0
\$\begingroup\$

Background:

I am designing a small Arduino-driven circuit to control a ~3/4 hp AC hoist motor. The hoist motor includes a starter-capacitor. The Arduino drives a transistor which in turn drives a power relay that switches the motor on and off. I am not an electrical engineer, so I'm using a Factor of Safety of 2 or higher throughout the circuit to avoid component damage/burnout.

The particular relay I had in mind is supposed to be capable of handling up to 30A sustained (more than 4 times the max-load current of my motor) at double the nominal operating voltage. My intent is to run two of them in series for safety (to handle one failed-closed relay) and for wear-leveling of the contacts.

Consulting an ampacity chart, it looks to me like I need 10AWG copper wire (about 5.3mm^2 conductor area) to handle 30A. But as you can see from the relay datasheet, the load pins are ridiculously tiny: each is roughly equivalent to 17AWG (1.6mm^2). Unfortunately Digikey does not carry a socket with solder cups or terminal lugs for this particular relay, and I'm having a very hard time soldering 10AWG wire to a 17AWG pin.

Questions:

  1. How can the relay pins carry so much current with such a tiny conductor?
  2. Is 10AWG really the right size of wire to carry 7A AC with an inductive load?
  3. How should I go about wiring this relay to the rest of my circuit?
\$\endgroup\$
  • \$\begingroup\$ This is all a matter of voltage drop. You will have the most voltage drop at the connector anyways, it doesn't matter if 10cm length of wire is 10AWG or 17AWG. \$\endgroup\$ – Janka Feb 25 '18 at 20:46
  • \$\begingroup\$ Surge current for motors can be as much as 500% for AC and relays are derated about 30% of resistive rating. Ampacity is based on heating, voltage drop affects current limit. Your relay is expect to last 1x10^3 cycles on 230Vac , possibly less with heavy hoist loads. \$\endgroup\$ – Sunnyskyguy EE75 Feb 25 '18 at 20:48
  • \$\begingroup\$ Spade lugs are easy to fit. \$\endgroup\$ – Sunnyskyguy EE75 Feb 25 '18 at 20:53
  • \$\begingroup\$ That may be rated for 30 A. (I saw the datasheet.) But I would not even consider it for more than 10 A. Best case. (Which nominally might work for 3/4 HP.) Frankly, I just think the datasheet can't be trusted since PC-mount relays like this, with pins like that, cannot handle 30 A. If you want 30 A, then what you need is called a "medium current relay" and these don't look anything like your unit. To reduce wear from arcing, consider an SSR in parallel but operated with different timing control. \$\endgroup\$ – jonk Feb 25 '18 at 20:55
  • \$\begingroup\$ What voltage and surge current and acceleration time? \$\endgroup\$ – Sunnyskyguy EE75 Feb 25 '18 at 21:08
1
\$\begingroup\$

Since motors have a surge current starting up that is up to 5x the max load rated crrent, if you say the hoist motor is rated for 15A then expect up 75A starting surge which reduces while increasing in speed to match the load torque=current demand.

In this hoist motor case, I would be looking towards OMRON or PANASONIC Relays rated for 75 to 90A if you want to do more than 1k cycles, which I estimate from TE's datasheet.

https://www3.panasonic.biz/ac/e_download/control/relay/power/catalog/mech_eng_hepv.pdf?via=ok

read all the fine print.

The thermal terminals imply heavy wire needed to remove heat from relay contacts and extend life. AWG 10 will give you more torque and than 16 AWG but it depends on length and voltage drop for surge currents. So you must look up this value for Ohms/m * 75A. A 10% drop may be acceptible for you, 5% or 20% drop for others, but the lower the torque and start voltage, the longer it takes to reduce the current.

\$\endgroup\$
  • \$\begingroup\$ Thank you for this info and the suggested alternative relay. Unfortunately, this particular relay requires about 3.5 times more coil current per unit than I can supply from my Arduino without adding some significant additional power electronics. I double-checked the hoist specs, and it's actually rated for a max of 7.1A steady-state. It includes a starter-capacitor but I don't have any details on that. \$\endgroup\$ – MikeB Feb 27 '18 at 23:18
  • \$\begingroup\$ SS 7.1A is still minimum 25A inductive surge and likely more. This is why your relay is a poor choice only rated for resistive current @30A with 99.9% life reduction from arcing. ~(1M to 1K cycles rough est.) Many simple solutions include a small FET to drive coil current or even a cheap $5 opto relay board to drive power relay to ensure EMI spikes dont get back into Arduino lines with twisted pairs. \$\endgroup\$ – Sunnyskyguy EE75 Feb 28 '18 at 1:35
  • \$\begingroup\$ Even my LG stove top has improper rated/protected relay contacts dying in 2yrs from a slow cycle coil heater under ceramic. \$\endgroup\$ – Sunnyskyguy EE75 Feb 28 '18 at 1:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.