# Protection to Micro lathe power supply?

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I have not read this in full. But it seems to me that a simple calculation to study ratings of fuses and breakers is to use the Isquare R value.
SIMPLY:
Determine the current drawn when the motor stalls = 6.25AMPS.
Continuous CURRENT rating should be nearest (just over) 120% of that = 7.5Amps.
MAX. Current rating should be not exceeding 150% of the continuous current rating = 9.375A. (Perhaps break the rules and use a 10A fuse!) - So an 8A SLOW BLOW fuse should manage this.
But please be aware that fuses are simply resistors, with a thermal "inertia" - so take time to heat to failure point.
So the energy needed to blow a fuse is time dependant. For starting motors - or resisting stalling - or preventing stall currents from cooking the motor, you have to be aware of the time element.
Here's a graph of I versus Time.
View attachment 155128
I presume the motor will take maybe 5 or 10 seconds of stall without vaporising the internal insulation? (Burn-out).
But if you had a momentary stall on the lathe and quickly hit the stop button - say within 2 seconds - you would not want the motor to burn-out or fuse to blow?
That leaves you (say) between 3 and 10 seconds of "safe" use - but something that will blow before the Motor fries and lets all the smoke escape! - Then that can give you an "IsqR" value for the fuse.
This is what manufacturers of proper equipment do for "the layman" so he can but his drill, lathe, washing machine or TV and it "works OK".
But you are asking about doing all that yourself.
You can see from the graph that at RATED current, the fuse relating to the graph will virtually NEVER blow. But at 150% of rated current it will blow in something like 2 seconds. So following this logic, you want something that is rated a bit closer than 150% of rated current, maybe 130% of rated current? - which is close to the 8A rating for a fast blow fuse - like regular cheap fuses...
A slow-blow fuse is a different curve to a fast blow fuse - this graph is for industrial fast-blow fuses. You could try a 6A slow-blow, as this will probably manage your use as the motor isn't actually drawing rated current when cutting, (stop-start). But will blow in seconds (not minutes I reckon) on a stall. Just time for a quick button hit ?? - and if it blows it is cheaper and quicker to replace than the motor.
A 10A fuse "regular" (= fast-blow) - I reckon - will put the motor at a lot of risk as it will take the starting current for your motor if starting on full load. But if the motor fries in >10 seconds at less than 10A then the fuse won't give you the protection you need.
Hope this doesn't sound too confusing or technical?
Conclusion - try a 5A or 6A SLOW-BLOW. - to be safe.
K2
I like your chart on slow blow fuses
24VDC 150W 6.25 amps.
That gives a over load of 250% 15.625 amps for 10 seconds before blowing.
https://www.amazon.com/MDL-6-25BC-S...sc=1&msclkid=e94ab938259f193c78dcdf2ba486802c

There is also thermal fuses if motor is over heat it will blow it works on temperature. If fuse gets to 200° it blows there are different temperatures too.
Using both types will save the motor. But still give great start ampage too.

Here one pack from Amazon 200°F or 93°C
Saves the motor if starts to over heat.
https://www.amazon.com/dp/B08NTXJBV3?starsLeft=1&ref_=cm_sw_r_apan_dp_CKZXS6DMD18C2C2MCQBB

Dave

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I figured there would be someone who understands these things to explain what is needed....
K2

I figured there would be someone who understands these things to explain what is needed....
K2
I agree
I think to simple but works the best

Dave

This what order a thermometer for my mini lathe motor.
Most motors on mini lathes are DC and can run at low speed for a long time. The cooling fan is not turning very fast

The advantage is I see the temperature rising and let lathe run higher speed to cool the motor down .

All the other ways only till you you to hot .

The dial on the thermometer tells me I may have problems.

Insulation Class
Insulation temperature (insulation class). The MAX temperature rises so that the winding of the motor can handle normal operation. Below is the Insulation class table. Found the motor name plate

Class A. 221°F / 105 Degrees C
Class B 266°F / 130 Degrees C
Class F 311°F / 155 Degrees C

Class H 356°F / 180 Degrees C

Dave

This what order a thermometer for my mini lathe motor.
Most motors on mini lathes are DC and can run at low speed for a long time. The cooling fan is not turning very fast

The advantage is I see the temperature rising and let lathe run higher speed to cool the motor down .
View attachment 155187

All the other ways only till you you to hot .

The dial on the thermometer tells me I may have problems.

Insulation Class
Insulation temperature (insulation class). The MAX temperature rises so that the winding of the motor can handle normal operation. Below is the Insulation class table. Found the motor name plate

Class A. 221°F / 105 Degrees C
Class B 266°F / 130 Degrees C
Class F 311°F / 155 Degrees C

Class H 356°F / 180 Degrees C

Dave
I would like to discourage you from operating that way. Because when heat builds up in a motor they tend to hold that heat longer, a lot longer then you realize and its better to nip it in the bud by cutting off the power before it gets hot in the upper range. I have experienced that only twice but in those cases the motor had more than adequate air cooling and in the other the generator had a cooling systems. If you have a thermal switch they should be set to disconnect at a temperature much lower than maximum design. Also when you work with equipment you usually are not concentrating on how hot it is unless alarms are going off. Theory is great until you end up fixing stuff.

I would like to discourage you from operating that way. Because when heat builds up in a motor they tend to hold that heat longer, a lot longer then you realize and its better to nip it in the bud by cutting off the power before it gets hot in the upper range. I have experienced that only twice but in those cases the motor had more than adequate air cooling and in the other the generator had a cooling systems. If you have a thermal switch they should be set to disconnect at a temperature much lower than maximum design. Also when you work with equipment you usually are not concentrating on how hot it is unless alarms are going off. Theory is great until you end up fixing stuff.
Some types of motor protection works on the temperature of motor.
It is a thermostat that turn off the motor when gets to hot

It is one best protection but cost more than other.

Dave

I see the problem as being at some hot-spot within the core of the windings of the motor. Usually a manufacturer's thermo-switch is buried inside the windings.... but obviously motors then cost more and you are paying for their expertise in making a safety system that saves money against cooked motors...
I am sure some sort of exterior temperature measurement indication or control can indicate when cooling air is rising in temperature. But this won't respond as quickly as a session buried in the windings. So it is quite likely that in some circumstances, the motor will overheat.
My experience:
• Hand drills: heavy drilling and overloaded in less than 1 minute.
• Angle grinder cutting through a 3mm steel bar. Using a cutting fixture so the cut was very precise, c onto label and I removed the cut every 10 seconds or so. The grinder speed was maybe 2/3 of free speed. The grinder simple stopped with burnt armature at just over half way through the bar. No smell, no smoke, but armature completely open circuit.
• Small lathe motor after fitting new bearings, I was running the lathe, on no-load when I was called by my wife. 15 minutes later, I returned and the lathe had stopped - cooked armature... I should have stopped it when I was a called away..... Armature = 3/4 of windings open circuit. No smoke no smell.
The simple facts are that correctly cooled motors are made to work reliably at high speed. But the heat-flow characteristics of every motor design are unique, and slow speed always generates hotter windings with the same current but lower cooling. What a motor can manage before being damaged is always an unknown.
I have fitted an additional fan to my lathe to blow cooling air constantly, in addition to the motor's in-built fan. And take great care with all tools to take short cuts (<5secs?) and long pauses (>20 secs?) when below 50% of rated speed. It seems to keep my motors safe.... But larger diameter lathe work, and parting-off do take a lot longer as a result. Also use of a tap or die when running the slowest speed and the lathe is working hard or stalling! Hit the stop urgently! Hand powered is slow and doesn't cook motors.
Take care, enjoy doing jobs that the machine can handle, and beg, borrow or steal a bigger machine when necessary.
K2

I see the problem as being at some hot-spot within the core of the windings of the motor. Usually a manufacturer's thermo-switch is buried inside the windings.... but obviously motors then cost more and you are paying for their expertise in making a safety system that saves money against cooked motors...
I am sure some sort of exterior temperature measurement indication or control can indicate when cooling air is rising in temperature. But this won't respond as quickly as a session buried in the windings. So it is quite likely that in some circumstances, the motor will overheat.
My experience:
• Hand drills: heavy drilling and overloaded in less than 1 minute.
• Angle grinder cutting through a 3mm steel bar. Using a cutting fixture so the cut was very precise, c onto label and I removed the cut every 10 seconds or so. The grinder speed was maybe 2/3 of free speed. The grinder simple stopped with burnt armature at just over half way through the bar. No smell, no smoke, but armature completely open circuit.
• Small lathe motor after fitting new bearings, I was running the lathe, on no-load when I was called by my wife. 15 minutes later, I returned and the lathe had stopped - cooked armature... I should have stopped it when I was a called away..... Armature = 3/4 of windings open circuit. No smoke no smell.
The simple facts are that correctly cooled motors are made to work reliably at high speed. But the heat-flow characteristics of every motor design are unique, and slow speed always generates hotter windings with the same current but lower cooling. What a motor can manage before being damaged is always an unknown.
I have fitted an additional fan to my lathe to blow cooling air constantly, in addition to the motor's in-built fan. And take great care with all tools to take short cuts (<5secs?) and long pauses (>20 secs?) when below 50% of rated speed. It seems to keep my motors safe.... But larger diameter lathe work, and parting-off do take a lot longer as a result. Also use of a tap or die when running the slowest speed and the lathe is working hard or stalling! Hit the stop urgently! Hand powered is slow and doesn't cook motors.
Take care, enjoy doing jobs that the machine can handle, and beg, borrow or steal a bigger machine when necessary.
K2
It would be better to be in hot-spot within the core.

But that is not option so do next best thing.
Place on outside and use a lower temperature for max.

At this point the bub is less than ½" of aluminum and steel from coil.

Dave

This what order a thermometer for my mini lathe motor.
Most motors on mini lathes are DC and can run at low speed for a long time. The cooling fan is not turning very fast

The advantage is I see the temperature rising and let lathe run higher speed to cool the motor down .
View attachment 155187

All the other ways only till you you to hot .

The dial on the thermometer tells me I may have problems.

Insulation Class
Insulation temperature (insulation class). The MAX temperature rises so that the winding of the motor can handle normal operation. Below is the Insulation class table. Found the motor name plate

Class A. 221°F / 105 Degrees C
Class B 266°F / 130 Degrees C
Class F 311°F / 155 Degrees C

Class H 356°F / 180 Degrees C

Dave
All windings are insulated from the laminations by the wire insulation, the insulation liner of the slot and the fact that the hottest wire are in the middle of the bundle in a slot. The only placement of the probe that will get even close to a good temperature is potted in the end turn, however; the temperatures on the table is adjust for the above losses and the losses of the laminations. Basically for someone that worked with motors and generators for aerospace. We used the actual insulation temperature. The reason is simply the huge assumptions that had to be made. When you look at the assumptions I suspect this table only is for induction and brushless motors and thus for the stator winding.
For brush DC motors the rotor has wound field. Their is a huge additional heat resistor of the air gap between the rotor and stator and the rotor shaft and then the bearing, and the brushes add heat. This is why high power DC motors pump air to flow around the rotor. A locked rotor means the temperature of the rotor will be increasing because cooling is gone.

All windings are insulated from the laminations by the wire insulation, the insulation liner of the slot and the fact that the hottest wire are in the middle of the bundle in a slot. The only placement of the probe that will get even close to a good temperature is potted in the end turn, however; the temperatures on the table is adjust for the above losses and the losses of the laminations. Basically for someone that worked with motors and generators for aerospace. We used the actual insulation temperature. The reason is simply the huge assumptions that had to be made. When you look at the assumptions I suspect this table only is for induction and brushless motors and thus for the stator winding.
For brush DC motors the rotor has wound field. Their is a huge additional heat resistor of the air gap between the rotor and stator and the rotor shaft and then the bearing, and the brushes add heat. This is why high power DC motors pump air to flow around the rotor. A locked rotor means the temperature of the rotor will be increasing because cooling is gone.
A lock rotor will high ampage will a lot higher than running amps. Now the heat on outside of motor of will be to hot to touch.

A temperature probe inside now to costly to install.

You can only next best install on outside and use lower temperature for maximum on dial .

Dave

Some types of motor protection works on the temperature of motor.
It is a thermostat that turn off the motor when gets to hot

It is one best protection but cost more than other.

Dave
Well, let me present a counter argument. Motors will get to hot when they experienced a high current draw above their rating. So which protective device sees that condition first? I think that would be a fuse or circuit breaker. But on a side note if a DC motor got hot at lower speeds I would think it might it might be too small. However a DC motor is also dependent on its control system so both have to be protected. And if I understood the issue the power supply went south but the motor is ok. You might think about a cheap little DC meter in the circuit.

Opinions only change when it costs money to repair.

That is my understanding, the power supply failed, not the motor.
I think sizing the fuse for the motor surge using a slow-blow fuse may not be the correct approach.

The power supply is current sensitive, and electronics typically don't take overcurrent well, so use a fast fuse, and cut the current to the motor before the power supply gets into an overcurrent situation.

Not a perfect solution, but a fuse I think may be lesss expensive than a power supply.

.

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Well, let me present a counter argument. Motors will get to hot when they experienced a high current draw above their rating. So which protective device sees that condition first? I think that would be a fuse or circuit breaker. But on a side note if a DC motor got hot at lower speeds I would think it might it might be too small. However a DC motor is also dependent on its control system so both have to be protected. And if I understood the issue the power supply went south but the motor is ok. You might think about a cheap little DC meter in the circuit.

Opinions only change when it costs money to repair.
The main problem is low speed turning .
My lathe did come slow blow fuse.
Did think of axillary fan but did know temperature.

So when the way cars and trucks have a temperature gauge.

This way I stop work long before it gets real hot .

It is to costly to put inside so doing the next best on motor and use a lower maximum temperature for safety.

Dave

That is my understanding, the power supply failed, not the motor.
I think sizing the fuse for the motor surge using a slow-blow fuse may not be the correct approach.

The power supply is current sensitive, and electronics typically don't take overcurrent well, so use a fast fuse, and cut the current to the motor before the power supply gets into an overcurrent situation.

Not a perfect solution, but a fuse I think may be lesss expensive than a power supply.

.
My lathe came with a slow blow fuse.
I think lot like electronics way fixing things.

Dave

It still begs the question though, should the fuse to be added to the configuration in question be fast-blowing in order to protect the power supply?

A slow-blow fuse is typically for motor and transformer inrush currents, but we are trying to protect the power supply.

Would a fast-acting fuse allow the motor to start?

Edit:
I think someone posted some fuse curves previously.
Typically for industrial power studies, the fuse curve is plotted against the motor curve, the dry-type and/or pad mounted transformer damage curves, etc., and so in our case you could plot various types of fuses against the motor curve and the power supply curve, assuming you had these things.
.

Last edited:
It still begs the question though, should the fuse to be added to the configuration in question be fast-blowing in order to protect the power supply?

A slow-blow fuse is typically for motor and transformer inrush currents, but we are trying to protect the power supply.

Would a fast-acting fuse allow the motor to start?

Edit:
I think someone posted some fuse curves previously.
Typically for industrial power studies, the fuse curve is plotted against the motor curve, the dry-type and/or pad mounted transformer damage curves, etc., and so in our case you could plot various types of fuses against the motor curve and the power supply curve, assuming you had these things.
.
The fast blow fuse is mainly for electronics.
The slow blow is used on motor for starting load.
It also on welders because start needs more amp for few seconds.
Note most just use a large breaker that does not offer protection need.

I work on oil rigs. There I have experienced a peculiar problem with DC motors.
We have many treadmill in our gym. These are used mostly on lower speed for walking. The motors have only motor driven cooling fan. So at slow speed they become quite hot.
We had many treadmill breakdown due to motor insulation melting and jamming between rotor and stator. We tried to fit an external cooling fan but do not have enough space for it.
Maybe its the cheap treadmill that they supply the real problem.

Regards
Nikhil

I work on oil rigs. There I have experienced a peculiar problem with DC motors.
We have many treadmill in our gym. These are used mostly on lower speed for walking. The motors have only motor driven cooling fan. So at slow speed they become quite hot.
We had many treadmill breakdown due to motor insulation melting and jamming between rotor and stator. We tried to fit an external cooling fan but do not have enough space for it.
Maybe its the cheap treadmill that they supply the real problem.

Regards
Nikhil
I did not think of other uses for low speed DC motors .
I first put a DC motor on a bench lathe in 1982. On that lathe I could touch the motor and check the temp.

I did not think of Treadmills had this problem too. Bynow I would think they had overheating work out on treadmills.

Good luck
Dave

A lock rotor will high ampage will a lot higher than running amps. Now the heat on outside of motor of will be to hot to touch.

A temperature probe inside now to costly to install.

You can only next best install on outside and use lower temperature for maximum on dial .

Dave
Dave, the model I posted for a bush DC motor which applies to all DC motors shows the relationship between speed and current. And the highest current occurs at zero rpm. So a fuse designed for motors will disconnect the motor when locked. Consumer product such as a garbage disposal that expect to have a locked rotor have a bimetal thermal circuit switch that is located in the end turns to disconnect the motor, and I believe the water heaters for coffee pots also choose this part. Retro-fitting a motor I agree maybe difficult.
However; there is a reason that consumer companies choose this solution. It would be much easier to mount the device on the motor housing in the connection box or region on the housing. The reason is simple, there is a lag in the temperature rise of the housing and a reduction in the peak temperature if failure of the windings didn't occur, stayed generating heat at the locked current level. It is slow and inaccurate. Temperature measurement is useful to determine with experience what temperature the housing needs to reach before turning on a fuse or breaker tripped motor and the minimum time due to lag.

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