Magnetic Switch

[Josh Hulit]

Rewiring my jointer. The motor is 3hp, 1 phase, 1740 rpm, 220v, 17.3 amp.

The switch I'm looking at is rated 18-26 amps.

Is it a problem having the amp rating on the switch higher than the motor?

Thank you.

[Adam Herman]

amps is like flow rate. you only use as many amps as you need. as long as it is rated for the max of the motor or higher, you are good to go. the motor will run at some current less than its max for nearly all of its life.

[Josh Hulit]

Thanks Adam. Thought I would be safe, but wanted to check with somebody that knew more about this than me.

[Bill Dufour]

If this is just a switch no problem. If it has overloads they can not be adjusted low enough to protect your motor.
Bill

[Adam Merritt]

I thought that a motor's amperage had to fall in the range of the magnetic switch, otherwise, it wouldn't properly energize the magnetic coil and allow it to continue running. I can't find the post I read on this not too long ago, but I'm almost positive I saw this when looking for a safety switch for my new router table.

[Rod Sheridan]

I thought that a motor's amperage had to fall in the range of the magnetic switch, otherwise, it wouldn't properly energize the magnetic coil and allow it to continue running. I can't find the post I read on this not too long ago, but I'm almost positive I saw this when looking for a safety switch for my new router table.

Hi Adam, that's not correct.

The magnetic coil is energized by the supply voltage, not load current. You can energize the starter and have it close the contactor with no load connected if you wish.

The current rating of the motor must fall within the current adjustment range of the starter so that the motor can be protected from overload (over current).

Regards, Rod.

[Rod Sheridan]

Rewiring my jointer. The motor is 3hp, 1 phase, 1740 rpm, 220v, 17.3 amp.

The switch I'm looking at is rated 18-26 amps.

Is it a problem having the amp rating on the switch higher than the motor?

Thank you.

Hi Josh, does your mode have a service factor rating? If so your starter minimum range of 18 amperes would be OK.

The starter is normally set to the motor full load current, which in your case is 17.3 amperes, you're within 5% at 18 amperes, I honestly would use that, especially if the motor has a service factor, coupled with the fact that the machine is hand fed and supervised in use...........Rod.

[Josh Hulit]

Not sure. Is the service factor rating on the motor plate?

[Adam Merritt]

Great info. Thanks for letting me know!

[Malcolm McLeod]

Hi Josh, does your mode have a service factor rating? If so your starter minimum range of 18 amperes would be OK.

The starter is normally set to the motor full load current, which in your case is 17.3 amperes, you're within 5% at 18 amperes, I honestly would use that, especially if the motor has a service factor, coupled with the fact that the machine is hand fed and supervised in use...........Rod.

There always seems to be a lot of confusion about motor 'starters'. I won't try to write a dissertation to explain it all, but I think most of the confusion revolves around what functions a 'starter' provides and the semantics we use to describe them.

In my world, a starter, like a relay or switch, is just a device to close one or more electrical contacts (poles). It can have either a manual actuator (a button or switch), or an electrical actuator (coil). In this capacity, a starter does not have any overload protection. However, a starter can, and sometimes does, pair with some type of 'overload' device. The overload protects both the starter and the motor from excess current or phase loss (if needed) - - while a CB protects the wiring, as has been pointed out in other threads eleventy-seven times.

You may see references to 'integral starters', which have both functions in the same device. (I don't take every WW'ing machine I pass apart for inspection, but suspect integral starters are rare in the hobby world...?)

I have a typical, 1-phase hobby workshop and tools up to 5Hp, but none have O/Ls paired with the starter. (Several have starters and some have O/Ls on the motor, but only my dad's (old & sold) 5Hp Belsaw planer had O/Ls with the starter.) - - YMMV

[Josh Hulit]

Service factor on the motor is 1.15.

[John Lanciani]

You may see references to 'integral starters', which have both functions in the same device. (I don't take every WW'ing machine I pass apart for inspection, but suspect integral starters are rare in the hobby world...?)


All of my European machinery, as well as my General International drum sander, have integral starters. As have every imported machine that I have owned that had 2hp+ motors.

[Bill Space]

Rewiring my jointer. The motor is 3hp, 1 phase, 1740 rpm, 220v, 17.3 amp.

The switch I'm looking at is rated 18-26 amps.

Is it a problem having the amp rating on the switch higher than the motor?

Thank you.

Back to the original question.

Since your motor is single phase phase chances are good it has a thermal overload built in and you do not have to worry about thermal overload protection in a magnetic starter. Does your motor have a button on it to reset a thermal overload trip? Often it is a red button...

If your motor is "self protected" you can either use a double pole switch or a magnetic starter. As someone said above, the magnetic starter has the advantage of preventing the motor from restarting after a power loss or perhaps when the motor's thermal overload is reset after it trips. This could be an important consideration to you but it is apparently something you have not needed to date.

As as long as the switch, or magnetic starter you use, is rated above the full load current of your motor you should be fine. Going larger is OK; going smaller is not. Make sure you use a device (switch or starter) which breaks both legs of the 220 volt single phase supply.

Discussing thermal overloads in a magnetic starter, when using a single phase motor that has a built in thermal overload, does not have much value. It is Important to consider motor protection, but it may not be important with respect to the original question, if the OP's motor has built in thermal overload protection.

Bill

[Rod Sheridan]

Not sure. Is the service factor rating on the motor plate?

Yes it would be............Rod.

[Rod Sheridan]

Service factor on the motor is 1.15.
Set your starter O/l at 18 amperes, as a conservative setting.

You can set it to 125% of 17.3 amperes (21.5 amperes) due to the service factor of 1.15 in Canada, I expect the USA will be similar.

Regards, Rod

[Josh Hulit]

Thank you all for the many helpful responses. Should have my switch this weekend and hopefully get things up and running.

[Josh Hulit]

Fired up the jointer last night, everything seems to be running smooth, surprised me how quiet the machine runs. Thank you all for your help! I can't seem to post pictures on here anymore, but if I figure that out I'll put some up.