PM66 3hp 1ph powered by gas generator?

[GalenSinkey]

I want to move my PM66 out to a shop that doesnt yet have power.  Is there any reason why I wouldn't be able to power it with a 240v generator? Anyone have any recommendations on such a generator?

[TDKPE]

A 3 hp motor is going to draw on the order of 80-100A at 240V to start, which is 19-24kVA.  If the generator can’t deliver that while maintaining adequate voltage for the magnetic control to not drop out then it won’t start.  Running isn’t the problem; it’s starting that’s the problem.  

My 4kVA running, 6kVA starting Generac portable can’t start my Unisaw (not even remotely close), and it has a lower running current at 12.4A than most 3 hp motors like the Baldor in the PM66.  

If you want to know what the starting current is on your motor, write down the “Code” or “kVA Code” or just “kVA” letter on the motor’s nameplate, look up the multiplier associated with that Code letter (either the range, or the mid-value), and multiply by the motor’s rated hp to get kVA while starting.  

Divide that kVA number by 230V (the motor’s nameplate design voltage) to get max amperage while starting.  Also know that while the current drawn while starting tails off as the motor picks up speed, it generally only tails off slowly (current remains high) for most of the starting event, and drops off fast as it nears design speed.  So it’s not just a blip - that current stays very high for most of the start, which admittedly is fast, but it won’t be so fast on a generator that can’t hold it’s voltage with such a big draw, and that’s assuming the magnetic control doesn’t chatter like a clumsy electro-mechanical door bell mechanism.  

https://www.engineeringtoolbox.com/locke...d_917.html

Oh, and welcome back, GalenS.  

[GalenS]

A 3 hp motor is going to draw on the order of 80-100A at 240V to start, which is 19-24kVA.  If the generator can’t deliver that while maintaining adequate voltage for the magnetic control to not drop out then it won’t start.  Running isn’t the problem; it’s starting that’s the problem.  

My 4kVA running, 6kVA starting Generac portable can’t start my Unisaw (not even remotely close), and it has a lower running current at 12.4A than most 3 hp motors like the Baldor in the PM66.  

If you want to know what the starting current is on your motor, write down the “Code” or “kVA Code” or just “kVA” letter on the motor’s nameplate, look up the multiplier associated with that Code letter (either the range, or the mid-value), and multiply by the motor’s rated hp to get kVA while starting.  

Divide that kVA number by 230V (the motor’s nameplate design voltage) to get max amperage while starting.  Also know that while the current drawn while starting tails off as the motor picks up speed, it generally only tails off slowly (current remains high) for most of the starting event, and drops off fast as it nears design speed.  So it’s not just a blip - that current stays very high for most of the start, which admittedly is fast, but it won’t be so fast on a generator that can’t hold it’s voltage with such a big draw, and that’s assuming the magnetic control doesn’t chatter like a clumsy electro-mechanical door bell mechanism.  

https://www.engineeringtoolbox.com/locke...d_917.html

Oh, and welcome back, GalenS.  

Thanks man! Super helpful!

[TDKPE]

As a workaround, you might  be able to utilize a single-phase soft starter.  $350, and out of stock at the moment, but probably doable.  

https://www.microair.net/collections/eas...0231884884

Another alternative would be to find a lower hp 3-phase 3600 rpm motor and run it off a VFD, which will allow ramped starting and very low start current.  I don't know for fact, but I believe the PM66 uses a standard NEMA C-face motor (145TC perhaps?), so a used motor should be easy enough to find.  The magnetic motor control would not be used in that case.

In the end, though, it might be cheaper to just run temporary power, on poles if you have to, assuming that's even available.

[jteneyck]

Not to hijack this thread, but your responses are such a wealth of info., TDKPE, that I want to ask a couple of somewhat related questions. I have a 17 KW Generac with whole house transfer switch.  It hasn't been needed much since I had it installed but I worry far less now when I'm away from home.  The power has gone out a few times when I've been down in my shop working so I continued working after the generator came on line.  I noticed my old 1.5 hp Dewalt RAS didn't sound the same as normal.  Is the wave form from a generator different than grid power and, if so, could that be the reason the motor sounded different?  If not, what might have been going on?  

Second question.  The wires coming from my 5 hp bandsaw look tiny, like maybe 14 gage.  How is it they are able to carry the amps necessary to start or even run the saw?  

We are lucky to have folks with such a depth of knowledge here, and willing to share it.  Thanks.   

John

[TDKPE]

I noticed my old 1.5 hp Dewalt RAS didn't sound the same as normal.  Is the wave form from a generator different than grid power and, if so, could that be the reason the motor sounded different?  If not, what might have been going on?  Frequency might be off, and/or dirty power if the generator has brushes.  Voltage while starting will certainly be lower, causing extended start times, as the output impedance is much higher for a small generator than it is for a utility service.  You can easily get thousands if not more amperes during a short or ground fault with the utility while the generator can only output so much and that's it, which is why even small CB's are 10kA or 22kA short-circuit rated.

Second question.  The wires coming from my 5 hp bandsaw look tiny, like maybe 14 gage.  How is it they are able to carry the amps necessary to start or even run the saw?  The saw is probably UL or CSA listed, and the motor would most likely be UR (UL Recognized component that's intended to be part of a UL listed assembly) or something similar.  As such, the conductors are not sized out of the NEC Table 310.16.  But even if they were, 14-gauge copper is good to 25A with 90°C insulation per that table, and there are conductors with higher temperature insulation that are shown at 54A with 200°C insulation for 14-gauge copper.  The point being that it's the insulation and termination temperature ratings that govern ampacity, and with utilization equipment that's UL listed, the leads and internal wiring don't necessarily conform to the common tables in the NEC, nor would they have to.  The internal wiring and leads in an electric oven, for instance, are usually pretty puny considering they're connected to a 40-50A circuit, and are typically a noticeably different type of insulation.

Wire gauge and conductor chemistry (Cu vs Al, for instance) will, of course, have an effect on voltage drop over length, but in a motor or appliance, that's a short distance, so they can use smaller wire.  And even with that puny wire, you can still pass thousands of amperes in a direct short-circuit or ground-fault as long as it's a short enough run.  Which is why even little CBs have such high short-circuit interrupt rating (see above).  

[jteneyck]

Thanks very much TDKPE.   

John

[Stwood_]

My 6500W gen will not start my 3hp 1023 Grizzly saw.

[lincmercguy]

Kind of apples and oranges, but I was able to power my 2hp 240v dust collector and my 1.5hp 120v table saw with a Champion dual fuel generator. I forget the wattage. But I also wasn't starting the full load all at once. The dust collector did draw it down quite a bit on startup.

[fixtureman]

A 3 hp motor is going to draw on the order of 80-100A at 240V to start, which is 19-24kVA.  If the generator can’t deliver that while maintaining adequate voltage for the magnetic control to not drop out then it won’t start.  Running isn’t the problem; it’s starting that’s the problem.  

My 4kVA running, 6kVA starting Generac portable can’t start my Unisaw (not even remotely close), and it has a lower running current at 12.4A than most 3 hp motors like the Baldor in the PM66.  

If you want to know what the starting current is on your motor, write down the “Code” or “kVA Code” or just “kVA” letter on the motor’s nameplate, look up the multiplier associated with that Code letter (either the range, or the mid-value), and multiply by the motor’s rated hp to get kVA while starting.  

Divide that kVA number by 230V (the motor’s nameplate design voltage) to get max amperage while starting.  Also know that while the current drawn while starting tails off as the motor picks up speed, it generally only tails off slowly (current remains high) for most of the starting event, and drops off fast as it nears design speed.  So it’s not just a blip - that current stays very high for most of the start, which admittedly is fast, but it won’t be so fast on a generator that can’t hold it’s voltage with such a big draw, and that’s assuming the magnetic control doesn’t chatter like a clumsy electro-mechanical door bell mechanism.  

https://www.engineeringtoolbox.com/locke...d_917.html

Oh, and welcome back, GalenS.  

Why would it draw that many Amps.  My 5 HP compressor draws less then 30 amps. I only have a 60 amp service in my shop.