Difference between revisions of "Motor repair"
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A dead motor means no current flow and no operation. This is usually caused by an open circuit winding or worn out [[#brushes|brushes]]. | A dead motor means no current flow and no operation. This is usually caused by an open circuit winding or worn out [[#brushes|brushes]]. | ||
| − | With a faulty winding, the motor conducts in most rotor positions, but goes open circuit in one position. This is easily checked with a [[multimeter]], turning the rotor by hand. | + | With a faulty winding, the motor conducts in most rotor positions, but goes open circuit in one position. This is easily checked with a [[multimeter]], turning the rotor round gradually by hand. |
| − | The repair option for a dead winding is [[#rewind|motor rewind]] or motor replacement. Simply [[#Turn the motor|rotating the motor]] is also an option in some cases. | + | The repair option for a dead winding is [[#rewind|motor rewind]] or motor replacement. Simply [[#Turn the motor|rotating the motor]] to get it started is also an option in some cases, if not an ideal one. |
Also see [[#Brushes|brushes]] | Also see [[#Brushes|brushes]] | ||
| − | |||
==Stalled== | ==Stalled== | ||
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Less often it may also be caused by incorrect wiring. This tends to occur when scrap motors are reused for home made projects, or when historic equipment in poor condition is repaired. See [[#rewiring|rewiring]]. | Less often it may also be caused by incorrect wiring. This tends to occur when scrap motors are reused for home made projects, or when historic equipment in poor condition is repaired. See [[#rewiring|rewiring]]. | ||
| + | |||
| + | Rarely a stalled motor can be caused by a failed run capacitor. | ||
Repair options depend on the cause, and include: | Repair options depend on the cause, and include: | ||
| Line 25: | Line 26: | ||
* [[#Rewiring|Rewiring]] | * [[#Rewiring|Rewiring]] | ||
* [[#Starter capacitor|Starter capacitor]] | * [[#Starter capacitor|Starter capacitor]] | ||
| − | + | * [[#Run capacitor|Run capacitor]] | |
==Squeal== | ==Squeal== | ||
| Line 31: | Line 32: | ||
==Shaft movement== | ==Shaft movement== | ||
| − | Lateral movement of the motor shaft is due to [[#New bearings|bearing failure]]. In some cases the motor may continue to run like this for a limited time. | + | Lateral movement of the motor shaft is sometimes due to [[#New bearings|bearing failure]]. In some cases the motor may continue to run like this for a limited time. |
| − | Note that many small fan motors are designed to operate with longitudinal shaft movement. Mounting position or | + | Note that many small fan motors are designed to operate with longitudinal shaft movement. Mounting position or the force generated by the fan blades keep the rotor in the right position during operation. These motors must be operated the right way up. |
The repair options are [[#New bearings|bearing replacement]] & motor replacement. | The repair options are [[#New bearings|bearing replacement]] & motor replacement. | ||
| − | |||
==Stiffness== | ==Stiffness== | ||
| Line 54: | Line 54: | ||
* partial winding short - [[#rewind|rewind]], replace or add a [[#dropper|dropper]] | * partial winding short - [[#rewind|rewind]], replace or add a [[#dropper|dropper]] | ||
* mechanical overloading - reduce loading | * mechanical overloading - reduce loading | ||
| − | * 220v motor on 240v - use a [[#dropper|dropper]] | + | * 220v motor on 240v - use a [[Droppers|transformer]] or [[#dropper|dropper]] |
| − | * 110v motor on 240v - use a [[ | + | * 110v motor on 240v - use a [[Droppers|transformer]] |
* loose or broken [[#self cooling fan|self cooling fan]] blades | * loose or broken [[#self cooling fan|self cooling fan]] blades | ||
* [[#centrifugal starting switch|centrifugal starting switch]] not opening at speed | * [[#centrifugal starting switch|centrifugal starting switch]] not opening at speed | ||
| − | * partly blocked air path in vacuum | + | * partly blocked air path in [[vacuum cleaner]]s |
| − | * Motors designed for intermittent use being run for longer than rated (eg blenders, drills etc) | + | * Motors designed for intermittent use being run for longer than rated (eg [[:Category:Appliances|blenders]], [[:Category:Tools|drills]] etc) |
| − | |||
==Smoking== | ==Smoking== | ||
| Line 69: | Line 68: | ||
Noisy operation is normally due to bearing wear. This can cause noise in any frequency range from rumble to squeal. Sometimes the noise sounds normal but much louder. High noise levels occur not long before [[#New bearings|total bearing failure]]. | Noisy operation is normally due to bearing wear. This can cause noise in any frequency range from rumble to squeal. Sometimes the noise sounds normal but much louder. High noise levels occur not long before [[#New bearings|total bearing failure]]. | ||
| − | In some cases the noisy bearings are in the load rather than the motor itself. This often happens with washing machine drum bearings. | + | In some cases the noisy bearings are in the load rather than the motor itself. This often happens with [[washing machine]] drum bearings. |
The only repair option is [[#new bearings|new bearings]]. | The only repair option is [[#new bearings|new bearings]]. | ||
| − | High noise levels can also be down to design, often the result of cost cutting. See noise reduction options. | + | High noise levels can also be down to design, often the result of cost cutting. See [[noise]] reduction options. |
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==Live case or earth leakage== | ==Live case or earth leakage== | ||
| − | A live case or earth leakage is often due to insulation failure in the motor. | + | A live case or earth leakage is often due to insulation failure in the motor. Now this tends to cause repeated [[RCD]] trips. |
| − | + | Leakage can be caused by a few things. Buildup of carbon dust from the brushes is one, the carbon conducts electricity from commutator to motor case. [[Cleaning]] resolves this if sufficiently thorough. | |
| − | It may also be caused by water in the motor, in which case prolonged drying can solve the problem. | + | It may also be caused by [[water]] in the motor, in which case prolonged drying can solve the problem. |
| − | + | The final cause is winding insulation breakdown. The normal option for this is a [[#rewind|rewind]]. In a minority of cases its possible to remount the motor so its insulated from everything else. Care is needed to ensure it meets all the usual [[safety]] requirements, and that's often impracticable. | |
| − | + | Swapping mains polarity was sometimes used in the past to enable such motors to function without [[fuse]] blowing or shock, but this is not totally [[safe]] practice as the supply neutral is still touchable, and can rise to a risk voltage during a [[fault]]. Experience in the US with 220v neutral chassis cookers has proven an excellent safety record, but this only applies when the appliance can't be plugged into a wrong polarity socket or lead by mistake. Such practice is '''not''' compliant with modern wiring [[regulations]]. | |
==Commutator uneven== | ==Commutator uneven== | ||
| − | Commutators should be accurately cylindrical. Uneven segment height is usually due to damage caused by a faulty winding. The result is brush bounce, and a motor that only runs at low speed, if at all, or possibly runs fairly fast but behaves erratically | + | Commutators should be accurately cylindrical. Uneven segment height is usually due to damage caused by a faulty winding. The result is brush bounce, sparking, and a motor that only runs at low speed, if at all, or possibly runs fairly fast but behaves erratically. |
| − | |||
| − | |||
| − | + | Such damage requires [[#rewind|rewind]]ing and a new commutator. | |
| + | A rarely chosen option is to file the commutator flat by running the motor on low voltage (via a transformer) and using a file, then using a [[#dropper|dropper]] to limit current in use. | ||
=Repairs= | =Repairs= | ||
| − | Replacement of motor or appliance are of course options. Appliance replacement is the most common approach, but in many cases is unnecessary. | + | Replacement of motor or appliance are of course options. [[Appliance]] replacement is the most common approach, but in many cases is unnecessary. |
==Brushes== | ==Brushes== | ||
| + | [[image:Motor brushes 0625-5.jpg|right|185px]] | ||
Worn out brushes cause arcing, intermittent operation, then failure. | Worn out brushes cause arcing, intermittent operation, then failure. | ||
| Line 115: | Line 114: | ||
===Variacs=== | ===Variacs=== | ||
| − | A few DIYers use variacs. These are designed to take a specific and uncommon design of brushes, made from a sandwich of high and low resistance material in bands. This gives good longitudinal conduction with higher lateral resistance, reducing winding shorting. Its recommended to use the correct type of brush, the wrong type | + | A few DIYers use variacs. These are designed to take a specific and uncommon design of brushes, made from a sandwich of high and low resistance material in bands. This gives good longitudinal conduction with higher lateral resistance, reducing winding shorting. Its recommended to use the correct type of brush, the wrong type causes increased power consumption and increased heating of the variac. |
| − | |||
==Cleaning commutator== | ==Cleaning commutator== | ||
| − | Sometimes excessive current draw is simply due to carbon accumulation in commutator slots. The dust is easily | + | Sometimes excessive [[Electrical|current]] draw is simply due to carbon accumulation in commutator slots. The dust is easily [[clean]]ed out once the commutator is accessed. |
| − | |||
==Cleaning bearings== | ==Cleaning bearings== | ||
| − | Bearings may be cleaned to some extent by running a solvent into them and operating the motor. The bearing should be relubricated with oil. | + | Bearings may be cleaned to some extent by running a solvent into them and operating the motor. The bearing should be relubricated with [[oil]]. |
Disassembling the motor and cleaning out the bearing throughly is much more effective, but the above option is simple and sometimes sufficient. Bearings should be grease packed once cleaned out. | Disassembling the motor and cleaning out the bearing throughly is much more effective, but the above option is simple and sometimes sufficient. Bearings should be grease packed once cleaned out. | ||
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==Replacement== | ==Replacement== | ||
| − | Motor replacement is often a quick easy fix, and is a good option when motor repair isn't worthwhile. It can be a no cost option when a similar donor scrap appliance is available. | + | Motor replacement is often a quick easy fix, and is a good option when motor repair isn't worthwhile. It can be a no cost option when a similar donor scrap [[appliance]] is available. |
| Line 139: | Line 136: | ||
Replacement bearings are fittable on some motors. | Replacement bearings are fittable on some motors. | ||
| − | Some tools can suffer very quick bearing wear when exposed to plaster dust. | + | Some [[tools]] can suffer very quick bearing wear when exposed to [[plaster]] dust. |
==Starter capacitor== | ==Starter capacitor== | ||
| − | Replace with the right capacity and voltage. Its recommended to use a capacitor designed for motor starting, as these are designed to survive dielectric puncture, which can occur in this application. | + | Replace with the right capacity and voltage. Its recommended to use a capacitor designed for motor starting, as these are designed to survive dielectric puncture, which can occur in this application. |
| + | |||
| + | Some variation in capacity works ok when the load has low starting torque, but if the capacitor is too far out the motor either won't start or will run backwards. | ||
| + | ==Run capacitor== | ||
| + | Replace with exactly the right capacity and voltage. Using the wrong capacity causes loss of torque and overheating. | ||
==Centrifugal switch== | ==Centrifugal switch== | ||
| − | + | Switch contacts can be cleaned with very fine sandpaper, eg wet & dry or emery. Also usable: a fine file, scrape with the back of a [[knife]]blade. Switch cleaner solvent is sometimes adequate. | |
==Belt adjustment== | ==Belt adjustment== | ||
| Line 156: | Line 157: | ||
Correct belt tension is typically when the belt is reasonably easy to twist by 90 degrees on its longest free run. Higher tension makes this difficult, and low tension makes greater twisting easy. | Correct belt tension is typically when the belt is reasonably easy to twist by 90 degrees on its longest free run. Higher tension makes this difficult, and low tension makes greater twisting easy. | ||
| − | Most domestic appliances that use belts have no self adjusting belt tensionsing mechanism. In these, the motor position is moved to achieve correct belt tension, and tension on the motor is maintained while the fixings are done up tight. This often requires 3 hands, or something just the right size to jam in to hold the motor while doing up the fixing | + | Most domestic appliances that use belts have no self adjusting belt tensionsing mechanism. In these, the motor position is moved to achieve correct belt tension, and tension on the motor is maintained while the fixings are done up tight. This often requires 3 hands, or something just the right size to jam in to hold the motor while doing up the fixing [[bolt]]s. Use a lever (prybar etc) to apply pressure to the motor to move it into final position. |
| − | Note that | + | Note that motor designs are not fully rigid, and rely on the motor mountings to hold the motor casing firm. With these, applying leverage to the end of the motor away from the belt can cause the motor casing to twist. In these cases the lever should be applied as near to the belt as possible, and its necessary to fully tighten all the motor mounting points. |
| Line 164: | Line 165: | ||
Some appliances with a dead winding can be started by turning the motor a little manually to get it to run, or by rotating the appliance a little so the motor turns enough to get going. Direct turning of the motor is of course done with a stick rather than a hand. | Some appliances with a dead winding can be started by turning the motor a little manually to get it to run, or by rotating the appliance a little so the motor turns enough to get going. Direct turning of the motor is of course done with a stick rather than a hand. | ||
| − | This is ok for small appliances where moving it is practical, and occasional failure to self start is acceptable. | + | This is ok for small appliances where moving it is practical, and occasional failure to self start is acceptable. This is often ok as a temporary fix. |
| − | |||
==Rebuild== | ==Rebuild== | ||
| Line 172: | Line 172: | ||
==Dropper== | ==Dropper== | ||
| − | [[Droppers]] are sometimes used to limit current on motors drawing excess current, to enable them to continue in service safely and effectively long term. This | + | [[Droppers]] are sometimes used to limit current on motors drawing excess current, to enable them to continue in service safely and effectively long term. This is a bodge, but since it meets all the necessary requirements of working acceptably, being [[safe]] and being reliable, its an acceptable option at times. Its occasionally used to [[repair]] domestic appliances where motor [[#rewind|rewind]] or replacement is uneconomic. It sees more use in industrial situations, where down time is expensive. |
The addition of a series resistance with the reduced impedance of the motor results in reduced top speed and power. The result is normally a functional appliance with reduced performance. Final performance depends on the damage. | The addition of a series resistance with the reduced impedance of the motor results in reduced top speed and power. The result is normally a functional appliance with reduced performance. Final performance depends on the damage. | ||
| Line 179: | Line 179: | ||
If in doubt about how to achieve the right motor operating conditions, assistance may be sought in news:sci.electronics.basics. | If in doubt about how to achieve the right motor operating conditions, assistance may be sought in news:sci.electronics.basics. | ||
| − | |||
==Rewiring== | ==Rewiring== | ||
| − | If there is any doubt over the correct motor wiring, running the motor via a [[Droppers|dropper]] prevents damaging excess current flow if the wiring is wrong. It also limits heat dissipation and prevents fuse blowing. | + | If there is any doubt over the correct motor wiring, running the motor via a [[Droppers|dropper]] prevents damaging excess current flow if the wiring is wrong. It also limits heat dissipation and prevents [[fuse]] blowing. |
When testing with a dropper, a motor should only be powered up for seconds at a time, as overheating can occur in 10 or 20 seconds when there's no rotation. | When testing with a dropper, a motor should only be powered up for seconds at a time, as overheating can occur in 10 or 20 seconds when there's no rotation. | ||
| Line 192: | Line 191: | ||
If loose or detached it can be reattached. | If loose or detached it can be reattached. | ||
| − | If the fixing thread is damaged the fan blade may be welded, or glued with [[Adhesive|epoxy resin]], with caveats. The glue may see high operating temperatures and/or high vibration levels, so high temp epoxy is required | + | If the fixing thread is damaged the fan blade may be welded, or glued with [[Adhesive|epoxy resin]], with caveats. The glue may see high operating temperatures and/or high vibration levels, so high temp epoxy is required, the metal should be solvent degreased first, the glue should be symmetrical to minimise imbalance, and it has to be accepted that [[glue]] repairs don't always survive. |
If the fan is badly damaged, a replacement part may be fashioned from sheet metal. | If the fan is badly damaged, a replacement part may be fashioned from sheet metal. | ||
| − | In principle an external fan can be attached to take over the function of the internal one. However in practice | + | In principle an external fan can be attached to take over the function of the internal one. However in practice its difficult to generate enough airflow. The fan generally needs to run as fast as the motor, and this is often a much higher speed than most off the shelf fans. |
=See Also= | =See Also= | ||
| + | * [https://groups.google.com/forum/?fromgroups#!topic/uk.d-i-y/xnaaEIymFcw Motor lubrication] | ||
| + | * [https://groups.google.com/forum/#!topic/uk.d-i-y/CMt21EnCe20 Shaded pole motor doesn't start] | ||
| + | * [https://groups.google.com/forum/#!topic/sci.electronics.design/vu1DALnmyxY PM DC motor faults & testing] | ||
* [[Multimeter]] | * [[Multimeter]] | ||
* [[Droppers]] | * [[Droppers]] | ||
Latest revision as of 04:05, 7 March 2015
Motor repair is in many cases practical for DIYers. Not needing to buy a new appliance can be a handy saving.
Faults
Dead
A dead motor means no current flow and no operation. This is usually caused by an open circuit winding or worn out brushes.
With a faulty winding, the motor conducts in most rotor positions, but goes open circuit in one position. This is easily checked with a multimeter, turning the rotor round gradually by hand.
The repair option for a dead winding is motor rewind or motor replacement. Simply rotating the motor to get it started is also an option in some cases, if not an ideal one.
Also see brushes
Stalled
A stalled motor has current flowing but doesn't turn. When there is no jammed rotor its often caused by a failed starter capacitor.
It can also be caused by the load torque requirement exceeding the motor's starting torque. This happens in home designed equipment and equipment where the mechanical load has increased, maybe due to muck in load bearings or mechanical overload.
Less often it may also be caused by incorrect wiring. This tends to occur when scrap motors are reused for home made projects, or when historic equipment in poor condition is repaired. See rewiring.
Rarely a stalled motor can be caused by a failed run capacitor.
Repair options depend on the cause, and include:
Squeal
Squeal is due to failing bearings or an overly tight drive belt.
Shaft movement
Lateral movement of the motor shaft is sometimes due to bearing failure. In some cases the motor may continue to run like this for a limited time.
Note that many small fan motors are designed to operate with longitudinal shaft movement. Mounting position or the force generated by the fan blades keep the rotor in the right position during operation. These motors must be operated the right way up.
The repair options are bearing replacement & motor replacement.
Stiffness
Stiffness is due to muck in the bearings or bearing failure.
Jammed
Motors can jam due to bearing dirt or failure.
Motor rotors can be locked in place due to a jammed load. If in doubt, separate the motor from its load to determine where the fault lies.
Jamming can also occur if a motor with intentional longitudinal movement is mounted the wrong way up. This is easily spotted by moving the rotor longitudinally and rotating; pushed to one end it will rotate heathily, moved the other way it will either jam fully or rotate part way only.
Overheating
Overheating may be due to:
- dust clogged vents - clean
- partial winding short - rewind, replace or add a dropper
- mechanical overloading - reduce loading
- 220v motor on 240v - use a transformer or dropper
- 110v motor on 240v - use a transformer
- loose or broken self cooling fan blades
- centrifugal starting switch not opening at speed
- partly blocked air path in vacuum cleaners
- Motors designed for intermittent use being run for longer than rated (eg blenders, drills etc)
Smoking
Smoking is overheating, see above section. Smoking quickly damages and destroys winding insulation. When testing a smoking motor, adding a series dropper can prevent rapid overheating and avoid further damage.
Noisy
Noisy operation is normally due to bearing wear. This can cause noise in any frequency range from rumble to squeal. Sometimes the noise sounds normal but much louder. High noise levels occur not long before total bearing failure.
In some cases the noisy bearings are in the load rather than the motor itself. This often happens with washing machine drum bearings.
The only repair option is new bearings.
High noise levels can also be down to design, often the result of cost cutting. See noise reduction options.
Blown fuses
A partial short on one of the windings causes high current draw in one rotor position. This blows fuses or causes rapid and/or severe overheating.
The repair options are motor replacement, rewind, or a series dropper.
Live case or earth leakage
A live case or earth leakage is often due to insulation failure in the motor. Now this tends to cause repeated RCD trips.
Leakage can be caused by a few things. Buildup of carbon dust from the brushes is one, the carbon conducts electricity from commutator to motor case. Cleaning resolves this if sufficiently thorough.
It may also be caused by water in the motor, in which case prolonged drying can solve the problem.
The final cause is winding insulation breakdown. The normal option for this is a rewind. In a minority of cases its possible to remount the motor so its insulated from everything else. Care is needed to ensure it meets all the usual safety requirements, and that's often impracticable.
Swapping mains polarity was sometimes used in the past to enable such motors to function without fuse blowing or shock, but this is not totally safe practice as the supply neutral is still touchable, and can rise to a risk voltage during a fault. Experience in the US with 220v neutral chassis cookers has proven an excellent safety record, but this only applies when the appliance can't be plugged into a wrong polarity socket or lead by mistake. Such practice is not compliant with modern wiring regulations.
Commutator uneven
Commutators should be accurately cylindrical. Uneven segment height is usually due to damage caused by a faulty winding. The result is brush bounce, sparking, and a motor that only runs at low speed, if at all, or possibly runs fairly fast but behaves erratically.
Such damage requires rewinding and a new commutator.
A rarely chosen option is to file the commutator flat by running the motor on low voltage (via a transformer) and using a file, then using a dropper to limit current in use.
Repairs
Replacement of motor or appliance are of course options. Appliance replacement is the most common approach, but in many cases is unnecessary.
Brushes
Worn out brushes cause arcing, intermittent operation, then failure.
Brushes look like rectangular carbon rods, and are spring mounted. Brushed motors have 2 brushes (a lot of motors don't use brushes).
Brushes don't wear all the way down before they are spent. Normally the available brush movement is restricted, and once the limit of travel is reached, proper contact is no longer made.
There are various ways to mount brushes, an inspection of the motor should reveal all. Some new brushes require a release tab to be bent out the way to enable them to engage.
Variacs
A few DIYers use variacs. These are designed to take a specific and uncommon design of brushes, made from a sandwich of high and low resistance material in bands. This gives good longitudinal conduction with higher lateral resistance, reducing winding shorting. Its recommended to use the correct type of brush, the wrong type causes increased power consumption and increased heating of the variac.
Cleaning commutator
Sometimes excessive current draw is simply due to carbon accumulation in commutator slots. The dust is easily cleaned out once the commutator is accessed.
Cleaning bearings
Bearings may be cleaned to some extent by running a solvent into them and operating the motor. The bearing should be relubricated with oil.
Disassembling the motor and cleaning out the bearing throughly is much more effective, but the above option is simple and sometimes sufficient. Bearings should be grease packed once cleaned out.
Replacement
Motor replacement is often a quick easy fix, and is a good option when motor repair isn't worthwhile. It can be a no cost option when a similar donor scrap appliance is available.
Rewind
Rewinding the motor is used to repair winding failures when the rest of the motor is in good condition, and motor replacement would be more expensive.
New bearings
Replacement bearings are fittable on some motors.
Some tools can suffer very quick bearing wear when exposed to plaster dust.
Starter capacitor
Replace with the right capacity and voltage. Its recommended to use a capacitor designed for motor starting, as these are designed to survive dielectric puncture, which can occur in this application.
Some variation in capacity works ok when the load has low starting torque, but if the capacitor is too far out the motor either won't start or will run backwards.
Run capacitor
Replace with exactly the right capacity and voltage. Using the wrong capacity causes loss of torque and overheating.
Centrifugal switch
Switch contacts can be cleaned with very fine sandpaper, eg wet & dry or emery. Also usable: a fine file, scrape with the back of a knifeblade. Switch cleaner solvent is sometimes adequate.
Belt adjustment
Too little tension causes slipping and localised belt burning.
Too much tension causes accelerated bearing wear, and can cause noise and sometimes stiffness (which increases power consumption and motor operating temperature).
Correct belt tension is typically when the belt is reasonably easy to twist by 90 degrees on its longest free run. Higher tension makes this difficult, and low tension makes greater twisting easy.
Most domestic appliances that use belts have no self adjusting belt tensionsing mechanism. In these, the motor position is moved to achieve correct belt tension, and tension on the motor is maintained while the fixings are done up tight. This often requires 3 hands, or something just the right size to jam in to hold the motor while doing up the fixing bolts. Use a lever (prybar etc) to apply pressure to the motor to move it into final position.
Note that motor designs are not fully rigid, and rely on the motor mountings to hold the motor casing firm. With these, applying leverage to the end of the motor away from the belt can cause the motor casing to twist. In these cases the lever should be applied as near to the belt as possible, and its necessary to fully tighten all the motor mounting points.
Turn the motor
Some appliances with a dead winding can be started by turning the motor a little manually to get it to run, or by rotating the appliance a little so the motor turns enough to get going. Direct turning of the motor is of course done with a stick rather than a hand.
This is ok for small appliances where moving it is practical, and occasional failure to self start is acceptable. This is often ok as a temporary fix.
Rebuild
A motor rebuild includes new bearings when necessary, new commutator and a rewind. This can be an attractive option for larger or higher priced motors with multiple faults.
Dropper
Droppers are sometimes used to limit current on motors drawing excess current, to enable them to continue in service safely and effectively long term. This is a bodge, but since it meets all the necessary requirements of working acceptably, being safe and being reliable, its an acceptable option at times. Its occasionally used to repair domestic appliances where motor rewind or replacement is uneconomic. It sees more use in industrial situations, where down time is expensive.
The addition of a series resistance with the reduced impedance of the motor results in reduced top speed and power. The result is normally a functional appliance with reduced performance. Final performance depends on the damage.
Motor current must be limited to a safe value in use, ie no higher than the motor's rated current. Motors using self-driven fan cooling will need to run at slightly reduced current to compensate for reduced cooling due to reduced speed. Where motors have unknown ratings, operating conditions can be determined by motor temperature. Its best practice to run the motor for some time to double check that it reaches a safe reliable working temperature.
If in doubt about how to achieve the right motor operating conditions, assistance may be sought in news:sci.electronics.basics.
Rewiring
If there is any doubt over the correct motor wiring, running the motor via a dropper prevents damaging excess current flow if the wiring is wrong. It also limits heat dissipation and prevents fuse blowing.
When testing with a dropper, a motor should only be powered up for seconds at a time, as overheating can occur in 10 or 20 seconds when there's no rotation.
Self cooling fan
Many motors have inbuilt fan blades to cool themselves.
If loose or detached it can be reattached.
If the fixing thread is damaged the fan blade may be welded, or glued with epoxy resin, with caveats. The glue may see high operating temperatures and/or high vibration levels, so high temp epoxy is required, the metal should be solvent degreased first, the glue should be symmetrical to minimise imbalance, and it has to be accepted that glue repairs don't always survive.
If the fan is badly damaged, a replacement part may be fashioned from sheet metal.
In principle an external fan can be attached to take over the function of the internal one. However in practice its difficult to generate enough airflow. The fan generally needs to run as fast as the motor, and this is often a much higher speed than most off the shelf fans.