Induction Motor Switch-off Transients

[Rowan Sylvester-Bradley]

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

 

[Rich Grise]

Rowan Sylvester-Bradley wrote:

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?)?

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

 

[whit3rd]

On Tuesday, June 14, 2011 3:53:38 AM UTC-7, Rowan Sylvester-Bradley wrote:

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 ...
possibly the transformers are being damaged by transients on the mains
supply caused when the induction motor switches off.

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.

 

[Phil Allison]

"Rowan Sylvester-Bradley"

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?

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

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

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

** Possibly a brief, large voltage spike.

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

** Maybe.

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?)?

** 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