When you buy an electric motor, ordering the motor alone leaves half of the investment unfinished. If the hardware that protects the motor against overload, phase faults and rising winding temperature is not planned together with the motor, missing components surface on commissioning day; worse still, a motor run without protection often announces its first fault with a burnt winding. As a company that has been manufacturing and selling electric motors since 1979, we at HEM Motor always give our customers the same advice: request the protection hardware inside the same quotation when you order the motor. In this article we cover the protection devices that make sense to buy together with a three-phase electric motor, which type of fault each one guards against, and the information you should pass to the supplier so the selection matches your panel.

Why a Motor Should Never Run Without Protection

Asynchronous motors are designed to run for decades under the right conditions; in the field, what shortens a motor's life is usually not the motor itself but the supply-side and load-side stresses it is exposed to without protection. When a winding burns out, the plant's loss is not just the price of the motor: the production halted during rewinding or replacement, the equipment dismounted and refitted, and the unplanned maintenance hours are the real cost. The main threats waiting for a motor are these:

  • Overload: A jammed pump, a clogged fan or a strained conveyor pushes the motor above its rated nameplate current; the winding temperature can exceed the insulation limit within minutes.
  • Phase loss: A three-phase motor keeps turning on two phases, but the current it draws from the remaining phases rises; phase loss is the most classic cause of winding burnouts.
  • Voltage imbalance and wrong phase sequence: A voltage difference between phases causes the windings to heat unevenly, while a reversed phase sequence makes the motor turn in the wrong direction and damages the connected equipment.
  • Insufficient cooling: Frequent starts, a closed and hot panel room, or a fouled fan cover raise the winding temperature even when the current looks normal.

There is a separate protection device for each of these threats, and the right approach is to evaluate them all within the same order as the motor. On the manufacturing side the picture is identical: in most of the winding damage that reaches our service, we see the motor was run either with no protection at all or with an incorrectly set relay. When a burnt-out motor is assessed, the first thing examined is whether a protection arrangement was present; on a fault that carries the trace of phase loss, the complete absence of protection draws the process out against the buyer. In other words, well-chosen protection is a two-way investment that protects not only the motor but also your warranty rights.

Hardware That Makes Sense to Buy Together With the Motor

1. Thermal Overload Relay

The thermal relay is the most basic protection device, connected to the output of the contactor and continuously monitoring the motor current. When the motor starts to run above the set current value, the relay opens the circuit and the motor stops before the windings are harmed. The critical point when buying is to choose a relay whose setting range centres on the motor's nameplate current; for example, a motor with a nominal current of 14.5 A should be matched with a relay whose setting range comfortably covers that value, such as 9-18 A. When you buy the motor and the thermal relay from the same supplier, the current matching is done by the supplier; with separate purchases, the most common mistake is overlooking that in a star-delta start the relay must be selected according to the phase current, not the line current.

2. Motor Protection Circuit Breaker

The motor protection circuit breaker combines thermal protection, short-circuit protection and a manual on-off function in a single body. Especially on motors below 15 kW, it is a tidy alternative to the contactor-plus-thermal pairing and takes up less space inside the panel. The setting scale is chosen according to the motor's nameplate current, and the short-circuit breaking capacity must suit the supply rating of the facility. In applications where several small motors are connected to one machine, using a separate protection breaker for each motor ensures that only the relevant motor stops in a fault, preventing the whole line from going down.

3. PTC / Thermistor Option: Must Be Requested at Order Stage

The protections listed so far monitor the motor current; but in some situations the winding heats up while the current stays normal. Fouled cooling fins, high ambient temperature or frequent starts are typical examples. In these cases the real protection is provided by PTC thermistor sensors embedded inside the winding during manufacturing. When the winding temperature reaches the threshold value, the thermistor's resistance rises sharply and the thermistor relay in the panel stops the motor. The critical point is this: the thermistor is buried inside the winding while the motor is being wound; it cannot be added afterwards. For this reason the PTC option must always be stated at the order stage. We particularly recommend ordering with thermistors for fans expected to run continuously, for motors in hard-to-reach locations and for applications driven by a variable frequency drive. On our general-purpose industrial electric motor models, the PTC thermistor is a production option that only needs to be added to the order note.

Elektrik motoru panosunda termik röle ve motor koruma şalteri

4. Phase Protection Relay

The phase protection relay monitors phase loss, phase sequence error and voltage imbalance, opening the control circuit the moment a problem occurs. While the thermal relay only intervenes in phase loss after the current has risen, the phase protection relay catches the problem on the voltage side, before the motor is strained. For facilities in regions where supply quality is variable, for agricultural irrigation lines and for construction sites, the phase protection relay should be treated as a standard line item of the motor order. In multi-motor systems fed from a single panel, one phase protection relay placed at the main supply inlet can protect all the motors at once.

5. Complementary Items

  • Contactor: Together with the thermal relay, it is the switching element of the motor power circuit; the contact current is selected according to the motor power and the starting frequency.
  • Thermistor evaluation relay: If you are ordering a motor with PTC, remember that this relay must also be added to the panel; the thermistor alone cannot stop the motor.
  • Anti-condensation heater option: In humid environments or during long idle periods, a body-internal heater that prevents condensation in the windings is, just like the PTC, an option that must be stated at the order stage.

Starting-side solutions (star-delta, soft starter, frequency converter) are outside the scope of this article; we covered in detail how to manage the starting current, particularly on sites where the supply is weak, in our article on motor selection on a generator-powered site.

Which Protection Stands Out in Which Application?

Thermal protection is a basic requirement for every motor; but the additional protections that stand out change according to the character of the application. The practical matching based on our field experience is as follows:

  • Centrifugal pumps: Because of the risk of dry running and jamming, a finely set thermal relay; in well and booster applications, a phase protection relay is almost mandatory.
  • Fans and exhausters: Because of long continuous operation and blade fouling, the PTC thermistor is the most valuable protection; on roof-type fans, where reaching the motor is difficult, stopping before failure is critically important.
  • Compressors: On frequently starting piston compressors, a contactor suited to the number of starts and a thermistor should be considered together.
  • Conveyors and crushers: Sudden load fluctuations require the thermal relay to be selected in the correct class; for rapid tripping during material jamming, a protection breaker is a practical solution.
  • Agricultural irrigation: Voltage drop and phase loss are common on long supply lines; phase protection and voltage monitoring take priority.
  • Cranes and lifting: Because of the intermittent duty regime, contactor life and thermal class selection require expertise; always inform your supplier about the application.

As you can see, the right protection package depends far more on the nature of the application than on the motor's power. So when asking for a quotation, saying what the motor will drive instead of "a 7.5 kW motor and its thermal relay" earns you a much more accurate package.

Information to Give the Supplier for Panel Compatibility

For the protection hardware to arrive compatible with the motor, pass the following information to your supplier together with your motor request:

  • Motor nameplate data: Power (kW), speed, voltage and connection type (star/delta). If you are buying to replace an existing motor, a photo of the nameplate is the most reliable way.
  • Starting method: Direct, star-delta, or via a drive? The connection point and current setting of the thermal relay change accordingly.
  • Operating regime: Does it run continuously, and how many times per hour does it start? Frequent starting affects both the contactor selection and the thermistor need.
  • Ambient conditions: Panel room temperature, and the dust and humidity of the environment where the motor sits. In a hot panel room, the ambient temperature compensation of the thermal relay gains importance.
  • Supply condition: If there is a history of voltage fluctuation or phase loss, a phase protection relay should be added to the quotation.
  • Existing panel infrastructure: Free space in the panel, existing busbar current and short-circuit breaking capacity; the frame size of the protection breaker is determined accordingly.

A sample request can be this short: "11 kW, 1500 rpm, 400 V three-phase motor; direct starting; 3-4 starts per hour; closed and hot panel room; occasional phase loss on the supply; with PTC thermistor." These two lines are enough for the supplier to offer you the motor, a thermal relay in the correct setting range, a suitable contactor, a phase protection relay and a thermistor evaluation relay in a single package. A quotation prepared with this information eliminates field surprises such as "the relay can't carry the motor" or "the breaker didn't fit the panel." On delivery, do not skip checking that the motor nameplate exactly matches the order confirmation; we explained step by step how to do this check in our nameplate matching guide.

3 fazlı elektrik motoru için faz koruma rölesi ve termistör bağlantısı

After Installation: Commissioning the Protection Settings

Once the hardware arrives on site, apply a short commissioning routine to make sure the protection actually works. Verify that the setting dial of the thermal relay is brought to the nominal current on the motor nameplate; relays are often at their lowest setting from the factory, and if left as they are, they cause unnecessary tripping. While running the motor under load, measure the current of all three phases with a clamp meter and compare with the nameplate value; if the measured current is above the nameplate, you should investigate the load rather than raise the relay setting. Set the adjustment windows of the phase protection relay (voltage upper-lower limit, imbalance percentage, tripping delay) according to the supply character of the facility. On motors with thermistors, test that the thermistor leads are connected to the evaluation relay in the panel and that the relay output actually breaks the control circuit; the most common omission we see in the field is motors whose thermistor leads have been left unconnected at the terminal. All these checks are a half-hour job and ensure your protection investment does not remain on paper.

The Higher the Efficiency Class, the More Important Protection Becomes

In plants switching to high-efficiency motors, the protection hardware is often glossed over by keeping the old panel as it is; yet the new motor's nameplate current and starting characteristic may differ from the old one. Because an IE4 motor of the same power may have a different nominal current value than the old motor it replaces, updating the thermal relay setting to the new nameplate is essential. Reviewing the protection elements along with the motor replacement secures the lifespan of the high-efficiency motor investment. On all our IE4 motor models, the nameplate current values are clearly stated in the quotation document; this way you can complete the panel-side preparation before delivery.

The Advantage of Buying the Motor + Protection Package From a Single Supplier

When comparing electric motors, often only the motor price is considered; yet for a complete commissioning, the motor, the protection elements and the right engineering form a whole. When you buy the motor from a manufacturer, the protection from another wholesaler and the installation from a third company, finding someone accountable becomes hard the moment an incompatibility arises. When you request your protection hardware together with your motor from HEM Motor, the current matching is done with manufacturer data, production options such as PTC and the heater are built into the motor at the factory, and all items reach your site on a single delivery, in one shipment. Thanks to our stock strength across Turkiye, asynchronous motors in standard ratings and common protection elements are mostly ready for dispatch the same day. We also remind plants that keep spare motors: if you record the protection setting values of the spare motor you put on the shelf together with its nameplate, you can update the panel settings within minutes on the day of an emergency replacement.

Frequently Asked Questions

If there is a thermal relay, is a PTC thermistor still needed?

The two monitor different quantities: the thermal relay measures current, the thermistor measures winding temperature. In situations where the current stays normal but the winding heats up (fouled cooling, hot environment, frequent starting, low speed on a drive), the thermal relay does not act; only the thermistor provides protection. On critical and continuously running motors, using both together is the safest approach.

Should I choose a motor protection circuit breaker, or a contactor plus thermal relay?

If remote start-stop control is required, a contactor is mandatory; in that case a contactor plus thermal relay, or a contactor together with a protection breaker, is used. On small-power motors operated manually on the spot, a stand-alone motor protection circuit breaker is both sufficient and the more economical solution. The right answer depends on the motor power and your control scenario; stating your operating method when asking for a quotation is enough.

Can I buy the protection hardware separately, after the motor?

Panel elements (relay, breaker, contactor) can be bought later; however, the risk of a current-matching error then passes to you. Options placed inside the motor during manufacturing, such as the PTC thermistor and the winding heater, cannot be added afterwards. For this reason, at least the production options must be clarified at the moment of order.

Get a Quote

Buy your motor and its protection hardware in a single quotation, matched to one another. Tell us your operating conditions, and let us prepare a complete package covering the three-phase electric motor, thermal relay, protection breaker, phase protection and the PTC option. You can reach us by phone at +90 (532) 345 49 86 or send your request through our contact us page. HEM Motor: manufacturer assurance, strong stock and sound engineering since 1979.