Surge Protector for Induction Motors.

It depends on the quality of the power supplying the motor(s) and the incidence of lighting strikes.

If the power is of poor quality or there is a high incidence of lighting strikes causing excessive voltage surges, you must clean up the power with conditioning filters and/or use surge protection to protect the motor(s).

It is a common practice to use surge protectors on all medium to large motors.

Without knowing about the quality of your electrical supply, Regs, no-one can say.

If its expensive, it needs to be protected not only from lightning but for HV transmission wires touching the distribution line. Though motors winding is robust and passive from Temporary Over Voltage and Transient Voltages.

A direct lightning hit on your supply lines definitely will toast your motor and so as your all your wiring on your vicinity.

A recommended installation of SPD's will be at mains and nearest every equipment.

Some SPD's are warrantied 10 years.

Probably not a good idea. Motors are, by nature, highly inductive devices. When you interrupt the current flow in an inductor, the collapsing magnetic field will increase the voltage in order to keep the current flowing. If you surge protection is located on the motor side of the disconnect switch, the surge protector will die a silent death within a very short time.

Inductive circuits (even ballasts) can make it tough to break the circuit if the wrong types of interrupter is used.

Like how much current or voltage it could develop from a disconnect?

That is a good question. Of course, it depends on the size of the motor and when the current is interrupted. Adding to that, one must also consider the effects of the inertia of the rotor and its mechanical load, etc. Unfortunately, I can not provide you with any meaningful data.

What I can tell you is that motors are inductive and that means that they have to obey the laws of physics for inductors, but maybe not to the same extent as other inductors. The type of inductor that I am most familiar is a magnetic ballast. Of those, the type that interrupts current in order to start a fluorescent lamp generates voltages reaching as high as 1500 volts before the lamp starts to conduct current.

That being said, a fluorescent lamp absorbes the energy stored in the inductor as it changes from megaohms down to a couple of hundred ohms. If you apply that kind of energy to a surge supressor, it may not fail on the first or second disconnect, but the application is a recipe for burning out the device.

Perhaps what you want is called a "gas discharge tube" which looks like a fat diode made of ceramic. It is actually an argon filled tube with an electrode at each end. As the voltage rises, the gas in the tube becomes ionized and current flows until the voltage drops enough for the plasma to extinguish. Note that both gas discharge tubes and fluorescent lamps loose resistance as the current increases. An uncontrolled increase in current is a run-away effect that will result in the destruction of the device. The moral of this story is to choose your components carefully, and don't forget to limit the current or it will go "pop" and let the magic smoke out.

Yes I know very well MOV and GDT, but are you referring to a running motor or one that is idle?

I was referring to the brief event when the running motor is disconnected.

An idle motor is not going to produce anything.

As I read further down, it seems your concern is actually about a lightning strike.

I'll jump in again further down.

Surge protectors are usually used to protect electronic devices (mostly solid-state) from very short-duration events which are on the order of microseconds. Induction motors, being inductors, are not simple resistive devices, displaying a great deal of inductive reactance, thus, high impedance to short-duration/high-frequency events. Surge protectors which protect against long-duration disturbances on the order of tenths of a second are neither cheap nor common.

It does not limit to miniature electronic devices only. It also protects your wiring from toasting when direct hit by a lightning or HV transmission line touching distributor line in the event of typhoon or other else's.

SPD's have classes and types. Please see and read Class I type I Spd's.

That's true, but motors, by their nature, are more or lesss self-protecting due to their inductive reactance.

Should you mean, it is unnecessary protecting it from lightning.

Question is, when it hits direct by a lightning at say 1.5 kVA and 80kA, it won't be affected? Would you agree?

Get real! A lightning strike is more likely to be in the Mega-Amp region.

Are you talking about the "Perfect Storm"? Perhaps you are implying that the lightning will seek out the bearings and vaporize those? If this motor is so important, install some lightning protection!

In the perfect world.... oh forget it.

No, the motor is not likely to be affected. The key words here are "not likely". I'll take the bet that it won't be. How much would you wager that it will be?

This site is not about speculation. Lloyds of London will probably sell you an insurance policy unless your motor is on the top of a mountain. If that is the case, you can always enclose it in a Faraday cage.

Correction, the concept of Faraday cage is seemingly safe to human since in nature the cage is with high conductivity than the body so electricity flows through the conductor. However, it can not save your motor since winding and line & load side wiring are made of copper.

Also Tesla Coil Arc is way different than DC lightning sudden discharge.

Wrong, wrong, wrong!

Just because you made it up (and let it loose), does not make it true!

A Faraday cage will protect anything inside it provided the geometrical requirements are not violated. Even copper objects!

Please, don't speculate. It serves no purpose here.

Get real! A lightning strike is more likely to be in the Mega-Amp region.

You have that backwards. Lightning has mega voltage not amperage. Voltage is inversely proportional to amperage. It needs very large push to get from start to finish.