Smaller pulley or Potentiometer?

Thread Starter

JWRich

Joined Aug 8, 2018
3
This question may be in the wrong section/forum/site but I am lost trying to work this out.

I have a motor rated to .66hp and 940rpm which needs replacing. The motor I am replacing it with is 1.5hp and 2850rpm. Its used to run an old saw via belt and pulleys. I can get the rpm of the larger pulley near enough where it needs to be using a pulley on the motor 35% of the size of the original, but the increased HP will probably cause the belt to slip, plus it means finding a specific pulley for a motor from 1960.
If I use a variable resistor of some kind, I could slow the RPM down to the speed of the original, but I cannot work out what that will do to the HP.

Any help would be appreciated,

Jordan
 

MrAl

Joined Jun 17, 2014
11,389
Hello,

A variable resistor would eat up a LOT of power, and get as hot as a space heater.
In fact, a space heater might be the right resistor :)

What kind of motor is it? Maybe you can use a speed control.
 

LesJones

Joined Jan 8, 2017
4,174
You can't use a resistor the control the speed of an induction motor. I think fitting a smaller pully or if belt slip is a problem then add a countershaft and do the speed change in two steps. What speed does the saw blade rotate and what is it's diameter ?

Les.
 

Thread Starter

JWRich

Joined Aug 8, 2018
3
Could I still use a voltage regulator/speed controller though?
Its a Qualters Smith power hacksaw, so no rotating blade, just a con rod from the pulley onto the hacksaw blade.
 

shortbus

Joined Sep 30, 2009
10,045
You may have to change both pulleys. If the original motor was only 940 RPM, the motor pulley was probably bigger than the saw pulley to speed up the RPM on the blade. So I would find the ratio of the existing pulley configuration and then figure out what the ratio would need to be with the new motors RPM. To keep the blade speed the same or somewhere close, because in the end it is all about the blade speed of the saw.
 

LesJones

Joined Jan 8, 2017
4,174
There is no speed controller that I know of that would drop the speed of a single phase induction motor. If it was a three phase motor you could drop the speed to 1/3 rd but as the torque would still be the same the power would be reduced to one third. (So a 1.5 HP motor would giive 0.5 HP at one third of it's rated speed.)
I think buying a 940 RPM motor would be the simplest solution.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
Even a 4 pole motor over the 2850 rpm 2 pole motor you are trying to install would be better.
This would give you 1420rpm at least and a better solution.
Max.
 

Janis59

Joined Aug 21, 2017
1,834
My first circular saw machine I weld when was just stuck the teeneger age. After that I have produced that rather much to sing You a song about what is needed. If Your circular blade diameter is about one meter, the minimum power for very poor job is about 7,5 kW but for well work is about 15 kW at 35 m/s tip speed. The same obstacles for roughly 50cm diameter is 3,3 kW and 5 kW. The same for 25 cm is about 1,5 kW and 2,2 kW.
Just, if Your aim is to saw the safety matches, then 1,5 kW is absolutely okay, but if the firewood, then its all is dead-end. Even for woodcraft shop this power is OK if Your saw has about 25-35 cm blade. Its for cutting the small or slightly thicker twigs, no more, maybe 1 cm thick planks with method of ``push in, wait while loss inertia, then release``.
The 3,8 kW at 1500 rpm demands a two or better three parallel B size konical shape pulleys (widest place 18mm). The 5kW demands tthe same but 5 or 6 pieces. 1kW may be fed by A size one piece (width 12mm) or much better with AB (15mm). Length of pulley is not critical. If the pulleys are teethed, then numbers may be slightly reduced.

If use a frequency inverter, the force of motor will unavoidable diminish. Its bad idea. Best is to look for lathe and make a new pulley disc of correct diameter.
 
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