I'm designing a linear actuator for a particular application, and the electric motor I'll be using will be a 1/8 HP three-phase motor. I need to decrease rotation of the motor, and my first idea was to design a reduction drive. But I know that this can also be done with a Variable Frequency Drive (VFD). I don't know how VFDs work and neither how to install and use them, but for this particular project, what do you think would be the best option? Would a VFD be cheaper than a reduction drive? And how VFD affects torque?
Electrical – VFD or gearbox for Linear Actuator
actuatorvfd
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The positioning control (real industrial stuf) is done in cascade with multiple controllers: Position controller (P-regulator), Speed controller (PI-regulator), current controller (PI-regulator).
https://www.google.si/search?q=position+control+loop&biw=1391&bih=683&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwj0gMXLo-rJAhWDfxoKHaUmCTMQsAQIIA#imgrc=N1aiYN6Li7XBCM%3A
When you ommit certain stages, the control gets worse. For example if you don't have current transducer, you can control the motor directly from speed controller, that would be voltage control. If the load wants to move the motor at standstill, the PI regulator "will find" such voltage that, will feed such current into the motor, that will produce such torque at shaft, that will put the motor back to standstill.
If you have a current transducer, then you can make two PI-regultors. First is speed control which gives the setpoint to second one PI-reg. the current controller. Now the output of current controller has to control PWM and measures a current feedback with the current transducer. With extra current feedback the overall dynamics will be faster compared to previous method without transducer.
So the rough conclusion is: the torque control is actualy the current control of the motor.
VFDs used for HVAC applications are often special models designed for that industry. You should look carefully at your manual and other documentation to determine what features are provided in your VFD. Rather than braking, it may be equipped with a feature that only allows the load to decelerate as fast as it will decelerate without regenerating more energy than is absorbed by losses in the motor and VFD. It may essentially "sleep" when the speed command is zero. It may have a feature to find the motor speed and synchronize with it if it is turned on when the motor is already turning due to air flow produced by the other drive. Various features can be enabled or disabled using drive set-up parameters. It is essential to have full documentation for the VFD.
Edit re added information:
I believe the drive is configured as for sensorless vector control. When zero speed is commanded it will energize the motor with magnetizing current, but try to drive the motor to zero speed if the shaft is turning. It will not provide DC braking unless that is configured, and I suspect that it is not configured. The drive has a braking chopper built in, but apparently no braking resistors. There will be no dynamic braking unless optional external braking resistors are connected. When deceleration is called for, the drive will decelerate at the slower of the rate called for by the deceleration ramp adjustment or the rate that prevents regeneration in excess of losses.
This drive has a group of preset adjustments that are loaded by selecting a "Macro." The PFC macro is intended for use with fan and pump loads. It sets the control method to scalar frequency control rather than vector speed control. I am pretty sure that would cause the drive to completely de-energize the motor when zero speed is called for. Since centrifugal fans and pumps produce zero or near zero flow at some speed above zero, I believe this drive has a configuration setting that would de-energize the motor if any speed below some set minimum speed is called for. I don't think this is configured by the PFC macro.
I have written technical literature under contract for ABB USA, but I have not done that for quite a while, so I am not completely familiar with this drive.
Best Answer
VFDs can easily get a motor to produce full torque at any speed from full speed down to zero. The difficulty is in motor cooling. An inexpensive standard motor can probably not operate continuously at much less than half speed without overheating. It is generally always better to use fixed gearing so that the motor operates at full speed for the highest machine speed that is required. Use adjustable speed only to reduce from the highest machine speed to lower speeds. Don't use a variable speed drive just to operate at a reduced constant speed.
For 1/8 Hp, a DC speed controller and a permanent-magnet DC motor or even a commutator DC motor will probably be less expensive than A VFD. DC motors can also have difficulty with continuous low-speed operation.