SOFT START for Servo Motors

[Gymnast]

On 3/15/2021 at 12:01 AM, Kcstott said:

I've never seen a table saw with a brushed motor. The Universal brushed motors I've dealt with were fractional HP as in 1/10 HP. 
 

I think most of the smaller table saws are made with a brushed universal motor. Universal motors are used for most of the vintage household sewing machines and in a power input range of 50-250 W. A lot of Universal motors are used in tools, vacuum cleaners and other household equipment. They are good small motors with a high power, but with a limited operating time due to wear on brushes. They typically can run at high speed, so they will make more noise. A vacuum cleaner typically runs 25,000 RPM and can have an input power of 2000 W. Induction motors are very silent in operation, because their speed will be below 3,600 RPM and they got no brushes.

Both universal motors and induction motors got a higher inrush current, which can be reduced by a soft starter.

Edited March 16, 2021 by Gymnast

[Kcstott]

43 minutes ago, Gymnast said:

I think most of the smaller table saws are made with a brushed universal motor. Universal motors are used for most of the vintage household sewing machines and in a power input range of 50-250 W. A lot of Universal motors are used in tools, vacuum cleaners and other household equipment. They are good small motors with a high power, but with a limited operating time due to wear on brushes. They typically can run at high speed, so they will make more noise. A vacuum cleaner typically runs 25,000 RPM and can have an input power of 2000 W. Induction motors are very silent in operation, because their speed will be below 3,600 RPM and they got no brushes.

Both universal motors and induction motors got a higher inrush current, which can be reduced by a soft starter.

I don't know what size table saw you run but the one in my garage is a 10" blade and 2HP motor and it's far from small. 

[Gymnast]

On 3/15/2021 at 12:01 AM, Kcstott said:

I've never seen a table saw with a brushed motor. The Universal brushed motors I've dealt with were fractional HP as in 1/10 HP. 
 

 

On 3/16/2021 at 10:35 PM, Kcstott said:

I don't know what size table saw you run but the one in my garage is a 10" blade and 2HP motor and it's far from small. 

Your garage table saw may be with an induction motor. I argue that most smaller table saws at about 1800 W for home use or craftsmen got universal motors. This is three examples of that:

This is a link to a small Metabo 1700 W table saw. The small size of motor and gearing to motor clearly indicate, that it is a universal motor. The gear ratio is above 3 and no load saw speed is 4200 RPM, so the no load motor speed is about 12000 RPM.
https://youtu.be/2kVjOW-VMrk 

This is a 2000 W Dewalt DWE7491-QS. I can hear on the noise, that this is a high RPM motor.
https://youtu.be/I_uLAz7ferE?t=334

This is a Bosch GTS 10J table saw. In this video you can see the sparks for the brushes of the universal motor. An induction motor got no brushes. And you can hear the high speed of motor.
https://youtu.be/RDMBiUF80Gc?t=142

 

Saws in this size becomes cheaper and lighter, when you use universal motors. I guess that bigger and more industrial table saws may use induction motors. Perhaps older table saws for craftsmen got induction motors and you may have an example of that.

Edited March 17, 2021 by Gymnast

[Kcstott]

22 minutes ago, Gymnast said:

 

Your garage table saw may be with an induction motor. I argue that most smaller table saws at about 1800 W for home use or craftsmen got universal motors. This is three examples of that:

This is a link to a small Metabo 1700 W table saw. The small size of motor and gearing to motor clearly indicate, that it is a universal motor. The gear ratio is above 3 and no load saw speed is 4200 RPM, so the no load motor speed is about 12000 RPM.
https://youtu.be/2kVjOW-VMrk 

This is a 2000 W Dewalt DWE7491-QS. I can hear on the noise, that this is a high RPM motor.
https://youtu.be/I_uLAz7ferE?t=334

This is a Bosch GTS 10J table saw. In this video you can see the sparks for the brushes of the universal motor. An induction motor got no brushes. And you can hear the high speed of motor.
https://youtu.be/RDMBiUF80Gc?t=142

 

Saws in this size becomes cheaper and lighter, when you use universal motors. I guess that bigger and more industrial table saws may use induction motors. Perhaps older table saws for craftsmen got induction motors and you may have an example of that.

I would consider those portable saws and not a true table saw, light weight hobby stuff. My saw is a 1980's Craftsman, Cast iron and weighs about 400 pounds. 

 

[dikman]

I agree with Gymnast. You refer to them as hobby saws. Maybe, but by definition they are a table saw (a table with a saw mounted in it), there is no rule that states a table saw must be large and have an induction motor. And these smaller table saws inevitably have universal motors, which lend themselves to easy speed control.

[DonInReno]

There’s no arguing that a big 3hp, or 4hp, or even 5hp cabinet saw (heavy stationary table saw) is the Cadillac of saws and every cabinet shop and furniture maker should have one.  

However, for professional finish carpenters who build onsite there is nothing more ideal than one of the 15 amp portable saws with universal brushed motors.    A universal motor will draw more amps the harder it’s pushed and the little 15 amp Mikita 2703 saw that I prefer is direct gear drive between motor and blade and will easily trip a 20 amp breaker when ripping thick material.   When scratch building something like a big oak staircase, or custom cabinets for an entire house it may run 6-7 hrs a day for a week ripping material to size.   Other than changing cords and a set of brushes I tend to get 4 or 5 years out of them before the bearings for the blade get a little loose - I’m on the 5th one.  Once there were 6 of us cabinet makers building custom cabinets and bars for an entire year as part of a casino renovation - we all have our own style and equipment, but we all used similar portable saws.

I do admit it would be nice to have a big heavy cross cut sled, but the new track saws have been almost as accurate and fast if the user has a clear head and good technique.

Ave has a great video showing a 12 amp Dewalt grinder and how many amps it draws under load...at one point in testing it even trips a 20amp breaker.    After this tool review I picked up one of these grinders and it’s been a great purchase.

 

[Constabulary]

7 hours ago, dikman said:

there is no rule that states a table saw must be large and have an induction motor.

agree - same with sewing machines - domestic sewing machines are still sewing machine even when they do not suit our own requirements.

[dikman]

Yep, horses for courses, as they say.

Hmmm, soft start on a domestic machine?