MID-7654 Current Limits

[chadmac]

Hello,

I am using an MID-7654 drive to power DC servo motors with the following specifications:

- Continuous Current:  4.2A

- Peak Current:  21A

- Inductance:  1.92 mH

- Maximum Terminal Voltage:  50V

My original understanding was that the current limit DIP switches on the front panel changed the gain of the system such that a +/-10V input would correspond to the peak current limits (+/-) [Quote from the  MID-7654 manual pg 10 - "The MID-7654/7652 varies the gain applied to the input voltage so the maximum input voltage corresponds to a current output equal to the peak current limit, Ipeak."].  Being paranoid like I am, I wanted to verify this thought before doing anything.  For current settings of 0.85A/1.7A (cont/peak) I measured a gain of approximately 0.99A/V and for current settings of 1.25A/2.5A (cont/peak) the gain was approximately 1.0A/V (ie:  no altered gain).  In both cases the output current saturated at the specified continous current limit (for a continuous input voltage).

Based on the above measurements, I have the following questions:

- Do the DIP switches simply change the current limits without altering the actual gain of the MID-7654? 

- If this is indeed the case, is there a way to change the gain of the drive such that +/-10V corresponds to a desired peak current range (say 4.2A or 4.9A)?

Thanks,

Chad

[David S.]

Hi Chad -

The gain settings on the drive should be changing the actual gain of the device.  Reading through your numbers, I'm curious whether you saturated the device immediately.  What input voltages did you provide, and what output currents did you see?  What was the impedance load placed on the motor when measuring?

[chadmac]

Hi David,

I have attached an Excel spreadsheet with the voltage and current measurements I made.  I supplied the input voltage by directly accessing the DAC on the PCI-7344.  The actual voltage measurements were made from the AOUT block on the back panel of the drive.  The current was measured with a DC ammeter in series with the motor, while the output shaft of the motor was fixed in place to prevent motion.

As you can see from the measurements, the motor is not saturating right away and the gain is linear (~1A/V) with a slight DC offset.  Voltages higher than those shown in the Excel sheet simply saturated the output current at the level specified by the DIP settings on the front panel.  Since the output saturated around the selected limits, I would think that these current measurements are reasonably accurate.

Thanks,

Chad

[allen_h]

Hello Chad,

The voltages from the AOUT terminals of the MID-7654 are actually Pulse Width Modulated (PWM) signals (see document below). 
KB 37FC7745: Relation Between Maximum Voltage Spec and Power Amplifier (Drive) Voltage Spec

National Instruments Motion controllers output a Voltage from -10 to +10 V. The max voltage of 10 V corresponds to the peak current output of the MID as configured by the dip switches on the Drive. If you have selected that your drive have a maximum current output of 1.70 A, by using the dip switches on the drive, then a 10 V signal from the motion controller will result in 1.70 A output from the drive. The continuous current output of the drive with this setting is 0.85 A. A 5 V signal from the controller corresponds to this 0.85 A output on the drive.

However, the drive can will only output a current higher than the continuous current for a maximum of 2.7 seconds.  See the document below for more information.
KB 2WJD5L45: What is the Significance of 2.7 sec. for the Peak Current Value

Allen H.

[David S.]

Hi Chad -

We talked over your data and looked further into the current settings with our technical leads.  You're correct:  the current is limited by current limit code in the current loop on the drive.  There isn’t any gain applied to the output.

I'm sorry that our manual has the wrong information.  I'll work to get the next revision corrected with the proper information.  Thanks a lot for your help in revealing this mistake.