marklewry
Electrical
- Sep 9, 2001
- 10
Hi all,
I have recently built a drive board for a stepper motor. This is a unipolar drive and simply has 4 mosfets, 4 fast recovery diodes and 4 1 ohm 3watt resistors (2 in parallel = 0.5 ohm sense resistors). All was going well until I encountered a problem using a different motor. The current surges were very large and everything started getting very hot. The motor was not reacting very well either. The drive is controlled by a pwm chip L297 from ST. This has a comparator that looks for the voltage across the sense resistor and compares it to the preset voltage (Vref). When the voltages are equal the output turns off. I found that there is a small delay before it 'looks' ate the voltage. This can be changed by adjusting the value of the capacitor for the oscillator to the chip. It seems that if the delay is too long the current is allowed to build up too rapidly without control, so making this delay smaller helps.
Here is the question:
Why does the current build up in such a small period of time 2micro seconds when the theoretical build up should be much smaller L/R time constant. The motors inductance is 4.5mH, resistance is 1.5ohms and 3V. My chopper circuit uses a 40V supply. According to the L/R time in 5micro seconds it should only be 50mA? approx.
If anyone has any info on how to correctly work out he oscillator frequency and delay period for a particular type of motor I would appreciate the input.
Many Thanks
Mark Lewry
I have recently built a drive board for a stepper motor. This is a unipolar drive and simply has 4 mosfets, 4 fast recovery diodes and 4 1 ohm 3watt resistors (2 in parallel = 0.5 ohm sense resistors). All was going well until I encountered a problem using a different motor. The current surges were very large and everything started getting very hot. The motor was not reacting very well either. The drive is controlled by a pwm chip L297 from ST. This has a comparator that looks for the voltage across the sense resistor and compares it to the preset voltage (Vref). When the voltages are equal the output turns off. I found that there is a small delay before it 'looks' ate the voltage. This can be changed by adjusting the value of the capacitor for the oscillator to the chip. It seems that if the delay is too long the current is allowed to build up too rapidly without control, so making this delay smaller helps.
Here is the question:
Why does the current build up in such a small period of time 2micro seconds when the theoretical build up should be much smaller L/R time constant. The motors inductance is 4.5mH, resistance is 1.5ohms and 3V. My chopper circuit uses a 40V supply. According to the L/R time in 5micro seconds it should only be 50mA? approx.
If anyone has any info on how to correctly work out he oscillator frequency and delay period for a particular type of motor I would appreciate the input.
Many Thanks
Mark Lewry