Using a frequency converter, is it possible to run a 55 kW motor continuously at 1200 rpm and lowering the torque to it's rated torque?

Hi,

I'm trying to wrap my head around sizing motors.

let us say that we have a constant torque load of around 350 N.M and this load MUST to rotate at 1200 rpm. so I chose the attached 55 kW motor, and using a VFD we run it continuously at 1200 rpm rather than 1450 rpm.

from the attached graphs, at 1200 rpm the motor develops much larger torque than needed (930 Nm) and accordingly draws too much current at (675 A). so my question is:

Using frequency control, can the motor run continuously at 1200 rpm with the torque somehow reduced to it's rated/load torque? also how far down can drive's output voltage be lowered?

PISCEAN 2010

Hello,

Basic question - How you calculate the power required to rotate load?  

 well the right way of course is calculating the load torque and speed.

I stumbled upon an old tender document at my company that they ultimately didn't pursue, it's for replacement of DC motors with AC ones, the DC motor had around 350 N.m of torque at 300-1200 rpm, connected to a reducer, then the reducer was connected to two sprocket with a 22/48 or something teeth ratio, to ultimately create a stroking motion of a specified number of strokes/min.

of course, no moments of inertia are available, not for the gearbox nor for the load. which is why i got curious regarding using an AC motor with similar torque to the DC one, limiting the speed to 1200 rpm, and whether torque can be set to a constant 350 n.m from 300 to 1200 rpm.

for the sake of argument, i don't wanna change the reducer.

PISCEAN 2010

Ok.

Please check below points.

1. Please refer DC motor nameplate & find out DC motor KW/HP rated capacity. Check if DC drive is 4 quadrant or 2 quadrant. 

2. Please check if DC motor runs in field weakening mode. If yes, find out base speed, max speed, etc. etc. Plot a graph of motor power, speed, torque. You need to check ramp up, ramp down time, braking if external or internal etc. etc. 

3. You need to select an equivalent AC motor based on motor KW/HP, torque & speed variable range based on DC motor speed torque characteristics. 

4. Please derate (increase)  AC motor power as compared to DC motor power while selection.

Few are the above points which will help you to select equivalent parameters. For more detail you need to study application & calculate acceleration, deceleration, kinetic energy, torque, force etc. etc. 

Hope this helps. 

 thanks for the help, but let's say that the motor is rated at 45 kW, and runs in 2 quadrants (definitely no reverse), and no field weakening is used, since the technical drawings I've seen has something like:

[motor]------------[[[[[[[[[[[[[[[[[[[[[[[[[[[[[reducer]]]]]]]]]]]]]]]]]------------------[[[[[[[[[stroke]]]]]]]]]

1200/300 rpm >> 1/29.219 gear ratio ||| 41.07/10.27 rpm >> 22/48 teeth ratio ||| 18.82/4.71 Strokes per min

essentially: 1200 rpm x (1/29) = 41.07 rpm x (22/48) = 18.82 spm

can a replacement be sized based on this info?