PM66 3hp 1ph powered by gas generator?

[TDKPE]

Baldor spec panel on motor (Code: H)

That translates to 82-92A at start-up, assuming it has full voltage at the motor leads while drawing that current.  Common ammeters, especially digital ones without peak-reading capability, don't display fast enough to see this, though an analog meter may get close since you can watch the needle move, but it's there.  See Roly's post above.

If the voltage sags at the leads, the current will be lower.  Normally, induction motors draw more current under load as the voltage sags (constant power), but because while starting it's basically just a giant electromagnet or resistor, the current is reduced with saggy voltage until it gets up to speed.  

But since it is just a big resistor drawing huge current, it will pull the voltage way down on a generator that can't deliver the current and hold the voltage ('starting' amps or watts rating) at the same time, which isn't a problem if 1) the motor control doesn't drop out due to low voltage, and 2) the starting torque isn't so low that it can't get up to speed.*  

Inverter generators seem to be quicker to trip out on overload than non-inverter types.  This discussion topic rages regularly on RV forums when talking about soft starters for roof air conditioners and attempts to run them on little portable generators like a 2000W Honda where the roof A/C unit draws something like 50A or so on startup but the generator is only good for about 20A, where it goes into 'overload mode' right away (hint: it can be done using a soft starter on the A/C unit, which holds the starting current to less than 20A).  Normal running current is around 14A, so well within the capability of the 2000W generator, if you can get it started.  


*The entire torque curve sags with reduction in voltage as the square  of the actual voltage to design voltage ratio, or (Vact/Vdes)^2.  If the voltage sagged to 80% of nameplate voltage, the torque curve would sag to 64% across the whole speed range, until it's up to speed where the current gets real small real fast.  Starting on a small generator, it could easily pull down to 50% voltage or less, resulting in a torque curve that droops to 25% of it's original design values across the board, and is not likely to even start.  And that's assuming the generator hasn't tripped out on overload.

[handi]

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.  

Not to argue, maybe there is a difference between household current and a generator, but a 30 amp 220v breaker has always been sufficient for both my 3 hp Unisaw and my 5 hp Woodmaster when I had it. 

I did have to switch to a slow-blow breaker for the 5 hp motor when I got it, but the Unisaw has always been on a 30 amp breaker, nothing close to 80-100 amps.

[Roly]

Not to argue, maybe there is a difference between household current and a generator, but a 30 amp 220v breaker has always been sufficient for both my 3 hp Unisaw and my 5 hp Woodmaster when I had it. 

I did have to switch to a slow-blow breaker for the 5 hp motor when I got it, but the Unisaw has always been on a 30 amp breaker, nothing close to 80-100 amps.

A typical 30 amp breaker will hold 100 amps for about 2 seconds before it trips which give the motor time to drop the starting current.  Tom explained about the time/temperature curve for protective device when it is in the thermal tripping mode, after current is very high as in a short circuit it trips magnetically which is very fast.
 Here is a link showing the TTC for Eaton load center breakers download the LINK for the type C 30a 2 pole breaker and you can see how it works.   Roly

[handi]

A typical 30 amp breaker will hold 100 amps for about 2 seconds before it trips which give the motor time to drop the starting current.  Tom explained about the time/temperature curve for protective device when it is in the thermal tripping mode, after current is very high as in a short circuit it trips magnetically which is very fast.
 Here is a link showing the TTC for Eaton load center breakers download the LINK for the type C 30a 2 pole breaker and you can see how it works.   

 
Still not believing that a 3hp motor pulls 80-100 amps on start up. My entire service out to the garage in Murfreesboro was only 60 amps and I could easily have my DC running when I turned on the saw.

[TDKPE]

 
Still not believing that a 3hp motor pulls 80-100 amps on start up. My entire service out to the garage in Murfreesboro was only 60 amps and I could easily have my DC running when I turned on the saw.

60A is the continuous current rating of the breaker protecting the feeder to your garage.  It will carry a small overload for hours, a larger overload for a shorter period of time, and a heavy overload for seconds, per the trip curves I showed above and Roly linked to.  

The feeder size has little to do with how much current can pass for the sake of discussion.  A puny 14 gauge copper branch circuit cable can carry thousands of amperes if it’s short, which is why those little breakers have at least a 10,000A short-circuit rating (they don’t explode or vaporize when opening due to a short or ground fault).  

A set of 8-gauge or larger copper conductors to your shop panel will pass essentially unlimited current (for the sake of discussion), so they’re not going to limit how much current a motor or two can pull on startup, if that’s what you’re thinking.  Nor are the breakers going to trip on a normal startup since they’re designed to handle very large overloads for short periods of time.  The only thing limiting current in any practical sense is the impedance of the motor windings and the magnetic funny business that’s going on inside during startup (back EMF, which is what controls motor current once it’s up to speed).

[handi]

60A is the continuous current rating of the breaker protecting the feeder to your garage.  It will carry a small overload for hours, a larger overload for a shorter period of time, and a heavy overload for seconds, per the trip curves I showed above and Roly linked to.  

The feeder size has little to do with how much current can pass for the sake of discussion.  A puny 14 gauge copper branch circuit cable can carry thousands of amperes if it’s short, which is why those little breakers have at least a 10,000A short-circuit rating (they don’t explode or vaporize when opening due to a short or ground fault).  

A set of 8-gauge or larger copper conductors to your shop panel will pass essentially unlimited current (for the sake of discussion), so they’re not going to limit how much current a motor or two can pull on startup, if that’s what you’re thinking.  Nor are the breakers going to trip on a normal startup since they’re designed to handle very large overloads for short periods of time.  The only thing limiting current in any practical sense is the impedance of the motor windings and the magnetic funny business that’s going on inside during startup (back EMF, which is what controls motor current once it’s up to speed).

I am not an electrician, I’ll take your word for it.

But I am tempted to drag out my Generator and try my Unisaw on it just to see.

[TDKPE]

I am not an electrician, I’ll take your word for it.

But I am tempted to drag out my Generator and try my Unisaw on it just to see.

Give it a shot.  Just back off if the magnetic control chatters so you don’t break it.

It’s possible to start even a large motor on a generator, IF the voltage doesn’t get pulled down too far to where the motor can’t develop enough torque to start, and IF the generator’s circuit breaker doesn’t open, and IF, in the case of an inverter generator, it doesn’t go into overload or fault mode, and IF the voltage is high enough to maintain the motor control engaged.  

That’s what soft-starters are for, by the way, and we use them on induction motors in the 250-1000 hp range that have to run off a generator, since you’d need a monster generator to start them otherwise, and that brings all sorts of problems with it besides big rental dollars and excessive fuel use due to needing a load bank to keep it working hard enough to keep the emissions systems hot.  

I just did a quick check, and a 50 ft length of 6-gauge cable can pass about 4800A at 240V, while an 8-gauge cable of the same length will pass about 3200A, assuming a dead short and no circuit protection.  

The insulation will burn off in no time, and it’ll glow like a light bulb eventually, but that’s what the math says for the published DC resistance for those common sizes.  But it should give you an idea of how the current from even a small panel is virtually unlimited as far as how big a start-up surge can pass, at the detriment of voltage of course.

Edit: Oh, and check Roly’s post up the page a bit, where he measured the inrush with a fast ammeter with peak hold.  His measured result is consistent with common 3 hp motors.

Tom I have meter that reads inrush current.  Just went downstairs to check it.  On my 3hp Grizzly 1023 saw on 240 volt the inrush was 91 amps with just the blade and belts for a load.

[Don_M]

Thanks man! Super helpful!

My understanding is that it could be a bit more complicated. When starting a motor from a generator, the current surge causes a voltage dip. When voltage drops, the consumed current is reduced proportionally (from what I have read - somewhere around 30%). Can’t say I completely understand the physics – I think it has something to do with a motor powering another motor.

By example, I have a 10kW generator and wanted to see it would run my 3-ton A/C. The A/C condenser has an LRA of around 80 amps and so at first blush – it would take 19.2kW at start-up. But, apparently what actually occurs due to this voltage drop (and proportional consumed current drop) is the required start-up power is reduce to around 9.4kW (80*.7)*(240*.7). Just under my 10kW generator capacity. So, I gave it a try and it WORKED. Admittedly,  I can’t have much else operating when the A/C kicks in and is running off the generator – but I am good with that if it keeps me cool, Also, make sure to con sider your FAU fan – which would be needed to actually blow cool air – but it doesn’t take much power.

Just my $0.02