Three Speed Motor Won't Self Start / New Capacitor

[No Poke]

I am re-purposing an A.O. Smith Mod. F48SNS4L6 Ser. 7193 three speed motor from a squirrel cage fan assembly into a small carpenter's jointer. The motor is of unknown history. It's a 115V 1 ph. three speed motor Type UF 6 (MAX 10) amps 1625 rpm motor with a rated cap. of 15mf 370v. The motor has a neutral wire, and one for each of the three speeds. There are two wires leading to an umounted capacitor and a ground wire. Then there are a black and white leading to a two prong reversible plug for reversing the motor direction.

When I first got the motor, it had two controllers on the fan housing. I think one was a speed controller, the other perhaps a humidistat. They are gone now. When I bench run the motor, it spools up just fine. It runs at 1,800 rpm [by my old Smith's mechanical gauge] on the slow wire. Medium wire, same thing. Fast wire, same thing. 1,800 rpm. I thought there might be a short in the fields but then I read somewhere online that a three speed motor will always turn at it's fastest rated speed until you put a load on it. It is now wired to high speed and when I install it and switch it on, it simply hums, trying to turn over. Give it a push and away it goes. Lots of torque, speed is fine. It requires this nudge every time. I replaced the capacitor with a correctly rated, brand new one and it's the same thing. If I lift the motor to take the pressure off the pulley, it starts fine and lowering it on to the pulley, it will run the machine but it simply won't self start.

I can't imagine this little 4" planer with a 3" diameter cutter head offers any more resistive torque than the large 14" squirrel cage fan did so I don't think it's a resistance issue.

I removed the cap just to see what happens and it starts fine when unloaded, won't start when loaded. Incidentally, with the cap removed, sometimes it starts in one direction, sometimes the other. With the cap in place, it's always the same direction.

So, I can't think of anything I've missed.

Thanks

 

[dArsonval]

The motor being a Permanent Split Capacitor motor... it is pretty much good for driving a fan blade mounted directly to the shaft and that's about it.
It does not have starting torque to power a pulley/belt application etc.

If the motor served a previous life as a fan motor, the sleeve bearing(s) could possibly be wore out enough that during energizing of the device,
the rotor is pulled slightly off center and just sits and "hums".
One can check for this by holding the body of the motor fast, and strong arm the shaft side-to-side noting play or movement.

Up and down movement.
Not referring to end play... back-and-forth travel.

Any sign of looseness indicates the bearing(s) have lost their running tolerance and the device
is simply not going to work as you have discovered.

John

 

[waross]

This motor is a design compromise.
Normally a squirrel cage motor is designed for minimum slip and good efficiency.
On a small three speed motor efficiency is sacrificed for extremely large slip.
The normal slip for an induction motor is 40 to 60 RPM
A rated full load torque of 1625 RPM is a slip of 375 RPM
To allow the high slip, and even higher slip on lower speeds the motor is designed with relatively high resistance and impedance.
Normally, the motor back EMF limits the current.
In this design, the higher resistance and impedance limits the current.
I expect that the efficiency will be from 50% to 60%.
This is a two edged sword:
Low starting current and very low starting torque.
I imagine that your drive ratio is much higher with the planer compared to the fan.
The motor is unsuited to this service.
You will not be happy with the results driving a planer.

NO GUARANTEE
USE AT YOUR OWN RISK
About 20 years ago, I installed a pair of gate openers for very heavy rolling garage doors.
3/4 HP openers were ordered.
1/2 HP openers were received.
The openers would not start the heavy doors moving, but once started they would move the doors.
In Central America you are often stuck with what you get.
I doubled the starting capacitors on the motors.
With double capacitors, the motors were able to start and open the doors.
I am still in touch with the owner.
Whenever he is contemplating some electrical work, I get a call for advice.
The openers are still working, after 20 years.
BUT
These motors had starting switches and the capacitors were only in the circuit for a few seconds during each start.
I have no idea what the long term effects of oversized capacitors on a permanent split capacitor motor.
Your call.
NO GUARANTEE
USE AT YOUR OWN RISK

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[No Poke]

John,
The pulley end bearing does allow some deflection. I can feel it. Very very little but, it is a bearing and I can feel it. There is about 1/8" of end play in the shaft; it's able to travel along it's axis 1/8".

Bill,
I'm curious, did you add the capacitor in series or parallel? Well, I can get the machine running and it planes just fine but no, I'm not happy with the setup as it is. The pulley ratio is about 2:1, large on motor, small on cutter to get the RPM's up to about 3,500.

OK, can anybody tell me whether or not a 3 speed motor runs at max. rated speed on all three settings until loaded? I would like to confirm that.

I also read that some of these motors have a dual purpose capacitor. It serves as a bit of a start cap. when starting and then stays in play as a bit of a current regulator while running. Is that what you mean by a split cap John?

Thanks.

 

[davidbeach]

Unloaded it will run at close to the same speed on all three settings. With the intended load in place, the lower two settings result in equilibrium between motor output torque and fan load torque at the two lower speeds. A different fan blade and you'd get different lower speeds. The motor must be in the air stream as it will rapidly overheat and burn out if not provided with adequate cooling. Presented with a load that doesn't match the fan speed/torque curve the motor will not have three speeds. Presented with a load that requires much above zero torque at near zero speed the motor will never accelerate the load to speed.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[No Poke]

The motor is in an open environment so no issues there but isn't in an air stream, no. I do have other motors but they have way more hp than I need in this little unit and I built the cabinet to house this small one. Still, I may end up having to redo the cabinet to use a more appropriate motor.

 

[waross]

In parallel.
What is the HP rating of the motor?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[No Poke]

1/2 hp. The pulleys are a 5" and a 2 1/2".

 

[waross]

At full load the efficiency will be around 30% and a lot of heat will be wasted.
What is wrong with this picture?
An inefficient, hot running motor in an area subject to flammable and explosive wood shavings and dust.
Keep an extinguisher handy.
Bu sure to post pictures of the fire.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[No Poke]

For what it's worth Bill, the top 6" of this cabinet has a floor in it and only a slot for the belt to go through. With a 4" 2hp dust collector evacuating it, nothing falls into the motor space. But I appreciate the heads-up.

 

[waross]

A furnace fan is kind of a special application.
The fan drawing over the motor provides a lot more cooling then a self cooled motor.
A furnace is supposed to produce heat. If the inefficient motor means that a little of that heat is produced by electricity instead of the chosen fuel, that is a small price to pay for the advantage of speed control.
A furnace fan motor is one of the very few applications where efficiency is relatively unimportant.
I suggest that you keep an eye on the temperature of the motor when it is working hard.
Large grain elevators and terminals have very powerful dust collection systems and are classified and wired as a hazardous area.
There have been several notable dust explosions in major grain terminals.
Large lumber mills have very powerful dust collection systems.
There have been several notable dust explosions in major lumber mills.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[No Poke]

What the heck, I'll take it out. Thanks for all the advice.