Skip to main content

Featured Post

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...
Post a Comment
Disclosure: As an Amazon Associate, I earn from qualifying purchases.

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.

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 Torque (LRT).

For high-inertia loads like loaded conveyors, this instant acceleration acts like a hammer blow to your gearbox teeth and belt splices.

VFD vs soft starter torque speed curve for conveyor gearbox protection
Figure 1: VFDs maintain full torque at low speeds, while Soft Starters sacrifice torque to limit current.

2. Why Soft Starters Stall Conveyors

A Soft Starter works by "chopping" the voltage using thyristors. It gradually ramps up voltage to limit the inrush current.

The Problem: Motor torque is proportional to the square of the voltage (T ∝ V2).
If a soft starter reduces voltage to 50% to stop the lights from dimming, the motor only produces 25% of its torque. If your conveyor is fully loaded with rock or packages, 25% torque isn't enough to get it moving. The motor stalls, hums, and overheats.

Note: Some modern soft starters include torque boost or kick-start functions, but these still cannot match full low-speed torque under heavy inertia.

3. The VFD Torque Advantage

A Variable Frequency Drive (VFD) is fundamentally different. It doesn't just lower voltage; it changes the Frequency (Hz).

By keeping the ratio of Voltage-to-Frequency constant (V/Hz control), a VFD can deliver 100% Full Torque at very low speeds (e.g., 1 Hz). This allows a conveyor to start gently (over 5-10 seconds) without ever stalling, even when fully loaded.

This is why most manufacturers recommend VFDs for high starting torque conveyor applications and classify them as constant torque drives.

Advertisement

4. Comparison: Cost vs. Protection

This decision usually comes down to budget vs. physics. Do you need to protect the grid (Soft Start) or the gearbox (VFD)?

Feature Soft Starter VFD (Inverter)
Starting Torque Reduced (Risk of Stall) Full 100% Torque
Mechanical Shock Reduced Eliminated
Cost $$ (Moderate) $$$ (High)
Best Application Fans, Pumps, Light-Duty or Empty Conveyors Loaded Conveyors, Crushers

5. Final Verdict

Based on our inertia calculations:

📢 Engineering Rule of Thumb

Use a Soft Starter if:
You only need to limit inrush current to satisfy the power company, and the conveyor starts empty.

Use a VFD if:
You need to protect the Gearbox. A VFD's ability to ramp up slowly (S-Curve) eliminates the "hammer blow" that shears gear teeth and splices.

Recommended Brands

While we discuss engineering theory here, for hardware, stick to reputable brands available via industrial suppliers or check availability below:

About the Author:
This article is written by a mechanical design engineer specializing in industrial automation, motor control, and conveyor system troubleshooting.

As an Amazon Associate, I earn from qualifying purchases.

Labels:

Comments

Post a Comment

Popular posts from this blog

Dowel Pins & Locating Pins: The Basics of Fixture Design

Figure 1: A typical fixture setup. Notice how dowel pins (silver) provide precise location, while bolts (not shown here) provide the clamping force. In the world of Precision Engineering , the difference between a high-quality product and a scrap part often comes down to microns. While bolts hold parts together, they are terrible at positioning them. This is where Dowel Pins and Locating Pins become essential components in industrial tooling . Advertisement What is a Dowel Pin? Dowel pins are precision-ground fasteners used to secure the relative position of two parts. They are typically machined to extremely tight tolerances (often within 0.0001 inches) and are available in materials like: Hardened Steel: For high-wear applications in CNC fixtures . Stainless Steel: For corrosion resistance in medical or food processing machinery. Plastic (Thermoplastic): For lightweight, non-conductive, low-load alignment. The ...
Post a Comment
Read more about this Mechanical Design Article »

Roberts straight-line mechanism

Figure 1: A modern linear ball slide (like this THK model) is the contemporary solution for precise straight-line motion. Many modern engineering applications require components to move in a precise linear fashion, known as " straight-line motion ". Today, we take this for granted. We can simply purchase an off-the-shelf Linear Motion Guide that moves a device accurately along a rail with low friction. The Historical Challenge: Making a Straight Line However, in the late 17th and early 18th centuries—before the development of high-precision milling machines—it was extremely difficult to manufacture long, perfectly flat surfaces. Creating a sliding joint without significant backlash was nearly impossible. During that era, engineers had to rely on Linkages . Much thought was given to the problem of attaining a straight-line motion using only revolute (hinge) connections, which were much easier to manufacture. The most famous early result was...
Post a Comment
Read more about this Mechanical Design Article »

Engineer's Guide to Ball Detent Torque Limiters

Figure 1: The ball detent mechanism provides precise overload protection by disengaging instantly when the torque limit is exceeded. The First Line of Defense: Overload Clutches In high-speed automation and heavy industrial machinery, a "jam" is not a matter of if , but when . Whether it is a cardboard box getting stuck in a packaging machine or a tool crashing in a CNC lathe, the resulting torque spike can destroy gearboxes, twist shafts, and burn out expensive servo motors in milliseconds. A torque limiter (or overload clutch) is the mechanical fuse of the drive system. While electronic monitoring (current limiting) is common, it is often too slow to prevent physical damage from the massive kinetic energy stored in the system inertia. A mechanical torque limiter provides a physical disconnect that operates in a fraction of a second. Search for Torque Limiters & Safety Couplings Advertisement Why Choose ...
Post a Comment
Read more about this Mechanical Design Article »