# Comparing Required and Available Power – February 16, 2021

### Follow the Rulmeca Corporation Blog for conveyor drive application “success stories” and updates on Motorized Pulley features and upgrades.

When driving a conveyor drive motor with a variable frequency drive (VFD) it is essential that the operator/designer be aware of all required powers and all available powers at all relevant rates.

We supply Motorized Pulley belt conveyor drives, serving equipment manufacturers directly and end users through a network of independent distributors & local service providers throughout the United States and Latin America.

We will examine rates and powers in four examples. In continuous flow, we will examine required and available powers when a given belt speed is reduced. In metered flow, we will examine required and available powers when a given belt speed is increased.

In continuous flow, material handling rate remains constant between conveyors. If, for example, conveyor A handles 450 TPH of stone at a 330 FPM belt speed, then conveyor B will handle 450 TPH at a 165 FPM belt speed, as long as the conveyor width is adequate.

Analyze conveyor A as follows:
• Material Handling Rate = 450 TPH
• Conveyor Belt Speed = V = 330 FPM
• Power Supply = 460v/3ph/60Hz
• Motor is optimized to run on 460v/3ph/60Hz
The CEMA Conveyor Design Manual shows that required belt pull, Te, may be calculated using the equation below.
Te = LKt (Kx + KyWb + 0.015Wb) + Wm(LKy + H) + Tp + Tam + Tac
Required Power = Te x V
Required power to drive conveyor A = 23.6 HP.
A conveyor drive consists of an electric motor, gear reducer, and pulley. If the operator installs a 25 HP drive system, designed to run on a 460v/3ph/60Hz power supply, and the power supply is 460v/3ph/60Hz, the drive will not fail.
Required mechanical power = 23.6 HP
Installed power = 25 HP
Available power = 25 HP
23.6 HP is less than 25 HP

Analyze conveyor B powers as follows:
• Material Handling Rate = 450 TPH
• Conveyor Belt Speed = V = 165 FPM
• Power Supply = 460v/3ph/60Hz
• Motor is optimized to run on 460v/3ph/60Hz
If the operator uses the installed 25 HP drive system and the changes the supply frequency to 460v/3ph/30Hz to decrease belt speed to 165 FPM, then the drive will fail.
Required mechanical power = 20.3 HP
Installed power = 25 HP
Available power = 12.5 HP
20.3 HP is greater than 12.5 HP

Two metered flow applications show the challenge of attempting to significantly increase belt speed with a VFD. In metered flow, material handling rate is linearly proportional to belt speed, as long as conveyor belt is sized appropriately. If conveyor A handles 400 TPH at a belt speed of 50 FPM, then conveyor B will handle 800 TPH at a belt speed of 100 FPM.

Analyze conveyor A powers as follows:
• Material Handling Rate = 400 TPH
• Conveyor Belt Speed = V = 50 FPM
• Power Supply = 460v/3ph/60Hz
• Motor is optimized to run on 460v/3ph/60Hz

Required power to drive conveyor A = 15.4 HP. If the operator installs a 20 HP drive system, designed to run on a 460v/3ph/60Hz power supply, and the power supply is 460v/3ph/60Hz, the drive will not fail.
Required mechanical power = 15.4 HP
Installed power = 20 HP
Available power = 20 HP
15.4 HP is less than 20 HP

Analyze conveyor B powers as follows:
• Material Handling Rate = 800 TPH
• Conveyor Belt Speed = V = 100 FPM
• Power Supply = 460v/3ph/120Hz
• Motor is optimized to run on 460v/3ph/60Hz

Required power to drive conveyor A = 29.9 HP.
Installed power = 20 HP
Available power = 20 HP
29.9 HP is greater than 20 HP
The drive system will fail. It is impossible to drive the motor at 120 Hz.