- #1
KLoux
- 176
- 1
Hello! We (myself and the other folks at my company) have a couple of questions about the proper way to compare electrical energy to mechanical energy. Our application involves holding a load at zero speed, as well as moving it back and forth at varying speeds. Most of the time, our motor will be at zero speed, holding a constant torque. We have some profiles that we are expected to run, and we wanted to calculate the RMS power requirement of the profile, to ensure that it is below the RMS power that can be supplied to the motor from the drive.
Our EE tells us that the power required to hold the load in place is very small, and probably won't contribute much to the RMS power. His claim is that the power draw is only what is required to overcome the electrical losses. Presumably, this is still P=VI, where I is the stall current. His argument is that there is no mechanical work being done (obviously), thus there will be little electrical work being done.
My feeling is that this is not necessarily the case. Will the current increase as the motor starts to turn, even if the torque is constant? If there is a torque constant, does that mean the current will have to increase, or is stall a special case?
Our EE performed a test on a small motor a little while ago and monitored the power while varying the load and the speed. He observed that the power requirement to hold a load at zero speed was small. When the speed increased and the load was reduced, the power draw increased significantly (this is how he justifies his above claim).
Our ME has a story to support his argument as well: Many years ago, we built a machine that employed a motor to hold a load at zero speed as well as to move the load dynamically (very similar to what we're doing now). He said that the drive saturated the RMS power while holding the load in place, and when they commanded some acceleration after holding the load for some time, it didn't respond because the drive's overload protection was limiting the juice going to the motor.
Who's wrong? Are they both right? I agree with some of what both of them say, but I can't quite form a complete opinion on it. I want to repeat the test that our EE performed, but I also want to know what is happening. Does it come down to efficiency/power factors, or is something else involved?
Thanks!
-Kerry
Our EE tells us that the power required to hold the load in place is very small, and probably won't contribute much to the RMS power. His claim is that the power draw is only what is required to overcome the electrical losses. Presumably, this is still P=VI, where I is the stall current. His argument is that there is no mechanical work being done (obviously), thus there will be little electrical work being done.
My feeling is that this is not necessarily the case. Will the current increase as the motor starts to turn, even if the torque is constant? If there is a torque constant, does that mean the current will have to increase, or is stall a special case?
Our EE performed a test on a small motor a little while ago and monitored the power while varying the load and the speed. He observed that the power requirement to hold a load at zero speed was small. When the speed increased and the load was reduced, the power draw increased significantly (this is how he justifies his above claim).
Our ME has a story to support his argument as well: Many years ago, we built a machine that employed a motor to hold a load at zero speed as well as to move the load dynamically (very similar to what we're doing now). He said that the drive saturated the RMS power while holding the load in place, and when they commanded some acceleration after holding the load for some time, it didn't respond because the drive's overload protection was limiting the juice going to the motor.
Who's wrong? Are they both right? I agree with some of what both of them say, but I can't quite form a complete opinion on it. I want to repeat the test that our EE performed, but I also want to know what is happening. Does it come down to efficiency/power factors, or is something else involved?
Thanks!
-Kerry