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Is there a difference in terms of operation between switching a relay connected to mains for an AC motor versus opening/closing a switch on the motor? Would the internal protection circuitry containing the switch be useless if a relay was used to switch power instead? I'm asking for how something like this would affect operation of something like start/run capacitors and thermal protection.

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  • \$\begingroup\$ +1 for using "affect" instead of "effect". \$\endgroup\$
    – Transistor
    Jun 25, 2020 at 21:29

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Protection circuitry would not actually "contain" a switch that could start and stop a motor. A switch could be packaged as a manual motor starter that contains motor protection components or circuitry and possibly the means of controlling a starting capacitor.

If a switch that is controlled by protection circuitry is wired in series with a relay is not removed, but just left in place and connected in series with a relay it would still protect the motor. If the protection circuit causes it to open, the relay will be unable to start the motor until the switch is re-closed. The method of controlling the starting capacitor would need to be evaluated, but is likely that would respond to motor current and perform normally.

If the manual motor starter is removed or bypassed, then everything that it contains would also be removed or bypassed.

A manual motor starter is a product that has features and functions that should be described in detail in literature provided by the manufacturer. There is no way to completely determine how it functions without examining the literature for the specific product. This answer provides only a general idea of what to expect.

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Let's consider a hair dryer as an example.

It has an inbuilt on/off switch and a thermostat to shut it down, should it overheat.

There would be no difference in its functioning or functioning of its thermostat, when power is fed to it, direct from a wall socket or through a relay.

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However, the relay would provide additional 'zero-volt' protection, which would prevent the hair dryer from restarting automatically, on resumption of power after a power failure.

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