Q: I have a conveyor application. There we have a Motor connected to Gear Box. Motor is driven by a variable frequency drive (VFD). Details are as follows
Motor Power - 2.2 kW, 415V, 50 Hz, 1460RPM
VFD - 2.2 kW
Gear Box - Ratio = 58.25, RPM - 25 Rev/Min ,
According to the Above details, gear box output is 25 Rev/Min When the Motor Runs at 50 Hz at 1460 RPM. But I need to increase the Gear box output (Conveyor Speed) from 25 RPM to 53 RPM. So, as a temporary solution, VFD Frequency was increased up to 100 Hz & Motor is running at 3000RPM. Now Conveyor is running at required speed. But I need a permanent solution to run the conveyor at 53RPM at motor Speed 1460 (50 Hz). What is the best solution for that..?
A: As a designer of conveyor systems for many years, the question more appropriately is how many feet per minute do you need the belt to run to convey the product. The simplest and cheapest method is a fixed frequency motor and gear box for gross refinement, if you need refinement, a sprocket and chain reduction or increase in ratio will give the belt speed required. The belt capacity has many variables, belt material and thickness, width, speed, distance between support rollers, trough angle (between vertical Centerline of the belt and the face of the belt) determining the depth of the material on the belt, the density and clumping factors ( usually tied to moisture content) determining the amount of stack up above safe belt depth (normally 80% for most solids) 60-70% for slurry mixes .
A conveyor is - essentially - a constant torque load. What that really means is that both power and speed move together - when one goes up, the other does as well.
A relatively simple means to get to the higher speed is to change gear ratio - but that may mean slightly higher gear losses and less actual power transmission to the belt. (Note that this is typically not a factor in the conveyor application, since steady-state loads are fairly low.)
Another choice is to do what you did: "crank up" the VFD output frequency, thereby increasing motor shaft speed. If it lasts, good for you! Check with motor manufacturer for possible motor (and bearing) issues at higher speed. Also check with VFD manufacturer for potential problems related to continuous operation at high output frequency.
Last choice - obtain a motor that will get there using the "normal" line frequency of the original.
As always, determination of "best" choice comes down to cost vs acceptable risk.