Nimh-0030 H-bridge Questions

[ND_Stryd3r]

I am controlling a 6v DC motor with a nimh-0030 h-bridge using PWM and GPIO from an ISOPod. Two Questions/Issues:

1. What is the optimal PWM period for this h-bridge? Should I set it around the max switching frequenct of 2kHz? Or, does this even matter?

2. Using a multimeter to test the MA pins and MB pins that the DC motor connects to, a PWM duty cycle of 50% correctly changes the supply voltage from 6v to 3v. Note: this is while the motor is NOT connected. Once the motor is connected, a 50% duty cycle gives anywhere between 4v and 5v and sometimes the motor does not even start to drive??? (Although it is most definitly getting voltage). Perhaps the current is somehow limited and isnt enough to start the motor (the motor is a faulhaber and actually quite efficent).

Any suggestions on either?

Note: the 6v supply for the motor is at the very lower end of the operating range for this H-bridge. Could this be a factor?

[Dave]

PWM frequency should be looked at as what is most optimal for the motor being driven, up to the limits of the h-bridge driver being used. So for :

1. doesn't matter, depends on motor electrical time constants.

2. What you are seeing with the meter is the inductance of the motor creating a higher voltage when the current is cut off. An inductor attempts to maintain the current going through it, and as such, raises the apparent voltage. I

In looking at the motor voltage on an oscilloscope, you should see a spike every time the voltage is switched off, created by the collapse in the induction of the motor. Easiest method is to simply try out differeing frequencies, up to the capabilities of the h-bridge, to see what most effectively drives a particular motor. There are people that claim good results in the 100s of hertz and others (coreless motors) that operate best above 20 kHz.

[ND_Stryd3r]

Thanks much, dave.

[ND_Stryd3r]

Just one more quick question, can this hbridge (nimh-0030) support pwm frequencies of 20kHz. The manual says the pwm switching frequency is 2kHz, but doesnt specifiy if this is a max or min. Thanks!

[RMDumse]

The TI controller chips in this family are nice for high current in a small package, but they are certainly limited in speed. The 2K is an upper limit, not a lower one. They will not do 20KHz.