3 phase motor inicial torque

[rodrigopires]

Hi everyone! A friend of mine built a wood chipper running of a 2HP 3 phase motor, but the inicial torque is too much for the axis and it might break. We want to reduce the inicial torque, just to start spinning the axis, and then apply the normal power. He has a vfd that would be great for this, but it its only single phase. We thought on modify the motor to run on single phase and then connect it to the vfd, but since we are modifying it, we could also had a starting capacitor and a running capacitor. However, i dont know the capacitor value that i should use, i tried 30uf and it was too low to start. After doing some research i found the the appropriate value is around 140uf to start and 20 to run. Is this correct? is there another way to do it?

Thanks!

[T3sl4co1l]

Get a "motor starter": little more than some resistors switched in series with the motor, to limit starting current (and, as a result, torque).

Tim

[woodchips]

Oh cummon!

There is no chance of the shaft breaking on a chipper, how much of a shock load will there be when chipping?

[rodrigopires]

it doesn't chip throw big pieces only small press wood sticks, but the machine is homemade and he is worried about the shaft, wants to add a slow start.

[promacjoe]

Are you sure The VFD is not single phase in, 3 phase out. Give us the brand and model number of the VFD. Maybe someone can help you with it.

[johansen]

worried about the shaft breaking due to the initial torque?
that is not a problem. you are more likely to burn out the motor overloading it, or trying to run it on single phase. you can easily Google the online calculators needed to calculate the torque needed to sheer a 5/8" diameter shaft.. it might be as high as 400 foot pounds.. and the motor might only generate on the order of 6-12 foot pounds of starting torque.

btw, the high tensile tool steel used for the 1/2 inch square socket drive takes about 800 foot pounds to sheer it. but motor shafts are usually soft steel..

you may however exceed the bearing's capacity, which is only a thousand pounds or something like that for the 6203 bearing that is on most motors of that size.

many cheap 5 hp gasoline engines are directly coupled to the chipper’s 20-30 pound shredder plate and i am surprised the sleeve bearing (1 inch diameter crankshaft bearing rotating against an aluminum casting) holds up at all. but it does.. 

[mjkuwp]

There isn't any torque unless there is a load.  What is the load on the chipper when you start it?  Is there a giant mass hanging on the motor? 

[Brutte]

We want to reduce the inicial torque

Not sure you understand what you are asking about. 99.99% of the problem is that the torque is not high enough. You are asking us how to decrease a starting torque 
Just limit the current by dropping the voltage. Delta and wye have different starting torques but not all inductive motors have all 6 connectors exposed. If you have them then try delta/wye switch. If that is a squirrel cage type with 3 connections then use drop resistors (symmetrically).
Do not try to load it with any unsymmetrical load or with DC content (like diode rectifying etc) and mind prolonging start extends the time when winding heats up.
Alternatively rivet something heavy to the other side of the shaft. The heavier it is the more time it takes to spin up.

[Circlotron]

Maybe something like three 1kW electric radiator heating elements, one in series with each motor wire, and a contactor to short them out once the motor is up and running. This is assuming the motor is started with no load.

[albert22]

As  somebody said you probably dont need to reduce the torque of the motor. But if you really need it you may consider a mechanical solution by using something similar to a shaft damper or a torsional shaft damper. (just google to get the idea)

[rodrigopires]

Are you sure The VFD is not single phase in, 3 phase out. Give us the brand and model number of the VFD. Maybe someone can help you with it.

The VFD is a Watt Drive L2000-007HFE, but now the i see, i think it might be 3 phase after all, is it?

There isn't any torque unless there is a load.  What is the load on the chipper when you start it?  Is there a giant mass hanging on the motor? 

The motor shaft is connected directly to the "shiper" part that rotates, it has some discs or something to do its job.

Not sure you understand what you are asking about. 99.99% of the problem is that the torque is not high enough. You are asking us how to decrease a starting torque 

I know, it strange, but that's what he asked me to do...

[T3sl4co1l]

There isn't any torque unless there is a load.  What is the load on the chipper when you start it?  Is there a giant mass hanging on the motor?

I would assume it has a big flywheel...

Tim

[cs.dk]

The VFD is a Watt Drive L2000-007HFE, but now the i see, i think it might be 3 phase after all, is it?

According to the manual; http://www.inverter.co.uk/applications/manuals&datasheets/Watt-Drive-L2000-Manual.pdf

"H" means 3 phase 400V.

Input voltage
(N: 200V single / three phase)
(H: 400V three phase)
(L: three phase 200V only)

I've been building and servicing woodchippers for a living. I've never seen a chipper with less then 15 kW motor. 2HP would only chip toothsticks.
Is it a fast rotating drum chipper, or a slow going with many knives?

[rodrigopires]

So the vfd is 3 phase input and output, thats great news, however, this model can only handle up to 0,75KW motor, he's motor has 1,5KW (2HP), so it cant be used right?
What can we do right now? Does that star delta switch work for my purpose? Star/wye to start slow and Delta to run fast(normal) all at 380V?

[johansen]

you can program the vfd for 70% of the nameplate volts per hz on the motor, and get the full 3/4th hp available from the vfd out of the motor without tripping the vfd.

however, a 3/4hp wood chipper is not worth your time