New to bladesmithing and need grinder help

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Piper2328

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Feb 26, 2020
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I’m needing to know how to make my dc motor run on 120 hertzSo the speed will increase 100%? It’s an 1800 rpm motor and I have a dc motor controller that I plug into a 110v outlet. Does anyone know how to make this happen or do I need a different motor to get the faster belt speeds I need?
 
Bill DeShivs said:
Increase the size of your drive pulley.
Click to expand...
I know I can go up to a 7” drive wheel which will get me approximately 3300 sfpm which is better than my current 1900 sfpm on a 4” drive wheel, but I am wondering if there is any easy way to double the rpms of the motor by doubling the hertz somehow?
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/303162387650
https://www.automationdirect.com/ad...ors_(up_to_2hp)/general_purpose/mtpm-1p5-1l18 Here is links to the motor and controller I’m using.
I’m not very smart on electrical stuff but I think I read somewhere about doubling the rpms of a dc motor somehow but I can’t find it.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/303162387650
 
If the motor is not designed to run over 1800 rpm you would probably ruin it. Some are designed as variable speed but most are not. You would need a special motor and drive controls. Or if you just want 3300 rpm, but that motor.
 
I just ordered a 8” drive wheel off Etsy. It will double my speed. Thanks for your input
 
Piper2328 said:
I know I can go up to a 7” drive wheel which will get me approximately 3300 sfpm which is better than my current 1900 sfpm on a 4” drive wheel, but I am wondering if there is any easy way to double the rpms of the motor by doubling the hertz somehow?
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/303162387650
https://www.automationdirect.com/ad...ors_(up_to_2hp)/general_purpose/mtpm-1p5-1l18 Here is links to the motor and controller I’m using.
I’m not very smart on electrical stuff but I think I read somewhere about doubling the rpms of a dc motor somehow but I can’t find it.
Click to expand...

Didn't see you had two threads about this. PWM control of a DC motor can't increase motor speed above its nominal rating, only decrease it. So at say 90 VDC, your motor runs at its rated speed of 1800 rpm. This is going to be at a solid 90V, with no PWM. From there the motor can be slowed by rapidly pulsing the voltage down to zero and back up again. The width, or length of time, of this pulse determines how much the average voltage is decreased. So basically, the PWM simulates a lower constant voltage by pulsing the output so that a lower average voltage is seen by the motor.

As I mentioned in the other thread, the only way to increase the motor's speed would be to increase the voltage, but if the motor isn't rated for that higher voltage you will shorten its life, possibly drastically. Good move going with the larger pulley.
 
Thanks so much for your input. Yeah I’m new to this forum thing too. That’s why there where 2 treads.
So let’s just say my motor was rated for 180 vdc instead of 90. But it was still rated at 1800 rpms. If I powered that motor with 220v ac “instead of 110v” to a dc motor controller would the motor only turn 1800 rpms or would it run at 3600 ?
 
No, you would fry that controller and possibly the motor too and probably in a dramatic and fiery mess DONT DO IT!!!!
 
This is similar principal to overclocking a computer. It is possible often but will void a warranty and without other modifications to account for the increased stress on the device it will fail quicker and with less warning. In theory you could do it but in practice the gains capable will be costly in modifications needed and/or minimal considering the costs.
 
I really appreciate everyone’s input!
I have no doubt this is not the route I am going to take. 8” drive wheel ordered and maybe in the future just buy a 3600 rpm motor.
 
Yeah, sickpuppy knows what he's talking about. Increasing the AC input voltage would be asking for trouble. To simplify the circuit a bit, the DC controller makes DC power from AC by using a rectifier, which cuts off or blocks the negative voltage side of the AC, leaving all the positive voltage in a series of sinusoidal pulses. Smoothing capacitors are then used to level out these pulses to a closer to constant DC voltage, then the output is controlled by using PWM to reduce voltage to slow the motor.

Increasing the AC voltage in would also increase the DC out, but again, just like the motor itself, the components in the DC control circuit are built for the rated input voltage. They normally have some degree of safety margin for over-voltage, but nowhere near 200%! As already mentioned, you'd be looking at instant catastrophic failure of the circuit if you tried that. Not probable, definite.

What I meant by the "increased voltage" I mentioned earlier was increasing the DC output voltage, which would require a higher voltage output DC controller. So if you had a 90VDC rated motor, you'd purchase a 180VDC rated controller. The motor would run just like normal from 0-90VDC and achieve it's rated 1800 RPM. Turning the DC output up from there would increase the RPM by the same percentage as the voltage increase. So taking a 90VDC rated motor thats RPM rated at 1800 RPM and instead running it at 180VDC would result in 3600 RPM. However, it could also fry the motor, and would at the very least cause severely accelerated brush and commutator wear. Would get a lot hotter than it should also, and if it got hot enough would short the armature wiring and probably fry the controller.
 
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