stepper motor get overheated

[momin]

hey everybody,
I'm using a PM35L-048 stepper motor for my final year project on mobile robots. the motor datasheet mentions that the rated voltage is 24 V. when using this voltage the motor works well but get over heated as long as the voltage is applied to it. this will cause a problem since i need to apply voltage to the stepper for a very long time but it keeps get hotter. i need to know what is the right procedure to follow.

 

[Ubergeek63]

do you actually need the holding torque?

 

[rahulshah]

the motor gets overheated not due to 24 v supply but due to the current. try to keep 5ohm, 10 W fusible resistors in series of each winding.

 

[rahulcet]

What shah told u is ok but it will also reduce the torque ouptut. When a stepper motor phases is switched sequnetially due to rate of chnage of flux volage is induced in the windings and the current is thus reguated to a safe value, which according to the motors ratings will not produce much heating. If u do not want holding torque u should not give voltage to any of the phases whne U dont want the motor to run.
Hope this helps
Good LUCk

 

[Sceadwian]

Seeing as how this is for a robot and you're likely already using a micro controller or other control system you could simple pulse width modulate the power going to the stepper when you're not running through the stepping cycle, this will allow you to maintain a variable holding torque you can adjust but alter the PWM duty cycle to any value that will keep your robot in place.

 

[momin]

thank u guys for responding;
i do need the holding torque, the motors are used to drive wheels and the holding torque will help as a brake.
when i tried to connect resistors in series with the motor coil in the computer simulator (ISIS Proteus) the motor rotor moves much slower. and I'm concerned about the power loss due to these resistors since I'm supposed to use batteries to power the system.
I'm using a microcontroller to control the motors but PWM technique seems complicated because i did use the same circuit with different stepper motors (PM55L-048) and it works without heat.

 

[Sceadwian]

How is the PWM technique complicated? It's simpler than the stepping routine used to drive it... I'm asuming you have variable(s) set aside for the stepping routine. A byte -127 to +127 with 0 being brake, just set up a conditional if brakeing to PWM the stepper port from 0 to it's normal state with a 1/8 1/4 or 1/2 duty cycle. If you want to be advanced about it you should have a thermistor attached to the motor to adjust the PWM dynamically based on motor temperature, that will give you maximum torque and keep your motors from going up in smoke or wearing out too fast and would be applicable to any motor you attached to it.

 

[AlainB]

Hi,

I have many of these motors. I used them at the beginning to learn about stepper driver. It is true that they are heating very much. However, it is not surprising considering that the coil resistance is 30 Ohms per phase for the unipolar witch translate to 19,2 watts at 24 volts. If you use them full step (that is 2 phases always energised) it would be almost 40 watts.

I was using them at 12 volts. They work not so bad at 12 volts, consuming only 4,8 watts per phase. Full step 9,6 watts. Still heating but less.

Alain

 

[Ubergeek63]

if they are overheating you are overdriving them. PWM drives are great for increasing the speed, but the torque is still current limited

 

[momin]

thanx guys for replying;
I'm using a delay of 500ms between each step, hence each coil is energized for about 1s since I'm using full steps. would it help to reduce the delay given that I'm not concerned about the speed?

 

[Sceadwian]

You're running a stepper but you don't care what speed it runs at? That's a bit odd. If you're worried about the heat so much, simply use a lower voltage power supply like AlainB mentioned. Or better yet, use a higher voltage power supply but one that's current limited to what the stepper is rated at. Steppers are not voltage driven devices they're current driven, as long as you don't use a voltage high enough to break down the insulation.

 

[AlainB]

These motors run at 48 steps per revolution. Do you want to "spin" your motor at 1,25 revolution per minute? That is what I understanded from your last post.

Anyways, the slower you spin them, the stronger they are but the warmer they get. Spin them at 300 RPM as long as you want (at 12 volts) and they will stay just warm. Stop them with 2 phases on and they will get hot in a few minutes. Stop them with 2 phases on at 24 volts and I suppose they will get very hot extremely fast.

Alain

 

[momin]

I'm spinning the motor at 1,25 RPM for testing purposes. thats why i don't care about the speed that much. but at advanced stages we may need to control the speed, so what is the range of speeds that these steppers can handle without getting very hot. and what is the maximum speed if the motor is off load.

 

[AlainB]

Sorry, I don't know much more about those motors. But there is a datasheet available.

Alain

 

[Sceadwian]

Steppers don't typically run very fast, they're used for precision feeding or high torque not really high speed. You'll know you're trying to spin it too fast when you suddenly lose torque or the motor starts 'tweaking' instead of running in a constant direction. Like Alain said, the heat comes with slower speeds not higher ones, when you try to spin them at a faster rate the inductance in the coils will self limit the current.

 

[rahulcet]

Yes, Steppers are not really meant fro fast speed operations. again the speed is not proportional to heating, the heating and the torque being proprtional to current in each phase. jus try running ur motor at gradually decreasing <delays between energising the next phase> and find the max speed at which ur motor can be run. Good luck

 

[be80be]

If you switch a stepper to fast and have high current it will over heat I would check the current going to the stepper and limit it to what it is rated at

speed is not proportional to heating

But the longer it is on can happen two way holding and over driving

 

[shingadaddy]

"thank u guys for responding;
i do need the holding torque, the motors are used to drive wheels and the holding torque will help as a brake."

Ever think about using worm gears and acceleration and deceleration profiles for smooth start and stop? With a worm setup, the motor is not needed for holding torque.

 

[blueroomelectronics]

"thank u guys for responding;
i do need the holding torque, the motors are used to drive wheels and the holding torque will help as a brake."

Ever think about using worm gears and acceleration and deceleration profiles for smooth start and stop? With a worm setup, the motor is not needed for holding torque.

I was just reading the post and thought the same thing

 

[shingadaddy]

Great minds think alike maybe! Or maybe you're just thinking like a newbie
And would you mind closing the door up there. It's snowing here

Gear ratio is easy to determine too. Just = the number of teeth on the output gear. Can always add a makeshift slip clutch if desired too.