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I'd like to create something similar to a hydraulic press. I know I need some sort of motor, but I was curious about a fundamental question about motors when used in a system where the force required to continue motion increases.

General question: if a motor is under stress and the intended motion is impeded to some degree, does the motor get damaged?

Literal example: what happens to the motor of a ceiling fan if you grab the fan to immediately stop it? Does this damage the motor? Are there motors that expect resistance and either increase the torque or stop immediately?

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The answer depends on the specific type of motor you intend to use, but in general: when a motor is loaded down, it draws more current, which means its windings get hotter at the same time that the cooling fan attached to the motor shaft is being slowed down. if this heat is allowed to build up, it will eventually cause the insulation on the windings to fail and short-circuit themselves, in which case the motor is ruined and may catch fire.

A better answer can be provided for you if you can get a little more specific about what type of motor you are thinking of. Brushed DC? Brushless DC? AC?

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It depends on the specific type of motor, as @nielsnelson mentions, but it also depends on what you use to drive the motor.

In general, a motor that's designed to run continuously isn't going to take well to being stalled for any length of time, unless its drive voltage (and frequency, if it's an induction or synchronous machine) is reduced.

There are motors that are designed to operate under continuous stall conditions under full voltage; there are motor drivers (VFDs for AC machines, servo drivers for DC motors) that can be set up to protect motors under continuous stall conditions.

So you can certainly design for the possibility of the motor stalling, but you have to do so from the outset.

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The fan starts to act like a resistor and soon will heat excessively, till the breaker cuts the power off.

Almost all of the electrical motors have built-in fuses and breakers which will act to protect them.

Combustion engines usually are tougher and if the torque demand increases beyond theire output they just stop cranking and you need to reignite them. However before that they could damage transmission and bearings and mechanical links along the power train.

Jet engines if forced to work outside their power parameters will stall, which used to be catastrophic. Modern jets have built-in smart electronics to protect them.

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There can be another case: if the motor is expected to stall time to time or experience an overload condition as a part of its routine, motor shaft can be equipped with a ball detent torque limiter instead of a fixed gear. That way, the motor can keep spinning in idle mode without notable damage while its working piece is blocked-by.

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  • $\begingroup$ How does that work without reversing the rotation? And: Welcome! $\endgroup$ – Volker Siegel Nov 5 '19 at 21:59
  • $\begingroup$ @VolkerSiegel, it is the same way as power screwdrivers do: set the dial to the max torque limit you need and start working. Once the torque is not enough to push the screw into a material, the ratchet comes into play. $\endgroup$ – Yury Schkatula Nov 6 '19 at 10:34
  • $\begingroup$ I see. But a ratchet is a device that can only turn in one direction, using a latch. You mean some kind of torque limiter, I think? A ball dent torque limiter is what is used in a power screwdriver, I think. $\endgroup$ – Volker Siegel Nov 6 '19 at 15:18
  • $\begingroup$ Yep, maybe I used incorrect naming, but idea is the same. $\endgroup$ – Yury Schkatula Nov 6 '19 at 15:54
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    $\begingroup$ Yes, except for the name, the idea is perfectly right. (You can edit the answer, if you want?) $\endgroup$ – Volker Siegel Nov 6 '19 at 16:07
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Consider that the torque converter in an automatic transmission is explicitly designed to support this scenario. Whereas, a limited-slip differential is designed to handle the opposite scenario, where insufficient load is encountered (on one of the drive shafts).

Every motor can be protected from overload conditions in a motor-appropriate way. Apparently, hydraulic presses have their own set of techniques for overload protection, often involving shunting of the hydraulic fluid via valves to remove power. Note that the motor is not the only or even most expensive component which can be damaged by overload conditions. In the context of hydraulic presses, the die is obviously a valuable component that you generally want to protect, even if your motor could easily survive abuse.

Conclusion: design safety in from the start, rather than thinking about how badly you can abuse one component or another.

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On a typical hydraulic press, there is negligible danger of motor overloading. However your instincts are correct. Consider the operating sequence of a press. An electric motor drives a pump. The pump drives oil into a cylinder. The cylinder extends until no more motion is possible. What happens after that? Oil must escape somewhere, or pressure will build continuously until some part of the system fails, whether that is a (1) hose bursting, (2) pump housing cracking, (3) cylinder seals rupturing, (4) electric motor overloading, etc...

All modern hydraulic presses include a relief valve for this reason. Once hydraulic pressure reaches some maximum setting (typically between 100 - 350 bar), the relief valve opens to allow oil to escape back to the reservoir. Thus all components are protected from overloading, including the electric motor. The relief valve setting is typically adjustable - therefore it can be set appropriately for many sizes of electric motor within a wide range of kW ratings. Happy to discuss further if you have questions.

Here is an example of a typical press circuit. https://www.coalhandlingplants.com/hydraulic-system/. Ignore the brief explanations for the symbols in that article. They are not very clear. I only link that article because the first circuit shows the 8 basic components correctly laid out. Here are some better-explained symbols: https://www.hydraproducts.co.uk/Blog/how-to-read-hydraulic-circuits

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