Drive Question

Could anyone tell me : is it possible to control both speed and current of a motor at the same time ? If it is could you please explain. Your help is appreciated.

Pick one. Current is a direct function of the LOAD on a motor. Speed is a direct function of the FREQUENCY given to the motor. There is a relationship between speed and load. Put those issues together, and you come up with this:

A VFD can limit current to a motor, and does so by modulating the frequency to reduce the load on it. That's why we use the term "current limit" not current control. You can program a current limit into the drive, which sets up an override of the frequency command. So IF you give a frequency command that does not make the motor try to pull more current than the limit, fine. But if you tell it to go faster, which increases the load, which increases the current the motor needs to move the load that fast, then the current limit function takes over and stops accelerating the motor in order to prioritize the current limit function.

If you would describe the application we may be able to suggest other solutions. Please state the drive model number (if you have selected it) for an even more precise reply.

With some servo motors you can have torque control, which is likely better than current control.

Russ is correct about torque control and, actually, even current control, but in both cases, there would be no speed control. As jraef says, current and torque is the result of load on a motor so it cannot be directly controlled as long as you are controlling speed.

You said should be at the same time of load change, namely torque speed and current control in some applications at the same time, also is the vector control ATV71 can satisfy your requirement.

jasper007 said:

You said should be at the same time of load change, namely torque speed and current control in some applications at the same time, also is the vector control ATV71 can satisfy your requirement.

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No, it can't. It's no different that the rest of them, it has no ability to change the laws of physics. If you perform torque control and thereby current control, it has to override speed control. Like I said in the beginning, pick one, but only one.

Hi jraef , I've tried a couple of 18.5 kW Dellta inverters recently and they seem to use a current limit and adjust the power by varying the voltage to the motor . For instance at 30Hz the indicated current on the inverter was 34A but the absorbed motor power was 3.5 Kw . I,ve looked at the parameters , but can't see a way of setting output current externally . with your experience , you've probably come across this type of control before .

Paul

pal said:

Hi jraef , I've tried a couple of 18.5 kW Dellta inverters recently and they seem to use a current limit and adjust the power by varying the voltage to the motor . For instance at 30Hz the indicated current on the inverter was 34A but the absorbed motor power was 3.5 Kw . I,ve looked at the parameters , but can't see a way of setting output current externally . with your experience , you've probably come across this type of control before .

Paul

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That's slightly different, it's done for energy savings. The drive monitors the loading (kW) on the motor and if it detects that the motor is less than about 50% loaded, it decreases the voltage to the motor, which decreases the magnetizing current losses. It also decreases the torque capacity, but the drive only does it WHEN IT ALREADY KNOWS the motor doesn't need all of the available torque. The reason you can't manually adjust it is because the only way it works is if it is done automatically, because if the load suddenly increased and the energy saving technique was not immediately disabled, the motor might stall.

Pretty must all VFDs do this, even some soft starters and is also the basis behind many of the "Energy Saver" boxes sold by scam artists. The energy it saves is not spectacular and is not really enough to warrant adding a device to do that alone, but it's so simple to implement on a VFD that it's kind of a "freebie" because everything you need to make it happen is already there just to make the VFD work in the first place.

If you only want to cap a maximum unloaded speed of a vector drive/motor, often you can set it to speed mode, set your desired maximum allowable speed, and then control the torque limit(s).

That doesn't "Control Both" at the same time, but limits the maximum speed to something you set in the event of no load.

For unwinders as an example, I generally set the speed command to zero, and then control the torque limits, whereas for winders, I'll calculate the required surface speed, add 15 or 20%, send that as a speed command, and again, limit the torque to develop tension.