Speed change

[Big Red]

There are 12 conveyors all running on vfd's. Turn six off and the other six increase in speed and yet the vfd setting has not changed. Why?

 

[telectrix]

reduction in volt drop???

 

[DPG]

reduction in volt drop???

Increase in voltage drop?

 

[snowhead]

When the 6 are turned off are the other 6 now reaching the set speed (which they weren't doing when there were 12) or are they going overspeed?

 

[Maji236]

I believe it may have more to do with current, than voltage. I'd have a testing done on at least that/those circuit(s).

 

[SheffSimon]

There are 12 conveyors all running on vfd's. Turn six off and the other six increase in speed and yet the vfd setting has not changed. Why?

All on one VFD or a VFD per motor?

 

[Big Red]

When the 6 are turned off are the other 6 now reaching the set speed (which they weren't doing when there were 12) or are they going overspeed?

Thanks to all who have answered. The vfd's overspeed but not by much.

 

[Simon47]

Poor quality VFDs so when the load is reduced and voltage increases (less volt drop in the supply), they don't compensate ? So same frequency, but higher voltage to induction motors = slightly faster speed due to reduced slip.

 

[ElectroChem]

When you say the VFD setting has not changed what do you mean exactly?
Have you checked each VFD's display to record the frequency output?
If they are all on a common speed control then possibly dropping out 6/12 drives could affect the voltage input to the others speeding them up slightly?

 

[Big Red]

Poor quality VFDs so when the load is reduced and voltage increases (less volt drop in the supply), they don't compensate ? So same frequency, but higher voltage to induction motors = slightly faster speed due to reduced slip.

I am using Allen Bradley Powerflex 525 VFD's. What are your thought about maybe a harmonics problem?

 

[SheffSimon]

I am using Allen Bradley Powerflex 525 VFD's. What are your thought about maybe a harmonics problem?

Where does the speed setpoint for each drive come from? PLC? Operator controls? Internal drive parameter? Any load sharing functionality in the drive?

Hard to see how any relatively minor change in supply voltage to the drive would affect the output, VFDs dont work like that.