Motor Run Capacitor Size

I am assuming that these fans are of a plug in type. You shouldn't be tripping breakers on something like that. Do the fans work on your fire house power supply? If so you may want to check the breakers and make sure that they aren't some sort of fast trip variety. Also, you will want to check the generator to make sure it is supplying your full voltage at a load. Are you making sure if you are using both fans to have the fans on separate circuits? and also to wait for the first fan to get up to speed before you start the second. Either of these can also trip the breaker. Also, are there any other devices running off of the circuit that could be already placing a load on the circuit breaker? I'm not usually in favor of placing a band aide over the root problem. Also, a capacitor will not change the current required to start the motor.

Do they trip immediately on start up or later on?

Be sure that the breakers are rated as motor start. Not fast blow.

A 20 amp breaker is a little shaky for your application. Some thermal breakers are adjustable to compensate for large inrush starting currents. I don't know if you're generator is equipped with this type a breaker or could be retrofitted. Some generators allow you to adjust the output voltage if this one does increasing the voltage toward the upper end of the operating range of the equipment you have attached could help.

Many 1.5 hp motors are dual voltage motors 120 V or 208/240 V. if this is the case with your fan motors check and see if the 220 V outlet on your generator is proportionally larger (anything above 20 amps would be larger ,if both fans must be connected through the same to 220 breaker, even a 20 amp breaker should allow you to run both fans provided you start them one at a time). If so change the connections on the motors and run them on the to 220 outlet. The change I'm referring to is internal to the motor and will be listed on the wiring diagram on motor, if it is a dual voltage.

While I agree with a previous poster in that I wouldn't like jury rigging life safety equipment, there is one more possibility if none of the other suggestions work. While increasing the size of the start capacitor would not necessarily help, you might try placing a capacitor across the load. Sizing would just be a guess, I would probably start with about a 50 µ . This could help by correcting the power factor in storing additional energy on startup. Hard start kits are available for air conditioning compressors that may be adaptable for your application. These last suggestions I would consider a Band-Aid until the problem can be properly corrected by the purchase of a different generator. I generally favored cheap quick and easy solutions . Life safety is obviously a different issue. But certainly it would be better having the fans operational with a Band-Aid approach than not operating.

A caveat capacitors can store a dangerous charge. Capacitors have to be rated for voltage , whether they're AC or DC, electrolytic DC capacitors make a rather loud pop when connected to AC, any capacitor left in the circuit must be rated for continuous duty. A colloquial phrase comes to mind, nothing but love for you, volunteer firemen perform a crucial and at times dangerous service with little or no compensation, you're the last guys I want to see riding in the back of a ambulance.

Can the fan motor be replaced? If so, an inexpensive 3 phase motor may be installed and a single phase to three phase Variable Frequency Drive can convert the the generator power to three phase. Now you can run the Fan without a large starting inrush current. The VFD will allow a very soft start, and you could vary the fan speed for best effect when in use. VFDs also will run on poorer quality power and will self-protect by shutting down if power quality or load become too unstable or large. The three phase motor has no start windings, start switch, or start capacitor to fail, making the three phase motor cheaper, lighter, and more reliable than an equal hp single phase motor.

Hello sparkchaser: changing the motors would probably be prohibitively expensive and is not necessary . Your suggestion of the VFD was excellent, as I'm sure you know VFD prices have been falling, I've heard rumors that they will even be producing VFD's for fractional horsepower motors. I don't usually work with a small VFD,s so I did a quick google http://www.wolfautomation.com/ProductList.aspx?CategoryID=5103&gclid=CPSM0ZaLhZECFRUmawod2mwKGQ.

The first site I check listen a 1.5 hp 115 V single phase VFD, for about 150 bucks. I agree with you that this should cure their starting problems. I thought you might be interested in taking a look at this link, the small VFD world is moving much faster than I thought it would, I can certainly make use of smaller VFD's to control fans, they have been available for a while but the prices dropped considerably.

Great link for the VFDs

A 1.5 HP motor 56C motor is pretty cheap compared to single phase motor of equal hp and frame. These guys have one at a real bargain of a price, $60. Might cost more than that just to ship it. I just purchased a new 1.5hp 56C 3ph motor for $160 just last month.

http://www.surpluscenter.com/item.asp?UID=2008012011212936&item=10-2002&catname=electric

I know it is a 2hp motor, but the frame and RPM's are correct so it should fit fine. A VFD will only send the power needed for the work the motor is doing. The fan is designed for 1.5hp at 3600 rpm, so that is all the energy a 2hp VFD will supply to the motor. HVAC guys use VFDs to control air flow by slowing and speeding up fans instead of opening and closing air louvers, saving a huge amount of energy. The application is very similar. I have a VFD that will work, if anyone has a 56C 1.5 hp 3600 RPM 3ph motor to loan for testing. We can do the testing in Albany New York at the NY State Bureau of Fire Prevention and Control. I can arrange a time...

I think those "single phase" VFD will take single phase input but output 3 phase and must drive a 3 phase motor.

Quite correct Groundrodman. The motor would need to be changed to a 3ph unit to use the advantages a VFD offers.

It sounds to me like the fans are tripping on start-up.If so, you can change the breakers to HACR type breaker.These breakers are designed for Air Conditioners/Heat pumps because of the heavy current draw at start up.

If the motors are tripping while running, make sure you do not have excessively long drop cords run to the fans.A # 12 guage is minimum up to 25 feet.#10 guage should be good up to 100 feet.

When the cord size it too small, the motor receives a reduced voltage.This causes the rotor to lag the rotating field excessively.This is equivilent to increased load from the motor perpective, and will result in HIGHER current draw, and excessive heating of the motor.As current increases, so does the heat in the conductors, which increases resistance, which strangely enough, causes the motor to draw more current, and eventually the breaker trips.

If your fans have variable sheaves, to adjust speed, set these for the smallest diameter on the motor, and the largest diameter on the fan, then check the amp draw of the motor.Adjust speed up till you reach nameplate amp rating of motor.

Some of the sheaves have a setcrew and a threaded pulley.Simply loosen set crew,turn sheave to change diameter, and retighten setscrew.

Not sure if this is the type you have or not, so this sheave info may not apply.

Hope this helps

HTRN

Another thought just occured to me: If the inlet is accessable, you can try blocking the inlet to the fan before starting.This will actually reduce starting load because the blades are turning in a partial vacuum.This is like taking a boat prop out of the water: less load.Some people will argue that the amps will increase if you block the inlet, but try it, you will see.Seeing is believing.

Try this at home:Block the inlet to a vaccum cleaner.The motor speeds up, the amps go down.

Hope this helps

HTRN

You can also block the outlet and reduce the load! We do it all the time on our blowers. We run blowers from 5 horse -power to 75 horse-power. We use these blowers to provide combustion air.

We shut the outlet gate and go by an ampmeter to adjust the air.