How to slowly ramp up RPMs on motor

[piedmontg]

I am building a small cut off saw from a set of castings. The motor supplied cranks 10000 rpm with enough torque to have jump off the table. My concern is the four 10-32 mounting screws are not going to hold this very long. I have used VFD’s in a number of applications for “soft” or “slow” start not sure terminology is correct but the result is nice easy start ie one example is 7 ½ HP air compressor.

Is there a simple circuit that will just ramp up the rpm’s? It is a universal motor and I have speed controls that will ramp up slowly as I turn the knob, but that is really not practical.

I did some searches on line and here but really did not see anything simple. Maybe just some Loctite and hope for the best.

All advice is welcome

Plate reads: volts 115, amp 5.1, hp 1/2, rpm 10000, cyc dc/60

 

[Phil in Montana]

Just need a veriack you have a universal motor, a vfd will not be usefull for a soft start...Phil

 

[David Ferguson]

I think Phil is suggesting a Variac, here is a link to the company's website:

VARIAC Variable Transformers

That is what your current speed controllers do.

 

[rons]

SCR phase control. All it take is one SCR and a circuit that adjusts the phase angle for full conduction after a few seconds.
I've made one before, seen one working on a 10" Bosch chop saw.

 

[nihilistic]

Why can't you use bigger screws/bolts?

 

[johansen]

Google soft start.

https://www.amazon.com/Hitachi-371468-Soft-Start-C10Rj/dp/B07DTBTS9D

 

[plastikdreams]

Google soft start.

https://www.amazon.com/Hitachi-371468-Soft-Start-C10Rj/dp/B07DTBTS9D

Google has a soft start function now!?!??!

 

[rons]

Looks like a SCR with a real simple phase firing control. Probably a chip resistor/capacitor circuit.

 

[Strostkovy]

You can use a VFD to soft start a universal motor. The frequency isn't very relevant the torque will be pulsey at low frequency but it totally works fine if you have the parts on hand.

I'd just use a contactor and a timer to short a soft start resistor or capacitor after a few seconds. Two speed soft starting is good enough for most applications, and is very reliable. Plus, universal motors draw very little current as they get up to speed, which makes the last bit of speed gain when the contactor engages very little, while still significantly limiting starting torque.

Just two parts, a standard industrial timer and a contactor, both rated to operate on 115VAC.

 

[SomeoneSomewhere]

As the motor in question isn't very large, I'd be inclined to use the soft-start module linked above and just put it inside the motor terminal box. It's clearly designed for 115V universal motors in power tools and will likely be cheaper, more reliable, more effective/efficient, and easier to replace than anything DIY.

 

[JST]

I don't see the problem that is to be "solved".

You have a 1/2 HP 10k rpm motor. The torque is going to be about 0.25 ft-lb, which the mounting screws will hold.

Four 10-32 screws will hold a pretty chunky piece of rack mount equipment to the rack. Your motor with 0.25 ft-lb is equal to about 3 inch-lb torque. Four screws often hold a 20lb or more piece of equipment in a rack, and they hold it while it gets jounced around in a truck. Equipment often weighs more than 20 lb.

The rack with 20 lb equipment is 5 lb in shear for each, with some tensile loading on the lower screws. Your motor will load each screw under 1 lb in shear, less if the lever arm is over an inch, and it will have a very large margin of safety, even including the motor weight.

That is not surprising, as the motor manufacturers were not idiots. Bolt the motor in place and stop worrying.

 

[johansen]

Most power tools are held together by 4 #10 ballpark screws on a 4 inch diameter ..in plastic. Up to about 1hp at 10 to 20k rpm.

 

[piedmontg]

Thanks for all the input, I knew I would get a solution(s) that would work out. I did not make it clear that this is a portable tool. The VFD and VARIAC solutions would be fine as well as my speed control unit, just a bit cumbersome for my application portability.

That little $10 module seemed like the best solution, I need to add a switch box of some kind since there is no control box on the motor just two wires coming out of it. All would fit nicely.

The unit is not yet built and assembled, my concern was based on the motor jumping out of my hand while trying to hold on the bench to test it. After reading JST’s explanation I took the motor and mounted it on the casting. Found out the screws were tapped 8 – 32 not 10-32 as indicated. I set the casting with the motor mounted on it on the bench and started it. Virtually no kick at all just a little bump. Once it is all built and mounted to the base the kick will likely be less.

If when completed I still think there is too much “jump”, the small soft start module will be added.

I have a chunk of 1144 mounted on the lathe to make the spindle. Then pulleys, wheel washers, and a small vise.

Again thanks for all the input, leading to a solution

Bob

 

[Strostkovy]

I don't see the problem that is to be "solved".

You have a 1/2 HP 10k rpm motor. The torque is going to be about 0.25 ft-lb, which the mounting screws will hold.

Four 10-32 screws will hold a pretty chunky piece of rack mount equipment to the rack. Your motor with 0.25 ft-lb is equal to about 3 inch-lb torque. Four screws often hold a 20lb or more piece of equipment in a rack, and they hold it while it gets jounced around in a truck. Equipment often weighs more than 20 lb.

The rack with 20 lb equipment is 5 lb in shear for each, with some tensile loading on the lower screws. Your motor will load each screw under 1 lb in shear, less if the lever arm is over an inch, and it will have a very large margin of safety, even including the motor weight.

That is not surprising, as the motor manufacturers were not idiots. Bolt the motor in place and stop worrying.

I don't think that's really a fair assessment of torque. The jump is near stall, and the stall torque of series wound motors can be pretty substantial. I wouldn't be surprised to see 1-2 foot pounds or more, but that's a gut feeling.

Regardless, the threaded holes are fine so long as the mount is made appropriately. If using something like standoffs to space for a shaft coupler, then there will be problems.

 

[Jraef]

Just get a 500W rheostat to use as the control device.

 

[JST]

I don't think that's really a fair assessment of torque. The jump is near stall, and the stall torque of series wound motors can be pretty substantial. I wouldn't be surprised to see 1-2 foot pounds or more, but that's a gut feeling.

Regardless, the threaded holes are fine so long as the mount is made appropriately. If using something like standoffs to space for a shaft coupler, then there will be problems.

2 ft-lb would be 800% stall torque, which is pretty high....... 1 ft-lb would be 400%, which is more possible. And, it isn't a stall, it is inertial loading.

Regardless, the screws should be fine with 2 ft-lb. Yes, spacers could begin to get in trouble, depending on how they are done.

 

[piedmontg]

No spacers, motor machined face mounted to machined surface of casting. Nice and solid.

 

[piedmontg]

Just an update. As I mentioned above I did purchase that little module. Hooked it up and it did noticibly make a difference on startup. It took the snap out of it. Still came up to speed quickly it just did not have the initial jump.

Bob