Induction Motor Switch-off Transients

I have been experiencing some problems with a power supply unit failing due to transformer faults. These 230V AC input 9V DC output PSUs are installed close to an induction motor which can be from 1/2HP to 10HP depending on the installation. It's been suggested that possibly the transformers are being damaged by transients on the mains supply caused when the induction motor switches off. There is no surge suppressor or filter in the PSU.

Can anyone comment on this?

What do you see on the mains when an induction motor is switched off?

Should the motor itself be fitted with some sort of surge absorption device?

Should I add a surge arrestor to the PSU? If so, what sort of surge does it have to deal with (how long, how big?)?

Thanks - Rowan

Reply to
Rowan Sylvester-Bradley
Loading thread data ...

I'd recommend doing a little analysis to find out what exactly is the failure mode of these transformers. One very seldom hears of a transformer being harmed by transients or even large external magnetic fields (although I wouldn't discount the possibility of magnetic fields affecting a transformer).

Have you determined that it's a "transformer fault," or is it merely "failure of the wall wart?"

First, I'd look a little deeper into how, exactly, the PSU(s) is(are) failing.

Good Luck! Rich

Reply to
Rich Grise

The inductance of the wiring between the motor switch and the main breaker panel is the source of transients (after all, the motor switch is OFF in this scenario), and a surge protector might be appropriate. It needn't be a large protector, it's not absorbing the motor inertia.

Reply to

"Rowan Sylvester-Bradley"

** This is a 50/60 Hz tranny - right ?

Is the primary side insulation failing - ie the tranny develops shorted turns ?

** Possibly a brief, large voltage spike.
** Maybe.

** A well made transformer should cope with 1kV or even 2kV spikes across the primary. However, some modern winding methods are not so good at this and allow spots to exist were spike voltages appear between adjacent wires in the primary. Very bad.

Aside from replacing all the trannys with better made ones - maybe try a

470nF class X2 capacitor across the primary PLUS a decent size ( say 25mm dia ) 275VAC rated varistor too, for good measure.

If the tranny is of low VA rating as I expect - add a series resistor of say 15 ohms 5W, WW.

It will help the X cap do its job better.

... Phil

Reply to
Phil Allison

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.