechanism itself can tolerate the deceleration, because a reversing drive stops the load faster than friction would, and anything loose on the load side becomes a shock load.
Where the load must be held after stopping, a reversible motor is not enough. The damping brake slows the shaft; it does not guarantee holding torque. That requires an electromagnetic brake motor, and the difference is worth stating plainly: a gear motor is not a safety device.
Duty rating and torque rating are separate constraints, and a selection has to satisfy both. The 40 W 5RK unit on the 90 mm frame illustrates how the two interact: it is a reversible 30-minute model, offers ratios from 3:1 to 500:1, and has a maximum allowable load of 9.81 N·m regardless of how far the ratio goes.
| Constraint | What sets the limit | How it fails |
|---|---|---|
| Duty rating (30 minutes) | Winding temperature rise | Insulation ages; motor dies in months, not minutes |
| Allowable load (9.81 N·m) | Gear tooth and bearing strength | Gear damage, usually sudden and under shock |
| Service factor margin | Load character and starts per hour | Premature failure on shock or frequent starting |
Running a reversible motor at half its allowable load does not extend its duty rating automatically, and improving cooling does not raise the allowable load. The two limits move independently. The service-factor logic that sits on top of both is set out in the service factor and gearbox duty guide.
| Mistake | Result | Correction |
|---|---|---|
| Using a reversible motor for continuous one-way duty | Winding fails after months of unnoticed overheating | Order the continuous-duty (induction) version of the same frame |
| Treating 30 minutes as a hard timer | Unnecessary motor changes on machines that were fine | Measure case temperature; the limit is thermal |
| Reversing without dwell | Plugging current and gear tooth shock | Add a standstill delay in the control logic |
| Assuming the brake holds the load | Load drifts or drops when power is removed | Specify an electromagnetic brake motor where holding is required |
| Ignoring starts per hour | Thermal failure at what looks like light load | Count reversals as starts when estimating duty |
For machines where the reversing duty is the whole point — door operators, gates, indexing stations, sorting diverters — the practical selection order is duty first, then torque, then ratio. Applications where that sequence has already been worked through include material handling drives and sorting system drives, both of which depend on knowing exactly how many cycles an hour the motor will see. Frame dimensions, shaft sizes and load ceilings for the models under discussion are in the gear motor size chart, and the mounting hardware that goes with a reversing installation — torque arms, flanges and couplings — is covered in the accessories guide.
The frame choice usually comes down to two neighbours in the same family. If the cycle is intermittent and light, a 40 W reversible unit with a 9.81 N·m allowable load covers a surprising amount of machinery. When the load grows past that, the 90 W unit on the same 90 mm frame takes the allowable load to 19.6 N·m — and the 120 W version shares that same 19.6 N·m ceiling, so the step up is worth taking for efficiency and thermal margin rather than for more load.
The 30-minute rating on a reversible gear motor is a thermal rating, and it is honest. It says that at rated load, in still air at 40 °C, the winding reaches its temperature limit in about half an hour. Run lighter, run cooler, run with longer pauses, and you can run longer — but run continuously at full load in one direction and you have simply bought the wrong motor.
Decide the duty before the model. If the machine needs to reverse and the cycle is intermittent, a reversible 30-minute unit is the right tool. If it needs to run all day, order the continuous-duty version on the same frame and use the money you save on a better brake. Tell us the cycle — run time, rest time, reversals per hour — and the load torque, and we will confirm which rating fits.
No. The rating describes where the winding reaches its permitted temperature rise at rated load in a 40 °C ambient. Run below rated load, in cooler air, or with pauses, and the motor can operate well beyond 30 minutes without exceeding that limit.
Mechanically yes, but you are paying for a damping brake you do not use and accepting a duty limit you do not need. The continuous-duty induction version of the same frame is the better and usually cheaper choice.
From the model suffix. GN and GU indicate reversible motors in this family, which are 30-minute rated. A model without the reversible lettering is an induction motor with continuous duty.
Warm to the touch is normal; case surface around 60–70 °C is expected under load. Above roughly 90 °C at the centre of the housing is the point to investigate load, ambient or duty.
Not by bolting one on. Order an electromagnetic brake motor, which combines the continuous induction motor with a proper holding brake rated for the load.
Yes, and it is a harder start than a cold start because the winding is already warm. When estimating duty, count every reversal as a start.
