# Vacuum pump problems



## edog3000 (Mar 4, 2012)

I have a 10 hp 240v baldor motor running a maple sugaring vacuum pump. The motor is rated at 41 fla. We haved supplied power from a pto driven 25kw generator. the gen will run our lights and smaller 120v motors with no problem,but when we turn on the vacuum pump, it starts to cycle up and down and the voltage drops from a steady 240 to around 150-160 with amps spiking at 146! So we got 2 other portable generators because we were told that the transformer in the gen needed to be shimmed,but the portables are doing the same thing. I need suggestions like yesterday. Sap is flowing and we are down and out!


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## garfield (Jul 30, 2009)

Probably undersized but it sounds to me like calling an electrician would be money well spent.


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## MDShunk (Jan 7, 2007)

Sounds a lot like the genset is undersized to start your motor. Seems like it never gets started, and stays pretty much drawing starting current. That nearly 150 amps is what I would expect to start a 10hp single phase motor. 

Can you bump up the throttle on the PTO generator long enough to get the motor started?


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## Southeast Power (Jan 18, 2009)

Can you operate the pump without a load until it starts?


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## JRaef (Mar 23, 2009)

edog3000 said:


> I have a 10 hp 240v baldor motor running a maple sugaring vacuum pump. The motor is rated at 41 fla. We haved supplied power from a pto driven 25kw generator. the gen will run our lights and smaller 120v motors with no problem,but when we turn on the vacuum pump, it starts to cycle up and down and the voltage drops from a steady 240 to around 150-160 with amps spiking at 146! So we got 2 other portable generators because we were told that the transformer in the gen needed to be shimmed,but the portables are doing the same thing. I need suggestions like yesterday. Sap is flowing and we are down and out!


Several issues here.

In theory, it could have worked. A general "rule of thumb" on generator sizing is 2-1/2 to 3X motor HP as kW size, so your 25kW generator fits in the low end of that rule. I've seen that work, but it is on the ragged edge. But...

Generator sizing is one thing, capacity is another. SIZE has only to do with the windings of the generator (alternator) itself. Capacity has to do with the size AND the prime mover. If you use a packaged gen*set* then the prime mover is matched to the generator. But you are using a PTO driven generator, so the prime mover is something else. If you are taking the PTO from an engine that does not have 25kW (about 33HP) in RESERVE capacity, then the rating of the generator is compromised. Reserve capacity means over and above whatever else that engine needs to do, including running itself. So if this is a PTO on a tractor with a 50HP engine, it might be fine but if it is a 25HP engine, you effectively have only an 18kW generator, and that's too small.

3rd and most likely issue is the vacuum pump itself. If it's a positive displacement vacuum pump (i.e. piston, gear, lobe) as most of them are, then it is CRITICAL that the pump vacuum chamber must be vented to atmosphere before starting, otherwise you are going to have essentially a locked rotor. There is likely an "Unload Valve" in the vacuum pump itself somewhere, usually near the head. It would be powered by a solenoid that is wired to the main starter and a timer or sometimes directly to a pressure switch. If any part of that unloader system is not functioning correctly, you are never going to start that compressor. I would start there. If it's working correctly, it should be open to atmosphere when the vacuum pump is off-line and you should be able to turn the shaft relatively easily. Sometimes after a period of being shut down for a season after being put away without proper maintenance, like oiling, it gets stuck closed.



> ..we were told that the transformer in the gen needed to be shimmed,


 :laughing:
1) There is not likely any transformer in a small generator like this.
2) A "shimming" inductor is something that would have possibly been done in the initial transformer design (if there was one) to improve stability, but is not something that changes or "goes bad" and has to be re-done. Besides, this is something that is almost NEVER done in things like portable generators, it applies to power converters. The only other possibility is if he was referring to shimming the DC exciter portion of the generator, i.e. it is misaligned. But if that were the case, it's unlikely your generator would work at all. Whomever said this was a nincompoop.

When you look at these other issues and solve the problem, go back to that guy and tell him that you "realigned the retroencabulator". If he nods in agreement you know he's a poser.


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## SteveO. (Oct 17, 2011)

jrannis said:


> Can you operate the pump without a load until it starts?


^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This right here may band-aid your issue until you can get a properly sized generator, or get a soft start for your motor to reduce the inrush current. 

This is why with motor banks, they always suggest starting the biggest motors first.

Start the motor before you put any other load on the generator and then start adding other loads. I'd grab a small electronic soft start if the generator can handle all the loads once the motor is started and use that. You may need to increase your system capacity soon though.


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## micromind (Aug 11, 2007)

A 10HP motor will draw about 8KW when running, but about 40KW when starting. 

A 25KW generator is capable of producing 40KW for a short time, provided the prime mover (in this case, a tractor) can provide enough power.

In this case, you'd need about 50 PTO HP from the tractor. 

If the tractor cannot provide enough power, it'll lug down. This will result in lower voltage at the motor, making it less likely to bring the vacuum pump up to speed. 

If the vacuum pump can be vented during starting, so there's no load on it, there might be enough mechanical inertia in the tractor and generator to get it started. Once it's up to speed, the vent valve would be closed and the tractor and generator will accept the load of the vacuum pump.


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## erics37 (May 7, 2009)

If you get yer s**t going, can I have some maple syrup?


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## Wirenuting (Sep 12, 2010)

Some unloading valves are on the pressure/start switch. It's small and mechanically operated. 
Just takes a little oil blow by and some dirt for it to stick closed.


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