ROJEK SH1 / TB Woods SE1 AC single phase input 120 VAC. Output is 230v 4.3 amp. 3

[Roy Ward]

I have an opportunity to purchase a used ROJEK SH1 MA lathe which has had a TB Woods SE1 AC single phase input 120 VAC. Output is 230v 4.3 amp. 3 phase convertor variable frequency driver installed on it. The plate on the ROJECK indicates the motor is 1,3kw / 3 x 230v / 5,3amp / 60hz and I believe it was manufactured in 1993/5.

I am not much of an electrical expert and I am not familiar with ROJEK lathes so I would appreciate and comments you might have. The asking price is $1500 and I understand that the lathe currently sells for $3700US in the UK.

I note that VFD is rated at 4.3 amps while the motor is rated at 5,3 amps so I am wondering what impact that will have. I have seen websites that indicate the VFD amps should be higher than the motor rating. I am wondering if the lower VFD amps is deliberate way of derating the motor to make sure the motor does not overheat.

 

[Mark Mandell]

Something doesn't fit here, Roy.

ROJEK doesn't list that lathe in their product line. In fact, the only lathe they show is something that looks like a clone of the JET 1236 and even that on is not listed on the ROJEK USA site.

The ROJEK brand is applied to a number of European-made single and multi-purpose/combination woodkworking machines, and, from what I remember, has a descent reputation.

 

[Roy Ward]

Rojek sh1 ma

The European website (Czech Republic) still shows a ROJEK SH1 MA model although the specs are slightly different from the machine I am looking at which probably about 10 years old

 

[Mark Mandell]

Ah Ha.

Configured as a copy lathe. You want to make a lot of legs?

Can't speak to the electricals.

 

[Roy Ward]

legs or bowls

actually I am more of a bowl turner. I had an old "American" lathe that did almost everthing I wanted it to until I lost it in a fire that destroyed my shop and home. I have bulit a new shop but have not yet replace the lathe. There are very few used lathes that come up for sale in my area and I cant justify buying a new Oneway which is what I would really like to have. The only other used lathe optio
n I have right now is a General 160 but I think the ROJEK is a better product even if I do not need all the copy features.

 

[Bill Boehme]

I have an opportunity to purchase a used ROJEK SH1 MA lathe which has had a TB Woods SE1 AC single phase input 120 VAC. Output is 230v 4.3 amp. 3 phase convertor variable frequency driver installed on it. The plate on the ROJECK indicates the motor is 1,3kw / 3 x 230v / 5,3amp / 60hz and I believe it was manufactured in 1993/5.

I am not much of an electrical expert and I am not familiar with ROJEK lathes so I would appreciate and comments you might have. The asking price is $1500 and I understand that the lathe currently sells for $3700US in the UK.

I note that VFD is rated at 4.3 amps while the motor is rated at 5,3 amps so I am wondering what impact that will have. I have seen websites that indicate the VFD amps should be higher than the motor rating. I am wondering if the lower VFD amps is deliberate way of derating the motor to make sure the motor does not overheat.

Roy, European motor nameplates generally give motor mechanical output power in kilowatts while we use horsepower in the USA. However, in this case there is something that definitely stinks in the nameplate specs. If you calculate the electrical input power, it is slightly more than 1.2 kilowatts. Typically, for a general purpose motor the mechanical output power is between 0.6 and 0.7 (this term is the motor efficiency). While there are motors with efficiencies much greater than that, you will not find such a motor on a low cost lathe like that. Let's assume that the efficiency in this case is 65%. This means that he mechanical output power would be 0.78 kilowatts or 1 horsepower.

Now, you raised a good question about the inverter since it is not able to provide full load power to the motor. Using the same estimated efficiency number, the mechanical output power that you could expect is about 0.643 kilowatts or 0.86 horsepower. This is not much power for a bowl lathe, but it is usable if you do not plan on making large bowls and only taking light cuts.

So what happens when the motor is not able to draw the current needed to provide full output power. Depending on the inverter design and how it is programmed it might either slow down or shut down (I would guess it would shut down based on the inverter being a very old design). It sounds like that particular lathe is designed mainly as a spindle turning machine in which case running into this power limitation is unlikely. However, with bowl turning, it might turn out to be a major annoyance.

Something very important that you need to know about variable speed inverter drives is that the output power that is available to you is not the rated mechanical output power of the motor or the inverter (if rated lower than the motor), but only a portion of the rated power based on the motor speed. For example if the speed control is set so that the motor is running at half speed then the output power would also be limited to half of the full speed power, which in this case would be 0.32 kilowatt or 0.4 horsepower. At, 25% of full speed, the output power would be 0.16 kilowatt or 0.2 horsepower. With that simple old Woods inverter, you will not be able to operate the lathe very much less than 25% of full speed because the speed control will become not very stable and will be significantly affected by the load torque.

I don't mean to sound too discouraging, but I just want to make sure that you have a realistic expectation of what the lathe can do.

 

[Roy Ward]

Thanks for the information, what you have told me is what I was afraid of. it is very difficult to understand all of the impacts when using electronics to modify the original machinery.