VFD and Motor Selection

I need to select VFD and motor combination for an existing chain conveyor application. The existing motor is 3 HP, 970 RPM with reduction gear boxes (200:1 total reduction). The reduction gear output is presently connected to a mechanical variator to get a max/min speed ratio of around 20:1. Now the user wants a ratio of 40:1 and use VFD instead of mechanical variator.

From what I have read, I think the chain conveyor is a constant torque application and to get 40:1 speed ratio I may have to use Vector VFD and vector duty motor. I don't have much experience in VFD applications.

My questions are -
a) can I use Sensorless Vector VFD for this application?
b) do I need to change the existing motor with a inverter-duty or vector-duty motor?

Thanks,

SVN

yes you can use senserless vector or flux control they have better torque control vs the standard V/f drives. You would be better off using an inverter duty motor the PWM signal generation will reduce the life expectancy on standard motor windings but will work.

Hi,
Am willing to help you out.
Ron

svn,
I'm a little unclear about your requirements

svn said:

I need to select VFD and motor combination for an existing chain conveyor application. The existing motor is 3 HP, 970 RPM with reduction gear boxes (200:1 total reduction). The reduction gear output is presently connected to a mechanical variator to get a max/min speed ratio of around 20:1. Now the user wants a ratio of 40:1 and use VFD instead of mechanical variator.
SVN

Click to expand...

1. What is the input speed of the gearbox when the mechanical variator is at it's fastest?
2. What is the input speed of the gearbox when the mechanical variator is at it's slowest?
3. Does the customer want the slowest output to remain the same and the fastest output doubled? If this is the case then the motor may not have enough HP to get the job done.

Asking milldrone's questions in a slightly different way---If you remove the mechanical variator, what would the motor speed range be in order to get the desired speed range on the conveyor?

Thanks everybody for replies.

milldrone and DickDv, the user wants to keep the minimum speed same and double the maximum speed. The motor is connected to the fixed reduction gearboxes (1:5 and then 1:40) and the output of the fixed gear box is connected to mechanical variator. The motor runs at a fixed speed of 970 rpm. Speed variations are done only with the mechanical variator.

SVN

svn said:

The motor is connected to the fixed reduction gearboxes (1:5 and then 1:40) and the output of the fixed gear box is connected to mechanical variator the user wants to keep the minimum speed same and double the maximum speed.
SVN

Click to expand...

Well that cleared up a few things, and pointed out a misconception I had about your project. (I presumed the mechanical variator was inbetween the motor and the gear box).

1. Can you get an amp reading and operating voltage and number of phases when the conveyor is running under normal load at the fastest speed you can now run?

2. Are there any special considerations, like jogging? Or reversing? Or can the load can sometimes make the motor spin when it's not energised? Does the motor neeed to stop fast?

3. Some details about the mechanical variator are needed. We need the input shaft to output shaft relationship. You have stated that it has a 20 to 1 turn down. But we need to know when the mechanical variator is at it's slowest. What is the ratio between the input shaft and the ouput shaft? And just to make sure we also need to know the ratio when the mechanical variator is at it's fastest. What is the ratio between the input shaft and the output shaft?

4. How old is your original motor? Has it been rewound?

5. How long is your wire run inbetween the motor and controller?

I'm making a presumption that the mechanical variator is in need of repair (not the first time) and the customer is looking at options and improvements.

Thanks milldrone,

1. The system is in a different town and I wont be able to go to site at least in next 2 weeks. I shall check if the site techinician can provide this information.

2. Jogging or Reversing of the motor is not required. The load doesn't make the motor spin when the motor is not energized.

3. The mechanical variator is Carter Gear. I don't know the full specs presently but I have mentioned the working turndown ratio. i.e. I have been told that maximum output speed corresponds to 20 hanger/hr and minimum speed corresponds to 1 hanger/hr. I shall check with customer whether they have data on output rpm at max and min linear speed.

4. The motor is 6 years old and it is not rewound.

5. The wire run in-between the controller and motror is about 15 meters.

The customer wants to replace the mechanical variator because they want more flexible and easy-to-maintain system while increasing the max/min speed ratio.
SVN

svn said:

3. The mechanical variator is Carter Gear. I don't know the full specs presently but I have mentioned the working turndown ratio. i.e. I have been told that maximum output speed corresponds to 20 hanger/hr and minimum speed corresponds to 1 hanger/hr. I shall check with customer whether they have data on output rpm at max and min linear speed.

Click to expand...

The critical points that need to be answered are the present loading of the motor at low and high speeds. And if the mechanical variator increases or decreases the output of the gearbox (or both). Also one new question can the gearbox handle a higher input shaft speed?

Thanks milldrone,

I shall get the information from the customer and post.

SVN

I just went through this myself. we had a mechanical variator that was moved by a DC motor and the PCB for the motor has become obsolete so we just locked the mechanical system down at 80% and use the drive to control the speed. We went with a AB power flex drive (60HP) because in the future we plan on upgrading the system control and want to communicate to the drive. I have used ABB and Mitsubishi drives also and like both. I didn't replace the motor because of the cost and in the past I have made similar changes to smaller motors and have never had enough problems to justifiy replacing the motor.

Good Luck

dadalex, thanks for the information.

Did you use general purpose VFD or Sensorless Vector VFD for your application?

SVN

I used the Allen Bradley Powerflex 70 which is a general purpose drive but it is hard to tell with all the bells and whistles it comes with. It ended up being a couple of dollars cheaper than tha ABB equivilant drive which was a suprise to me. If you are not operating this motor below 10 Hz and don't need to hold full motor tourque in that range a general VFD should be just fine. If your motor is sized right for the application you would probably be ok with a general purpose drive. The only time I have used a vector drive is when I was using an encoder to allow my vector drive to follow a second drive.

Another good flexible inverter is the Omron G5 which can be programmed to use the V/f with auto torque boost, closed loop flux vector providing full torque control at zero speed and open loop vector for precise speed control with quick response and high starting torque as well as closed loop V/f for encoder feedback

the question you need to ask is how critical your starting torque
and how accurate you need your speeds at

I still think that answering my original question in this post would open the door to a real solution and set aside a lot of speculation. Unless, of course, its speculation that you are after!