Motor Overheating: The 10 Degree Rule, Voltage Unbalance and the Cooling Air Nobody Checks

• Insulation class sets the temperature limit: Class B 266°F (130°C), Class F 311°F (155°C) and Class H 356°F (180°C). Each 18°F (10°C) over the rating roughly halves insulation life.
• Voltage unbalance is the quiet killer. The extra temperature rise in the hottest winding is about 2 times the percent unbalance squared, so 2% gives about 8% and 5% gives about 50%.
• Blocked shrouds, packed fins and layers of paint starve a motor of cooling air. All of them are visible on a walk past.
• Judge a motor against a sibling on the same duty, not against an absolute number. Frame surfaces run well below winding temperature, and 120 Hz vibration points to an electrical cause.

A motor is too hot when its windings run above the limit of their insulation class. It is also too hot when it runs above what its rating allows for the room it sits in. The frame you can touch is not the number that matters.

Motors are typically rated for a 104°F (40°C) ambient and for altitudes up to about 3,300 ft (1,000 m). A hotter room or a higher site means thinner cooling margin, and the motor must be derated to stay within its insulation limit.

That makes room temperature part of every motor check. A pump motor that is fine in January can be over its limit in a July mechanical room with failed ventilation.

The winding temperature itself is hidden inside the frame. The surface runs well below it, so a frame that feels merely warm can sit over a winding that is close to its limit.

Motor insulation classes are NEMA ratings for the maximum temperature a winding's insulation system can withstand over its design life. The class letter is on the nameplate.

• Class B: 266°F (130°C) maximum winding temperature.
• Class F: 311°F (155°C) maximum winding temperature.
• Class H: 356°F (180°C) maximum winding temperature.

A common design practice is Class F insulation with a Class B temperature rise. The motor is built to run no hotter than a Class B motor would, while its insulation can tolerate Class F temperatures. That gap is margin for hot days, dirty fins and imperfect power.

The margin is not free capacity. Every condition in this article spends some of it, and several small ones together can spend all of it.

Running hot shortens a motor's life because heat chemically ages the insulation, and the rate of aging climbs steeply with temperature. The widely used rule of thumb is that insulation life roughly halves for every 18°F (10°C) above rated temperature.

The damage accumulates. A motor that runs 18°F (10°C) hot for its whole life has spent its insulation in roughly half the time, even if it never trips or alarms.

Aged insulation becomes brittle and cracks. Then turns short to each other, a winding shorts to ground, and the motor burns out. The result is a rewind or a replacement, plus the downtime in between.

The rule also works in reverse. Clearing a blocked shroud or correcting an unbalance that brings a winding back down toward its rating buys back insulation life.

Voltage unbalance is a difference between the three line voltages feeding a three phase motor. NEMA MG-1 defines percent voltage unbalance as 100 times the largest deviation of any line voltage from the average of the three, divided by that average.

A worked example

Take line voltages of 460, 467 and 450 volts (V). Their average is 459 V. The largest deviation from that average is 9 V, on the 450 V line.

Divide 9 by 459 and multiply by 100. The unbalance is about 2%, small enough to look harmless on a meter and large enough to matter inside the motor.

Common causes

• Single phase loads spread unevenly across the three phases.
• A blown fuse on one phase of a power factor capacitor bank.
• A loose or high resistance connection on one phase.
• An open delta transformer arrangement.

The US Department of Energy (DOE) describes voltage unbalance as probably the leading power quality problem behind motor overheating (DOE tip sheet). Most of its causes sit upstream of the motor, which is why the motor is where it shows up first.

The extra temperature rise in the hottest winding, in percent, is about 2 times the percent voltage unbalance squared (Nidec on NEMA MG-1). Because the term is squared, small increases in unbalance cost far more heat.

• 1% unbalance: about 2% extra temperature rise.
• 2% unbalance: about 8%.
• 3% unbalance: about 18%.
• 3.5% unbalance: about 25%.
• 5% unbalance: about 50%.

Why current unbalance is so much larger

Current unbalance runs about 6 to 10 times the voltage unbalance. A voltage difference that looks trivial drives a much larger current difference through the windings, and current is what makes heat.

The DOE gives an example of a 100 horsepower (hp) motor at full load with 2.5% voltage unbalance. Its line currents were unbalanced by 27.7% (DOE tip sheet reprint). The phase carrying the most current runs hottest, and that phase sets the motor's life.

What NEMA guidance says

NEMA guidance is to keep unbalance at the motor terminals below 1%. Above 1%, the motor must be derated using the MG-1 derating curve, which also voids most manufacturer warranties. Operation above 5% unbalance is not recommended at all.

Unbalance has a second signature besides heat. The DOE lists vibration at 120 hertz (Hz) as another sign. It is the same electrical tell described in Part 2. Vibration analysis that finds a strong 120 Hz component on a motor is pointing at the power supply, not the mechanics.

Blocked cooling air overheats a motor by stopping it from shedding the heat it makes at normal load. The motor can be healthy, correctly sized and well supplied, and still run hot.

Totally enclosed fan cooled motors

A totally enclosed fan cooled (TEFC) motor sheds heat through its finned frame. A shaft mounted fan under a shroud at the far end blows air along those fins.

Anything that cuts that airflow raises winding temperature. The common culprits are all things a person walking past can see:

• Rags, debris or lint over the shroud inlet.
• Fins packed with dust or grease, so air passes over a blanket instead of metal.
• Repeated layers of paint, each one adding insulation to the frame.
• A cracked fan blade, which also adds imbalance, covered in Part 2.

Open drip proof motors

Open drip proof motors pull room air through the windings themselves. They lose cooling when their screens clog, and they also pull in whatever dust the room holds.

Cooling checks cost almost nothing and belong on every preventive maintenance schedule. They are also among the easiest checks to skip, because nothing looks broken.

Some do and some don't, depending on the load. A motor on a variable frequency drive (VFD) loses cooling air as it slows. The load decides whether heat falls faster than cooling.

The shaft mounted fan on a TEFC motor slows with the motor. At low speed, it moves much less air over the fins.

Variable torque loads usually stay cool

Centrifugal pumps and fans are variable torque loads. Their power falls steeply with speed, so heat falls faster than cooling does, and they usually run cool when turned down.

Constant torque loads can overheat

Constant torque loads keep demanding torque, and so current, as speed drops. At low speed they make nearly full load heat with a fraction of the cooling air, and they can overheat.

Every motor has a manufacturer's turndown limit for drive operation. Check it against how slowly the drive actually runs, not how slowly it was meant to run at commissioning. Low speed operation can also bring a machine into resonance, covered in Part 5.

You read a motor's thermal pattern by where the heat sits, then by how it compares with a sibling motor on the same duty. Location tells you the kind of fault; the comparison tells you whether it is a fault at all.

• Whole frame evenly hot: overload, a voltage problem or lost cooling.
• One end hot: a bearing, covered in Part 1.
• Conduit box hot: a connection problem at the terminals.
• Coupling end hot: misalignment, covered in Part 2.

Why the sibling comparison matters

Frame surfaces run well below winding temperature, so an absolute surface reading says little on its own. Two identical pumps on the same duty should look alike, and the one that doesn't is the one to investigate.

This is the same principle as needing a reference in the same frame. Comparison against something known removes most of the guesswork from a surface reading.

Service factor and current

A 1.15 service factor lets a motor carry a short term overload. Running continuously into that service factor shortens insulation life, even though nothing trips.

Compare the drive's displayed current with the nameplate full load amps (FLA). Drive and panel readouts are often the quickest way to see a motor living above its rating, and they are read on a walk past.

Chiller and air compressor motors overheat for the same physical reasons, but they show it in different places. Knowing where to look matters more than the reading itself.

Chiller motors

Hermetic and semi hermetic chiller motors are cooled by refrigerant, sealed inside the compressor housing. Overheating shows up as a motor temperature alarm on the chiller panel, not as a hot frame.

Open drive chiller motors sit outside the compressor and cool with air like any other motor. They get the same shroud, fin and sibling checks as a pump motor. Compressor distress itself is the subject of Part 7.

Air compressor motors

Air compressor motors often sit in hot, dusty compressor rooms with poor ventilation. They get ambient and cooling problems together: hot intake air and dirty fins at the same time.

That room is worth a temperature reading of its own. The 104°F (40°C) rating assumption is easiest to break in a small enclosed compressor room in summer.

In both, overheating ends the same way: insulation breakdown, winding shorts and burnout, followed by a rewind or replacement and downtime. What differs is who feels the downtime.

In an office building

Air handling unit (AHU), pump and cooling tower motors carry most of the load. A failure means lost comfort for tenants, and unbalance wastes energy every hour before the failure comes.

These motors fall under routine heating, ventilation and air conditioning (HVAC) work, covered in our guide to HVAC maintenance in office buildings. Those routines rarely include a voltage unbalance measurement.

In a data center

A chilled water or coolant distribution unit (CDU) pump motor failure spends redundancy. The backup carries the load, and the site is one failure from trouble until the motor is back.

A chiller motor alarm can lock a chiller out at the worst time, often on the hottest day when demand peaks. Motor checks belong with the cooling loads nobody owns.

Several faults put heat on or near a motor without the winding being the problem. The location of the heat usually separates them.

• A hot bearing: heat concentrated at one end, with the rest of the frame normal. Bearings are covered in Part 1.
• Slipping belts: heat at the sheave and belt, not the motor frame. Covered in Part 4.
• A normal warm TEFC frame: a healthy TEFC motor runs warm by design. The sibling comparison separates normal from overheating.

This is why no single sensor is enough. Heat location, drive current, 120 Hz vibration and a look at the shroud each rule out a different suspect.

A walk past can prove that a motor's cooling is blocked, that it is drawing more than nameplate current, or that its chiller panel has alarmed. It cannot prove the condition of the insulation or the actual unbalance at the terminals.

• Directly observable: a blocked shroud, packed fins, a hot conduit box, a drive display reading above nameplate current, and a motor temperature alarm on a chiller panel.
• Strong indicator: a motor running clearly hotter than its sibling on the same duty, and 120 Hz vibration.
• Suspected: winding insulation condition and actual voltage unbalance. Both need electrical testing at the terminals by a qualified person.

That is the limit of any route based check. It finds the hot motor and the blocked shroud; electrical testing confirms what is happening inside.

Electrical findings belong in the records described in what NFPA 70B asks you to document. A camera records a blocked shroud, and a walkthrough record keeps the history.

Trends are what turn a single hot reading into a decision. Comparing each motor with itself and its sibling over time is the core of predictive maintenance.

• Fan shroud inlet clear, and frame fins clean.
• Room temperature near the motor.
• Frame temperature compared with a sibling motor on the same duty.
• Conduit box temperature compared with the frame.
• Drive display current compared with nameplate full load amps.
• Burnt varnish smell or discolored paint.
• Chiller panel motor temperature and alarm history.

Coming Soon

1. 1. When was voltage unbalance last measured at your largest motors?
2. 2. Do any motors run continuously into their service factor?
3. 3. What is the hottest ambient any motor on site sees in summer?
4. 4. Which VFD driven motors run below their manufacturer's turndown limit?
5. 5. When were motor fins and shrouds last cleaned?

How hot is too hot for an electric motor?

A motor is too hot when its windings exceed their insulation class limit. That is 266°F (130°C) for Class B, 311°F (155°C) for Class F and 356°F (180°C) for Class H. Motors are typically rated for a 104°F (40°C) ambient, so a hotter room requires derating. The frame surface runs well below winding temperature, so compare with a sibling motor instead of judging the surface alone.

What is a motor insulation class?

An insulation class is a NEMA rating for the maximum temperature a motor winding's insulation can withstand. The common classes are B, F and H. A common design practice is Class F insulation with a Class B temperature rise, which leaves margin.

Does running hot shorten motor life?

Yes. A widely used rule of thumb says insulation life roughly halves for every 18°F (10°C) a motor runs above its rated temperature. The aging is cumulative and ends in winding shorts and burnout.

What is voltage unbalance?

Voltage unbalance is a difference between the three line voltages feeding a three phase motor. NEMA MG-1 calculates it as 100 times the largest deviation from the average voltage, divided by the average. For example, 460, 467 and 450 V give about 2%.

How much voltage unbalance is acceptable?

NEMA guidance is to keep unbalance at the motor terminals below 1%. Above 1% the motor must be derated per the MG-1 curve, which voids most warranties. Operation above 5% is not recommended.

Can a VFD make a motor overheat at low speed?

Yes, for constant torque loads. A TEFC motor's shaft mounted fan slows with the motor, so cooling falls while a constant torque load still demands current. Centrifugal pumps and fans usually stay cool, but check the manufacturer's turndown limit.

Why is my motor hot when the current is normal?

The most likely causes are lost cooling or a hot room. A blocked shroud, packed fins, heavy paint or an ambient above 104°F (40°C) all raise winding temperature without raising current. Voltage unbalance can also overheat one winding while average current looks normal.

What is motor service factor?

Service factor is the overload a motor can carry for short periods, such as 1.15 times its rating. Running continuously into the service factor shortens insulation life. Compare the drive's displayed current with the nameplate full load amps.

Can dust cause a motor to overheat?

Yes. Dust or grease packed into a TEFC motor's fins, or lint over the shroud inlet, cuts the airflow that removes heat. On open drip proof motors, clogged screens do the same.

• Bearing Failure, Stage by Stage: Why Ultrasound Hears It Before Vibration Feels It and Heat Shows It
• Imbalance, Misalignment, Looseness and Soft Foot: Four Faults, Four Vibration Signatures
• Thermal Imaging: How It Finds a Failing Connection Before It Fails
• Thermal Method: Why a Reading Needs a Reference in the Same Frame
• Electrical: What NFPA 70B Asks You to Document

• US Department of Energy: voltage unbalance tip sheet
• Nidec: NEMA MG-1 voltage unbalance and temperature rise
• ACHR News: DOE tip sheet reprint with the 100 hp example