Speed change

Big Red · Oct 21, 2020

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 · Oct 22, 2020

reduction in volt drop???

DPG · Oct 22, 2020

telectrix said:
reduction in volt drop???

Increase in voltage drop?

snowhead · Oct 22, 2020

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 · Oct 23, 2020

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 · Oct 23, 2020

Big Red said:
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 · Oct 23, 2020

snowhead said:
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 · Oct 27, 2020

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 · Oct 28, 2020

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 · Oct 29, 2020

Simon47 said:
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 · Oct 31, 2020

Big Red said:
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.