Am not getting the torque

[adroitguy]

Hello ,
I need some advice from the pros.
I have a bi-polar stepper which is rated for 12V / 0.67A/phase.
This motor is supposed to deliver 7Kgf.cm (97.21 oz.in).
I am driving this with a L297/L298 combination .
I attached a weight to the pulley and tried lifting a dead weight and found a maximum pulling torque of 4kg.cm @ 60rpm. I am inable to go beyond that with this.
I am using a 24VAC transformer with a bridge and a 2200uF cap to filter the AC.

Is this a valid design ?Where am I going wrong ? I need 7kg.cm atleast upto 120RPM .
Can anyone help me ?
I will supply any additional info as required .

Regards

[ger21]

That 7kg would be with a 1cm dia. pulley at 0 rpm. Once it starts spinning, the torque will start to drop. Sounds like you need a bigger motor, preferably with a lower V rating.

[CJL5585]

I second Ger21's recommendation.

Jerry

[ViperTX]

Yes, you have to look at the torque curves for steppers and determine the RPM range where you want to run.

[eidos]

Originally Posted by adroitguy Hello , I need some advice from the pros. I have a bi-polar stepper which is rated for 12V / 0.67A/phase. This motor is supposed to deliver 7Kgf.cm (97.21 oz.in). I am driving this with a L297/L298 combination . I attached a weight to the pulley and tried lifting a dead weight and found a maximum pulling torque of 4kg.cm @ 60rpm. I am inable to go beyond that with this. I am using a 24VAC transformer with a bridge and a 2200uF cap to filter the AC. Is this a valid design ?Where am I going wrong ? I need 7kg.cm atleast upto 120RPM . Can anyone help me ? I will supply any additional info as required . Regards

12 volts dc?
hummm
no good for 297 298 drivers!
these motors are high resistance phase ! + - 10 ohm ?!
I recommend you to measure the resistance of 1 phase, and itself is taller than 3 ohms, that motor are not all right for the pilotage in pwm 297 298 .
you will also have always a low couple feeding them to 40 volts.
visit my site!!
hi

[adroitguy]

Thanks Ger and all of you for your kind reply.

I have measured the torque at a pulley of 50 mm dia. ,where it is lifting about 1.5 kg , hence (1.5 * 2.5 = @ 4 Kg. cm ) .

Sadly I have already purchased the stepper and it cannot be exchanged for a lower voltage motor. My only recourse now seems to be to hack a solution.

Would increasing the voltage to 36V give me a better torque even at lower speeds ? As per my understanding the only way to get a better torque at a higher speed is to increase the voltage . Would this help for lower speeds too ?

[ger21]

Originally Posted by adroitguy As per my understanding the only way to get a better torque at a higher speed is to increase the voltage . Would this help for lower speeds too ?

It might help. but I think the bigger problem is that the motor is barely adequate torque-wise.

[adroitguy]

Originally Posted by ger21 It might help. but I think the bigger problem is that the motor is barely adequate torque-wise.

Hi Gerry ,

What I am trying to get across is that, I am trying to see if the motor is meeting its specs , which it is not . It does not deliver the rated 7kg.cm which it is supposed to deliver.

Eidos , unfortunately your page is in Italian , difficult for me to comprehend !!!

Thanks for your replies and that of others.

Regards
Thomas

[adroitguy]

Originally Posted by eidos 12 volts dc? hummm no good for 297 298 drivers! these motors are high resistance phase ! + - 10 ohm ?! I recommend you to measure the resistance of 1 phase, and itself is taller than 3 ohms, that motor are not all right for the pilotage in pwm 297 298 . you will also have always a low couple feeding them to 40 volts. visit my site!! hi

Hi Eidos ,
By couple I think you mean torque , so you say that with L297/L298 I will not get the torque ?
My motor's resistance is 17.5 ohms. Do you have any schematic posted on your site , can you give me a link ?

Thomas

[ger21]

Originally Posted by adroitguy Hi Gerry , What I am trying to get across is that, I am trying to see if the motor is meeting its specs , which it is not . It does not deliver the rated 7kg.cm which it is supposed to deliver. Regards Thomas

Like I said, the rated torqe is at 0rpm, without the motor spinning. You'll get less torque with it spinning. Will it hold 7kg without spinning?

[adroitguy]

Originally Posted by ger21 Like I said, the rated torqe is at 0rpm, without the motor spinning. You'll get less torque with it spinning. Will it hold 7kg without spinning?

Hello Gerry

Yes the motor holds 7 kg without spinning , but that does not help me much .
It is made by a company called srijan drives , but I am not able to get this data from them .
have to show it to a customer , where do you think I can get the torque / speed characteristics for this motor ?
Would this curve be more or less same for most bipolar motors ?

Regards
Thomas

[ger21]

Originally Posted by adroitguy Would this curve be more or less same for most bipolar motors ? Regards Thomas

With similar specs (volts and amps), torque curves should be similar.