I was hesitating to ask this but in many applications such as the ones with electrical machines or relays we always read such precautions or considerations: “If the load is inductive do that don’t do this ect.”

I know that some loads are very inductive such as electric motors, transformers, relays, any sort of coil operated stuff. I also know what power factor is and the phasor represation of AC currents and voltages. But in reality is there a way to categorize devices as inductive or not. Because no device is pure resistive or inductive or capacitive.

For example, a triac based SSR is not recommended to use with zero crossing with inductive loads. Alright so if I use a transformer as a load I can understand that zero crossing is no good for that.

I mean there are some loads such as power supplies where I dont know whether they are inductive enough.

As a load take an SMPS and a linear power supply for instance. Imagine you have a zero crossing type SSR. And your load could be one of those power supplies. Would you consider the linear power supply an inductive load? How about if the load were an SMPS?

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    \$\begingroup\$ For switching applications, I'm pretty sure the criterion is that a load is inductive if it produces inductive kickback. \$\endgroup\$ – Hearth Nov 3 '18 at 18:05
  • \$\begingroup\$ SMPSU is a caopacitative load, also typically negative resistance. - current draw draw drops as RMS supply voltage increases. \$\endgroup\$ – Jasen Nov 3 '18 at 18:56

IEC standard 947 defines various utilization categories for relay contacts. Some relay manufacturers summarize those categories in their literature. Some examples:

AC-1 Non Inductive or slightly inductive loads, example: resistive furnaces, Heaters

AC-3 Squirrel-cage motors: starting, swithces off motors during running time

AC-7a Slightly inductive loads in household appliances: examples: mixers, blenders

DC-13 Control of D.C. electromagnetics

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Relays conforming to USA NEMA standards have specified motor horsepower power ratings at typical motor voltages for which a given relay or contactor is suitable. Relays conforming to other standards often have motor power ratings in kW.

There is a lot of information available. You will probably need to find some of the information available from manufacturers and suppliers and study it and determine what is best for your needs.

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  • \$\begingroup\$ Is a linear power supply an inductive load for a zero crossing switch applications (having a transformer at its inputs)? \$\endgroup\$ – user16307 Nov 3 '18 at 20:12
  • \$\begingroup\$ The inductance of the load is not the only issue for switching applications. With transformers and motors, high inrush current is another important issue. A transformer with a load reflects the characteristics of the load impedance to the primary if the load is inductive, that is the steady-state impedance that appears at the primary. A power supply with a linear regulator is still a non-linear load because of the rectifier at the input. You have expanded the question with your comment. It would be better to ask another question about a specific type of power supply circuit. \$\endgroup\$ – Charles Cowie Nov 4 '18 at 3:28

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