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I have a Rasberry Pi running inside a device, which I want to connect to a relay to switch a 220VAC/20A load. The load 'spikes' to 20A when the motor is starting up, but then runs nominally at 7.7A when the motor reaches it's max.

I currently have an electro-mechanical relay rated for 10A contact current and am afraid that the 20A peaks from the motor startup (~7-8 spikes each for ~1.0 second) will cause the contact to 'arc and stick,' if not cause a worse failure.

I do not want to use a SSR, bc of the heat dissipation requirements, if I can help it. I was thinking I might use the 10A-rated relay to drive a contactor switching the higher-power load, but Im not confident in that design.

Thanks!

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    \$\begingroup\$ Why not just use a 20A (or better yet, 30A) relay? I'm not sure anyone here will tell you it's ok to use components that are under-rated for the job. \$\endgroup\$ – Dan Laks Jun 1 '17 at 20:02
  • \$\begingroup\$ And if the pi cannot drive the relay directly, let the pi control a transistor that drives the relay. \$\endgroup\$ – Dampmaskin Jun 1 '17 at 20:16
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    \$\begingroup\$ "10A contact ... 20A peaks". This shouldn't even be a question here. How is it not obvious that this is a dumb idea? Duh! \$\endgroup\$ – Olin Lathrop Jun 2 '17 at 11:00
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Rather than guess at how much to oversize the relay, it is best to use a relay that is rated based on motor power. The IEC motor standards include a rating system that rates relays on more specific application information. In the USA, relays that are suitable for use with AC motors have a horsepower rating.

Look at my more specific answer to this more specific question: Max allowed inductive load (pump) for a given relay

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  • \$\begingroup\$ Thank you, Charles - the motor is a 220VAC 50Hz 1100W motor w a 0.93 power factor. It's spec states 7.7A, but we are measuring an in-rush current of ~20A during startup (what I meant by 'peaks'). I am finding it difficult to find a 5VDC coil voltage relay that can switch a 220VAC motor that can also handle the ~20A in-rush current. \$\endgroup\$ – user8087995 Jun 6 '17 at 11:48
  • \$\begingroup\$ The relay rating highlighted in the question linked in my answer has a motor rating listed as 60 locker-rotor amps (LRA) and 10 full-load amps (FLA). For general use, it is rated 16 amps. It is available with a 5 volt coil. Note what I wrote there regarding life. Songle has a relay with a 5 volt coil rated 80 LRA and 30 FLA and 2 Hp (1500W). songle.com/en/pdf/2008414165561000.pdf \$\endgroup\$ – Charles Cowie Jun 6 '17 at 15:48
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Unless the data sheet specifies the relay can tolerate such spikes, don't use it. Using a under-rated relay is a fire and safety risk. Also the relay may appear to work, but die after some time.

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There's no reason why you can't use a small relay to drive the coil of a larger relay. It may be simplest to use a second relay with a 220V AC coil, if you can't find a suitable 5V one.

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  • \$\begingroup\$ Both the RPi and the motor are using the same power supply - a series of 12VDC car batteries with a 4kW power inverter. I believe our motor is drawing all of the power on startup (220VAC x 20A = 4400W > 4000W), so if we were to pull the 220VAC into the smaller relay contacts to drive the larger contactor, there wont be any/enough current on the contactor coil to keep it closed (during in-rush)? \$\endgroup\$ – user8087995 Jun 6 '17 at 11:58
  • \$\begingroup\$ @user8087995 I would only expect a contactor to draw a few milliamps at 220V. If the voltage from the inverter drops enough for the contactor to drop out, you've probably got other problems. Alternatively, if you have 12V available, look for a contactor with a 12V DC coil, and use a small 5V relay to drive that. \$\endgroup\$ – Simon B Jun 6 '17 at 12:07
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Use a motor starter relay. Those relays, often called contactors or motor starters, have a nominal rating and an overload rating.
You won't find a 5V coil motor starter relay, so you will need an small 5V coil general purpose relay connected to the Rasberry Pi to switch the motor relay.

Do not forget the buy the large contactor with snubber (attachment), otherwise your next question will be why the Raspberry Pi keeps restarting and why your control relay isn't working anymore.

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Your application targets directly a Solid State relay there is no question. The SSR's need very little current to switch the load since they work with light to trigger (inside the puck). You must always have a load for them to work properly and that is what you have, a motor. Note that there will always be a very small current flowing throught the SSR/motor, typicaly 150mA.

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