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VFD vs Soft Starter for Conveyors: Preventing Gearbox Failure

You selected the right AGMA Class gearbox . You calculated the belt tension perfectly. But the moment you hit "Start," the belt snaps or the gearbox makes a terrifying clunk. The culprit is likely your Starting Method . In conveyor systems, the starting torque profile matters more than steady-state power. Note: We previously discussed VFDs as Energy Savers for pumps and fans. For conveyors, however, the goal is not lowering your electric bill—it is preventing your gearbox from exploding. Table of Contents 1. The Physics of Shock Loads 2. Why Soft Starters Stall Conveyors 3. The VFD Torque Advantage 4. Comparison: Cost vs. Protection 5. Final Verdict Advertisement 1. The Physics of Shock Loads When an AC induction motor starts Direct-On-Line (DOL), it draws 600% to 800% of its rated current (Inrush Current). More importantly, it produces a sudden spike known as Locked-Rotor Torqu...
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Calculate Conveyor Motor Power & Torque

Designing a belt conveyor may look simple, but incorrect motor sizing is one of the most common causes of conveyor problems such as motor overheating, belt slipping, gearbox failure, or poor acceleration.

This article explains how to calculate the required motor torque and power for a belt conveyor using a clear step-by-step engineering method, including a realistic industrial example.

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1. Why Motor Power and Torque Calculation Matters

If the motor is:

  • Undersized → motor stalls, trips, or overheats
  • Oversized → unnecessary cost and poor energy efficiency

Correct calculation ensures reliable starting, stable running speed, acceptable motor temperature, and long mechanical life.

2. Basic Belt Conveyor Parameters

Diagram of inclined belt conveyor showing geometry and forces

Figure 1: Inclined Conveyor Geometry. The motor must lift the load (Gravity) and overcome resistance (Friction).
Conveyor geometry
  • Conveyor length, L (m)
  • Inclination angle, θ (degree)
  • Drive pulley diameter, D (m)
Load information
  • Material mass on belt, mload (kg)
  • Belt mass, mbelt (kg)
Operating conditions
  • Belt speed, v (m/s)
  • Coefficient of friction, μ
  • Gearbox efficiency, ηg
  • Motor efficiency, ηm

3. Step 1 – Calculate Total Moving Mass

The total moving mass on the conveyor is:

mtotal = mload + mbelt

In most medium-length conveyors, roller rotational inertia is either neglected or included later as a safety margin.

4. Step 2 – Calculate Required Conveyor Force

4.1 Horizontal Belt Conveyor

For a horizontal conveyor, the resisting force is mainly friction:

F = μ × mtotal × g

Where g = 9.81 m/s2.

4.2 Inclined Belt Conveyor

For an inclined conveyor, the required force consists of two components:

  • Friction force (acts on belt and load)
  • Gravity force (acts only on material load)
Total Force (F) = Ffriction + Fgravity

Ffriction = μ × mtotal × g × cos θ
Fgravity = mload × g × sin θ
Engineering note:
In a continuous loop belt conveyor, the belt mass on the return side counterbalances the belt mass on the carry side. Therefore, the motor only consumes power to lift the material load, not the belt itself.

5. Step 3 – Calculate Torque at the Drive Pulley

Torque at the drive pulley is:

Tpulley = F × (D / 2)

This is the minimum mechanical torque required at the pulley shaft.

6. Step 4 – Calculate Required Motor Torque

If a gearbox is used:

Tmotor = Tpulley / (i × Î·g)

Where i is the gearbox reduction ratio.

7. Step 5 – Calculate Required Motor Power

Motor power is calculated from force and belt speed:

P = (F × v) / (ηg × Î·m)

Power is expressed in watts (W). Divide by 1000 to obtain kilowatts (kW).

8. Worked Example – Industrial Belt Conveyor

Given:

  • Conveyor length = 12 m (horizontal)
  • Belt speed = 1.0 m/s
  • Drive pulley diameter = 0.25 m
  • Material load = 500 kg
  • Belt mass = 80 kg
  • Coefficient of friction (roller supported) μ = 0.03
  • Additional accessories drag Faccessories = 200 N
  • Gearbox ratio i = 20:1
  • Gearbox efficiency ηg = 0.95
  • Motor efficiency ηm = 0.90
  • Service factor = 1.4

1. Total mass:

mtotal = 500 + 80 = 580 kg

2. Friction force:

Ffriction = 0.03 × 580 × 9.81 = 171 N

3. Total force:

F = 171 + 200 = 371 N

4. Pulley torque:

Tpulley = 371 × (0.25 / 2) = 46.4 N·m

5. Motor torque:

Tmotor = 46.4 / (20 × 0.95) = 2.44 N·m

6. Motor power:

P = (371 × 1.0) / (0.95 × 0.90) = 435 W

7. Design power:

Pdesign = 435 × 1.4 ≈ 610 W

Selected motor: 0.75 kW motor with gearbox
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9. Important Engineering Notes

9.1 Coefficient of Friction (μ)

  • Roller supported belt: 0.02 – 0.05
  • Slider bed (UHMW): 0.10 – 0.20
  • Slider bed (steel): 0.20 – 0.35

Using roller friction values for a slider bed conveyor can undersize the motor by several times.

9.2 Starting Torque vs Running Torque

Static friction and loaded starts often require higher torque than steady running.

Design recommendation: Ensure the motor or VFD can deliver 150–200% of rated torque during startup.

For conveyors where startup conditions are critical, see our detailed guide on How to Calculate Starting Torque and Acceleration Power for a Belt Conveyor »

10. Conclusion

By separating friction and gravity effects and accounting for real-world losses, motor power and torque for a belt conveyor can be calculated accurately and reliably.

This step-by-step method is suitable for most industrial belt conveyors and provides a solid engineering basis for motor and gearbox selection.

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