For AC motors, the lower voltage might mean they do not have enough power to turn, so all the power they consume becomes heat in the motor windings (and since they're not turning, there's less cooling).
For electronic power supplies, they might either pull more current to maintain the same output (again leading to more heating), might output a lower voltage (and DC-DC converters which consume that voltage might in turn draw more current, again leading to more heating), or they might even misbehave and output an oscillating voltage; they might also detect the power dip and shut down, only to power up again moments later, repeatedly.
A device in an idle stand-by mode is probably the best case, since it's using little power from a supply sized for a much bigger load.
I disagree with the sibling comment that the only protection from voltage dips is a UPS or similar; a simpler protection is an undervoltage relay with a timer, to convert the voltage dip into a power outage (and also prevent the power from being restored too quickly).
* The only protection available as a consumer add-on.
Sure, if the manufacturer wants to increase the cost on the BOM for their PSUs, they can use additional components to survive dips, but who does that on disposable electronic equipment? Hell, even "expensive" TVs don't. I quote expensive since you can now get 65" 4K TVs cheaper than a mobile phone, but that's a tangent. So if you are wanting to protect your electronic equipment with another device readily available that is just plug an play ease of use with no knowledge of electronics, what do you suggest?
I have a friend who did that; he put one of these relays on the apartment's power panel, so the whole apartment was protected against voltage dips or too short outages. It might not be a common consumer item, but it's a readily available device, AFAIK commonly used to protect industrial motors.
It's not power dip, it's voltage dip. An appliance will often draw the same power at the lower voltage, so increase current, which can cause overheating and damage.
Typically in these dip situations, you can hear the PSUs being very unhappy about the situation. A cringe inducing high pitched squeal can be heard from any PSU that's doing work as they try to continue doing their job while in this undervolt condition.
If it's turned off or it is on and not being used, probably not much. But more and more equipment no longer has an off mode and is more of an idle stand-by mode, so current is still always being used by part of the system.
The point is that in electricity delivery, it is normal to have deviations in the voltage as acceptable. Most power supplies have tolerances that can handle these deviations. A dip is something outside of the deviations that most people don't consider as they are only focused on spikes and only use surge suppression devices. Since the only protection from dips is a more expensive battery solution, most people do not bother outside of computer related usages.
Ah, so e.g. something like a device flipping between “powered” and “not powered” states, which then wears out some part that doesn’t expect that state change to happen so often?
no, not quite. a dip is some lower than the accepted deviations from the expected value. if you have an accepted range of +/-10%, but the dip causes the voltage to drop 15%, that's a dip. some places define an outage as being less than 5% of expected value. so somewhere before outage, you have a severe undervolt situation where the equipment struggles. the longer the dip lasts, the worse things can get.
one of the shops that I worked at suffered a catastrophic situation when a nearby construction crew cut one phase of the 3-phase power coming into our facility. the poor transformer died, and took out the some of the more delicate PSUs attached to some very expensive equipment. we were down for weeks recovering from that.
