Overload Relays Explained: How They Protect Your Motors

Your factory’s conveyor belt, a water treatment plant’s pump, or even the HVAC system in a skyscraper—all rely on electric motors to keep operations running. But here’s the problem: motors can overheat, burn out, or fail catastrophically if they draw too much current. That’s where overload relays step in.

In this guide, we’ll demystify overload relays: what they do, how they work, and how to pick the right one for your motor. We’ll also share why CNC’s overload relays are trusted by engineers worldwide—and answer common questions to help you avoid costly downtime.

What Is an Overload Relay, and Why Do You Need One?

An overload relay is a safety device that shuts off power to a motor when it draws excessive current, preventing damage from overheating. Think of it as a “smart fuse” for motors: unlike a regular fuse (which blows once), an overload relay resets after the fault is cleared, making it reusable and cost-effective.

Why motors need overload protection:

• Overloads: A motor might draw extra current if it’s overworked (e.g., a pump clogged with debris).
• Phase Loss: If one phase of a 3-phase motor fails, the remaining phases overheat.
• Locked Rotor: If the motor’s shaft gets stuck (e.g., a jammed conveyor), it acts like a short circuit, frying windings.

Without an overload relay, these issues can destroy motors—costing thousands in repairs and days of downtime.

2 Main Types of Overload Relays: Thermal vs. Electronic

Not all overload relays are created equal. The two most common types—thermal (bimetallic) and electronic—work differently, and each has pros and cons depending on your application.

1. Thermal (Bimetallic) Overload Relays: Simple & Affordable

Thermal overload relays use a bimetallic strip (two metals bonded together) and a heating element. When current spikes, the heating element warms the strip. Since the two metals expand at different rates, the strip bends—triggering the relay to open the circuit.

Best for:

• Small to medium motors (1–100 HP).
• Applications with stable loads (e.g., fans, small pumps).

Pros:

• Low cost ($20–$100 per unit).
• No need for external power (works passively).

Cons:

• Less accurate at high temperatures (the strip can “remember” heat, causing false trips).
• Slower to reset (needs time to cool down).

2. Electronic Overload Relays: Precise & Smart

Electronic overload relays use solid-state circuits to monitor current. They measure current digitally, compare it to a set threshold, and trip instantly when overloads occur. Many models even include features like phase-loss detection or communication capabilities (e.g., 4-20mA signals for PLC integration).

Best for:

• Large motors (100+ HP).
• Variable-load applications (e.g., conveyor belts, industrial mixers).
• Facilities needing real-time monitoring (e.g., data centers, food processing plants).

Pros:

• High accuracy (±1% current measurement).
• Adjustable trip settings (customize for your motor’s needs).
• Faster reset (no cooling time needed).

Cons:

• Higher cost ($100–$500 per unit).
• Requires external power (24V or 120V).

Overload Trip Classes: How to Choose the Right Speed

Overload relays follow an inverse time curve—they trip faster as the current increases. This speed is rated by a “trip class,” which tells you how many seconds the relay takes to open at 600% of the motor’s full-load current (FLC).

Common trip classes and their uses:

Example: A pump with a jam-prone impeller (high inertia) needs a Class 30 relay—it won’t trip from short jolts but shuts off if the jam persists.

How to Select the Right Overload Relay for Your Motor

Picking the wrong relay can lead to two disasters:

• Nuisance Trips: The relay trips too often, halting production.
• Motor Burnout: The relay doesn’t trip fast enough, damaging the motor.

Here’s how to avoid both:

1. Match the Relay to the Motor’s FLC

Check your motor’s nameplate for its full-load current (FLC). The relay’s current range should cover 80–120% of the FLC. For example, a motor with 20A FLC needs a relay adjustable from 16–24A.

2. Consider the Load Type

• Constant Loads (e.g., fans): Thermal relays work fine.
• Variable Loads (e.g., mixers): Go electronic—they handle spikes better.

3. Environment Matters

In hot, dusty, or wet environments (e.g., steel mills, water treatment plants), choose a sealed electronic relay. CNC’s Extreme Duty Series is rated for -40°C to 70°C and dust/water resistance (IP65).

Why CNC Overload Relays Lead the Market

At CNC, we design overload relays for real-world reliability. Here’s what sets us apart:

• Thermal Relays (Model TR-200): Ultra-durable bimetallic strips with anti-aging coatings—last 2x longer than competitors.
• Electronic Relays (Model ER-500): Built-in phase-loss detection and 0-10V analog output for PLC integration.
• Global Certifications: UL, IEC, and CE approved—tested to meet strict safety standards.
• Free Support: Our engineers help you size, install, and troubleshoot relays—no extra cost.

FAQs About Overload Relays

Q: Can I use a thermal relay with a VFD (variable frequency drive)?

A: No—VFDs create harmonic currents that confuse thermal relays. Use an electronic relay with VFD-compatible settings (CNC’s ER-500 has a “VFD Mode” for this).

Q: How often should I test my overload relay?

A: Annually. Use a motor analyzer to check trip times and reset functionality. CNC offers free training webinars on maintenance best practices.

Q: Why does my relay trip even when the motor seems fine?

A: Common causes: loose connections, a mis-sized relay (too low for the motor’s FLC), or a failing contactor (sticking contacts increase current).

Final Take: Protect Your Motors, Protect Your Bottom Line

Overload relays are the unsung heroes of motor protection. By choosing the right type (thermal or electronic) and trip class, you’ll avoid costly breakdowns and keep operations running smoothly. And with CNC’s reliable, smart relays, you’re not just buying a part—you’re investing in peace of mind.