Replace Servo with AC motor

[Big John T]

Then there are many indexing type setups that could use induction motors or servos. The induction motor has the advantage of price. The A-B Ultra drive can be setup for induction motors but I'm not sure how costly that process is. I know that the ABB ACS800 has an offering with a positioning macro. It also would have communication modules to talk with an A-B PLC if necessary to change position presets. Or, this could be done through the keypad on the drive.

Also, don't always take for granted that the application was originally sized correctly. If possible, take some current readings to see what the actual HP and torque requirements are. I have done several retrofits where the original design was extreme overkill which also means very inefficient. Sometimes the production parameters have changed and less power is needed for the current process. I would certainly verify the requirements of the application since you are facing multiple retrofits.

On a servo drive will the amp draw tell me what percentage of power is used like an AC motor? It does look like an overkill but looks can be deceiving...

John

 

[swhite65]

On a servo drive, the current draw (of the motor) is directly proportional to torque since you have no magnetizing current so it is much easier to determine than with an induction motor. In the case of a move where the motor is at standstill, starts, accelerates and then decelerates to a stop.....you need to capture the torque or current profile for this event. Some drive software will sample quickly enough to give you enough resolution to determine what peak current draw is. Otherwise, you will need to have an instrumentation setup that is fast enough to capture the information you want. I'm sure there are many ideas available from this forum for doing this if you need suggestions. Your peaks will occur at the start of the move and at the start of deceleration where the inertia has to be handled. If you have the opportunity to experiment with the machine, you could decrease current limit to the point that the move is adversely affected. This will give you some idea of how much excess you have available.

 

[Big John T]

Is there a formula to calculate the torque or a table?

Can you just use a clamp on with peak hold to get max amps then convert to watts to get power used?

John

 

[swhite65]

Maybe. You may be able to use the 1 ms sample setting and peak hold for example on a Fluke Meter with a clamp-on plugged in. Servos are more sized on torque requirement and the motor should have the torque constant on the nameplate (Amps per Nm ).

 

[kamenges]

From post #6, Big John T says 6 lb-in/A. He also said the motor listed 145 lb-in, but he never said if that was continuous or peak. Given the motor frame, I'm guessing peak since that torque level would require just shy of 25 amps. It would be nice to know the continuous torque or current rating.


Keith

 

[Big John T]

Maybe. You may be able to use the 1 ms sample setting and peak hold for example on a Fluke Meter with a clamp-on plugged in. Servos are more sized on torque requirement and the motor should have the torque constant on the nameplate (Amps per Nm ).

All the printed data labels have long since been cleaned off by folks preparing the machine for painting... The only info we can read on the nameplate are the stamped numbers...

John

 

[Big John T]

From post #6, Big John T says 6 lb-in/A. He also said the motor listed 145 lb-in, but he never said if that was continuous or peak. Given the motor frame, I'm guessing peak since that torque level would require just shy of 25 amps. It would be nice to know the continuous torque or current rating.


Keith

The mains on the drive are fused with 25 amp fuses.

John

 

[JERA]

Unidrive SP from Control Techniques

Hi Big John T


Try to have a look on Unidrive SP from Control Techniques at this link: http://www.controltechniques.com/products/products_frame.htm?ac_drives.asp&mainFrame

This is a very good solution for your application and with The SM-Encoder Plus option you have the feedback control.

I have made a lot of solutions with this drive, but I will also use the SM-Applications module in application like the one you have. With the SM-Applications module in the unidrive you make the drive application in the drive, an only send the setpoint and start command to the drive, and the drive make / control the movement. In the way you can send a different setpoint to the drive, depending of the position you want and start the movement.

 

[Big John T]

The mains on the drive are fused with 25 amp fuses.

John

The mains are fused with 35 amp fuses not 25's. The current draw is 6.5 amps so it's using about 18.5%...

 

[Big John T]

Hi Big John T


Try to have a look on Unidrive SP from Control Techniques at this link: http://www.controltechniques.com/products/products_frame.htm?ac_drives.asp&mainFrame

This is a very good solution for your application and with The SM-Encoder Plus option you have the feedback control.

I have made a lot of solutions with this drive, but I will also use the SM-Applications module in application like the one you have. With the SM-Applications module in the unidrive you make the drive application in the drive, an only send the setpoint and start command to the drive, and the drive make / control the movement. In the way you can send a different setpoint to the drive, depending of the position you want and start the movement.

I have used a FM servo drive from them once...

Is the setpoint start and direction command set via digital inputs or via communications?

Thanks
John

 

[Big John T]

There has been a lot of replies and lots of help on this application. I have attached a picture of the motor and drive as a picture is worth a thousand words.

John

 

[Big John T]

Hi Big John T


Try to have a look on Unidrive SP from Control Techniques at this link: http://www.controltechniques.com/products/products_frame.htm?ac_drives.asp&mainFrame

This is a very good solution for your application and with The SM-Encoder Plus option you have the feedback control.

I have made a lot of solutions with this drive, but I will also use the SM-Applications module in application like the one you have. With the SM-Applications module in the unidrive you make the drive application in the drive, an only send the setpoint and start command to the drive, and the drive make / control the movement. In the way you can send a different setpoint to the drive, depending of the position you want and start the movement.

Hi Jera,

Getting info from them is like pulling teeth! I called the tech support and was in a round about way told to call back when I knew everthing about their products and had a specific question!

John

 

[milldrone]

The mains are fused with 35 amp fuses not 25's. The current draw is 6.5 amps so it's using about 18.5%...

Big John,

Is this reading peak (when servo is accelerating), or constant? Single phase? Three Phase? 240 volt, 480 volt, Or?

Just for gigles can you temporary install a 2hp @ 1750 145tc to the flange? And test it without breaking any thing?

Is the torque that can be applied to the shaft with the machine not running the same as when the machine is running? If so can you put a temporary lever on the shaft and attach an ordinary spring scale (like you weigh fish with) 1 foot from the center line of the shaft? Then pull on the scale at 90 degrees to the temporary lever. This will give you a foot Lb. torque reading.

 

[swhite65]

Finding out what the actual peak current is is important. Also, looking at this picture, the gearbox and shaft intertia need to be considered. Sometimes the actual torque requirement of an application is low but a large inertia mismatch can cause major problems. The rule of thumb is that the reflected inertia of the load should be no more than a 10 to 1 ratio to the inertia of the motor. The inertia of the load would represent the inertia of the gearbox and what is connected to the output of the gearbox(and technically the coupling also). The reflected inertia of the load connected to the gearbox is equal to that inertia divided by the square of the gear ratio. So if the gear ratio is 10 to 1, then the reflected inertia would be the inertia of the load divided by 100, plus the inertia of the gearbox. Compare this value to the inertia of the motor.

I apologize for the wordiness of this but if the peak current is acutally around 6 amps, then that would specify a much smaller motor from a torque standpoint. The inertia factor should also be considered.

If the cycle time is such that you have a "generous" amount of time to stop within position after a move, then the inertia factor will be less important.

AC motors usually help in cases where you need a lot of motor inertia due to the construction of the rotor. Servo motors come in many styles, some very low in inertia.

 

[JERA]

I have used a FM servo drive from them once...

Is the setpoint start and direction command set via digital inputs or via communications?

Thanks
John

In the applications I have made where the setpoint and command set by communication. I have tried Profibus, Interbus and Devicenet as communication for the Unidrive with no problems.
I am always using a home position and controling the movement with a setpoint and start command. When the movement was done the drive answered back with a done bit.

I have made applications with fixed positions where I give the drive a positions number and the drive go to the programmed setpoint for the position or with “floating” positions where I send the setpoint, all depending of the application.

When I am using Unidrive I save the high speed counter card in the PLC and program in the PLC for controlling the movement. The control of the movement is in the Unidrive and the only thing I need is to send is an setpoint / position and a start command.

I have also made one application where the setpoint first is active when an input on the drive goes high. This was used to set load on a “flat” conveyorbelt in correct position under a camera.