Why is does the motor spin at twice the specified/indicated speed?

Jeff McDevitt

Other Parts Discussed in Thread: DRV8301

I am getting started with a F28017F Launchpad with a DRV8301 Booster Pack. I am working through the labs in the InstaSpin Projects and Labs User’s Guide. I have the gMotorVars.SpeedRef_krpm set to 0.160. When I enable the system, the indicated speed (gMotorVars.Speed_krpm) closely matches this value. However, when I use an Extech Tachometer I measure the motor speed at 320 RPM. I am using an Anaheim motors BLWS232D_24V_4000 motor. My motor parameters from the user.h file are

#define USER_MOTOR_TYPE MOTOR_Type_Pm

#define USER_MOTOR_NUM_POLE_PAIRS (4)

#define USER_MOTOR_Rr (NULL)

#define USER_MOTOR_Rs (0.4221607)

#define USER_MOTOR_Ls_d (0.001263413)

#define USER_MOTOR_Ls_q (0.001263413)

#define USER_MOTOR_RATED_FLUX (0.07853003)

#define USER_MOTOR_MAGNETIZING_CURRENT (NULL)

#define USER_MOTOR_RES_EST_CURRENT (1.0)

#define USER_MOTOR_IND_EST_CURRENT (-1.0)

#define USER_MOTOR_MAX_CURRENT (3.0)

#define USER_MOTOR_FLUX_EST_FREQ_Hz (20.0)

Am I making a rookie mistake?

Thanks

over 7 years ago

0 Rick Duncan over 7 years ago

TI__Guru** 114905 points

Hi Jeff,

Your post is being moved to the C2000 forum for better support.

0 Ecologix 123 over 7 years ago

Prodigy 170 points

I reckon the BLWS232D_24V_4000 motor is a 4 pole motor which translates to

#define USER_MOTOR_NUM_POLE_PAIRS (2)
rather than 4 pole pairs which means your user setting is set for an 8 pole motor

0 ChrisClearman over 7 years ago in reply to Ecologix 123

TI__Guru** 118270 points

when you command 0.16 KRPM you are commanding 10.66 Hz
= 160 * USER_MOTOR_NUM_POLE_PAIRS / 60

If you are measuring 320 RPM with a tach then the actual USER_MOTOR_NUM_POLE_PAIRS = 2 =
10.66 Hz * 60 / 320

I'd guess you set the pole_pairs wrong

0 Jeff McDevitt over 7 years ago in reply to ChrisClearman

Expert 1775 points

It was indeed a rookie mistake - I had the number of poles wrong. Thanks for the help!

One more question - I am seeing approximately a +- 5 RPM variation in the motor speed with no load at 320 RPM. Is this typical or should I be able to get it tighter?

Thanks again.

0 Yanming Luo over 7 years ago in reply to Jeff McDevitt

TI__Guru** 111090 points

It's typical variation, and you could tune the speed and current PID parameter to improve the performance.

0 ChrisClearman over 7 years ago in reply to Jeff McDevitt

TI__Guru** 118270 points

10.6 Hz is pretty low speed for a sensorless algo, and +/- 5% is pretty good for a speed controller, but you can do better. With more load you will see better steady state performance (As there will be more current to monitor) and tuning the speed controller will help. You could also use a higher performance speed controller like the SpinTAC version in InstaSPIN-MOTION if want to get even better. But at 10 Hz the main limitation will be from the analog sampling and that effect on FAST.

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Why is does the motor spin at twice the specified/indicated speed?