motor not spinning

Hi, I have a 3 phase 380V 2.2kw vfd
and a 400v 50hz 2 pole, ac motor
the vfd turns it ok, in both directions.

the vfd speed is set by 0-10v input.

if the voltage drops to 0.7v, 0.6v
the motor stops turning, or turn very little every few seconds, and not continuously.

is it because its 2 pole ?

If the voltage is linearly related to frequency.... then 0.6 V is 6% speed, or about 2.5 Hz for a 50 Hz motor.

Most motors are not going to do very well at that speed. Basically 6% of RPM, 6% or less POWER, etc. Not very useful. It may also, if not set up quite right, suffer from an overcurrent at the slow RPM. That could account for it running a bit every few seconds.

MAYBE, if the VFD can do "vector control" it may do better.

Is such as low speed needed? Can it not be obtained with gears or belt? It is easier to run a motor FASTER than 50 Hz, than it is to run it that slow, especially since cooling will be poor at the slow RPM. By running at twice the RPM, and using a belted reduction, you might do better.

Induction motors require some amount of slip to make torque. If you have an old motor, especially one with a big difference between full load speed and synchronous speed then the amount of slip will be higher. For a 2850 rpm motor, the slip is 150 rpm to get full torque at 50 hz.

It appears you are trying to drive the motor at just over 100 rpm.

Best case scenario the motor has very little power, very poor efficiency, and no cooling. Worst case it won't move and will have no cooling.

Either way a gear reduction or belt reduction would seem like a better solution, but without further information I can't say.

The vfd must have a screen which can display the commanded frequency output of the drive at any given moment. What is it saying when the signal is set to .6v?

assaf said:

Hi, I have a 3 phase 380V 2.2kw vfd
and a 400v 50hz 2 pole, ac motor
the vfd turns it ok, in both directions.

the vfd speed is set by 0-10v input.

if the voltage drops to 0.7v, 0.6v
the motor stops turning, or turn very little every few seconds, and not continuously.

is it because its 2 pole ?

Click to expand...

What's been said, but yes, you want an 8-pole AC motor to even get close to that low an RPM, still with the limitations, plus some OTHER trade-offs you may not like.

IF/AS/WHEN you NEED really low RPM that can tolerate even sitting at-rest and still "pull", you need a servo with its specialized drive system, or at least an AC "torque motor" (think lift door closers), or a DC motor. Also serious external fans, most cases.

Search "blower duty", find lots of old DC motors that had to "sit still" so as to act as infinite springs, holding tension, then speed-up to move something in a manufacturing process of some sort. Making fabrics, rubber belting, sheet steel coil - lots of "stuff" needs that.

Otherwise, yes, the "generality" is that AC + VFD prevails for base RPM UPWARDS, DC prevails for base RPM DOWNWARDS, they have a useful overlap where there isn't much difference except that DC motors usually cost more than AC.

Servos just do WTF they are told to do. It's why we CALL them "serve o's"