3 replies to this topic

[AB2005]

My friend has been facing a problem that they have a concrete mixing machine having 150KW DC motor (2100rpm) with a speed reduced gear box from 650rpm to 16rpm. They faced some problem in gear box and tried to arrange same but could find 32rpm gear box. Now they installed that gear box and reduced motor speed at 325rpm but motor tripped again and again.
Can anyone explain where we have been mistaking?

[marke]

Hello AB2005

Long time no see!!

When you use a gear box, the torque increases with the reduction in torque.
Altering the gearbox ratio will alter the input torque (to the gearbox) to develop the same torque output at the same speed.
As the ratio is now half, the motor must produce twice the torque at half the speed to give the same performance.
While DC motors do not suffer the torque limitation of AC motors, they are not perfect constant power devices and it depends on the type of DC motor as to how the speed torque curve looks.

If the motor is connected to the gear ox via drive belts, you could change the pulley ratio to get the motor back up to the same speed again.

Best regards,
Mark.

[AB2005]

Thanks Mark for your valuable comments

I was busy on a project which is now completed. I tried many times to open the forum but faced problems and I did not had time to see where the problem. Yesterday, forum opened after spending some time and you seen me again.

Back to the topic, I have understood where we have been mistaking.

I have another question that if a 10kw DC motor is connected with a load which staring torque requirement is less than 150%, can we replace that motor with a 10kw AC motor and inverter?

[marke]

if a 10kw DC motor is connected with a load which staring torque requirement is less than 150%, can we replace that motor with a 10kw AC motor and inverter?

Yes, provided that the torque does not elevate above 150% at any point of the speed torque curve of the driven load, and the inverter is able to supply 150% torque.
If the load has a breakaway torque of 150% to get the load to begin to turn, then there can be problems with many inverters that just can not easily provide that level of torque at zero shaft speed. It is common for the torque to fall off when the frequency is less than 10 Hz or 5Hz. This is dependent on the inverter design.
A closed loop vector control system or a DTC system will be able to supply this level of torque into a motor at zero shaft speed, but open loop vector drives will not.

Best regards,
Mark.