Motor won't run in vector mode

[agarb]

Building a couple machines with 6 motors and 6 VFDs. All the motors are SEW and the VFDs are all ABB ACS355.

Had an odd situation where I couldn't get two of the smallest 1/4 HP motors to run on either of the two VFDs. Both motors were same part number and VFDs were same part number as well. Even after disconnecting the motors from driven equipment they wouldn't run on the bench.

Eventually discovered that they would run fine in V/Hz mode but in vector the drive display would just show 300-400% current draw and the shaft would twitch a bit until the drive tripped due to motor temperature. The motor would occasionally start, but only about 20% of the time.

Has anybody ever seen this before where a motor simply refuses to run in vector mode?

ABB didn't have good explanation. It is just a little 1/4 HP motor and the smallest VFD offered in the ACS355 series. ABB seemed to indicate the motor could be sized too small for the drive as it is only using about 40% of the drive's current capacity. But it is the smallest drive they offer in that series and I had no other choice.

The other 4 motors (5-10 hp range) all run fine in vector mode. These four motors have encoders, the other 2 did not.

[g.mccormick]

Did you do a static tune?

[jraef]

The general rule of thumb is that the less expensive drives (like the 355 and similar products) cannot accurately run in Vector modes if the motor is less than 50% of the Drive’s rating. The current sensors they use are below their accuracy range, especially at the lowest speeds and loads. So what happens is that the drive no longer “sees” the motor in the same way as it attempts to match it to the mathematical model it has in its memory (from the auto tune) and interprets that as something wrong. This is especially bad on really small motors.

There are better current sensors out there, but they are a lot more expensive and need more processing power, hence they are only available in the more expensive (and typically physically larger) series of drives.

[TheWaterboy]

I have seen it when there was a motor overload heater set still in the line.

This was in a config where there was an ATL motor starter parallel with the VFD drive, In case the VFD died, but the overload heaters were common to both. PF40 ran fine that way for years, New 700 series would not even start. Same exact symptom.

[agarb]

Just got back in town. Thanks for all the responses.

g.mccormick - Yes, I did a static tune.

TheWaterboy - Interesting, thanks for sharing.

jraef - Thanks for the explanation. Certainly seems reasonable. Especially when FLA on the nameplate is 0.54 but I can only enter 0.5 or 0.6.