Changing the DC-bus motor voltage on the MID-7654

[jmetcalf]

Hello,
I am new to using both Labview and Motion Control so these questions may seem somewhat simple. I have a Motion Control 7344 connected to a MID-7654 Power Drive. I am needing to use the MID-7654 to control some 12VDC PM servo motors (they have a 13VDC MOV built into them). If I understand the user manual for the MID-7654 correctly the servo motor drive controls to a current set by the analog out of the 7344 and the voltage output is 48VDC. My question is two part, first am I correct in my understanding that the Power Drive controls the current to the servo motor and not the voltage? Second, is there a way to adjust the maximum output voltage of the servo drive to 12VDC?

Thank you for any help that you can provide...


--Jon

[Dan_Quinn]

Hi Jon,

Thanks for using our discussion forum. First of all, yes, you are correct in assuming that the MID controls the current provided to the motor. Current is what in fact drives motors and not voltage. You will notice on your MID-7654 that you have a series of dip switches which are appropriately labeled on the top panel. Setting them will set the appropriate current for your motor. It sounds like you have concerns over not sending more than 12V to the motor, which is certainly a legitimate concern. However, your motor is probably rated based on its current. What I would suggest is getting the current spec from the motor manufacturer and setting the dip switches appropriately on the MID. If you can't find this spec, you might want to try getting the load provided by the motor. Then if you know 12V and the resistance, you should be able to figure out the max current to set based on Ohm's Law, V=IR.

I hope this helps you out. Good luck on your application!

Regards

Dan Q
Applications Engineer
National Instruments

[jmetcalf]

Dan,

[jmetcalf]

Dan,
First off thank you for your response. The motor that I am needing to use in my application is a 12VDC P.M. motor. The motor also has a 13VDC MOV built into it for EMI suppression. It's locked rotor current is 2Amps. This gives me an impedance of 12 ohms under maximum load (i.e. locked rotor). Unfortunately in my application I have a varying mechanical load there by causing the impedance of motor to vary. If the impedance of the motor goes above 6 ohms then at 2Amps I will have greater than 12V until the PID control brings the velocity back to the setpoint.

I also have an application in which I would like to run a P.M. DC motor(12VDC @2A max. With a 13VDC MOV)without an encoder attached (mechanically can not fit it in). Ideally I would run this motor at a fixed DC voltage. I have thought about using the ADC input as a feedback but this does not appear to be isolated from the servo output.


Thanks for any ideals...

--Jon

[Dan_Quinn]

Jon,

Could you send me a link to where I can find info on your motor? I'd like to check some of the specs on it.

Thanks!

Dan Q
Applciations Engineer
National Instruments

[jmetcalf]

Dan,
Unfortunately I can not provide the datasheet since the motor is a custom made part. However I can provide another datasheet for a similar motor by the same manufacture. The only major difference is that the one that I am using has a 13V MOV in it.

http://www.mabuchi-motor.co.jp/english/cat_files/rk_370ca.pdf

[jmetcalf]

Dan,
Reference part number RK-370CA-18260 in the datasheet.

[Dan_Quinn]

Jon,

Something to consider is that even if we were theoretically limiting the voltage at a given load, we would then be limiting the current to the motor and not getting the proper output. So realistically, we need to limit the current since that is what is possible on the MID-7654.

Also though, I've been starting to wonder about the inductance of your motor. I want to make sure that it isn't too low before it gets running. If you could get that spec for me it would probably help out.

Regards

Dan Q
Applications Engineer
National Instruments

[jmetcalf]

Dan,
I am going to need to get the inductance spec. from the motor manufacture. I am trying to get better specs for the motors directly from the manufacture but my email has been down since Friday (server crashed). As soon as I get better specs I will post them here.

Thanks again for your help.

--Jon

[jmetcalf]

Dan,
The manufacture representative has gotten back to me with the inductance spec. To quote him, "The inductance of the motor is about 4mH at zero speed."

--Jon