Motor Starter question

Your question doesn't make a lot of sense; to me anyway. It reads as if you have a 3-phase motor and starter and you want to run it off of single phase. That's a no-no. I suggest you secure the services of an electrician.

The first question which occurs to me is *why?* A starter is designed to shut down a three-phase motor when it loses one phase, and would otherwise burn up. If you lose one hot with single phase, the motor will stop and will draw no current anyway.

It depends on the motor in question. Assuming a 240V single phase which could also be strapped for 120V single phase, and assuming a normal three-phase reversing switch, yes, it can be done, but requires some lateral thinking.

Normally, a three-phase reversing switch would be set up to switch like this:

LINE side of switch MOTOR side of switch +------+ | | L1 0-----------------o o--> o +-------------0 T1 \ / \ / \ / =x= / \ / \ / \ L2 0-----------------o o--> o +-------------0 T2 | | +------+ +---------+ | | L3 0-----------------o o--> o +-------------0 T3 | | +------+

The funny bit with the "=x=" in the center is supposed to indicate that the wires cross by each other without connecting. :-)

Now -- this reverses the motor by exchanging two of the three wires -- which works fine in a three phase motor. The third wire is also opened in the center [STOP] position for safety, so no power reaches the motor when it is told to stop.

Now, a single-phase motor, wired for 240V (if that is the higher of two voltages will look like this:

L1 0--------------------------------------+ | 3 3 3 Run 1 3 3 3 Cent. Start Starter | SW 0---o--->o--|(---0----WWWWWWWWW-------+ Switch Cap Winding | 3 3 3 3 Run 2 3 3 | L2 0-------------------------------------+

And you can make it run one way by connecting the start winding (SW) to (L1), and the other way by connecting (SW) to (L2) Inside the motor's connection box, you will find two wires for each of the run windings, and two for the starter winding, and (probably) two which connect to the centrifugal starter switch and the capacitor somewhere inside the motor. There should be a data label inside the cover to the motor's wiring box with clues as to how to connect the windings for 240V operation, so all you have to do is to translate that into what I've shown here.

The 'WWWW' and '3333' sections represent the coils of the windings in the motor in different directions.

So -- to get full safety (switching power completely off to the motor in the stop position), and reversing (nothing that you will *not* get instant reverse with this kind of motor in any case -- you have to wait for the motor to coast to nearly a full halt, before switching it on in the other direction), you can wire it as follows (ignoring the details of the switch and the motor shown above):

Line Switch motor L1--------------L1 T1-----------L1

L2--------+-----L2 T2-----------L2 | +-----L3 T3 SW

(Note that it really does not matter whether the input side of the switch's L3 is connected to L2 or L1 -- other than which way the motor will run with the switch in the forward direction.

# # ### ## # #### ##### ###### ### # # # # # # # ### # # # # # # ##### # # # # # # # # # ## # # # # ### # # #### # ###### ###

I have *not* drawn in another wire which does not connect to the switch (normally) but does connect to the motor's frame and the power from the wall -- this is the safety ground which should *not* be switched --

*ever*!

Some drum switches actually have individual rocker switches with jumper wires making the 'X' and the straight across on the third wire. Others use wafer switching elements, where the connection is done by clever design of the switch wafers, so you may need to try to figure out which is which using an ohmmeter with everything disconnected from the switch.

Good Luck, DoN.

Peter,

Does the use of the word "trip" mean that it has heaters?

Does the use of the word "trip" mean that it has heaters?

"Heaters" are a "word of art".

These are devices which contain: 1) a heating element, selected as to their trip point, and 2) a separate trip mechanism which actually initiates the disconnection upon being heated by (1).

There was no need for more than two elements, usually labeled "OL" in one-line diagrams, because if any OL device detected an overload, then that OL's contacts would be opened, and since both (all) such OL contacts were in series connection, opening any one would immediately result in the the main contactor, usually labeled "M" in the diagrams, opening, thereby removing all power from the machine.

Now, it is more common to find three OL devices, one for each phase.

However, in the past, there was two- and three-phase power, and it was apparently thought that two heaters were enough, as two heaters could protect a three-phase machine (by protecting two of its three phases), and also a two-phase machine (by protecting its two phases).

NFPA 70 requires the motor controller to open all the supply conductors if it also serves as the "disconnecting means". You need to wade through about 4 pages to figure out what exactly constitutes a "disconnecting means" in this context.

But if you were to switch on1y 1 conductor on a 240V single phase motor, why not switch just 2 for a 3 phase motor?

Interestingly, NFPA 79 - Electrical Standard for Industrial Machinery requires that the controller open all conductors, with no exceptions. It's unlikely that this standard is strictly applicable here, but I find it's an excellent guide to good wiring practice for machinery.

Ned Simmons

Ah, not exactly.....your typical starter doesn't have 'loss of phase' protection, only indirectly in that single phasing a three phase motor under load will probably cause the OL's to open. If lightly loaded the motor will continue to run single phase. The heaters are sized to trip the OL's when full load amps are exceeded, thus protecting the motor from overloading during normal three phase operation.

There is also a significant safety advantage of starters over drum switches in that the motor won't restart when power is restored after a power failure when using a mag starter.

Not trying to nitpick here but larger motors and their starters often have loss of phase protection, as well as maybe phase imbalance and ground fault protection in addition to the standard overload protection. The larger the motor the more complex it's protective relaying.

If you looked at my ASCII schematics in the previous article, you would see that the three-phase drum switch can be wired to disconnect all line leads, even when switching a reversible single-phase motor.

I do not believe in leaving even one hot lead into the motor when it is switched off.

In the UK, switching one conductor on a 240 V motor makes more sense, as one side of the 240 V is grounded (just as one side of our

120 V is grounded in the US.)

However, I would not switch either only one conductor on a US single-phase 240 V motor, or only two on a three-phase motor. There is always the possibility of a winding short to ground, to complete the circuit if you leave any one lead hot.

Agreed. Since the industrial machines may have single-phase loads on the switched wiring (e.g. a lamp or a lube pump) as well as the three-phase motor. (And my CNC Bridgeport has single phase loads on all three phases (that is, between all three combinations of pairs). The stepper motor power is derived from a different phase for each axis.

Enjoy, DoN.

But it *will* trip if the motor is stopped, and then an attempt is made to re-start it -- perhaps by automatic equipment, perhaps by a remote switch. The starting current through two phases will be enough to open the heater switch and deprive the motor of power entirely.

That is a significant benefit, indeed. Depending on the setting, it can also be the case when a three phase motor is being controlled by a VFD -- but of course it does not give the certainty of disconnect that a contactor does.

Certainly.

Enjoy, DoN.

Right, no electrician would wire something that way. IIRC the term 'disconnecting means' in the code has a specific meaning, and it has to do with the ability to positively remove all power from a machine while it's being serviced. For example, all my machines at home have a disconnect means because they are connected via plugs and receptacles. Unplug the twist-lock and it's disconnected. I also seem to recall that this is allowed only up to a certain size - hp or ampicity or whatever.

I think that the code allows the starter to serve as the disconnect means for larger machines that are hard-wired in. But not a simple drum switch. At this point we are in the 'lock-out-tag-out' realm and it becomes more complicated.

Jim

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I need to get a motor starter for a 15hp air compressor. I plan on running

220v 3phase. My question is, when selecting a motor starter, what voltage coil should I get to power the contactor? They make 110v and 208~240. If I get a 110v coil, I would assume I need to run a neutral wire back to the panel? If I select a 220v coil, then I can just pull 2 leads L1 & L2 to power the coil?

You do have options, but I would.. Go with the 110 coil, that is the standard that 95% of all industry is using these days. Years ago it wasen't uncommon to see 208-240v and 480 coils in the control circuits of motor starters. If the starter you buy doesn't already have a control transformer to get 110 from your 220v source they are cheap to buy. Just remember to fuse the transformer. If you are using second had equiptment you might not have the option of choosing your voltage and many off brand starters are increasingly becoming harder to buy parts for. In that case you might have to buy a transformer to step up or step down your power to whatever your starter coil is rated for.

tim

You could go the control xformer route, but as the only control device is just a pressure switch and not normally accessed by personnel, just go with the 220 coil, it's how all the air compressors I'm familiar with do it.

Regards Paul