Motor Capacitor Question.

[volvoguy87]

I just gave a friend of mine a very large (and old) belt-drive squirrel cage blower to use in his shop. I don't know its origins, but I suspect it was once part of a furnace. The blower runs beautifully. It was stored for at least 20 years before I got it, but the bearings are free and I put some zoom spout in the motor's oil ports. This monster moves some SERIOUS air! When I first turned it on, I didn't know there were the remains of an old birds' nest inside. That thing shot out of the blower like a cannon and flew across the room.

The motor is a big thing that is rated as follows:

1725 RPM
115 Volts
1/2 Horsepower
8.1 Amps

The wiring is simple. There is a hot terminal, a neutral terminal, and a grounding screw.

The amperage draw has me a bit concerned. I've read that adding a capacitor can reduce the draw, but I don't know how realistic this is. I'd love to lower the amperage draw if possible, if for no other reason than saving my pal some money on his electric bill. Is it possible to lower the amperage draw by adding a capacitor? If so, how would it need to be wired in and what would the specs on the capacitor need to be?

What a handy piece of equipment to have,
Dave

 

[cfz2882]

adding run capacitor

split phase motors that have a run capacitor have that capacitor connected across
the start switch(and start capacitor if equipped)so that some current continues
to go to the start winding through the run capacitor after the motor is up to
speed and the centrifugal switch has opened.You could add a run capacitor to a
motor that did not originally have one,but i am not sure what the effect would be
as i have not tried this-chances are good i would reduce the partial load current
some...On a 1/2 hp motor with a run capacitor the capacitor is usually around 15-
20 uf/370v"oil"capacitor.If the motor on the blower is an older one-say mid 60s or
earlier vintage,those older motors will often draw quite a bit less than full-load
current when the motor is loaded to less than it's rated HP...
If i notice a splitphase motor handy(i have dozens of them)i'll add a capacitor
to the motor and see what the effect is!

 

[arbilab]

Efficiency wasn't much of an issue "back then" and many 1/4-1/2HP motors weren't designed for capacitors. I know of no way to add a capacitor to a motor that wasn't designed for one.

At 8A and 120V you're at 1KW (rounded). Which ideally should produce 1.3HP. So yeah it's inefficient. Whether you can retro-engineer it, I can't say. Those motors only had 2 windings and a centrifugal switch that turned off the start winding. A capacitor across that switch might improve efficiency. If you can demonstrate that to be true, do let us know.

 

[cfz2882]

tried it

had a 1 hp split phase motor in for repair,and since the start switch terminals
were handy,hooked a 15uf capacitor that was handy across the switch and ran the
motor to see what effect the capacitor had:no-load current(at 115v)dropped from
9.4 amps to 8 amps-capacitor dropped the current draw by 1.4 amp!
Next i will try this trick on "daily driver"washer #3-an'81 filter flo to see if i
can reduce the current of it's 1/3hp motor a little-very,very easy on this washer
as filter flo uses a relay start split phase and the relay is very easy to get at
to connect a capacitor-i'll try several different microfarad ratings to see what
works the best with the filter flo's motor.

 

[arbilab]

Fascinating! Even my latest window/box fan (extremely cheap) has a capacitor. None of the older ones did.

 

[supremewhirlpol]

Now, when yall are finished with playing with the capacitors, be sure to discharge them!! I think you all are missing the point why the current is reduced. When you have a load that the majority is inductive, adding capacitor in parallel with the load reduces the reactive power that the system consumes. The capacitor in parallel with the inductive load on an A.C. circuit creates a 90 degree phase shift. The charge stored in the capacitor is really reactive power. That being said, the current becomes lagging, power factor is improved and overall current draw of the system is reduced.

 

[arbilab]

That's 'why'. What we didn't know was if a capacitor would improve efficiency on a motor not originally equipped with one. Also consider that the winding being connected to the capacitor is by design, not used in 'run' mode at all.

 

[volsboy1]

Be Very careful when your messing around with Caps.Years ago a guy I knew died from messing with a old Microwave  and I told him and told him to be careful when messing around with gadgets I don't think he took me serious when I told him..Induction motor's all have a huge current in rush when first turned on and these really large ones they have motors to start the motors because of the in rush and stator flex it causes.

 

[tolivac]

Caps and motors---years ago in industrial power systems-equipment manufacturers deliborately oversized motors on equipment.Say if the machine needed a 5Hp motor-a 10 or even 15Hp was used instead-the thought was the machine could handle a larger load if required.-but for normal load-say for the 5Hp motor-the larger ones are UNDERLOADED-thus HIGH reactive power factor.So--"power factor" correction caps would be installed on the 3 ph line feeding each motor.then PF corrected.Nowadays-manufacturers of equipment use--say the 5Hp motor if the device calls for it.Now the 5Hp motor is properly loaded-say to the nameplate current rating-then the motor presents a less reactive load to its power supply-caps not required.For household single phase motors may work-if the motor is fully loaded-a clamp on ampmeter can tell you-then a cap wouldn't do any more good.At the tool repair place I did work on a masonry saw that had a 2Hp 120V motor.It was cap start-cap run.and it could run on a 15a 120V circuit.normally such a motor would require 20A circuit.The "run" cap in it failed-VERY rare for this to happen.The machine would start-but you tried to cut a brick with it-trips the breaker.when the run cap was replaced-no problem could slice bricks all day with that thing.Tool Repair-most of the fun was TRYING the machine when you were done!Cap start-run motors are quite common-you find them on the brick saw as above-and air compressors that run on 120V-240V single phase lines.another place for such a motor-was VERY large 240V single phase motors for crop dryer blowers-10-25Hp.And for oil companiewes thaey have used 240V-480V single phase motors on the "bobbing donkey" horsehead oil pumps-up to 60hp.Caps for AC srive-power factor correction usually have built in discharge resistors-as required by electric codes.But Nonetheless--be careful- large oil caps can store a charge that CAN KILL YOU!!Discharge them with a ground stick or VERY large insulated screwdriver---AFTER letting them set for several min in the circuit before touching them.I have gotten many a large SNAP from caps here at the transmitter.