Fixing a Harbor Freight motor

[snowluck2345]

I have a harborfreight 8x12 lathe and the lathe has been acting up. I've blown 2 startup capacitors in the last month, both times when the motor was geared down. I felt this was odd and I have a theory on whats going on. The increased friction slowed the motor down enough to prevent the centripetal switch from tripping therefore leaving the capacitor in the circuit.

Is this a reasonable explanation? If so, how would I fix it? Just add more weight on tho the switch or weaken the spring?

When the lathe is running when geared down there is voltage across the startup capacitor at all times.

 

[Tony Wells]

Unless I am misunderstanding, gearing the motor down would increase torque available at the spindle and decrease the likelihood of the actual motor speed dropping to the point that the switch brings the start capacitor back into the circuit. That is, unless you are loading the machine to the point where even in low gear the motor is unable to maintain speed. I'd think that if it were slowing the motor down that much, you would sense it and back down on the drill pressure, or feed rate....whatever is loading the motor.

However, if the motor has some sort of problem that is exhibited by unusually low power, your theory could well be right. If you have an Amprobe (or equivalent), try to monitor the current draw when using the machine under the same conditions where the capacitor fails. It should not exceed plate ratings in any case, but could give clues to the root cause.

I wouldn't recommend altering the mechanism to try and compensate for this. The switch is just trying to do its job. If you change the springs, for example, you change the timing on when the start winding drops out and probably before the run windings are capable of supporting full motor speed, or getting to that speed in a reasonable time, or perhaps even at all. It's conceivable that you could (in an extreme case) put the motor in an endless cycle of start windings in.....run windings unable to bring it to full speed, motor slowing until start windings cone back in, repeat.

 

[Codered741]

Centrifugal switches are generally set to disengage at about 1/2 of the motors rated speed. If you listen closely, you should be able to hear the switch click when the motor starts. You would have to put quite a load on the motor, especially in low gear, to make the centrifugal switch re-engage while running.

One thing to check, make sure that the switch is not stuck, and not disengaging. I have had debris keep a centrifugal switch from opening before.

-Cody

 

[flutedchamber]

Centrifugal switches are generally set to disengage at about 1/2 of the motors rated speed. If you listen closely, you should be able to hear the switch click when the motor starts. You would have to put quite a load on the motor, especially in low gear, to make the centrifugal switch re-engage while running.

One thing to check, make sure that the switch is not stuck, and not disengaging. I have had debris keep a centrifugal switch from opening before.

-Cody

I had a similar problem with a Grizzly drill press motor. I bought their G7948 drill press at their sale and never opened the box and assembled it for a year. The press ran fine for about a minute, then the centrifugal switch would switch on and off with no load on the motor other than spinning the spindle. There was one spot on the armature that was dead. If the motor stopped there, it wouldn't start, it would just hum.

I brought it to a local motor repair guy and he found that one of the legs on the centrifugal switch was higher than the others, causing binding, which he said would make the motor go in and out of start mode and cause the dead spot.

He must have been right. It's been four years and the motor has given me zero problems. Just poor quality control on the part of the manufacturer.

 

[rdhem2]

Hey guys. What's the chance that when going into back gear something is binding up, or not engaged correctly. My lathe will not even turn if you are not correctly in gear. Maybe the bull gear is hanging up or dry, just a guess.

 

[Ulma Doctor]

I would be very suspect of the centrifugal switch, and would not hesitate to remove the motor and tear it down to find out.
the nature of centrifugal switches requires a correctly functioning switch.
the starting circuit could be energized at the incorrect time.
Thereby creating unnecessary electrical imbalance in the motor leading to premature failure.
if the switch fails to disconnect the start capacitor from the motor the start cap will fail quickly.
start caps are optimally short duration devices that only stay in circuit for seconds, providing a short high energy boost to induce a high rotational force to get a motor started.
After the motor has gained sufficient speed,the switch is opened. the switch is actuated by a set of weights, the set of weights (in most cases) are are trying to close the centrifugal switch at all times, centrifugal force is trying to open the switch.
if the motor speed during operation is slowed sufficiently it can and will close the centrifugal switch and the capacitors charge will be discharged into the start circuit trying to get the motor up to speed.
too many start/run cycles too close together heat's the motor past normal operating temperatures and is generally not recommended.

 

[snowluck2345]

I couldn't think of any thing else that could be wrong with the motor than the centripetal switch. I think in my motor the springs are attempting to close the switch and the weights when brought up to speed open it.

Both times the caps blew when I had it geared down heavily which I thought was suspicious.

I have the motor open right now and I see nothing that looks like it is binding on the switch though. It opens and closes relatively easily.

The second time it blew, I can't remember the first time as it was several months prior, I had been switching the motor on and off alot, could that be the reason why? The centripetal switch was working properly, but the capacitor just was overworked and overheated.

 

[Ulma Doctor]

I couldn't think of any thing else that could be wrong with the motor than the centripetal switch. I think in my motor the springs are attempting to close the switch and the weights when brought up to speed open it.s

A) That's correct, the centrifugal switch opens when proper speed is attained!

Both times the caps blew when I had it geared down heavily which I thought was suspicious.

I have the motor open right now and I see nothing that looks like it is binding on the switch though. It opens and closes relatively easily.

A) Good, binding is not an issue that is good to know.

The second time it blew, I can't remember the first time as it was several months prior, I had been switching the motor on and off alot, could that be the reason why? The centripetal switch was working properly, but the capacitor just was overworked and overheated.

​A) Yes. the start caps are just tightly wrapped coils of conductive material that store a charge for a brief time. they heat up rapidly while in circuit and will burn up if abused! if you're lucky to find a run capacitor in the same value or create a capacitor bank from many smaller run caps, you could create a starter circuit that could run for longer durations of time without much consequence for the capacitors.
as for the motor itself... the motor may not last long under the same conditions and may not survive rigorous testing.

 

[dwdw47]

Hi: You never stated if the motor is a two speed or a single speed, That would make a difference in when the centrifugal switch opened being loaded. This being closer to the run speed and under a heavy load it would take longer for the switch to open. Possibly fluctuating in and out making up for the lower power of the motor. Another thing is the start capacitor is only reliable for about 15 starts per hour as over heating lowers the insulation break down voltage. This causes the start capacitor to flow the circuit power and overload the line current and acts as a conductor instead of a capacitor, not giving the quick surge to get the motor running.

You said you have the motor apart turn the lathe pulley to see if there is a tight spot or a just hard to turn, meaning a possible dry spot as a hidden oiler.
Hope this helps
dwdw47