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How do I run a small motor with a large PL/X?

The PL/X DC Drive allows a 33 to 100% range on the setting of 2)RATED ARM AMPS in CALIBRATION.

If you are testing a large drive on a small DC motor, it may not be possible to turn down the rated armature current enough. This is no problem. You have the facility to use a small motor to test a large drive.

On the power board just to the left of terminals 51, 52, 53 you will see resistors R100, 101, 102 and 103: These are the burden resistors for the ACCTs, and they set the actual current produced by the drive.

Just above these resistors you will see a jumper which can be in either the right or left position, or it can be removed. If it is in the right hand position (default) the unit is calibrated to the normal 100% rating (72A in the case of the PL/X30). If it is in the left position, the unit is calibrated for 50% of rating. If removed altogether, the drive is calibrated for about 6A in the case of a frame size 1 drive (different for other frame sizes). The drive does not know the physical burden resistance value, so if you change the position of the jumper you need to tell the drive you have done so by changing the value of a parameter called 680)Iarm BURDEN OHMS, otherwise displayed values on the MMI will be incorrect.

For example, on a PL/X30 (72A calibration, to be tested with a 15A motor) you need to proceed as follows:

Put the burden jumper in the leftmost position.
Go to CONFIGURATION, DRIVE PERSONALITY and in that menu set 680)Iarm BURDEN OHMS to 56 ohms (this is about 50% of the original value, to match the physical burden. You do not need to be very precise). This parameter is only read by the drive on power up, so you need to perform a PARAMETER SAVE having done this.

Turn off the power completely, so the display is blank. Turn on the power and go to CHANGE PARAMETERS, CALIBRATION. You will now find that you can set 2)RATED ARM AMPS to the value you need. You can then proceed as normal with other aspects of calibration and commissioning, AUTOTUNE etc.

Incidentally, if you had a very small DC motor and only needed 2 or 3A you would have removed the jumper altogether and set 680)Iarm burden ohms to 320 ohms – the highest value allowable (for a frame size 1 drive).

If you subsequently need to run the drive on a bigger motor you just reverse the procedure, setting 680)Iarm BURDEN OHMS back to its original value. This principle applies to all of our digital DC drives (unless they are very old) – this facility was introduced around January 2000. You can find further information in section 13.13.4 of the PL/X manual - there is also a chart, which gives theoretical values for the burden resistors on all frame sizes and current ratings.


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