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Topic: Relay Control of 240AC - Inductive loads? (Read 138 times) previous topic - next topic

Frappe

Hi all,

I am fairly new to the arduino and just done my 2nd little project. I have wire a short 240VAC extension lead via a 5VDC-240VAC relay and I can remotely control this 240 switch remotely no issues. I am wanting to plug a desk fan into this outlet.

However I have read briefly about fire risk due to inductive loads on motors. Since the desk fan technically has a motor, can someone explain to me briefly what that is and if I am at risk?

See the specs of the desk fan attached. By my calcs it's only 0.166 amps? Am I missing something obvious?

MarkT

You've missed that its likely to be a single-phase induction motor, so that the current is a lot higher
than if it was a resistive load due to the power factor not being unity, and that the startup current
is several times higher than the running current for these motors, and that's its an inductive load
which needs special handling.

Inductive loads typically need snubber circuits on the relay to reduce arcing at the contacts, and
need a relay rated for inductive load in the first place.

This link seems pertinent: https://www.electronicspoint.com/threads/resistive-or-inductive-load-and-relay-rating.9550/
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Frappe

Thanks for the response.

How can I calculate what the highest current would be? Surely it wouldn't be that high and certainly not higher than what the relay is rated to. It says on their it's a 240VAC and 10amp relay.

Is there a fire risk?

MarkT

Calculate?  That's something on the manufacturer's datasheet for the motor I'm afraid - easier to measure it
with a peak detecting current meter really.

The fire risk is not from the relay, they fail in safe ways and are usually reliable if not mistreated.

Induction motors can overheat if stalled, or run at too low a voltage leading to stalling.  Stall current
can be loads higher than running current, though it all depends on the exact motor design and
the construction of the rotor.

Good motors have thermal-cutout fuses embedded in the windings which melt if the temperature
gets too high and breaks the circuit - if you have such a motor its pretty safe...  Others are designed
to take continuous stall (normally called a "torque motor"). Others are not, however.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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