Mr. Vex: Miss Ori, one of our customer asked if we can offer a motor that can control speed during lowering operation. What can i recommend him?
Miss Ori:What is your idea, Mr Vex?
Mr. Vex:I know that a motor equipped with a power off activated electromagnetic brake can provide hiding torque to the output shaft when it's turned off. Can i say that all the speed control motors with electromagnetic brake are able to control vertical movement speed?
Miss Ori: No, not always. First, you will have to choose a motor after knowing about work feeding direction at customer's facility. Available motor varies depending on work feeding direction. Let''s talk about the two kinds of work feeding directions.
In figure A, the motor drives in upward direction. This is called lifting(roll up) operation. On the other hand, in figure B, the motor drives in downward direction. This is called lowering (roll down regeneration) operation. There are many motors which can control lifting operation speed. However, very few motors can control lowering operation speed. At this point , i would recommend brushless DC motor BX Series or BHF Series.
Mr. Vex: Why can't the others motors achieve speed control during lowering operation?
Miss. Ori: Other motors control speed by using voltage phase control. For instance, in this method, input from AC supply to motor increases when rotation speed is high, and it decreases when rotation speed is low. The problem is the gravity added to the movement during lowering operation. That makes the motor rotation faster than preset speed. If you use any other speed. If you any other speed control motor than the two i recommended above, it automatically decreases input to the motor because it judges that the motor is rotating faster than preset speed.Can you imagine what will happen to the work then?
Mr. Vex: Oh well, let see. The motor rotation speed wont decrease even if speed settling value is decreased, because the motor rotates using the power of gravity. In that way, almost no voltage is fed to the motor. So the shaft becomes free and the work migh drop because of that.
Miss. Ori: That is right ! contrary to that, brushless motor DC is good in lowering operating as well. The secret is it's structure. Mr Vex, do you know anything about the structure of a brushless DC motor. Well yes. The rotor has permanent magnet, the stator works as an electromagnet , and the rotor rotates by following the stators excitation sequence.
Mr. Vex: Well , yes. The rotor has permanent magnet, the stator works as an electromagnet, and the motor rotates by following the stators excitation sequence.
Miss. Ori: Correct. In other words, brushless DC motor keeps preset speed by switching the polarity of stator's electromagnet quick and repeatedly. That is why stable speed control is possible even during lowering operation. However, there is one problem. The shaft of the motor is pulled by gravity and becomes rather unstable during lowering operation. Thus permanent magnet of the rotor generates electric power, and consequently, a reserve voltage runs from the motor to the driver. Such electric power is called "Regenerative Power". If the driver cannot absorb Regenerative Power, the driver will damaged during lowering operation.
Mr. Vex:I see. And is brushless DC motor BX series able to absorb regenerative power?
Miss. Ori:Yes, however, regeneration units offered separately is required to absorb regenerative power. By attaching it to the device, regenerative power is discharged as heat. However, as capacity of regeneration unit is limited, you need to estimate regenerative power to be generated in advance by calculating it from torque, speed, duration and cycle of the operation in advance.
Mr. Vex:I understand. Customer might be able to estimate the value in advance by calculating themselves. But if they contact our motor sizing staff of the customer support centre, we can do that for them free of charge.
Miss. Ori: Exactly, the customer can save time and effort that way.
Mr. Vex:All right. I will visit our customer right away and ask about the data required for the regenerative power estimation including the torque, speed and operation cycle.