Speed Controller

[Robotstar5]

I would like to run an angle grinder (12000rpm) at lower speeds (2000rpm upwards) to use wire cup brushes and possibly buffing / polishing mops.
Would one of these work, mainly concerned it would still have torque to do the job.
I realise it would have a low duty cycle due to heat build-up but could live with that.
Cheers, Stuart.

 

[Hitch]

Not sure on the polishing/buffing side of things, but normally, small brushes suitable for small grinders have a higher speed rating than the bigger counterparts. A 100mm brush rated at about 8000iirc, so no problems in a 7" or 9" grinder. Smaller 65mm ones are rated at 12500 or so normally.

A 100mm brush in a 5" grinder would soon kill even the best ones i reckon.

Polishers, down to 3000 rpm isnt it?

 

[Robotstar5]

I've "found" some 80mm brushes but only rated to 6500rpm

 

[gavuk]

iI bought one of these .disposable for the money ,comes with spare brushes ,,,,,£10 grinder........Gav

 

[rtbcomp]

That grinder's cheaper than the cheapest motor controller!

 

[gavuk]

Its a bit of a handfull one handed,because of the length of handle,but i've used the company many times had another cheap grinder that failed within 6months contacted them they sent out another without asking for old to be returned....thumbs up

 

[Robotstar5]

As I appear to have 4 grinders already I thought I could experiment with one of them. Good link Gav but I don't want a 5th!

 

[R Kraft]

I would like to run an angle grinder (12000rpm) at lower speeds (2000rpm upwards) to use wire cup brushes and possibly buffing / polishing mops.
Would one of these work, mainly concerned it would still have torque to do the job.
I realise it would have a low duty cycle due to heat build-up but could live with that.
Cheers, Stuart.

Do you suppose, just putting a diode in series with the grinder power would work well enough.
Robert

 

[Robotstar5]

Do you suppose, just putting a diode in series with the grinder power would work well enough.
Robert

I can't see how a diode would allow me to vary the speed, it would just half wave rectify the supply.

To clarify, I want to vary the speed between 2000 and 6000rpm of a grinder with a fixed speed of 12000rpm without losing torque.

 

[R Kraft]

Okay variable speed, not just reduced speed.Sorry
The largest of the speed controllers should be plenty large , if your grinder is 750-1000 watts.

The other thing would be if the fan on the grinder motor would move enough air at the reduced speed, or if the reduced power would make up for the lesser speed.
Robert

 

[Robotstar5]

The other thing would be if the fan on the grinder motor would move enough air at the reduced speed, or if the reduced power would make up for the lesser speed.
Robert

I think it would just be a case of keeping an eye on the body temperature to work out a reduced duty cycle.

 

[rikrobson]

Hi,

The torque would be reduced if you use a speed controller.

The one you showed used a Thyristor. this is what is used in a light dimmer and in effect will give a lower RMS voltage to the motor. This will have reduce the power that the motor is running on and thus the speed at which it turns.
Torque is proportional to the power and speed of the motor

Torque = Power/Speed

The power is proportional to the square of the voltage and the speed is likely to be directly proportional to the voltage making the torque proportional to the voltage.

Basically the lower the speed the less the torque

To preserve the torque you would need to alter the gearing as opposed to the power.

Hope this helps.

R

 

[shenion]

My small lathe came with a similar speed controller. Had no torque at low speeds.

I replaced it with a proper speed controller. This used feedback to monitor the speed of the motor (by the back EMF.) It made a huge difference.

A good speed controller will cost far more than a cheap grinder. I found my speed controller on ebay under industrial speed controls. Was just an open frame contraoller that needed cabling, potentiometer and plugs added. These only work on brush-type motors as they output DC PWM.

 

[Robotstar5]

I found my speed controller on ebay under industrial speed controls. Was just an open frame contraoller that needed cabling, potentiometer and plugs added. These only work on brush-type motors as they output DC PWM.

Good job I asked the question before wasting my money then, thanks guys.

We have a few inverter drives kicking around at work, I wonder if I could use one of them would they work on brush type motors?

 

[shenion]

Inverter drives are fro 3-phase brushless (induction) motors.

 

[Robotstar5]

Inverter drives are fro 3-phase brushless (induction) motors.

Ah well back to the drawing board!

 

[R Kraft]

The torque of the motors is proportional to amps, the rpm is proportional to voltage.
A motors resistance increases as the rpm get higher, so the amps are steady with increasing rpm..
The resistance goes down as the load increases, so if you maintain voltage as the load increases, the rpm stay the same.

Some of the triac type controls have an IC that regulates the voltage to keep the rpm steady under load.
A treadmill control for brush type motors is the most common, you would need one without the tach circuit.

I don't know if the speed control out of a variable speed drill, is wired across the whole motor, or if it just controls the windings.
They seem to maintain torque with loading.
It may be an option.
Robert