Abel Anada

Electric motor burnout happens when the insulation materials used in the motor burn due to the overheating of wires or copper windings inside the motor. 

These are the factors that cause overheating and burning of an electric motor.  The following are also the reasons why amps go so high beyond what is written on the nameplate.



1. Overloading the motor

2. Single phasing of power supply

3. Wrong motor connection

4. Motor component failure

I also Include:

• What burns inside the motor when it overheats
• What happens when the motor is overloaded.
• Steps to prevent electric motors from burning  due to overloading.
• What happens inside a 3-phase motor when it runs on a single-phase power supply by mistake.
• Ways to prevent the motor from burning due to ‘single phasing’ of the power supply.
• Factors that contribute to the motor being connected wrong
• What happens when a Wye connected motor is hooked up to Delta by mistake
• Ways to prevent electric motor burns due to wrong motor connection.
• How the single-phase motor starting works:
• What will happen when the motor runs without the starting winding.
• Ways To Prevent Electric Motor Burns Due To Failure Of Its Electrical Component
• Difference between Over-current protective device and Thermal protective device

In this post, you will learn how and why electric motor burns and how to avoid them.

Let’s start with what’s burning.

Inside the motor are coils wound using magnet wires or insulated copper wires.  These wires are sized according to the horsepower of the motor and the full load current it draws.  

So what happens if the amps are too high? 

When the electric motor draws a current way more than its nameplate rating and exceeds the ampacity or current-carrying capacity of the wires used in the windings, it overheats.

This over-current condition takes place when the motor is operated beyond what it is designed to operate and do.  

You cannot go over the rated amps without damaging the insulations inside the motor. 

Overload protection is set at 125% of the rated current, so the motor can be subjected to that increase in amps but only momentarily.  A few seconds.

Motor winding insulation can only handle certain temperatures depending on the insulation class. 

Insulations within motor windings susceptible to burning are: 



• Insulation between wire turns
• Insulation  between phases
• Insulation between the windings and the steel laminations inside the motor
• Insulation coating of the copper wire

These insulations will fail if subjected to temperatures above their rating.  For example, an electric motor with class F insulation can withstand 155˚C of heat as per NEMA standards. 

When conductors overheat, the temperature reaches 400˚C and up.  This is enough to burn these insulations.

Motor failure is costly not only in terms of having your motor repaired or replaced but also losses in terms of production output or product delivery, manpower hours, and lost profit.  

Knowing these reasons and taking preventative measures makes electric motors last longer and is beneficial in the long run. 

 In this article, I will provide you with information on how each of these reasons affects your motor and how to prevent it from happening. 

Let’s have a look at number one.



1.  Electric Motors Burns Because Of Overloading



Motor overloading happens when the load applied to the shaft of the motor becomes greater than the output torque of the motor itself. 

These are what’s going to happen when the motor is overloaded.



a) The current will go past the rated amps

b) The motor produces an abnormal humming

c) Rpm or the rotation of the motor at times will go a bit slower

d) Clicking sound inside a single-phase motor because of starting switch kicking in and out.

e) Frequent tripping of a circuit breaker or motor overload protection.

f) The increase in amps, when it goes beyond the current-carrying capacity of the windings, will result in damaging heat

g) The heat created by the motor coils deteriorates and burns the insulation of the motor windings 

e) The motor smokes and burn

f) What happens inside the motor when it is connected wrong.

From the formula;  

Torque=(Volts x 1.732 x Efficiency x Power Factor x 5250) / (RPM x 746)  

Torque is directly proportional to the current and inversely proportional to RPM or speed of rotation. A 5 horsepower, 1750 rpm motor can only produce 15 pounds foot of torque.

If the torque required to turn the motor shaft increases and is beyond what the motor can deliver, the rotation decreases, and the current increases.  This increase in current burns the motor.

Examples of ways a motor can get overloaded and burn:

• Steel bar pressed too hard on the stone of a pedestal grinder
• Saw blade jammed on the wood or steel it is cutting
• Hardened chemicals at the bottom of the agitator
• Dough too thick for the mixer
•  hard material stuck in the pump impeller
• Pressing the cut-off saw too hard

Steps to prevent electric motors from burning  due to overloading:

Overloading is preventable though some are just bound to happen.  Lack of maintenance doesn’t help either.  Here are some suggestions on how to prevent motor failures due to overload.



1. Use the motor protection device that comes with the motor

2. When buying a new motor or sending one for repair,  request that a protection device be installed and have the electrician hook it up during motor installation.

3. Use a magnetic contactor that has a thermal overload relay

4. Do regular inspection and maintenance not only of the motor but also the equipment it runs.

5. Follow proper operating procedures of types of equipment. 

6. An infrared temperature scanner is cheap.  Use it to detect over-heating motors

7. Do not disable the overload relay or other motor protection.  These devices trip for a reason so find out why

8. When fuses or circuit breakers trip, don’t replace them with a bigger one.  It’s very dangerous

9. When production and load requirement has increased over time, consider upgrading the motor

10. Do regular maintenance not only of the motor but also the machines they are running because that is the source of the overload.  Most often the motor is fine.

Maintenance not only of the motor but also the equipment it is running reduces the amps to an acceptable level and will also make the motor runs cooler.  This includes:



• Regular overhaul of the motor
• Properly greased or lubricated bearing and pillow blocks of equipment the motor is running
• Regular maintenance and cleaning of the equipment like fans. conveyor rollers, contaminant build-up, clogged chutes, etc.
• Protecting the motor from water



2.  Electric Motors Burns Because Of The “Single Phasing” Of The Power Supply. 
 

Three-phase motors have 3 sets of winding coils supplied with 3 lead cables each of which is carrying voltage. 

If you lose power on one of the cables, depending on the motor connection whether WYE or Delta, one or two sets of winding will burn inside the motor.  

This is because the motor is trying to run the load using only a part of its winding.

In this figure, when the power on line 3 for some reason dropped out, only phase A is experiencing full line to line voltage. 

At full load, it will be impossible to run the motor with only one phase fully functional.  The result will be:



a) The motor will not run

b) Humming noise is very pronounced

c) Current will be a few times higher than normal on two lead cables

d) The wires of the motor winding will be red hot

e) Insulations inside the motor burns

f) Smoke starts to appear from the motor



This figure shows the winding connection inside the motor.  Polarities are shown by the arrows and the proper layout of the three phases.  Yellow lines are used to connect a group of coils on the same phase.  

If the cable of line 3 is disconnected, only phase A is energized with the full voltage. Only the coils colored red are experiencing the actual voltage.

And again, since the rotor is locked, the current will shoot up creating heat that will burn the insulation on that group of coils. 

This is why on ‘single phasing’ when you look at the actual winding, you will notice a pattern of overheated coils.  In this example, it will be burnt – good – good – burnt – good – good.

Repair shops can tell right away if the particular motor fails on “single phasing’ or not.  If it did they will deny you a warranty even if the motor is brand new because it is the customer’s fault why the motor failed.

To prevent the motor from burning due to ‘single phasing’ of the power supply;



1. Do a regular inspection of connections.  Check for the following:

a) Inside the motor terminal box, look for signs of over-heating connection screws.  Tighten if necessary. 

b) On the magnetic contactor inside the motor control panel look for signs of over-heating connection terminals.  Loose terminal screws create high resistance contacts that produce heat.

c)  If the magnetic contactor shows dust contamination, clean them with contact spray cleaner or very light compressed air.

2.  If it is economically possible, replace the fuse disconnect switch with a 3 phase circuit breaker.  Most of the time, the disconnect switch blows only one fuse leaving the motor susceptible to ‘single phasing’.

 3. If the motor is plugged in a three-phase receptacle, make sure it is securely plugged and “twist locked”

Warning:  Inspections of motor and control units must be done while all the main circuit breakers and fuse disconnects are in the OFF position to avoid shock hazards.



3.  Electric Motors Burns Because Of Wrong Motor Connection

Three-phase motors have three sets of windings connected into a circuit or circuits of either Wye or Delta to in order run.   It could be one circuit, two circuits, four circuits, and so forth.  You cannot just put these three sets of winding in parallel and expect it to work.  

All the windings of the motor are equally divided and distributed to the three phases of the power supply namely phase A-B, phase B-C, and phase A-C,  so they will have more or less equal resistances.

The marked leads of the motor have to be connected properly at the j-box or terminal box.  You miss the right connection and the motor will burn. 

The following contribute to the motor being connected wrong:

•  Illegible or no motor lead markings
• Illegible or no nameplate
• Absence of connection diagram



a)  Illegible or absence of motor lead markings or identification can lead to a wrong motor connection that will eventually burn the motor. 

For example, a 5 HP, 460 volts, 1725 rpm motor has 6 leads with no markings or some color codes.  We know from the electrical diagram that the end of T1 is T4, the end of T2 is T5, and the end of T3 is T6.  For the IEC standard, the end of U1 is U2, the end of V1 is V2 and the end of W1 is W2.



The first figure shows the standard numbering of a Wye-start Delta-run motor.   Let us say we have a motor to be connected but doesn’t have lead markings. 

Looking at the diagram, since we know which leads are for Phase A, B, and C, we assign T1, T2, and T3 respectively on them.  After that, we label the opposite of T1 as T4, the opposite of T2 as T5, and the opposite of T3 as T6.

 If the lead numbering on any of the phases is interchanged say T1 to T4 and T4 to T1, there will be bucking of polarities, and soon as the motor runs, the current shoots up, and the motor burns.  

The same is true with a 9 lead or 12 lead motor.  The numberings and connections have to be right.

This figure shows the connection diagram with the correct lead marking.

 When Line 1 of the power supply is connected to T1 and T6, Line 2 to T2 and T4, and Line 3 to T3 and T5, It illustrates the right flow of alternating north and south magnetic field on every coil inside the motor.



This last figure shows when T1 and T4 are mistakenly interchanged.  

When the motor is connected wrong, these will happen; 

a) The coils will create “bucking” of coil polarities, as shown on coils 6, 1, and 2 or coils 3, 4, and 5.

b) The shaft will either seize or turn slow,

c) The motor will create an unusual humming sound, 

d) The current will shoot up way beyond the rated amps.

d) The coils inside the motor will eventually overheat burning the insulations. 

b)  Illegible or absence of a nameplate creates confusion on the part of the installer and gets a higher chance of mistake in connecting the motor wires to the power supply.  

For example, a motor with 6 lead cables all numbered T1 to T6.  The motor also has either worn out or no nameplate. The problem is, that other kinds of motors like two-speed variable torque, constant torque, or constant horsepower are numbered T1 to T6 also. 

The same with a Wye start-Delta run a motor or a dual voltage Delta-Wye motor all have the same lead numberings T1 to T6. Without the nameplate, it is harder to figure out what the right connection is.

Let us say we have a motor with the following condition:

•  The nameplate voltage is unreadable
•  The electrician knows that the motor works at 460 volts 3 phase. 
• The lead numberings are T1, T2, T3, T4, T5, and T6. 
•  It was assumed that the motor is 460 volts, 3 phase, 60 hertz
• Because of this information, the motor was connected delta across the line. 

If the motor is a single voltage, the motor will work because it is a Wye start-Delta run the motor.  But we don’t know.

What if by chance the motor is 380/460V dual voltage which is a possibility?  The motor has to be connected to WYE to work properly.  This is because, on a dual voltage 6 lead motor, the connection should be Delta for a lower voltage and Wye for a higher supply voltage.

But then the motor was connected delta and the motor overheated.  They thought it was overloaded.

On a dual voltage 6 lead motor, this is what happens when a Wye connected motor is hooked up to Delta by mistake:



• First, the right connections should be Wye for higher voltage and Delta for lower voltage. Example 380D/480Y
• Since it was connected delta, the motor was given higher across the line voltage instead of the lower value
• There is an overvoltage condition when the motor is run.
• At full load, the motor will experience saturation current
• The increase in current will lead to overheating of the windings inside the motor eventually burning the insulation.



c)    Absence of a connection diagram also leads to burning a motor.  There is no problem connecting the motor, especially the basic ones like 12 lead motors, 9 lead motors, Part Winding Start motors, or even 6 lead Wye start-Delta run motors.  

On the following motors, you might need a connection diagram to hook up the motor leads.

 

o   Triple Rate or Three Horsepower- Two Winding motor

o   Triple Rate or Three Horsepower-One Winding motor

o   Two Speed-One Winding motor

o   Two Speed-Two Winding 9 lead motor

The reason for this is that some manufacturers have their version of how to connect their motor depending on how it was connected on the inside of it.  You are running the risk of burning the motor if they are not connected right.

Ways to prevent electric motor burns due to wrong motor connection: 

Pay attention to the nameplate information when hooking up a motor.  Indications that you have to be careful connecting the motor are:

1. When there are two or three horsepower
2. 9 or 12 leads that might indicate that it can be a Wye or a Delta connected
3.  When the motor is rated at 120 cycles per second, or anything other than 50 or 60 hertz
4. When you have a two-speed or three-speed motor with multiple leads
5. When you have a two speed motor with 8 leads it means you have a center tap for the brake

On new installs, use a clamp-on ammeter to monitor the current the motor is drawing.  A value above the rated current especially at no load is an indication of a wrong connection.

Check the actual supply voltage and don’t guess.  It has to be the same as the nameplate voltage of the motor.  


4.  Electric Motors Burns Because Of Failure Of Its Components.

Some examples of these components are:

• Starting switch assembly
• Starting capacitor
• Motor overload or over-current protection protection
• Motor cooling blower



1. Starting Switch Assembly

A single-phase capacitor start induction motor has two sets of windings, a starting winding, and a running winding. 

The inductive property of the running winding makes it unable to run by itself.  So, a starting winding with a lesser number of turns and smaller wire size is added or wound on top of the running winding inside the motor.  

This is how the single-phase motor starting works:



a) When the motor is energized both the main or running winding and the auxiliary or starting winding will operate

b) The starting winding, with the aid of a capacitor, creates approximately 3 times the torque of the running winding.  It will then run until it comes 3up to speed.

c) Although it has the torque suitable for starting the motor, it cannot run continuously or it will burn. 

d) A centrifugal switch is connected so that soon as the motor reaches 75 to 85% of the speed, it will disconnect the starting winding leaving the main winding running the motor.

e) The centrifugal switch wears out over time and if they fail, will eventually burn the motor windings.  The centrifugal switch has two parts:



Several times I have seen a motor with a broken rotating switch disc.  If they break, it jams onto the stationary switch breaking the contact mechanism.  But most of the time, it is the contact in the stationary switch that wears out.  

The constant stop and go for example of an air compressor creates arcing on this contact point.  Sometimes they just fused together keeping it from opening the starting circuit. 

If the switch doesn’t cut the starting winding out, It will burn the starting winding for sure. 

Another case is when the contact point breaks or blew out.  This is what will happen when the motor runs without the starting winding:

o   The auxiliary circuit will always be open

o   The starting winding will not function

o   The result is that only the main winding is energized

o   Motor will just hum unable to rotate the shaft and eventually burning the main winding. 




2. Capacitors

A worn-out or defective capacitor can also hinder the normal operation of the starting circuit.  Its job is to suppose to help the starting winding produce the needed torque to start the motor by making the current through auxiliary winding lead the supply voltage by a considerable angle. 

 This is what will happen if the starting capacitor is defective:

a) There will not be enough torque to start the motor

b) The motor will just hum and will not rotate or if it is not fully loaded, the motor might run slow.

c) Since the motor doesn’t come up to full speed, the windings will eventually burn



3. Motor Protective Devices 

Motor protection stops the motor in events of winding overheating before it burns the windings.

Two kinds of motor protective devices  are:

a) Over-current protective device – mounted either on the J-box of the motor or the load side of the magnetic contactor controlling the motor.

 It employs a bi-metallic element or snap-acting bi-metal disc rated in amperes.  As the motor experience the over-current condition, the bi-metal disk or element opens the power supply connection stopping the motor.  After it cools down, the disc snaps back allowing the motor to run again. 

b). Thermal protective device –  mounted directly into the motor windings and their leads connected to the motor control circuit. When they sense a temperature above their preset setting, it opens the control circuit of the motor shutting it down.

Constant arcing wears out these devices.  If for example it got stuck in a close position, the motor is already over-heating and the protection is still not responding.

Ways To Prevent Electric Motor Burns Due To Failure Of Its Electrical Component:

Proper testing and maintenance is the best way to extend the life of these electrical devices.   They are not expensive, and if you have to replace them, it is just a small fraction of the cost of a brand new motor. 



1. Check if the electric motor protective device is working properly by monitoring the current of the motor.  

To do this:

a) Make sure the power to the motor is in the OFF position

b) Hook the clamp meter in one of the cables supplying the motor. Let it hang by itself.

c) Operate the motor and monitor the current.

d) If the amp is normal and the motor protection trips, replace the motor protection device.

e) The motor overload protection is good if the amps went way above the nameplate current and the motor protection trips.  The motor is experiencing an overload condition.  Find out why and fix the problem. 

f) Soon as the motor cools down, press the red reset button either in the side part of the j-box of the motor or the small reset button at the magnetic contactor of the motor.

Note: Do not in any way disconnect or disable the motor protective device.

Warning: 

Working close to energized cables or rotating machines is dangerous and should be done by a trained technician only.

2. Schedule regular maintenance of the electric motor.  Every six months or yearly depending on how often a single-phase motor is running on and off.

To check these components, do the following:

a) Test the capacitor by using a capacitor tester or a multimeter that can test a capacitor.  Check if the capacitance value in microfarad (μF) is within the range of what is written on the capacitor.

b) Remove the motor end shield and inspect the operation of the rotary switch. It should smoothly travel in and out if you press it with your finger. Check for any crack.

c) Inspect the stationary switch and check for burnt or worn-out contact points due to arcing.  Use a smooth file or a grade 220 emery cloth to smoothen the contact surfaces.

Warning:  

Disassembling a motor is a job for a trained technician.  If you have to do it yourself, be sure that the motor is completely disconnected from the power supply.  

Conclusion:

These are the four common reasons why electric motors burn. In short. Keep your current even and at bay and your motor will last longer.  

Your motor protection is like a body thermometer that says “hey I’m not feeling well.  Do something about it”.

I did not include the reasons like bearing failure, electrical surge,  insulation breakdown, or winding contamination to name a few since it is more on coil blowing in the slot, phase to phase, or coil to coil short and not a complete burnout.

The more you understand your motor the more you know what to do.