Converting an AC synchronous gearmotor mechanism
An AC synchronous gearmotor gives you line-locked timing and a benign stall for free. Converting to DC means paying for both, and matching nameplate watts gets you neither.
- Wedge
- Obsolescence, second source, and reshoring
- Catalog reviewed
- 2026-08-18
- Configurations screened
- 32 published
An AC synchronous gearmotor drives a cam, a damper, a timer, a valve or a dispenser at a speed that is set by the line frequency and nothing else. The rotor is a permanent-magnet or hysteresis type that pulls into synchronism and then turns at 120·f/p rpm regardless of load, up to its pull-out torque. Behind it sits a high-reduction spur train, often with a slip clutch. The mechanism was designed around three properties of that motor: exact timing, a torque that does not fall off with load, and a stall that simply stops without damage.
The mechanical load model is whatever the mechanism is: a cam profile, a damper reaction, a valve seat. The relation that matters in a conversion is that the AC unit's synchronous speed is n = 120·f/p, so on 60 Hz a four-pole rotor turns 1 800 rpm and behind a 300:1 train the output is 6 rpm, exactly, forever. A DC motor has no such property. Its speed falls with load along its torque-speed characteristic and with supply voltage, so the output speed becomes a tolerance rather than a constant, and any timing the mechanism relied on has to be recreated by a switch, an encoder or a controller.
What normally controls the selection
Continuous output torque at low output speed, and what replaces the timing. The reason AC synchronous gearmotors persist in these mechanisms is that they deliver substantial torque at a few rpm from a small package without a thermal problem, because the rotor is doing very little work and the reduction is doing all of it. A small DC gearmotor asked for the same output torque at the same speed runs into two ceilings: the gearhead's rated output torque, and the motor's own continuous torque referred through the ratio. In this catalog the second ceiling is the binding one, and it sits at 319.2 mN·m.
Continuous, peak, starting and abnormal
Sizing on one of them is how a mechanism gets a part that works on the bench and fails in the field.
The specifications that get compared are rarely the ones that decide the outcome.
What follows is what actually fails, and which published number sends people the wrong way.
| Failure mode | Physical cause | What it looks like in the field |
|---|---|---|
| The mechanism's timing goes out of tolerance | Output speed was line-locked and is now load and voltage dependent. Nobody wrote down that the timing came from the motor. | Dwell or fill times that vary between units, with temperature, and with supply voltage, on a mechanism whose drawing has no speed tolerance on it because it never needed one. |
| Gearhead or mechanism damage on the first overload | The mechanism has no torque limiting because the AC motor was the torque limit. The DC replacement has none. | A broken cam, a stripped output stage, or a bent linkage on an event that the previous drive absorbed without a mark. |
| Overheating in continuous service | Sized on nameplate watts. The AC nameplate is input power and the mechanism's actual output may be a tenth of it, so the substitution can be undersized on continuous torque while looking generously oversized on watts. | A drive that works on the bench and fails after an hour of continuous duty, with the winding as the failure site. |
| Direction and holding behaviour changed | Some AC synchronous units start in either direction unless a mechanical anti-reverse is fitted, and some rely on residual holding torque. Neither behaviour transfers. | A mechanism that runs backwards on some power-ups, or one that no longer holds its position between cycles. |
| Specification | Why it misleads in this application |
|---|---|
| Nameplate watts | It is input power on the AC unit and rated mechanical output on the DC unit, and the two are separated by the efficiency of a high-reduction train. In the worked example the mechanism's actual mechanical output is 0.157 W and no nameplate on either side says so. |
| Rated speed | On the AC unit it is exact. On the DC unit it is the speed at one specific torque and voltage. Comparing the two numbers as if they mean the same thing is the most common error in a conversion. |
| Torque quoted at the AC unit's rated point | AC synchronous gearmotors are commonly rated at pull-out or at a maximum permissible torque. That is a limit, and sizing the DC replacement to it oversizes the whole conversion. |
| Frame size and shaft dimensions | They make the substitution look like a drop-in. Envelope compatibility says nothing about torque, timing, stall behaviour or the supply the mechanism now needs. |
The tradeoffs that matter here
Spur or planetary reduction
Brushed coreless or brushless
How timing is recreated
Worked example
Convert a cam mechanism driven by a 60 Hz AC synchronous gearmotor at 6 rpm output, and see whether the published catalog can do it.
- Existing output speed
- 6 rpm, line-locked at 60 Hz
- Running torque
- 250 mN·m at the output
- Breakover torque
- 400 mN·m
- Duty
- continuous rotation while the machine runs
- Design factors
- 1.4 on continuous, 1.25 on peak
| Step | Expression | Result |
|---|---|---|
| Mechanical output power | 2π × (6 ÷ 60) rev/s × 0.250 N·m | 0.157 W |
| Synchronous speed check | 120 × 60 ÷ 4, and 120 × 50 ÷ 4 | 1 800 rpm at 60 Hz, 1 500 rpm at 50 Hz: the mechanism runs 17 percent slower on a 50 Hz line |
| Requirement after design factors | 250 × 1.4 and 400 × 1.25 | 350 mN·m continuous, 500 mN·m peak |
| Highest published continuous output torque | MM-C1736-P02560-060B | 319.2 mN·m, 8.8 percent short of the requirement |
| Second highest | MM-B2419-P03610-120A | 304.3 mN·m, 13.1 percent short |
| Where that continuous limit comes from | 2.9 mN·m motor rated torque × 256 × 0.43 | 319.2 mN·m, so the limit is thermal at the motor, not the gearhead |
| Gearhead rating against the peak requirement | 785 mN·m against 500 mN·m | The gearhead is not the constraint |
| Ratio that would satisfy the continuous requirement | 256 × 350 ÷ 319.2 | 281:1 on the same motor and gearhead platform |
| Motor speed at 6 rpm output with that ratio | 6 rpm × 281 | 1 684 rpm, comfortably inside the 4 900 rpm no-load speed |
Result
This one does not resolve to a published configuration. The requirement is 350 mN·m continuous at the output and the highest published continuous output torque in the catalog is 319.2 mN·m, 8.8 percent short. Both of the configurations that come closest have a 785 mN·m gearhead rating, so the gearhead is not the problem: the limit is the motor's own thermal rating referred through the ratio. The arithmetic points directly at the fix. Raising the ratio from 256:1 to about 281:1 on the same 22 mm platform satisfies the continuous requirement, stays inside the gearhead rating, and puts the motor at 1 684 rpm against a 4 900 rpm no-load speed. That is a configured part, not a catalog part, and it goes to a review with the mechanism's measured torques attached. Separately, the conversion still owes the mechanism two things the AC motor supplied free: a speed reference to replace line-locked timing, and a torque limit to replace the benign synchronous drop-out, because a DC drive stalled at this ratio delivers 1 310 mN·m, 1.67 times the gearhead rating.
What would change it
The measured torques are the whole calculation and they are the thing most often assumed. If the 250 mN·m running figure was taken from the AC motor's rating rather than from the mechanism, the real number could easily be half of it, and the requirement would land comfortably inside two published configurations. A lower design factor on continuous torque, if the duty is genuinely intermittent rather than continuous, has the same effect. If the mechanism's timing tolerance turns out to be loose, the encoder disappears from the conversion and with it most of its cost. Measure the mechanism first.
Evidence, in the order it is worth collecting.
Each step names the measurement, not the intention.
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Measure the mechanism, not the old motor
Output torque against angle through a full cycle, with a torque transducer on the mechanism's input shaft, at the temperature extremes.
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Write down the timing requirement
What the mechanism actually needs, stated as a position tolerance or a speed tolerance. The AC motor's exact speed is not automatically a requirement.
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Define the overload behaviour
What used to happen when the mechanism jammed, and what is now required to happen. This becomes the torque limit specification.
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Confirm the supply and the control
Available DC supply, its regulation and its tolerance, since output speed now depends on it. Include the behaviour at brown-out.
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Run the mechanism, not the motor
Life test on the assembled mechanism at the real duty, with timing measured throughout, not a motor bench test.
Tools, products and articles for this problem
Send the AC gearmotor and the mechanism data.
Send the existing part, a nameplate photograph, or a drawing, with the mechanism's measured torque and timing. You get back a screening against the published catalog and, where nothing published fits, the configured path and what it needs.