Selection Of Ac Motor To Replace The Dc Motor

[AB2005]

Hi;

 

In a “Single Facer” unit of our Corrugator machine, the “take up conveyor” has a DC motor 5.9KW 2700RPM. We want to replace this DC motor with AC motor and inverter. On the driven side, there is a gear box with two sprockets which rotate two rolls on which a belt is installed. As there is no high starting torque required, so I am going to select a 10KW 2pole AC motor. What do you think, is it ok?

[marke]

Hello AB2005

 

Without a lot of detailed information on KW and torque requirements against speed, it is hard to be definitive, but is the motor is to be running close to rated speed, you should be OK.

 

Quite often, DC motors will spend considerable time running close to zero speed. You must remember that as the speed reduces, the KW capacity of the motor also reduces.

Also, if the speed is very low, then there is little cooling on the motor and it may need additional cooling.

 

If the speed is low for part of the cycle, and there is a reasonable torque requirement at low speed, consider selecting a lower speed motor (more poles) and operating it above rated speed as well as below.

 

Double the size in a lack of other information, is a good starting point.

 

Best regards,

Mark.

[AB2005]

Hi Marke;

 

As the machine most of the time runs above 70% speed, so for me cooling of motor would not be an issue. For other concerns, lets see.

[AB2005]

Dear Marke;

 

If i select 10KW 4pole motor instead of 2poles and run it up to 3000rpm (to get max speed of machine) via inverter, would it be feasible? What precautions would be taken in to account?

[AB2005]

Dear Marke;

 

If i select 10KW 4pole motor instead of 2poles and run it up to 3000rpm (to get max speed of machine) via inverter, would it be feasible? What precautions would be taken in to account?

 

Dear Mark;

 

Do you please reply against my above question?

[marke]

Hello AB2005

 

Sorry, I missed your post.

 

If you over speed the motor, there can be issues with bearings, so you need to be sure that the bearings are OK at the higher speed.

 

The voltage waveform will become less sinusoidal, increasing the losses in the motor, especially in the rotor.

As the frequency rises above the rated frequency, the voltage will be restricted, (assuming that you have a motor rated at line voltage at rated frequency) and this will cause the flux in the gap to reduce as the frequency rises. The net result is that the torque reduces with frequency above rated frequency and so the output power is theoretically limited to the rated KW minus increasing losses.

 

Another option is to use a motor rated at less than line voltage at rated frequency, set the V/Hz down (rated motor voltage in the parameters) and then you will have an increasing power rating above rated frequency and a more sinusoidal output.

 

I would usually try to limit the overspeed to around 75Hz (150%) rather than double. This will raise the torque at low speed without damaging the bearings.

 

Best regards,

Mark.

[AB2005]

Its mean we should select 10kW 2pole motor.

[AB2005]

Dear Mark;

 

The max speed of the unit is 300M/min and at that speed, the RPM of DC motor is 2560. But normally this DC motor runs between 2000-2360RPM correspond to unit speed according to different jobs. Its mean if we select a 4pole motor, the max frequency of this motor would be around 85HZ and the normally it would run between 67HZ to 78HZ (2000-2360RPM). So what do you think now, should we start with a 4pole motor? If yes then what will be its rating, still 10KW or 7.5KW will be enough?

[AB2005]

Mark;

 

Please reply.

[marke]

Hello AB2005

 

While I can tell you what happens and what characteristics I would expect from the information that you give, I am not a paid consultant to you and so I can not make your decisions for you.

 

Operating the induction motor below rated speed will yield a maximum torque equal to the rated full speed torque of the motor. As power is torque times speed, this means that the maximum output power of the motor reduces with speed.

Operating the induction motor above rated speed will yield a maximum power rating equal to the full speed rating of the motor, therefore, as power equals speed times torque, the torque will reduce with speed.

Arrmed with this information and the power and or torque at different speeds, you can map the AC induction motor performance against your requirements and determine the best fit.

 

Best regards,

Mark.

[AB2005]

Thank you Marke. Always co-operative.