Schneider ATV12 help

Hello Gents,


I am working with some Schneider VFDs and was reading the manual of one of them and read this:

In case of higher size of motor, possible up to 2 sizes (example is using a 4 kW (5.5 HP)
on a 2.2 kW (3 HP) drive) it is necessary to ensure motor current and actual motor power will not pass over nominal power of drive.

Does "not pass over nominal power of drive" mean "is not higher than nominal power of drive? Or do they mean something else?

The point I don't understand is that the paragraph seems contradictory, on one hand it states that it is possible to use a motor with higher rated power than that of the drive and then not.


Thanks

Oh, do they mean that the motor will not be operated at a load that will exceed the power and current rating of the drive?

kalabdel said:

Oh, do they mean that the motor will not be operated at a load that will exceed the power and current rating of the drive?

Click to expand...

It means that you can select a bigger motor (think higher service factor) as long as you limit its hp output to match the drive.

There's a common confusion that a 100hp motor is going to consume 100hp worth of power. Nope, the motor is going to operate at whatever hp it needs to get the job done.

You could use a 50hp motor to drive a 10hp load (as long as you could get its higher rotational mass to spin up).

We had a 150hp VFD "Self-Disassemble" (blew up), and the only VFD we had available was a 100hp unit. Since the original VFD was operating at well below its max, it was a no-brainer to stick in the smaller one. No problem, and as far as I know, it's still being used there.

Thanks rootboy, much appreciated.

Cheers
Kal

As pointed out, it's not a problem to use a VFD on a motor that is begger (in an emergency basis*) as long as you ensure the motor current is never allowed to exceed the rating of the drive. In some drive you have the ability to program in a "Current Limit" function. What that typically will do is to set a current value that the drive will never exceed. What you need to understand though is that the WAY a drive limits current is to either trip off line or limit the output speed of the motor. So if you have that feature, it will override your commanded speed to limit current if it has to. As a rough estimation, let's say you have a 10HP drive and it is connected to a 20HP motor. You can set the speed to be 100%, but as you approach that current limit value, likely around 50% speed, the drive will no longer accelerate, even if you need it to.

The ATV12 has a function it calls "current limit", but there is no description of how it does that, whether it limits the output speed / torque or trips off line. You would need to ask them for clarification. If it trips, then you have an alternative; you are going to need to do that manually. So what you will do is experiment using the current readout and increase the speed until the output current reaches the rating of the drive. That then is programmed back into the drive as the maximum frequency limit. So if your 5.5kW motor hits the drive's max current at 36Hz, you program the frequency upper limit to be 34Hz (gives you a little wiggle room for variables).

If, however, you need the ability to go full speed, you are not going to be able to get there in all likelihood with a VFD that is under sized.

* Footnote: Technically, it could be considered a violation of the NEC to use a device not rated for the purpose (110.3, "suitability for use..."), I'm pretty sure the CEC has a similar clause.

Thanks jraef.

Kal