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One of the most insidious enemies of an electric motor appears not while it runs but while it stands still: condensation. When a motor stops, its frame cools, the air inside releases its moisture and a thin water film forms on the winding surface. Even if invisible, this moisture lowers the insulation resistance of the winding, weakens the insulation over time and can lead to leakage current or even winding burnout on the first start. The simple but vital option used to eliminate this risk is the anti-condensation heater, that is the space heater. At HEM Motor, supplying the anti-condensation heater option at the right power and voltage on motors in high humidity, outdoor environments or that stand idle for long periods is a critical decision that extends winding life and prevents commissioning problems. This article covers why condensation forms, how it damages winding insulation, heater power and voltage, the automatic activation logic, which environments require it and the right option selection, all focused on the purchasing decision.
Why Does Condensation Form and Why Is It Dangerous in a Stopped Motor?
Condensation is the conversion of water vapour in the air into liquid water on a cold surface. While a motor runs its windings heat up and the air inside stays warm; when it stops the frame slowly cools to ambient temperature. When the ambient temperature drops at night or the motor stands idle for a long time in a humid environment, the moist air inside the motor falls below the dew point and water vapour condenses on the winding, terminal and frame surfaces. This is especially pronounced in environments with a high day-night temperature difference, such as the seaside, around cold stores, open sites and tropical climates.
Condensed water forms a conductive path on the winding's insulation surface. The insulation resistance (the value measured with a megger) drops; in serious cases the motor must be dried before it is started, otherwise leakage current and short circuit risk arise when power is first applied. Repeated condensation-drying cycles fatigue the insulation material over time and shorten the motor's life. We covered the relationship between moisture and commissioning in a stopped motor in our storage and long-term standstill article, and the durability of the winding insulation class in our winding and insulation class (F/H) article.
The insidious side of condensation is that it usually progresses unnoticed. The motor looks dry from outside, the terminal box is closed; but the moisture built up inside the frame overnight is already on the winding surface when the motor is started in the morning. For this reason, condensation-related failures are often interpreted as the motor burning out for no reason; yet the root cause is moisture during standstill. The risk is highest in motors that wait as standby and run once or twice a month, because they both stand idle for long periods and, since they do not run, do not dry out from their own heat. The anti-condensation heater exists precisely to eliminate this invisible risk.
A drop in insulation resistance is not only a failure risk but also time lost during commissioning. A motor with a low megger value due to condensation must be dried by heating or special drying methods before it is started; this process can take hours or even days and delays project delivery. In a motor with an anti-condensation heater this problem is prevented from the start, because the winding has already been kept continuously dry and the motor is ready to run at any moment.
How Does the Anti-Condensation Heater Work?
The anti-condensation heater is a low-power heating resistor placed inside the motor (usually under the winding or inside the frame). Its job is not to heat the motor but only to keep the temperature inside the motor a few degrees above ambient so that it stays above the dew point. This way, even while the motor is stopped, the moisture in the air inside cannot condense and the winding surface stays dry. The heater usually energises when the motor stops and switches off while the motor runs (since its own heat is already sufficient).
This automatic logic is set up via a contactor or auxiliary contact: when the motor contactor is pulled in (while the motor runs) the heater supply is cut; when the motor stops the heater is automatically energised. This means the operator does not need to switch the heater on and off manually, and condensation protection is provided continuously. The heater has a separate supply and its own protection (fuse); its power is completely independent of the motor power.
There is an important safety rule here: the anti-condensation heater must always be de-energised before any work is done on the motor. Even if the motor's main supply is cut, the heater's separate supply may remain energised; so during maintenance you must be sure both the motor and the heater supply are cut. This warning is usually indicated with a label in the motor's terminal box. In a correctly set-up system, thanks to the automatic logic, the heater quietly does its job every moment the motor is stopped without adding any extra workload to the operator.
The anti-condensation heater is part of the motor's overall protection strategy. While protection elements such as PT100 and thermistors that monitor winding temperature during operation protect the motor against overheating, the anti-condensation heater protects the motor against moisture and cooling while it is stopped. The two complement each other: one manages the risk of the running moment, the other of the standstill moment. This holistic approach ensures the motor is both long-lived and reliable in humid and harsh environments.
How Are Heater Power and Voltage Selected?
The anti-condensation heater's power is determined by the motor's frame size; a larger frame means more internal volume and surface, therefore requiring a higher-power heater. The voltage is selected according to the plant's auxiliary supply voltage; the most common values are single phase 230V and, in some applications, 110V. The table below summarises the typical heater power range by frame size (values are approximate and may vary by manufacturer).
| Frame Size (IEC Frame) | Typical Heater Power | Typical Voltage |
|---|---|---|
| 71-100 | ~15-25 W | 230V (or 110V) |
| 112-132 | ~25-40 W | 230V |
| 160-200 | ~40-65 W | 230V |
| 225-280 | ~65-100 W | 230V |
| 315-355 | ~100-200 W | 230V |
As shown, the heater power is very small compared with the motor power; the aim is not to heat but to create just enough temperature difference to prevent condensation. When ordering, the heater power and voltage must be specified, and separate terminals for the heater supply must be provided in the terminal box. Terminal box sealing is also critical in a humid environment; we covered this topic in our terminal box gasket and sealing article.
Which Environments and Applications Require an Anti-Condensation Heater?
Not every motor needs an anti-condensation heater; a dry, enclosed and continuously running motor may not need one. But if one or more of the following conditions is present, the heater is the right and often mandatory option:
- High humidity environment: Seaside, water facilities, around cold stores, textile dyehouses, paper mills.
- Outdoor and open sites: Open or semi-open installations with a high day-night temperature difference.
- Long standstill: Standby pumps, seasonally operated drives, rarely used motors.
- Marine and ship applications: Salty humidity and long standstill together; the heater is almost standard.
- Cold climate and outdoor storage: The condensation risk is high at low ambient temperature.
In these environments, the anti-condensation heater, when evaluated together with a high IP protection class and tropicalised winding, makes the moisture protection complete. We detailed the IP upgrade in humid and dusty environments in our IP65/IP66 protection upgrade article, and tropicalisation and moisture protection in our tropicalisation and moisture protection article. These three measures complement each other: IP manages water ingress from outside, tropicalisation the winding surface and the heater the internal condensation.
The Right Option Selection and Ordering Notes
The anti-condensation heater is an option that is hard to add later; the heater is placed inside the motor at the factory. So if the need is known, it is best to specify it at the ordering stage. For the right order, three pieces of information must be clarified: heater power (by frame size), supply voltage (usually 230V) and separate connection terminals in the terminal box. Also, setting up the automatic activation logic in the panel ensures the heater energises only when the motor stops; this control is done on the panel side and connected to the heater terminals in the motor's terminal box.
- Power: Suitable for the frame size, at a level that prevents condensation.
- Voltage: Suitable for the plant auxiliary supply voltage (230V common).
- Automation: Automatic switch-on when the motor stops (via a contactor auxiliary contact).
- Labelling: The heater terminals must be clearly marked in the terminal box (usually with a separate warning label).
During the incoming and acceptance inspection of the motor, the presence of the heater terminals and the adequacy of the insulation resistance should be checked. We described the acceptance inspection steps in our incoming and acceptance inspection article. A correctly selected anti-condensation heater is an investment that, at very low cost, extends winding life and seriously reduces failure risk especially in humid environments.
Frequently Asked Questions
Does an anti-condensation heater use a lot of electricity?
No. The heater's purpose is not to heat the motor but only to create enough temperature difference to prevent condensation; its power is therefore very low (typically between 15 and 200 watts depending on the frame). It also runs only when the motor is stopped and is off while the motor runs. This low consumption is negligible compared with the winding failure and loss of life it prevents.
Is an anti-condensation heater needed if the motor runs continuously?
A continuously running motor's own heat already prevents condensation; in this case a heater is usually not needed. However, if the motor stops from time to time, runs seasonally or stands idle for long periods as a standby, the heater is recommended because condensation will form during standstill. The decision is made according to how long the motor is not running and the ambient humidity.
Can an anti-condensation heater be added later?
Because the heater is placed inside the motor's internal structure, adding it later is not practical and usually requires opening the motor. The best approach is to request the heater as an option at the ordering stage if the moisture risk is known. This way the motor is produced with a heater at the right power and voltage and with separate terminals in the terminal box; no extra intervention is needed in the field.
At HEM Motor, for your high humidity, outdoor and long standstill applications we ship your motors with an anti-condensation heater (space heater) option at the right power and voltage, with separate connection terminals in the terminal box. When needed, we plan complete moisture protection together with high IP protection and tropicalised winding, and offer fast delivery from stock. Share your application's ambient humidity, operating regime and frame size; request a quote for the right anti-condensation heater option and protect your winding against condensation from the start.
