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Brush DC Motor control - problem with low speed regulation

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Lisa

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Hi everyone,
I'm Lisa from Italy, I'm new to this forum, I hope I can find help.

I have a problem in driving a brush DC motor I'm using to make a drill.

The motor I'm using is a Portescap 23GST2R82-216P: **broken link removed**.
I'm driving it using Intersil HIP4086 driver (I need this driver because on the same machine I need to optionally plug in a brushless motor), and STD20NF06 ST mosfets with embedded diodes. My micro is a PIC 24FJ256GA110, which provide PWM peripherals.
I'm using sign-magnitude drive, with high side mosfet closed and pwm on the low side mosfet.

I need to give to the motor an overvoltage (32V) for short usage periods to reach higher rpms.

Anyway my problem at the moment is with low speeds: if I drive the motor with VM=24V and PWM Duty cycle 1% I would expect to see it running slowly, but I see it still.
I also tried to start it with a higher duty cycle and to reduce it gradually once the motor is in movement, but it stops as soon as it reaches 1% duty cycle.

Moreover the speeds I measure with higher PWMs (with >2% duty cycle the motor starts) are significatively higher than the ones I would expect. In example, with VM=24V, duty cycle 10% I measure no load speed of 9020 rpm, more than 12VDC nominal value reported on the datasheet!

I'm using 15.6kHz PWM frequency, but the problem is exactly the same even if I slow down the pwm frequency to 244Hz.

I provided the circuit for current and back emf measurements, but I still didn't embed speed and torque regulation in firmware, this was the first "rough" attempt to see the motor running, and I would have expected a speed proportional to the values reported on the datasheet (like 8700rpm with VM=24V, 50%duty).

It's my first attempt in working with motors, I studied a bit of theory, but really I can't understand what I'm wrong in practice.

Can anyone help me?
Thank you very much :)
 
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Lisa

New Member
Sorry, a rectification about what I wrote above:
it's probably right that with 1% of duty cycle I get no rotation, as even with 0,24VDC directly applied to the motor it stays still.
So the problem is rather that with 2% of duty cycle I get a shaft speed of 688 rpm!
 

smanches

New Member
Do you have a cap between the drive and motor? It will reduce the impedance slighty so that more current can flow at the lower duty cycles, which the motor will use during the off cycle and smooth out it's operation. Make sure it's not polarized so the back EMF spike doesn't cause it issues.

BTW, those mosfets do NOT have a protection diode built in. The diode they are showing is the body diode. Be careful that you take that into consideration. I would suggest adding an actual protection diode as well.
 
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