UPS output affected by transient disturbance on input

[davva]

Is it possible that UPS output can be upset by a large transient on the UPS input, e.g. a large induction motor start? It should switch to the batteries and give maintained power.

What should you look for in a UPS specification if this is a problem

 

[DanEE]

You give no details of a UPS topology in mind, so below is a general answer based on how a ferro transformer based UPS would handle it, but all should do more or less the same.

Generally companies will publish an isolation ratio e.g. 2000:1 .. i.e. if it is hit with a 2000 volt spike, 1 volt will get through. But that is a description of transient protection and not undervoltage.

The condition you state describes an undervoltage condition with some duration and of course there may be inductive spikes which the above isolation ratio would apply.

As soon as the UPS voltage line input goes down to the undervoltage set point, the inverter activates to pick up the load until line voltage stabilizes and then an additional setpoint delay times out to transfer back to line voltage. This second delay is one of several values set in the UPS to avoid nuisance transfers back and forth on dirty power.

Again using a ferro-resonant transformer based UPS as a model to describe, the tank circuit in the ferro can store up to 1 cycle of energy and totally buffer the transition.. i.e. the output will not see the drop in voltage and, the inverter will fire up can carry the load before the energy storage has been exhausted..

 

[waross]

A large motor start may cause a voltage dip. That's why we use a UPS. To protect equipment from "Out Of Spec" power conditions. If it is staying on the motor may be causing the voltage to drop below the set point of the UPS. Hire an expert.

Bill
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"Why not the best?"
Jimmy Carter