How to Find Out If Your Motor Starter Has Heaters Inside
Finding out if your motor starter has heaters inside is a critical diagnostic step for any technician dealing with motor protection issues. Checking the overload relay confirms whether the starter uses traditional thermal elements or modern solid-state technology. This guide provides a clear, step-by-step method to visually and electrically verify the presence of heater elements safely.
Simply put, you can find out if your motor starter has heaters inside by opening the starter enclosure, locating the overload relay, and visually inspecting it for small rectangular or square metal blocks (heater elements) bolted between the relay terminals. Checking the device nameplate for model numbers or using a multimeter to test for continuity through the overload block also confirms their presence.
Key Takeaways
- Motor starter heaters are thermal overload protection devices located inside the overload relay block and are essential for preventing motor burnout.
- The most reliable method to confirm their presence is a direct visual inspection of the overload relay after safely opening the enclosure.
- Always enforce strict Lockout/Tagout (LOTO) procedures before touching any motor starter components to ensure safety.
- If you see small metal blocks or melting alloy units connecting the relay terminals, those are the heater elements.
- Modern solid-state overload relays do not contain traditional physical heater elements, which changes how you test them.
What Does a Motor Starter Heater Do Exactly?
A motor starter heater is a thermal overload protection device. Its primary job is to monitor the current flowing to the motor. When the motor draws excessive current over a sustained period, the heater element generates enough heat to trip the overload relay, opening the control circuit and stopping the motor.
This action protects the motor from damage caused by prolonged overcurrent conditions, such as a mechanical jam or a stalled rotor. The heater element is designed to match the specific full-load current of the motor it protects.
Types of Thermal Overload Heater Elements
There are a few distinct types of heater elements you might find inside a motor starter. Knowing the difference helps you identify them during inspection.
| Heater Type | Common Brand/Name | Visual Look |
|---|---|---|
| Eutectic Alloy (Melting Alloy) | Allen-Bradley, Cutler-Hammer | Small square block with holes that melts at a specific temperature. |
| Bimetallic Strip | Siemens, Telemecanique, ABB | A strip of two metals bonded together that bends when heated. |
| Tin Alloy | General Electric (Legacy) | Often a small wire or loop embedded in a metal casing. |
Motor starter heaters are typically categorized by “trip class” (Class 10, 20, 30), which defines the maximum time the relay takes to trip under a locked rotor condition. The National Electrical Code (NEC) requires overload protection for all motors, and the heater element is the core component fulfilling that requirement.
Important: According to the IEEE, approximately 30% of motor failures are directly related to overload conditions that were not properly protected against. This is why correctly identifying and sizing your heaters is so critical.
- Visual inspection is the first step; look for small components installed in the power circuit terminals of the overload relay.
- Heater elements are always connected in series with the motor contactor and the motor itself.
- They work by converting electrical current into heat (I²R heating).
- The heat actuates a mechanical mechanism (melting alloy or bimetallic strip) to trip the relay contacts.
- Without a properly sized heater, the overload relay cannot accurately protect the motor.
Why You Need to Check for Motor Starter Heaters
Checking for heaters is not just a routine step—it’s a troubleshooting necessity. If a motor is tripping repeatedly or failing to start, the heater element might be the culprit. A missing or incorrectly sized heater can lead to motor burnout or nuisance tripping.
Here are the primary reasons why you need to perform this check:
- Verifying Overload Protection: You must confirm the starter has the correct protection element installed to match the motor’s Full Load Amps (FLA).
- Troubleshooting Nuisance Tripping: Heaters can degrade over time or become too sensitive for the application.
- Diagnosing Motor Burnout: If a motor burns up, checking the heater can confirm whether it was the correct size or if it was bypassed.
- Retrofitting or Replacing Starters: When changing a motor, you need to know if the heater elements can be reused or if they need replacement.
- Safety Compliance: Many safety audits require documented proof of proper overload protection settings.
Warning: Never operate a motor starter with a missing heater element in a critical phase. Doing so creates a single-phasing condition, which will destroy three-phase motors quickly. Always replace damaged or missing heaters with the exact size specified for your motor.
The 5-Step Inspection: How to Find Out If Your Motor Starter Has Heaters Inside
Follow this rigorous step-by-step process to safely and accurately determine if heaters are present. This method works for NEMA and IEC type starters commonly found in industrial and commercial settings.
Step 1: Safety First – Lockout/Tagout
Before you even touch the starter cover, you must isolate the power source. Locate the disconnect switch feeding the panel or motor control center (MCC). Turn it off and apply a personal lock and tag.
Test for the absence of voltage at the line side of the starter using a rated voltmeter. The NFPA 70E standard mandates this step to prevent arc flash incidents.
Step 2: Open the Enclosure and Identify the Overload Relay
Once the power is verified off, remove the enclosure cover. The motor starter usually consists of a contactor on top and an overload relay attached to the bottom. The heater elements, if present, will always be housed within the structure of the overload relay.
Look for the device that has terminals labeled T1, T2, T3 (load side) coming out of it.
Step 3: Look for the Heater Elements Physically
Now you are looking at the overload block. On the front face, you might see a reset button and a current adjustment dial. The heaters themselves are typically located on the front face of the relay or at the power connection points.
- Front-Mounted Heaters (Common on NEMA starters): You will see small metal blocks inserted into slots. These are the eutectic alloy heaters. They are often labeled with a number (e.g., N79, B61).
- Direct-Connected Heaters: Some bimetallic relays have the heater soldered directly onto the bimetallic strip. You may see a small loop of wire connecting the terminal to the strip.
- No Visible Heaters: If you see a flat block with no removable elements or a digital display, you likely have a solid-state overload relay.
Step 4: Check the Nameplate and Part Number
If you cannot immediately see the heaters, check the nameplate on the side of the overload relay. It often lists the heater type or the specific heater part numbers that are compatible. You can then match this to a manufacturer catalog.
Here is an example of how part numbers correspond to heater sizing:
| Manufacturer | Overload Series | Heater Part Number Prefix |
|---|---|---|
| Allen-Bradley | Bulletin 592 / 509 | N (e.g., N56) |
| Siemens | 3UF7 / 3RV10 | 3UF or 3RV (specific element) |
| Cutler-Hammer (Eaton) | A10 / XT Series | B or L (e.g., B34) |
Step 5: Use a Multimeter for Verification
If visual inspection is inconclusive, use a multimeter. Set your meter to the resistance (Ohms) setting. Place your probes across the line (L1) and load (T1) terminals of the overload relay.
A heater element will have a very low resistance, typically less than 1 Ohm. If the meter reads infinite resistance (OL), the heater is either missing or blown open.
- Measure between L1-T1, L2-T2, and L3-T3.
- A reading of 0.2 – 0.8 Ohms indicates a heater element is present.
- If you measure a direct short (0.0 Ohms), there might be a bypass or a solid-state relay.
- If you measure OL, the heater is likely missing or open.
Tip: When testing resistance, touch your meter leads together first to know your lead resistance. Subtract this from your reading to get an accurate heater resistance value. This helps you determine if two heaters are paralleled.
What If Your Motor Starter Doesn’t Have Heaters?
If you have completed the visual and electrical inspection and found no physical heater elements, your starter likely uses a solid-state overload relay. This does not mean the motor lacks protection; it means the protection logic is handled electronically.
Here is how solid-state relays differ from traditional thermal heaters:
- Protection Mechanism: Uses a current transformer (CT) and a microprocessor to measure current and simulate the heating effect (I²t curve).
- Adjustability: You can often adjust the FLA setting over a wide range (e.g., 4:1 or 5:1) without buying physical parts.
- Additional Features: Solid-state relays can offer ground fault protection, phase loss detection, and jam protection.
- No Thermal Memory: Some cheaper solid-state relays lack the “cooling” memory feature that bimetallic relays inherently have.
- Cost: Higher upfront cost but eliminates the need to stock spare heater elements.
To confirm you have a solid-state unit, look for a digital display or a dial with a wide current range (e.g., 10-40 Amps). Traditional thermal heaters have a very narrow range (e.g., 12.5-14.3 Amps for one specific part).
Common Mistakes to Avoid When Checking Motor Starter Heaters
Even experienced electricians can make mistakes during this process. Avoiding these common errors will save you time and prevent equipment damage.
- Skipping Lockout/Tagout: The most dangerous mistake. Always verify zero voltage before opening the enclosure.
- Confusing the Contactor Main Contacts with Heaters: The contactor’s main contacts look like large copper pads. The heaters are smaller and located on the overload block, not the contactor.
- Assuming All Overloads Look the Same: NEMA starters usually have front-accessible heaters, while IEC starters often have them integrated inside the case. Don’t force a part out without seeing how it mounts.
- Mixing Heater Sizes: Never install different heater sizes on different phases. All three phases must have identical heater elements for balanced protection.
- Overtightening Heater Terminals: Heater elements are made of specific alloys. Stripping the threads or cracking the block requires a costly replacement of the entire overload relay.
Warning: Do not use a fuse or a piece of wire to “bypass” a missing heater element. This removes all thermal overload protection from that phase and represents a serious fire and equipment hazard.
Frequently Asked Questions
Can a motor starter work without heaters?
Yes, a motor starter can physically close its contacts and start a motor without heaters. However, this leaves the motor completely unprotected against overloads. If the motor jams or draws excessive current, no protection device will stop it, leading to rapid motor burnout or fire.
How do I know if a heater element is bad?
A bad heater element often shows physical signs of heat stress such as discoloration, blistering, or a melted appearance. Electrically, a meter should show continuity. An open heater (infinite resistance) is a clear sign it has failed and needs replacement.
What is the difference between a motor starter heater and a fuse?
A fuse provides short-circuit protection and acts very quickly (instantaneous). A heater element provides overload protection and has a time-delay characteristic, allowing momentary high currents during motor startup. They serve different purposes and are usually used together in a motor circuit.
Do all motor starters have overload relays?
Not all “switching devices” have overload relays. A simple contactor or a magnetic switch might not have one. However, a “motor starter” by definition includes an overload relay.
If you are looking at what you think is a starter, it must have an overload relay to technically qualify as such.
How do I size a motor starter heater?
Heater sizing is based on the motor’s nameplate Full Load Amps (FLA). You must select a heater element that trips at 125% of the motor’s FLA for continuous duty motors (per NEC guidelines). Always consult the manufacturer’s heater selection chart for your specific overload relay model.
Final Thoughts
Once you know the visual and electrical cues, figuring out if your motor starter has heaters inside becomes a straightforward diagnostic task. Always prioritize safety by following proper lockout procedures before inspecting any high-voltage equipment. Whether you are verifying protection levels, troubleshooting a frequent trip, or replacing a motor, knowing the state of your heater elements is key to maintaining reliable motor operation and extending equipment lifespan.
