Which Thermal Relay Matches Your Motor and Contactor?

You're specifying a motor starter, and the thermal relay needs to match both the motor's full load current and the contactor's frame size. A thermal relay that's mismatched to either component won't provide adequate protection—or worse, won't fit at all. The JR28-630 thermal relay from Dalier was engineered with compatibility as the priority: multiple current setting ranges that match motors from 30A to 630A, and compatibility with CJX2 contactors for integrated motor starter solutions. This article walks through what to consider when selecting the right thermal relay for your motor and contactor combination—from the current setting that matches your motor's full load current to the contactor frame size that determines which JR28 model you need.

The First Step in Selection — Sizing the Relay to Your Motor

Why Current Setting Range Is the Starting Point

The JR28 series is available in current setting ranges from 30-50A up to 380-630A. The first step in selecting the right thermal relay is identifying the full load current (FLA) of your motor and choosing the range that covers it. A relay set too high won't protect the motor; one set too low will nuisance-trip during normal operation.

Covering the Full Range of Motor Sizes

From smaller motors in the 30-50A range to large industrial motors in the 380-630A range, there's a JR28 model that fits your specific application. This means you can standardize on one relay family across multiple motor sizes, simplifying inventory and maintenance.

The Second Step — Matching the Relay to Your Contactor

Why Contactor Compatibility Matters

The thermal relay must be mechanically and electrically matched to the contactor it's protecting. The JR28-630 is compatible with a range of CJX2 contactors, ensuring that the relay and contactor are properly matched for both mechanical and electrical performance.

Which JR28 Model Matches Which Contactor

The JR28-200 (up to 150A) is compatible with CJX2-115-185 contactors. The JR28-630 (132-220A) is compatible with CJX2-225-265. The JR28-630 (200-500A) is compatible with CJX2-225-500. And the JR28-630 (380-630A) is compatible with CJX2-400-630. Matching the relay to the correct contactor ensures proper mechanical fit and reliable electrical connections.

Understanding the Trip Characteristic — Why Class 10 Matters

What Class 10 Means for Motor Starting

The JR28-630 features a Class 10 time-current characteristic curve. During motor starting, inrush currents can reach 6-8 times the full load current. A Class 10 trip curve allows the motor to start without nuisance tripping, while still providing protection against sustained overloads.

Protection Once the Motor Is Running

Once the motor is running, the Class 10 curve provides reliable protection against overload conditions. If the motor is overloaded, the relay will trip before the motor windings overheat, preventing damage and extending motor life.

Two Types of Protection — Overload and Phase Failure

Overload Protection — Preventing Heat Damage

When a motor draws excessive current for an extended period, the windings overheat and eventually fail. The thermal relay detects this condition and trips the contactor, removing power from the motor before damage occurs. The adjustable current settings allow you to match the protection level to your specific motor.

Phase Failure Protection — Detecting Single-Phase Conditions

A phase failure—when one phase of a three-phase supply is lost—causes the motor to draw excessive current on the remaining phases. This condition can quickly destroy a motor. The JR28-630 detects phase failure and trips the contactor, protecting the motor from this damaging condition.

Specifications at a Glance — JR28 Series Thermal Relays

Where the JR28 Series Fits in Motor Control Applications

Industrial Motor Control

In industrial settings, the JR28-630 protects motors driving pumps, fans, conveyors, compressors, and other equipment. The wide current range covers most industrial motor sizes, from small pumps to large compressors.

HVAC Systems

Heating, ventilation, and air conditioning systems rely on motors for fans, pumps, and compressors. The JR28-630 provides the protection needed to keep these systems running reliably.

Commercial and Civil Electrical Scenarios

The JR28-K variant is suitable for industrial, commercial, and civil electrical scenarios. This includes building services, water treatment, and other applications where motor protection is required.

Straight Answers on Thermal Relays

Q: What current ranges are available for the JR28-630 thermal relay?

A: The JR28 series offers setting ranges from 30-50A up to 380-630A.

Q: What is the trip characteristic of the JR28-630?

A: The relay features a Class 10 time-current characteristic curve, providing the right balance between motor starting and overload protection.

Q: What contactors are compatible with the JR28 series?

A: The relay is compatible with CJX2-115-185, CJX2-225-265, CJX2-225-500, and CJX2-400-630 contactors.

Q: What protections does the thermal relay provide?

A: The JR28-630 provides overload protection and open-phase (phase failure) protection for motors.

Q: What is the design philosophy of the JR28 series?

A: The JR28 series features multi-specification models with precise current settings for quick protection.

Q: Is the thermal relay easy to install?

A: Yes. The relay features a compact, standardized mounting design for easy integration.

Request a Quote for Your Motor Protection Needs

A thermal relay that's properly matched to both your motor and your contactor is the foundation of any reliable motor protection system. The JR28-630 thermal relay from Dalier delivers adjustable current settings from 30-50A up to 380-630A to match your motor's full load current, compatibility with CJX2 contactors for integrated motor starter solutions, Class 10 time-current trip curve for the right balance of starting and protection, and overload and phase-failure protection for comprehensive motor safeguarding—all in a compact, standardized mounting design.

Whether you're protecting motors in industrial plants, HVAC systems, or commercial buildings, the JR28 series offers the compatibility and performance that electrical engineers require. With multiple models covering the full range of motor sizes and contactor frames, you can standardize on one relay family across your entire facility.

Contact Dalier for a quote or technical consultation → their team can help you select the right current setting range and contactor combination for your specific motor protection requirements. Get a thermal relay that fits your system—and protects your motors.