gear ratio change - how does it affect the motor ? 1

[edison123]

My client, a car tire manufacturer, has a rubber mixing mill (called Banburry) driven by a 1500 HP/1000 RPM DC motor. The present rubber mill runs at 40 RPM with a 3-stage reduction gearing from the 1000 RPM motor. He tried to run the mill at 50 RPM (to increase productivity !!) but landed up in problems with the field weakened operation of the motor (vicious sparking, vibrations etc).

Now, he wants to change the gear boxes ratio from 1000 RPM/40 RPM (3 stages) to 800 RPM RPM/40 RPM (3 stages). With this arrangement, he figures he can increase the DC motor field current to reduce the motor speed to 800 RPM to get 40 RPM and then run it at the original 1000 RPM to get 50 RPM at mill end.

My query - if the gear box ratio is changed from 1000/40 to 800/40 RPM, what will the torque and horsepower implications to the DC motor ? More torque / More HP required at the reduced speed ?

 

[haggis]

Edison123

The first set up wasn't great to start with. Running the motor at full speed of 1000 rpm. His second thought is half way there if he runs the motor at 800 rpm and changes the gear ratio to achieve 800/40 rpm but then he would defeat the purpose by cranking the motor back up to 1000 rpm.

A variable speed DC motor is most efficient when run at 65 - 70 % of its rated speed. So why doesn't he run the motor at 650-700 rpm and change the gear ratio to achieve say 700 input/50 output. He had intended to change the gearbox anyway.

 

[edison123]

Haggis,

Thx for your post.

The present DC motor has a base speed of 1000 RPM. To reduce this to 650-700 RPM will require a large increase in field current which will lead to motor flux saturation issues.

I agree not much of planning seems have to been made in choosing the present set up.

 

[sacem1]

I think the problem is being viewed from an incorrect point.

The owner of the mill has his machine working at 1000 rpm motor speed with a reduction gear box (25:1 ratio)that gives him 40 rpm output speed and gets a production volume X, he tries to change the speed reducer gear box to get 50 rpm output speed (20:1 ratio) and his set up begins sparking and being overworked well the reason is that the power (1000 HP) of the motor has not increased and obviously he was using the full potencial in the motor, no amp draw has been given, when he tries to run tyhe mill at 50 rpm the motor is being overstressed and is apparently not able to cope with the increase in power requierements from the mill.

In rubber mills power drawn from the motor is almost directly related to speed, so if the original speed 40 rpm drew full amp from the 1000 HP motor then increasing the speed will reaquiere for 50 rpm a 1250 HP motor.

The only way that you could stick with the old motor and increase the speed would be if the motor had a 25% unused capacity to begin with.

Cheers

SDACEM1