Detect if 3-phase 110VAC motor is running via 24VDC PLC input

Question

We have an industrial machine with 3-phase ~250W AC (~2.5A @ 110V) motors on it. I want to non-invasively detect if the motors are running. That detection signal needs to feed into the 24V input of a Koyo Click PLC.

After some Googling and part-searching, it seems like I could use a non-invasive current-sense transformer, but the output looks like it's AC with a scaled current. I still have to convert an AC -> DC signal.

I've also looked at various relays, but their input current requirements are generally very low (< 50mA).

Am I on the right track here by using a current-sense transformer that drives a solid state relay with AC-driven coils? Or is there some easy-to-grab part that I'm missing? I'm looking for something that is in-line, chassis mount, or DIN rail basically (no custom soldered circuit please). My background is in PC software development, so this mini-project has been giving me fits.

Answer 1

Such things exist. (It's very hard to come up with an original question these days.) For example, the Crouzet Current Monitoring Relay with SP-NO/NC Contacts, 24 → 240 V ac/dc.

Figure 1. A current sensing relay. See RS Online.

Specifications for this unit are:

Operating voltage:              24 V → 240 Vac / Vdc (-15% / +10%)
Measurement range:              2 → 20 A
continuous overload (°25@C):    100 A
Measured signal frequency:      40 → 70 Hz sinusoidal
Measurement cycle (max.):       30 msec / Measurement in true RMS
Threshold setting:              10 → 100% of the range
Response time:                  200 ms
Start delay:                    500 ms
Output:                         1 single changeover relay

The relay contact can be used to switch your PLC input.

A websearch for current monitoring relay will give you plenty of options.