protecting ups from back emf when using as an inverter

[vortexwizard]

Hi i use old ups(uninterupted power supplies) the type used in offices made by APC with larger battery banks for very cheap off grid inverters, one problem with this i have read is the damage caused by back emf caused when a motor stops. How is it best to protect against this?
Many thanks

 

[jraef]

I think there is something missing or wrong in your basic premise. If you are talking about an AC induction motor, "stopping" is generally done by opening a contactor, so there would be no back EMF issues at all.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[vortexwizard]

hi it is just what i have read, i am mainly using them with power tools and a water pump, so you think there will be no problem with this? I am not using any contactors just the inbuilt trigger switches on power tools and unplugging the water pump to turn it off, if i were to turn off the power at the ups(inverter) would that be a problem? I think as they are designed to run computers and not inductive loads there maybe some additional protection missing or are people over worrying? I am mainly using the older grey apc smart ups series. Maybe back emf is the wrong terminology but what i am refering to is the current surge when the induction motor is switched off.
Many thanks for reply, i hope i am being clear

 

[ScottyUK]

More likely a voltage disturbance caused by the arcing at the contactor as it interrupts an inductive load. Most UPS designs won't be particularly happy with that type of load.

 

[vortexwizard]

thanks, so is there any circuitry i can add to minimise this like the equivalent of a snubber circuit and how do normal inverters deal with this? Thanks

 

[jraef]

Inverters don't like AC motors as a general rule, but it depends on how much bigger the inverter is than the motor in question. But portable power tools usually use "Universal" motors, a DC motor designed to accept AC input. They don't have the same sort of inrush current profile of an induction motor.

I have started a 5HP 230V 3 phase induction motor for an A/C unit on the output of a double conversion UPS, but the UPS was 25kVA with maybe 10kVA of total load on it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden