DRV8711EVM output pulse width too narrow

Hi John,

Can you provide the settings that you are using, the motor parameters, and scope shots of the gates of the FETs?

With the many settings available with this device, it may provide a clue.

Thank you.

Hi John,

Have you determined the cause of the short output pulse? If not, please send the information requested.

Thanks.

Rick:

Unfortunately I can't provide the info that you requested because I don't have access to the DRV8711EVM board now, as it's in the hands of the motor developer.

However, I fear I may have incorrectly chosen the DRV8711EVM board for our application as the motor is a 2-phase Switched Reluctance or Variable Reluctance motor, not a permanent magnet motor. I cannot find info that indicates if the DRV8711EVM is suitable for a SR or VR (not PM) motor. Can you advise me?

The motor developer is considering changing his design to a 3-phase SR or VR motor. Can you recommend possible EVM board(s) for such a motor.

Thanks, John

Hi John,

Can you provide a link to the motors that you are trying to drive?

Also a link to the 3 phase motors being considered will help with an EVM recommendation.

Thanks.

Rick,

Thanks for your prompt reply and questions.

Here are links to info about the SR and VR motors that the motor developer plans to make:

http://en.wikipedia.org/wiki/Switched_reluctance_motor

Here is more info from fleadh.co. The original motor that my client has developed is a two-phase one but I believe he will be working on making a 3-phase motor because the 3-phase motors are easier to start and produce less ripple:

http://www.fleadh.co.uk/srm.htm

and a simulation paper at:

http://www.opal-rt.com/sites/default/files/technical_papers/EPE-PEMC-2010-Opal-RT-FINAL.pdf

and a TI article at:

http://www.ti.com/lit/an/spra420a/spra420a.pdf

The motor designer hopes to not have to use Hall sensors in his motor to keep costs low.

Thanks, John

Hi John

The SR motor need an asymmetric bridge to do the driving. The idea is to energizing the current with one direction by two MOSFETs and then providing a fast decay path for the recirculation of the phase coil.

So you can use a full H bridge in an asymmetric way by just control two diagonal MOSFETs and only use the other two's body diodes as the recirculation path. Although you just left two MOSFETs of a full bridge never controlled in this way, but you can quickly get the hardware done with existing EVM boards.

If the voltage and current level is meet, DRV8711 EVM will be good to control a two phase SR motor just in PWM mode (without on chip index). But as you said if we are trying to control three phase SR motor, we need three full bridge. We have DRV8823 which have four full bridges inside but it is a serial interface. Anyway, you can choose other two or three devices of DRV8x serials with proper voltage and current rating to get three full bridges with phase/enable or IN/IN interface to open the logic control with external MCU.

Best regards, 

Wilson:

Thanks but I confess that I don't completely understand your reply.

1) You said that the DRV8711EVM board is suitable for the 2-phase SR motor, I believe and

2) I think you said that the DRV8823 chip would be suitable for a 3-phase motor (a 6/4 motor I believe) but you lost me after that. Would the DRV8823 EVAL board, the DRV8823EVM, be suitable for a 3-phase motor?

Or were you saying that I would have to make a board with two or three of the DRV8823 chips plus some MOSFETs?

Thanks, John

Hi John

I mean for each coil of the SR motor, we will need a full bridge to control it. And it is just a workaround method to drive a SR motor in case you don't want to build a dedicated asymmetric driving hardware (including two MOSFETs and two diodes just like the examples in your reference link)

DRV8823 has 4 full bridge(No external MOSFETs needed), so it is possible to drive a 3 phase SR motor. But the SPI interface will affect your MAX switch speed and the running speed of the SR motor. We need more verify according to the application.

All above is for SR motor. DRV8823 is never intend to drive a normal 3 phase BLDC.

Best regards,

Thanks very much Wilson. Very helpful! When you say DRV8823, I assume you mean the DRV8823EVM board, but I will study up on it to make sure I understand how to use it for the SR motor. It looks very promising for our 3-phase (6/4) SR motor.

John

Hi Wilson

My client is designing his 3-phase SR motor and for his testing purposes will need to rotate the shaft at up to 2000 RPM, 24 steps per revolution. Later on he plans to operate the motor at speeds up to 4000 RPM. And he expects to operate the motor at up to 48VDC and several amps, probably up to 3A.

The DRV8823EVM looks problematic for my client's needs with the SPI interface to the chip.

What do you think? Are there any other TI possibilities?

Sorry for the delay getting back to you. I was busy with another project.

Thanks, John

Hi John,

Sorry for my late response. Since you need to get three full bridges, I think it is hard to find one part solution.

Below choices may be not extactly matching, but worth trying.

1. Using two DRV8412. One device drives two phases of the SR motor and the left phase is driven by the other device with only 1/2 of the part used.

2. Using three DRV8840. But the MAX voltage is 45V.

Actually we haven't got any real experience to drive a SR motor using above device. So the suggestion here is just preliminary proposal.

Best regards,

Best regards,

Wilson,

I'm still hoping for an EVM board and am wondering if the DRV8312-C2-KITmight be suitable. Note that our motor will be sensorless and will be a 3-phase SR motor.

Thanks,

John