PDA

View Full Version : Electric motor question



Hamilton
06-05-2010, 08:17 PM
I have an older 3 PH 1 HP motor for a drill press. The three wires used for hook up are all the same color. I do not know which wire does what. Is there a way to figure this out without damaging the motor? :confused:

doctor demo
06-05-2010, 08:34 PM
All three wire are hot . L1 L2 L3 . If You have the correct voltage supply for the motor's needs you are good to go. If the motor runs the wrong way , swap any two leads.
Of course You have to have three phase power not single phase.

Steve

HSS
06-05-2010, 08:45 PM
And, wire it thru 3 phase protection.:eek:

Mcgyver
06-05-2010, 10:51 PM
And, wire it thru 3 phase protection.:eek:

what is three phase protection, vs say other current sensitive protection like heaters or fuses? My motors are protected but i don't know what '3phase protection" is :confused:

RobbieKnobbie
06-06-2010, 01:30 AM
Three phase protection links (mechanically) the three legs so if one leg trips out, the other two are de-energized as well so the motor doesn't try to single phase (run on 2 legs). Which it will do... but not for long.

1-800miner
06-06-2010, 02:07 AM
You do have three phase power available don't you?
It may sound like a stupid question. but you might be one of those green guys. Three phase wont work on single phase.

Tony Ennis
06-06-2010, 10:00 AM
If you don't have 3 phase power, you can get a VFD large enough for that motor for about $120. The VFD will convert 1ph to 3ph and give you a slew of other useful features.

J Tiers
06-06-2010, 10:29 AM
Three phase protection links (mechanically) the three legs so if one leg trips out, the other two are de-energized as well so the motor doesn't try to single phase (run on 2 legs). Which it will do... but not for long.

A) Three phase motors work fine on single phase once they are going..... Thousands of "static converters" prove that.

However....

1) the max power is reduced (this is why "single phasing" is bad in industrial pumps etc). For home machining, this is not a huge issue, it is rare to need full power.... (see above about static converters)

2) they do not START on single phase, a static converter has a "phasing capacitor" to give a start to the motor.

.

B) For a 1 HP or less, in the US, you are not required to have all the fancy overload protectors etc that a larger motor must have.

However:

1) you should switch all three leads if operating from 3 phase (other than from a VFD)

2) it is best to use a relay that will automatically disconnect from the mains if power fails. This is a safety issue to prevent an un-expected re-start if power comes back. (Note that many VFDs default to re-start when power is applied..... you may need to program the VFD to NOT do that)
.


C) your choices for power are:

1) real 3 phase..... if you have it

2) static converter.... cheap, is NOT 3 phase, still has all the problems of a single phase motor (tendency to chatter, primarily)

3) Rotary converter.... can be cheap if you make your own from a surplus 3 phase motor.... makes good 3 phase, and gives all the 3 phase advantages

4) VFD.... electronic device to make 3 phase. Works very well, gives you variable speed, most are accepted to function as an "overload" device to satisfy electrical codes (if properly programmed).

Hamilton
06-06-2010, 11:15 AM
I have a phaseomatic static converter to achieve 3PH. This has been running my lathe with no problems. I will wire this to a 4 prong twist lock receptacle.The 3PH protection sounds like a good idea. The converter wiring directions suggest not using fuses as stated above possible damage if one leg fails.As one question is answered another question is created. How do i wire in 3PH protection? Thanks, to all of you for the info.:)

J Tiers
06-06-2010, 11:23 AM
A static converter runs the motor on single phase power.

Don't bother with 3 phase protection, you do not HAVE 3 phase, and proper fusing is fine.

RobbieKnobbie
06-06-2010, 12:15 PM
A three phase circuit protection device will have three terminals on the input (line) side and three on the output (motor) side. You're essentially 'splicing' the breaker into the three lines. It's very straightforward.

The last thing you want to do is have only two - or worse yet one - legs feeding power into a three phase motor. The motor will run (poorly) for a while... but like I said: not for long.

I've replaced a number of motors from little single HP to 25 and 50 horse because one phase was lost and the operator didn't report that the machine was running badly until it overheated and quit completely. That turned a $50 quickie repair into several days lost replacing a motor.

J Tiers
06-06-2010, 05:11 PM
The last thing you want to do is have only two - or worse yet one - legs feeding power into a three phase motor. The motor will run (poorly) for a while... but like I said: not for long.




Robbie...........

Most static converters in the world does what you say is bad....... run the motor on single phase.

Some have a run cap for the 3rd phase, but it is not very effective in most cases, except for a truly fixed-power load.

You are referring to a 3 phase motor at full or close to full load, which "single-phases". The same power cannot come from ONE phase without more current, which overheats the motor.

For home shop purposes, that is not an issue 99% of the time. if it IS an issue, the person will probably already know it.

AlleyCat
06-07-2010, 11:18 AM
I have a Rivett 918 lathe with a three phase motor that runs great on a home made static converter. It has a start capacitor and a run capacitor that are basically wired in parallel. When motor power is applied a timer disconnects the start capacitor after about a second or so. The motor is then powered only through the run capacitor until power is disconnected. Like J Tiers mentioned, a system like this may not work in every situation. Most of the parts I make are small and the motor never carries a heavy load so it works fine for me.