Replacing with Brush or Brushless DC motor

When I was look adds on brush vs brushless I found a lot made up facts. It hard size motors just sell there motor and not giving true facts.

The brushless is a three phase motor using VFD.

Here is a torque graph for DC brush motor



This torque chart of single and three phase motors. This about same for brushless too.
Note the full load torque most time it is 5% to 10% slip


The advantage of brushless is longevity no brush to change every 1,000 to 3,000 hours.
The advantage of brush type great low in torque

Then pricing too between the two.

Please comment

The problem I see with DC motors is the electronics required, which is a point of failure in my opinion.

I have seen some magnetic drive cabinets that are built very robust, and they have DC power supplies in them, and seem to last forever.
These are for magnetically coupling large motors (1,500 hp and up) to sewage pumps, where the speed/flow rate needs to be modulated.

There are definitely some advantages to using DC motors, and applications where they really are a must.

I suppose an induction motor operated from a VFD is sort of a brushless DC motor, but I think the DC motor characteristic give it a lot of low speed torque without overheating ? Check me on that, I am not a DC motor person.
I have always thought of DC motors as best used for applications needing a wide speed control.
I think an AC motor torque will drop off at low speeds more than a DC motor ?

.

GreenTwin said:

The problem I see with DC motors is the electronics required, which is a point of failure in my opinion.

I have seen some magnetic drive cabinets that are built very robust, and they have DC power supplies in them, and seem to last forever.
These are for magnetically coupling large motors (1,500 hp and up) to sewage pumps, where the speed/flow rate needs to be modulated.

There are definitely some advantages to using DC motors, and applications where they really are a must.

I suppose an induction motor operated from a VFD is sort of a brushless DC motor, but I think the DC motor characteristic give it a lot of low speed torque without overheating ? Check me on that, I am not a DC motor person.
I have always thought of DC motors as best used for applications needing a wide speed control.
I think an AC motor torque will drop off at low speeds more than a DC motor ?

.

Click to expand...

I am same page about DC motors.
I remember pipe threader you thread a large pipe .
Most can relate to hand drill and slows down it give a lot torque at bottom speed.

Dave

GreenTwin said:

The problem I see with DC motors is the electronics required, which is a point of failure in my opinion.

I have seen some magnetic drive cabinets that are built very robust, and they have DC power supplies in them, and seem to last forever.
These are for magnetically coupling large motors (1,500 hp and up) to sewage pumps, where the speed/flow rate needs to be modulated.

There are definitely some advantages to using DC motors, and applications where they really are a must.

I suppose an induction motor operated from a VFD is sort of a brushless DC motor, but I think the DC motor characteristic give it a lot of low speed torque without overheating ? Check me on that, I am not a DC motor person.
I have always thought of DC motors as best used for applications needing a wide speed control.
I think an AC motor torque will drop off at low speeds more than a DC motor ?

.

Click to expand...

Here is schematic of a brushless motor


Dave

GreenTwin said:

The problem I see with DC motors is the electronics required, which is a point of failure in my opinion.

I have seen some magnetic drive cabinets that are built very robust, and they have DC power supplies in them, and seem to last forever.
These are for magnetically coupling large motors (1,500 hp and up) to sewage pumps, where the speed/flow rate needs to be modulated.

There are definitely some advantages to using DC motors, and applications where they really are a must.

I suppose an induction motor operated from a VFD is sort of a brushless DC motor, but I think the DC motor characteristic give it a lot of low speed torque without overheating ? Check me on that, I am not a DC motor person.
I have always thought of DC motors as best used for applications needing a wide speed control.
I think an AC motor torque will drop off at low speeds more than a DC motor ?

.

Click to expand...

One thing I noticed was some the manufacturer lathe using a brushless motor has 4 steps from 300 to 2,500 RPM.

The DC brush used only 2 steps 1,000 next step 2,200 or 2,500 RPM.

When saw that different it made me wonder on the decision on type.

I like a wide range control.

Dave

The cost of brushless is the only downside I can see. Brushless are slightly more efficient, No need to maintain brushes and comm's, and slightly better control. The controllers for AC and Brushless more complex and more expensive.
If I was changing motors and drives on a lathe or mill I would go DC brushless - DC brushed, in order of personal choice. Otherwise it would depend on the use and frequency of use.
I personally used AC variable speed to upgrade my lathe based on overall cost. There is still some degree of gearing required this way.

On my lathe - as manufactured 2 speed with DC brush motor - the speed change was mechanical!
- In my limited and simple understanding: With a 2-brush motor, you only get a single speed range with the pairs of magnets and commutator just able to speed up and slow down with a variable voltage (Which in turn varies the current) and the power of the motor varies with the Square of the voltage change. I.E. double the voltage, quadruple the power (and torque adjusts accordingly as speed changes).
I've not configured the maths in my head... , because it is more complex than power = V-squared / resistance, as the resistance varies with increased current, there is back-EMF from the inductance, etc.
But I think the Torque increases linearly with the current, as B = nI :
I.E. the magnetic field (B) in the armature increases linearly with Current (I) in the wires (number of wires = n)... so as Torque is simple directly proportional to B (flux strength), so it is directly proportional to current (I).
And as the power is Torque x Revs, the revs must increase linearly with Power (V x I) rising as a Square of voltage, and divided by the torque being linear with current, speed is linear with voltage...
Thus making DC motors easy to control both speed and torque with a variable voltage supply. (Thyristors).
I do not thing the electronics are "fragile" or short-lived, but simply cannot tolerate the poor design INSIDE the brush-holder zone that is intolerant of carbon dust - which causes electrical faults that can fry the electronics. Or an overheated motor that shorts the electronics to Earth...
However, in the "simpletons' debate" of how the DC motors are applied to lathes, etc, at 100rpm (low voltage limit) the applied voltage - and torque - will be 1/10th of the torque at 1000rpm.... By my reckoning? - And my experience is that I can stop the lathe with hand torque (holding the chuck), or on a tap wrench at less than 5 inches radius. (as it is only 10inches long!). - e.g. threading a 3/8th inch brass fitting with fine thread... Easy by hand in the vice, but the "1 HP" lathe could not cope turned down to minimum speed.. And as there is virtually no forced cooling below maybe ~1/3rd rated speed these motors can be cooked - as mine has eventually succumbed to my abuse...
On running ANY motor below about 1/3rd rated speed (as low as 11% of rated COOLING!) - perhaps still with 1/3rd rated power? - the power-in and cooling heat out imbalance becomes 3 times the "rated" ratio, so maybe that is a simple reason why motors cook at low speed? e.g. on a 1HP motor at around 1/3rd speed is is getting 1/3rd rated heat and 1/9th rated cooling! I.E. = 1/3rd of the cooling it needs.... So it overheats.
My thoughts on variable frequency (3-phase, etc.) AC motors, is that the power supply has to deliver whatever frequency suits the motor at whatever speed it is set to run at.. and will supply full voltage and full current at that frequency. So the motor gets FULL POWER and torque even at low speed settings... But as above the cooling drops off as the square of speed (of the motor fan), so an EXTERNAL fan should be installed to keep the motor cool at any and every speed below full speed, to avoid cooking the windings' insulation... (letting the smoke out!).
ANY experts able to correct my assumptions here? I am simply trying to figure it out from the simple rules of O-level physics and maths, not the complexity of what really happens!
Thanks,
K2

Smith-Door: Thanks for the graphs, as they do explain a lot about the differences in the motor designs. - very useful - but I am not sure how? We rarely slow the motors down much with cutting loads on lathes, etc. We set an appropriate speed and feed for the job and materials, then stay running at that fixed speed... providing the machine does not stall. These graphs take a motor with presumably constant voltage input and describe how the motor manages against a variable load from starting to full speed.
They do not describe the torque when controlled to "max torque " at a particular speed, as we are doing with our various machines. - e.g. at "half speed" my DC lathe motor was being controlled to 1/2 torque and 1/4 power. I think a Variable frequency drive on an AC motor will give full torque and 1/2 power at half speed? - which is why users say they are the best thing to have... Also the torque increases a lot when the speed just slips by 5%. A DC motor "slowed by the load" to 5% below max does not have quite the torque increase...
But my interpretation of this may be wrong, so please correct me where wrong?
Thanks,
K2

The true and original DC brushless motor is a motor with fixed magnets embedded in its rotor and a series of coils placed around the stator.
The series of coils are in 3 groups spaced 120Deg apart. Each group is pulsed with DC current in succession with an electronic switching controller giving a rotating motion. The speed of the motor can be controlled by varying the speed of the pulses to the coils.

So does it manage full current at all speeds? Speed control by switching frequency?
I. E. Full torque at all speeds?
K2

Steamchick said:

So does it manage full current at all speeds? Speed control by switching frequency?
I. E. Full torque at all speeds?
K2

Click to expand...

With brushless/ ac most speed curve (rpm) has same torque.
You see that on the graph.

The brush motor is very close or could be the graph for motor on the mini lathes. As motor slows down the torque increase a lot from full load speed. If motor says at same voltage and motor slow down the torque goes way and ampage goes up.
So if control keeps the speed same at same time the torque goes up.

At high speed it not a big difference between brush and brushless. But when need low low speed the torque goes way up for brush.
The brushless at lock rotor is about 1.6 times full load.
Brush at lock rotor is about 4.4 time full load .

Dave

Steamchick said:

So does it manage full current at all speeds? Speed control by switching frequency?
I. E. Full torque at all speeds?
K2

Click to expand...

There lot of place where DC / universal motor are use because of it torque curve.
The surprise to me was high end rotor rooter uses a DC / universal motor. But the brush life before replacing is over 7,000 hours.

FYI Most graph on brushless is only to sell not for engineering

Dave

Steamchick said:

Smith-Door: Thanks for the graphs, as they do explain a lot about the differences in the motor designs. - very useful - but I am not sure how? We rarely slow the motors down much with cutting loads on lathes, etc. We set an appropriate speed and feed for the job and materials, then stay running at that fixed speed... providing the machine does not stall. These graphs take a motor with presumably constant voltage input and describe how the motor manages against a variable load from starting to full speed.
They do not describe the torque when controlled to "max torque " at a particular speed, as we are doing with our various machines. - e.g. at "half speed" my DC lathe motor was being controlled to 1/2 torque and 1/4 power. I think a Variable frequency drive on an AC motor will give full torque and 1/2 power at half speed? - which is why users say they are the best thing to have... Also the torque increases a lot when the speed just slips by 5%. A DC motor "slowed by the load" to 5% below max does not have quite the torque increase...
But my interpretation of this may be wrong, so please correct me where wrong?
Thanks,
K2

Click to expand...

I have a variable torque set up. High end treadmill motor with a built in encoder. The result is that if I start a ⅛ deep cut in mild steel, my lathe's max, the controller will up the juice to maintain the speed. It's very useful.

The Ignoble Troll said:

I have a variable torque set up. High end treadmill motor with a built in encoder. The result is that if I start a ⅛ deep cut in mild steel, my lathe's max, the controller will up the juice to maintain the speed. It's very useful.

Click to expand...

The DC motor have about 6 adjustments .
Most mini lathes I seen only has 3 you can adjust
1. Low speed
2. High speed
3 . Max Torque.
The rest are per set on board like
4. Torque acceleration
5. Breaking
6. Slope speed increase

There some boards 6 adjustments

1. Low speed
2. High speed
3 . Max Torque.
4. Torque acceleration
5. Breaking
6. Slope speed increase

There are are few more but almost never seen or better way not need a engineering going wild.

The brushless has about same thing but a little different
Like speed increases is slowly increasing the frequency.
I know more but have had must do with brushless.

Dave

The Ignoble Troll said:

I have a variable torque set up. High end treadmill motor with a built in encoder. The result is that if I start a ⅛ deep cut in mild steel, my lathe's max, the controller will up the juice to maintain the speed. It's very useful.

Click to expand...

Here photo of my board if look phoc see MAX the other one see in photo it is low.
The torque limit does not on photp

Dave

Steamchick said:

So does it manage full current at all speeds? Speed control by switching frequency?
I. E. Full torque at all speeds?
K2

Click to expand...

I do not know.
I remember reading a long time age on VFD had problems as the frequency is reduced the torque would drop. I have no idea this true or maybe fix problem.
The sales people and advertising has errors giving great troubles getting good information.

I have be careful about or I will get lot no so or worst because of adds

Dave

Hi SmithDoor,
I had the same type of board, - but an older version without low voltage DC supplies for the speed encoder, etc.
ADVICE FROM MY FRIENDLY TECHNICAL EXPERT who repairs these boards (mail business).
He told me that when he receives the boards, he checks the settings, and MOST - with very few exceptions, have been adjusted (by their owners) to supply MORE (higher) voltage, faster speed rise, etc. than the standard factory settings, So he reckons that is often the cause of the board failing.
But he will only do repairs if the owner supplies the motor as well,, and he has a very high correlation between boards that are blown and Motors that are arcing, shorting or whatever, as a primary cause of failure. Boards that are set to max at too high a voltage almost always have motors that have arced to earth - and blown thyristors.
And he always changes a resistor (high wattage low Ohms - I know no more!) for a double wattage version, or is it double ohms? - because he says the "factory" design uses a resistor "on the limit" - which can fail if the control pots are not set correctly. - And thus the board doesn't work.
He always sets the board - after replacing thyristors, etc. - using a 100W bulb as the load - to 180V DC MAX. - Any higher and the motor will arc or something and blow the thyristors - That is the check I was doing (but with a 60W bulb as the largest filament bulb I had) when it exploded the fuse holder! He told me it would do that as I raised the voltage to the point where the blown thyristor (form the shorted motor to earth) would be supplying AC voltage to the other thyristors, which would be switching to try and control the DC voltage - then all fail instantaneously due to the single blown one supplying AC volts where it shouldn't be... or something. It was frightening BANG! when it went and tripped the 30A Mains breaker. He reckoned the 10A fuse had no chance of handling the internal arc current the back EMF from the thyristors etc. would have generated as the fuse blew. I don't understand it, so don't ask "proper" (hard) questions, please.
My new servo motor and controller has been delivered by the postman,

so now I need to drill the mounting plate to suit the motor mounting PCD.

Then fit the bits into the lathe... including the bigger cooling fan... (12V 0.24A replacing 12V 0.1A). And I need to fit my replacement fuse holder, Emergency stop switch, and internal wiring for mains supplies to all the devices. ..
This may take a little bit of time....
K2

Steamchick said:

Hi SmithDoor,
I had the same type of board, - but an older version without low voltage DC supplies for the speed encoder, etc.
ADVICE FROM MY FRIENDLY TECHNICAL EXPERT who repairs these boards (mail business).
He told me that when he receives the boards, he checks the settings, and MOST - with very few exceptions, have been adjusted (by their owners) to supply MORE (higher) voltage, faster speed rise, etc. than the standard factory settings, So he reckons that is often the cause of the board failing.
But he will only do repairs if the owner supplies the motor as well,, and he has a very high correlation between boards that are blown and Motors that are arcing, shorting or whatever, as a primary cause of failure. Boards that are set to max at too high a voltage almost always have motors that have arced to earth - and blown thyristors.
And he always changes a resistor (high wattage low Ohms - I know no more!) for a double wattage version, or is it double ohms? - because he says the "factory" design uses a resistor "on the limit" - which can fail if the control pots are not set correctly. - And thus the board doesn't work.
He always sets the board - after replacing thyristors, etc. - using a 100W bulb as the load - to 180V DC MAX. - Any higher and the motor will arc or something and blow the thyristors - That is the check I was doing (but with a 60W bulb as the largest filament bulb I had) when it exploded the fuse holder! He told me it would do that as I raised the voltage to the point where the blown thyristor (form the shorted motor to earth) would be supplying AC voltage to the other thyristors, which would be switching to try and control the DC voltage - then all fail instantaneously due to the single blown one supplying AC volts where it shouldn't be... or something. It was frightening BANG! when it went and tripped the 30A Mains breaker. He reckoned the 10A fuse had no chance of handling the internal arc current the back EMF from the thyristors etc. would have generated as the fuse blew. I don't understand it, so don't ask "proper" (hard) questions, please.
My new servo motor and controller has been delivered by the postman,
View attachment 158362
so now I need to drill the mounting plate to suit the motor mounting PCD.
View attachment 158361
Then fit the bits into the lathe... including the bigger cooling fan... (12V 0.24A replacing 12V 0.1A). And I need to fit my replacement fuse holder, Emergency stop switch, and internal wiring for mains supplies to all the devices. ..
This may take a little bit of time....
K2

Click to expand...

It looks great.

Trying to repair board on business side you not make any money as cheap you boards for today.

Most control boards fail because either dust or pood design. Next is owner playing with board.
Voltage spikes can be problem in some location but power strip with protection should work.

I found yearly cleaning of boards exstens the life includes my AC on my home

Dave

Hi Dave,
A couple of comments re: above...
"But when need low low speed the torque goes way up for brush." = Not so on machines that limit the current. - Or reduce Voltage to reduce speed. On my lathe, I could easily hold the chuck STOPPED when set on the slowest speed (~80rpm), but at double that (150rpm and upwards) I could not. But that was too fast for cutting threads with taps and dies. Simply because at min speed, the controller gave about 12V = and max 5 A - so no more than 60Watts. NOT 750 Watts. = Hence the controller prevents the motor from giving a high torque at low speed. But I agree, at full voltage, if you load the motor to slow it down the back EMF drops and the full "starting current" is seen by the windings and gives a very high torque as you describe. But these *cheap" (£120+) lathe speed controllers don't exceed a certain current (mine was 5A. or 180V.DC.), so that doesn't happen.
"Voltage spikes can be problem in some location but power strip with protection should work." In the UK we have pretty stable mains voltage for domestic use, so the filters for spikes go between the controller and mains to stop the lathe motor speed controller generated spikes from going back up the domestic mains to a TV, Computer, or other household, equipment, Electric meter, etc. that may not like it. Motors are large inductances, so rapid switching of motors can generate large back-EMFs, and the fast switching of power Thyristors controlling motors can similarly cause large spikes.
Cheer,
K2

Steamchick said:

Hi Dave,
A couple of comments re: above...
"But when need low low speed the torque goes way up for brush." = Not so on machines that limit the current. - Or reduce Voltage to reduce speed. On my lathe, I could easily hold the chuck STOPPED when set on the slowest speed (~80rpm), but at double that (150rpm and upwards) I could not. But that was too fast for cutting threads with taps and dies. Simply because at min speed, the controller gave about 12V = and max 5 A - so no more than 60Watts. NOT 750 Watts. = Hence the controller prevents the motor from giving a high torque at low speed. But I agree, at full voltage, if you load the motor to slow it down the back EMF drops and the full "starting current" is seen by the windings and gives a very high torque as you describe. But these *cheap" (£120+) lathe speed controllers don't exceed a certain current (mine was 5A. or 180V.DC.), so that doesn't happen.
"Voltage spikes can be problem in some location but power strip with protection should work." In the UK we have pretty stable mains voltage for domestic use, so the filters for spikes go between the controller and mains to stop the lathe motor speed controller generated spikes from going back up the domestic mains to a TV, Computer, or other household, equipment, Electric meter, etc. that may not like it. Motors are large inductances, so rapid switching of motors can generate large back-EMFs, and the fast switching of power Thyristors controlling motors can similarly cause large spikes.
Cheer,
K2

Click to expand...

You do lot with adjustment on board.
The torque / amps can adjust.
From as add more torque the motor slows
To as add more torque the speed increase hard stop spindle and will give a lot of torque.

On our lathes should adjustment so speed stays the same at any torque.

Dave