3PH motor adapted to run single phase.

[fastline12]

We are going to have to repair a saw with a retro fit motor. 3PH designs are much easier to find. This is a saw application where starting torque is not an issue. I have experience building RPCs but have never dynoed a tuned in slave motor to see what they can do.

How might I optimize the performance of a 3PH motor on single phase? I realize we will be down on power but we can work past that. I would like to improve balance to all legs if possible. I realize this will be a function of load so we might have to choose a sweet spot.

 

[dpc]

Not sure what your background and experience is, but in general, a three-phase induction motor will not run on a single-phase supply. It just won't. If the motor is spinning when a phase is removed, it may continue to run, but will soon trip on overload or self-destruct. If you try to start a three-phase motor on a single-phase supply, it will not turn.

You can generate three-phase power from a single-phase source using an adjustable frequency drive. This has become very common as the price of the drives has come down. Maybe this is what you were planning - I'm not clear on that.

 

[pwrengrds]

Do you have 3 phase available? then use 3 phase to run the motor.
or
Do you want to run a three phase motor on single phase input? The easiest way to do that is use a variable frequency drive and make your own three phase with it.

 

[fastline12]

I have been capacitive starting 3PH motors in single phase for years. They will run on it.

I did not plan on a VFD drive for this application.

 

[dpc]

Well, good for you. If this is something you're doing for a living, you need to educate yourself on the problems with doing this. It's really bad for the motor.

 

[waross]

The right size motor with a VFD.
An over-sized motor with capacitors.
Or go all the way and use an ADD_A_PHASE.
That is an over-sized motor with a capacitor bank spinning free to act as a three phase induction generator to generate the three phase for your saw motor.
With the relatively little cost of a small VFD with single phase input, it may be cheaper than the labor to cut and try capacitors until you get it right.
Another option may be to look for some DIY sites. Someone there may have some guide lines for capacitor sizing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fastline12]

N, this certainly is not being done at any professional capacity and I know not to advise this. However, the "add a phase" mentioned above we call an RPC or rotary phase converter. Been making and running them for about 10yrs. They work perfect when tuned right. I have lots of experience and notes setting up the start cap bank and balancing run caps.

However, I have never really determined how much power I can get from a 3ph motor running on 1ph. By running run caps, I can get a pretty nice balance at a specific load but I have also run into issues with pump motors tripping overloads from too much imbalance.

We are replacing a 2HP 1ph motor. I planned to use a 5HP 3ph motor for this.


Obviously for any professional capacity, this machine would likely be deemed scrap because of the proprietary motor that failed. Or, a VFD.


DPC, what problems are you aware of in single phasing a 3ph motor?

 

[jraef]

You have hit the biggest core concept, that the motor capacity is severely reduced. 5HP motor for a 2HP output is about right. The potential damage comes from when you have the SAME load on the motor and you run it on a single phase source. The motor load on the remaining windings increases, but worse yet, the severe current imbalance sets up what are called "negative sequence current" in the rotor that create counter rotating magnetic fields which fight against the desired directional rotation. So your motor must work harder, i.e. pull more current and create more heat, do accomplish the same amount of output power. The heating effect is greater than the normal heating effect of current flow. At full load on the motor, this will cause the overloads to trip it off line. But one danger is that if the load on the motor is NOT full, and the current drawn is at or below the motor overload relay pickup threshold, the motor continues to run WITHOUT tripping the OL relay and the extra heating effect damages the insulation. That is why some types of overload relays will trip EARLIER on a single phase condition, the loss of a phase biases the OL trip threshold down to compensate for the heating effects of a current imbalance. If you then severely over size the motor for the load, i.e. your 5HP motor for a 2HP load, the current is still unbalanced and heating up the motor disproportionately, but it is causing no damage because the motor is over sized and can dissipate it.

The secondary issues are;

1) That you must now magnetize a larger set of windings to get a smaller amount of work out of the motor, so your line-to-load efficiency goes to hell in a hand basket.

2) That a 3 phase motor is not self starting with a single phase supply. So it's not just a matter of trying to balance the phases with capacitors, it's a matter of creating that pseudo rotation effect in the correct direction as well. And capacitors can balance the VOLTAGE between the phases, but not the current, and it is the CURRENT imbalance that causes the negative sequence currents to take place.

If you master all of that and you can live with the inefficiency, then you can do it. I don't know why you would want to, but it can be done.

"Will work for (the memory of) salami"

 

[fastline12]

Due to the madness of trying to keep this old saw going, we are actually looking at rewinding the little motor since it is proprietary. In looking at it, it might also just be possible to weasel the start winding out and replace it without touching the main windings but I know for sure a parts of the start winding bonded to a main. However, if I can find and isolate that, we might patch it up for a go... What a waste of time though.


Re the 3ph/1ph motor, wouldn't the negative currents actually be caused by the voltage sequence though in which the 3ph coil windings are not in the correct clocking to get the power factor anywhere close? IIRC, a 1ph motor has windings dance around the rotor exactly 180* parts and 3ph motors are triangulated or every 120*?

I thought the idea in mind with adding capacitors was to aid the power factor thus tuning the current? I know in once instance with an RPC, I had a driven 3ph motor run hot and trip overloads. I determined 1 leg was drawing too much but voltages were out of whack as well. As I tuned with caps to balance voltage, current came back in line as well and running perfect to this day. That is a hydro pump motor though that is already on the ragged edge by design.

 

[waross]

General Purpose Motor, 3-Phase, 5 HP, Nameplate RPM 1750, Voltage 208-230/460, 182T/184T Frame, Open Dripproof, Number of Speeds 1, Full Load Amps 15.0-13.6/6.8 $517.00 Plus Capacitors!!!

Prices from Grainger.
General Purpose Motor, 3-Phase, 2 HP, Nameplate RPM 1745, Voltage 208-230/460, 143-5T/56HZ Frame, Open Dripproof, Number of Speeds 1, Full Load Amps 6.0-5.8/2.9 $454.25 Plus a Drive.



AC Drive,Var Freq,2 HP,6.9A,230V,1Ph
Item # 6DWZ0
AC Drive, Variable Frequency, Max. HP 2, Max. Output Amps 6.9, 230 Voltage, 200-240 Voltage Output, Input Phase AC 1, Output Phase AC 3
$512.00



General Purpose Motor, Capacitor-Start, 2 HP, Nameplate RPM 1725, Voltage 115/208-230, 56HZ Frame, Open Dripproof, Number of Speeds 1, Full Load Amps 20.2/10.7-10.1

$406.00
Why not use a single phase, 2HP motor. It looks like the cheapest option.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Or go shopping on ebay. I picked up a brand new 5.5kW GEC motor for under £100 shipped, and an A-B 1336 Force 15kW drive for £46. Depends if you're prepared to be patient!

 

[dpc]

I think an adjustable frequency drive would make much more sense than rewinding the motor. Many hobbyist woodworkers and metalworkers are using drives to allow them to run the 3-phase motors in their "old iron".

 

[fastline12]

Again, I would highly prefer to swap out the motor BUT this motor is SPECIAL. The shape all the way down to the iron core is special to fit in the space. Highly ridiculous if you ask me but I guess a good time to do some learning about rewinding.


Scotty, you gotta tell me how you got such a smoking deal on that drive?! What voltage? I find high voltage drives in that size commonly but seems drives in the 208-240V are hard to find of any size. I shopped for months for low voltage 15KWs for a fan application and had to buy new.

 

[ScottyUK]

It was a 208V drive, which is a real oddity in the UK because our standard supply is 400/230V, so it probably came off an imported machine. The guy who was selling it listed it as a power supply. I almost felt guilty, but he was happy and so was I. It runs quite happily with 230V single phase input and a couple of big capacitors strung across the DC bus.

 

[jraef]

Never heard of a "208" only drive. Typically they come in three voltage classes; 300V class that covers 200-254V distribution systems, 400V class that covers 380-480V distribution systems, and 690V class that covers 575-690V systems (although there are a lot of 600V only versions, not 690). The issue is, drives cannot create voltage that isn't there (forget voltage doublers for the sake of argument), so people often REFER to a drive as being "rated" for the voltage rating of the MOTOR they use it on, not knowing that it really can accept a much wider INPUT voltage, as long as it is AT LEAST the voltage of the motor. So ScottyUK likely snagged a great bargain because his seller ASSUMED the drive only worked on 208V. The caps were probably just because it was single phase instead of 3 phase, right Scotty?

Fastline,
Many, if not most, 230V drives 3HP and under are now designed to accept 1 phase or 3 phase input and provide a 3 phase output without de-rating the drive. The reason is, modern power device designs are so much smaller and cheaper now, that over sizing the input rectifier devices doesn't really cost anything at that size and below, so the only difference is in the amount of DC bus capacitance to smooth the extra ripple caused by a single phase inpu, and again at that size, the cost difference is negligible. You can likely get a brand new 2HP 230V VFD that will take 1 phase input for $200 or less (I just saw one for $185). You will never be able to remove, redesign / rewind your 3 phase motor for single phase and re-install it any cheaper than that.



"Will work for (the memory of) salami"

 

[fastline12]

I very much agree and always value your input Jraef! I did not realize the smaller drives could do this but it makes sense. The only components that matter are the input rectifier and smoothing caps. I have heard of people just modding a 3PH drive for a little more to get max output rating.

That brings me to another question though, as Scotty has done, what about using oil caps between L1-3, L2-3 to move some of the load current to the 3rd leg for a single phase application with a bigger drive? Technically you "should" be able to simply monitor the input amperage compared to rated to ensure one will survive. I am not sure you could get MAX rated drive capacity but 90% would be nice.

Thoughts?

 

[ScottyUK]

The caps were probably just because it was single phase instead of 3 phase, right Scotty?

Yeah, exactly: it has way to little bus capacitance for it to be happy on 240V 50Hz single phase. A couple of 3300uF low ESR caps were lying about so they got added - not much science to the sizing I'm afraid, just stuff from the parts bin! We Brits don't have an low voltage multi-phase supply equivalent to the US 208/120, so my drive was likely ignored by most folks who would have been looking for a 400V drive to suit our standard industrial supply voltage.

fastline,

I figured the input rectifier of a 15kW drive was big enough to handle a single phase source connected to two lines, considering that it was only driving a 5.5kW motor. The third line input to the rectifier is left open circuit. The extra bulk capacitance is across the DC link to smooth out the high-amplitude low-frequency ripple from the single phase input.

 

[electricpete]

That brings me to another question though, as Scotty has done, what about using oil caps between L1-3, L2-3 to move some of the load current to the 3rd leg for a single phase application with a bigger drive? Technically you "should" be able to simply monitor the input amperage compared to rated to ensure one will survive. I am not sure you could get MAX rated drive capacity but 90% would be nice.

Thoughts?

I'm not sure I understand what you're proposing, but you probably don't want to connect caps between phases output of vfd. They'll have very low impedance to the higher harmonic content of voltage.


=====================================
(2B)+(2B)' ?

 

[jraef]

I think he meant on the input side, to try to balance the current draw out across the three sets of diodes on the bridge.

I think that by the time you get big enough caps to make a difference, you could have just bought a larger VFD. The simple rule is that for 5HP and up (or single phase input above 240V), double the VFD amps compared to motor full load amps. That over sizes the diodes PLUS provides more DC bus capacitance. Below 5HP at 240V though, it's generally unnecessary.

But as always, YMMV so RTFM.

"Will work for (the memory of) salami"

 

[Compositepro]

In reading this discussion it seems tome that there is a little confusion in some of the posts regarding the difference between using a rotary phase converter versus a static phase converter. A static phase converter contains only capacitors and a few other components to create the third phase in conjunction with the 3-phase drive motor. This method requires derating of the drive motor. A rotary phase converter contains a motor with capacitors that is used for generating the third phase. A 10 hp motor might be used in a RPC to drive a 5 hp load. The load motor itself does not need derating using a RPC. And an RPC can be used to power multiple motors within its capacity.