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AC Vs DC

23225 Views 107 Replies 10 Participants Last post by  johnsiddle
Is there a post on this wiki or in the main forum comparing the pros and cons of AC motors vs DC motors?
Thanks everyone!
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Honestly no experience with Fork Lift motors. Could not tell you if they are sealed or not.
Traditional "forklift" brushed series DC motors all appear to be open: you can see the commutator and brushes. Some builders use a commercially available or home-built shroud to close that opening, and provide a connection for a blower, with a filter on the blower intake. An appropriately placed filter will prevent problems with water.

Search this forum for "blower" or check suppliers such as EV West for a motor cooling kit, and you can see examples.
 

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FWIW If you were designing an EV from the ground up, you would never use a series DC motor. Either BLDC or 3-phase Induction motors. Otherwise the vehicle gets to heavy and expensive. Both BLDC and Induction motors have the best of both worlds of torque and speed. They produce very flat torque curves up to several thousand RPM, and operate at much higher RPM' up to 10,000 to 13,000 RPM's requiring no expensive transmission other than a fixed differential direct drive.Yep the motors and controllers are more expensive than a DC motor and Controller, but a heck of a lot less money and weight that a DC motor, Controller, and a transmission to make the DC motors work. Not to mention BLDC and Induction motors are more efficient.
If by "BLDC" you mean 3-phase AC synchronous motors with permanent magnet rotors... then yes, this is the nearly universal choice.

Typically torque is flat (at some level limited by current, which in turn is limited by who-knows-what) up to a few thousand RPM (I don't think I've seen higher than 4,000 RPM for a volume production vehicle), then power is flat from that point to near that high speed limit. Whether this is there result of AC operation, or another aspect of motor design, or the relatively high (compared to what an ex-forklift motor can handle) maximum voltage... I don't know.

There is one oddball exception that I've seen: Renault puts a 3-phase AC synchronous motor in the Zoe and Kangoo Z.E., but it has a powered rotor winding (instead of permanent magnets), using brushes and slip rings. Still no commutator.
 

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There is one oddball exception that I've seen: Renault puts a 3-phase AC synchronous motor in the Zoe and Kangoo Z.E., but it has a powered rotor winding (instead of permanent magnets), using brushes and slip rings. Still no commutator.
Does it also have a beefy resistor bank?

LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of.


Gives higher starting torque at startup apparently because of the lower phase angle, the tradeoff being the obvious complexity and cost.

But makes me wonder, is low starting torque an issue for EVs... anywhere? Seems like solving a problem that didn't exist. Or that's not the same thing that you meant.
 

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Does it also have a beefy resistor bank?

LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of.


Gives higher starting torque at startup apparently because of the lower phase angle, the tradeoff being the obvious complexity and cost.

But makes me wonder, is low starting torque an issue for EVs... anywhere? Seems like solving a problem that didn't exist. Or that's not the same thing that you meant.
Is this what you reference?

https://m.youtube.com/watch?v=JPn5Ou-N0b0

It is a control method used for mains applications. As such it is completely redundant and undesirable with an inverter driven induction motor in an EV.

And of course low starting torque would be an issue in EVs.

And beeswax???? Maybe 100 years ago. Why would you even mention it?

Regards,

major
 

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Does it also have a beefy resistor bank?
No, I'm sure that there are no resistors carrying rotor current. It's from 2018, not 1918. :)

LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of...
YouTube links like that no longer work in this forum. Use a URL link instead.

That video describes a real issues for induction motor powered straight across the line without a VFD ("mains applications", as major said), and an interesting solution for it; however, it is not an issue or a solution for a modern EV.

The Renault unit is not an induction motor of any kind, so this doesn't apply. It has a four-pole single-phase powered (so two slip rings) rotor winding, not shorted bars (a "squirrel cage") or a three-phase winding with current induced by the stator field (as shown in the video).

Link to previous Zoe motor thread (with an image of the rotor):
Renault Zoe motor synchronous motor

Gives higher starting torque at startup apparently because of the lower phase angle, the tradeoff being the obvious complexity and cost.

But makes me wonder, is low starting torque an issue for EVs... anywhere? Seems like solving a problem that didn't exist. Or that's not the same thing that you meant.
I'm not aware of any problem with low starting torque for EVs, even with induction motors, because as major noted in the previous post, EV motors are not powered by a constant-frequency supply like an industrial motor without a VFD.

It is important that an EV has high starting torque, but it's not a problem. For AC motors torque is roughly constant from zero speed through the lower part of their speed range. Strangely, in the Renault specs the peak torque does not go down to zero RPM (just 1500 to 3395 RPM, then peak power from 3395 to 10980 RPM), but I'm sure that torque is still near peak all the way to close to zero speed.


Stall (zero speed) is an interesting point to compare the various motor types (assuming that each is driven by a proper controller)... but it's really a synchronous versus asynchronous (induction) comparison, rather than DC versus AC:
  • In both brushed DC motors (whether the field is provided by windingf or permanent magnets), and in AC synchronous motors the stator and rotor magnetic fields are in synch, so both are stationary at stall (and the stator side of the DC motor is stationary at all times). All currents are continuous (the three phases of the AC motor are at different values from each other, but each stuck on one value). All of the power going into the motor is just being dissipated as heat due to wire resistance.
  • In an induction motor torque is only produced when the stator field is rotating faster than the rotor field; the stator's magnetic field must sweep through the rotor windings to induce current and a rotor field to react with. The 3 phases of the inverter output are all at a low frequency corresponding to the slip speed. All of the power going into the motor is still being dissipated as heat, but due to a combination of wire resistance and magnetic losses.
In a common AC synchronous EV motor the rotor's magnetic field is provided by permanent magnets; in the Renault wound-rotor motors the rotor's magnetic field is provided by DC current through windings, so it is still in a fixed orientation in the rotor (although presumably variable in strength by varying current to suit operating conditions). The rotor of the Renault motor is like the stator of a common DC motor, powered independently of the rotor as in a separately excited ("SepEx") DC motor.

Of course stall is not a normal operating condition for more than an instant. The vehicle immediately starts moving and the rotor speed is no longer zero. If someone is holding an EV on a hill with power - so stall is sustained - they should move their foot to the middle pedal!
 

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Discussion Starter · #66 ·
Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).
What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
 

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And beeswax???? Maybe 100 years ago. Why would you even mention it?
I'm sorry for discussing things on a discussion forum that I found interesting. I will endeavor to only reply like a robot from now on.

I've taken apart electronics from the 1980s that were dipped in beeswax.

Brian said:
YouTube links like that no longer work in this forum. Use a URL link instead.
Odd. They work just fine for me, including in quoted sections it still shows the embedded video player and the video player is functional.
 

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I'm sorry for discussing things on a discussion forum that I found interesting. I will endeavor to only reply like a robot from now on.

I've taken apart electronics from the 1980s that were dipped in beeswax.
...
You were replying to and answering a specific question and stated the motor core was dipped and vacuum impregnated with beeswax if it was really old. I doubt that was ever done. Oh, you once had some electronics that were dipped in beeswax. So you assume beeswax was also used on motor cores? Considering temperature and centrifugal forces? You don't know what you're talking about so please stop pretending you do.

if you want to share or discuss something which you have found to be interesting, use an appropriate thread or start one. Don't just throw irrelevant or off-topic items into replies to a member's question.

major
 

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YouTube

Odd. They work just fine for me, including in quoted sections it still shows the embedded video player and the video player is functional.
Okay, it probably depends on the browser. It's just a big blank space to me. major's comment suggested that he didn't see it, either.

After a bit of experimentation, I see that the embedded YouTube player uses Flash, which is no longer support by my browser; that's common, so it won't work for lots of people.

A simple URL link is much more compact (good for all of those quoted repeats), and works for everyone.
 

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Discussion Starter · #70 ·
I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
 

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He
I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
Hi Dr.G,

Yep, motor will overheat. Controller will limit current and help, but if you're lucky it will self protect and cut back to a current or power level it can tolerate. But it is likely to fail before the motor. That is why you need to use a proper gear ratio.



The black lines are the motor torque-speed curves for different drive voltages; the red line is an example of a LOAD torque-speed curve. Source.
The red line I added to the graph is for something like a fan, where the torque increases as some power of the angular speed. The running speed of the combined power supply-motor-fan system is represented by the blue dots marked 1 and 2. Point 1 is a higher-volt drive which causes higher motor current, hence higher torque and the fan runs faster. Makes sense. The engineering edge here is that if we have the actual, measured curves we can really predict the speeds.
From: https://nathotron.wordpress.com/courses/engineering-design-process-530-381/motor-selection/

The use of motor speed torque curves and load profile helps understand how to choose the right ratio. The above is just an example. The series motor will have a non-linear (curved) characteristic instead of the straight lines of the PMDC motor. The vehicle load curve will be similar shape as the fan curve.

Regards,

major
 

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I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
Major has answered this - but I would like to answer it a little differently

Your controller will control the Current - so if you have a 600 amp controller then full throttle = 600 amps and half throttle = 300 amps

In the forklift the controller will set to a certain maximum current - with my motor it was 200 amps

Rpm ------Motor Current ----Voltage ----Battery current ----- Battery voltage
0 -----------600 amps-------------10v------------42 amps-------------144v
This is well above the sustainable current - and will melt in a minute or so
1700--------600 amps------------144 v-----------600 amps-------------144v

NOT 5100 rpm - because You are using three times the current and will have three times the Back EMF

5100 -------200 amps -----------144 v -----------200 amps ------------144v

In order for the rpm to rise the current must go down - the controller will be on 100% trying it's best to meet your desired current

In practice the dropping torque will intercept the rising wind resistance before them


I did this with my car - changed the controller from a 500 amp and 150v model to a 1000 amp with the capability of 400v
And at the same time reconfigured my batter to get more kwh - but at a reduced voltage - 130v

It took off like a scalded rat - then the acceleration dropped and at about 3500 rpm and 100 kph the current had dropped to 200 amps - which balanced the increased wind drag - Maximum speed!
 

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Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
I think you might be confusing max ratings with an expected result. This is a fictional motor I presume, so it probably wouldn't be rated this way.

For almost any other motor type, you can easily predict the unloaded RPM of a motor in wide circumstances. For example, synchronous AC motors will rotate based on however fast the coils get pulsed, asynchronous will always be behind a bit but fairly close. Permanent Magnet DC motors will turn a certain speed based on the voltage you give them, and you can pretty much linearly plot that (double the voltage, double the RPM). You might sometimes hear the term "KV" applied to a motor which means how many RPMs it will spin to, per volt you feed it (so a KV of 100 means if you give it 1 volt it will spin 100 RPM, 12 volts it will spin 1200 RPM, etc. They're telling you the ratio of RPM:Volts). SepEx and Shunt (parallel) wound DC motors will have similar, predictable speeds.

But DC Series motors you can't tell. With any voltage and no load, they accelerate forever, very quickly. The only thing that slows them down is having a load on them, and their speed/current curve isn't linear. So for example, they're not a great choice for any drive system that has a chain, because when the chain inevitably snaps or hops off a sprocket, the motor will just about instantly accelerate until it rips itself apart.

So in your example, if the motor is "rated" for 1700 RPM, that probably means that is its mechanical ceiling before its physical structural integrity can't be relied on anymore (as per whatever conservative degree of its rating and circumstances). Or, perhaps that in that specific application with that load, at 48V it would be expected to spin 1700 RPM. But it would have to be a very specifically known load.

For example, if you give an unloaded DC Series motor 5v, it will reach 12,000 RPM and then maybe explode. If you give it 50v, it will reach 12,000 RPM in much less time, but then maybe explode. It doesn't really matter what voltage you give it, without some drag, it spins up and then explodes. :p

By tripling the voltage, you don't physically change anything about the motor. It becomes no more strong at holding itself together. Much like if a chain is rated for 1700 lbs, you can't make it be rated for 5100 lbs. It just is what it is.

Bigger motors will tend to be rated for a lower max RPM than smaller motors because the outside edge of the rotor is moving faster on a larger diameter.

TL;DR -
1 - Your (fictional?) motor is probably not rated for a max of only 1700 RPM. That's pretty slow. But if it was, max means mechanically the max before it blows up, nothing to do with voltage, and,
2 - Your (fictional) motor is probably not rated for 1700 RPM @ 48volts, because you say it's a DC Series motor and that's nonsensical since there isn't a "no load" speed for that type of motor.

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
In short yes. Demand too much of anything and it will fail. Almost all electronics have a thermal limit by which they fail (in addition to the RPM limit mentioned earlier which you probably won't reach).

That's probably not the easiest way to think about it, for any motor type.

For example, think about going down a steep downhill. You might barely have to push at all to make the motor spin at 5100 RPM. So it's not how fast you spin the motor, it's how hard it is to spin the motor that fast.

I would kind of abstract the whole "target speed" thing and not think about it that way.

Think about a normal gas car driving down a highway. You have the accelerator pushed down some amount. Let's say it's flat and not windy so nothing is changing. Because wind resistance increases as a cube of speed (it's increasingly hard to go faster), you will eventually come to a steady speed based on how far you're pushing the pedal.

You don't have to know the math or quantities behind how much gas is pouring into the engine every second, exploding and propelling the car. You know that if you back off the gas, the car will slow down and reach a slower steady speed. If you step on the gas, the car will speed up and reach a faster steady speed. You don't have a pedal position that dictates "speed" and half way down is half speed, all the way down is full speed, etc. It depends on wind, whether you're climbing, whether you're towing, whether you're accelerating, etc. More is more and less is less.

Same thing for electric motors. Tell the throttle to give you "more", it will demand more. Tell the throttle for "less", it will give less.

If you redline a gas engine, suppose you have a small engine and you're trying to tow a heavy trailer up a hill, or, suppose you're trying to go 200mph on a crappy car... you will be demanding More and More and More. At some point you will only be able to put so much gas into the engine and it can't give you move (pedal to the metal), but more troublesome is that with you redlining the engine it's going to be getting hotter than you can keep cool, temperatures will rise, and something will blow up.

Ditto for electric motors. If you have too small of a motor and you're trying to do too much to it... accelerate too fast, reach too high a speed, drag too much weight up a hill... it's going to overheat and something will blow up. Unlike with gas engines though, motors have a bigger swing in what you can demand from them. You can only shove so much fuel and oxygen into a piston, and chemistry can only happen so quick. But electric motors you could abuse to a wider rate, several whole multiples of their rated steady power for very short periods, hitting only magnetic limits in the core I suppose.

I would think more in terms of power.

How much power is required from your motor, and can it sustain that?

You can know about the power it takes to travel a certain speed, or accelerate a weight at a certain rate, or travel up a hill a certain speed from that calculator I linked earlier. Those are physical facts. It will take X amount of power to get a certain car of a certain weight and shape, to travel a certain speed up a certain incline in certain weather. That amount of power has to come from the motor.

When you press the accelerator, you're in the end asking for more power. The actual sequence will be something like:

1 - Pedal position tells a circuit what duty cycle to pulse the motor on/off at since changing the actual voltage is impractical.
2 - The power transistors do as instructed, and pulse the voltage to the motor with those proportions of on:eek:ff time.
3 - This averages a net voltage on the motor (i.e. 50% on/off time for 100 volts, appears like 50 volts and the motor would act the same if you gave it 50v steady).
4 - Voltage causes current (amps) to flow.
5 - Current makes the motor spin.
6 - The harder it is to spin, (not speed directly, but all the things that make a car difficult to move like wind, hills, and accelerating) the more current flows for a given voltage.
7 - The more current the more heat.
8 - Too much heat and something melts, either power transistors or motor brushes or motor wires or, something.

So you're not really telling it "Go this fast", you're telling it "Push this much harder". And pushing hard is what makes heat.

Think of perhaps, a toy electric car that you put your foot on. What happens? The motor can't pull you, it tries as hard as it can, and it overheats and melts. Ditto for an undersized motor in a car. It tries, it overheats, and melts.

By whatever method you're asking for "more", if it's more than the motor can provide, it dies.
 

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Discussion Starter · #74 ·
I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
Major has answered this - but I would like to answer it a little differently

Your controller will control the Current - so if you have a 600 amp controller then full throttle = 600 amps and half throttle = 300 amps

In the forklift the controller will set to a certain maximum current - with my motor it was 200 amps

Rpm ------Motor Current ----Voltage ----Battery current ----- Battery voltage
0 -----------600 amps-------------10v------------42 amps-------------144v
This is well above the sustainable current - and will melt in a minute or so
1700--------600 amps------------144 v-----------600 amps-------------144v

NOT 5100 rpm - because You are using three times the current and will have three times the Back EMF

Strewth!.. I didn't realise that back emf rises that way. I thought it falls as a fraction of input voltage as the input voltage rises..

5100 -------200 amps -----------144 v -----------200 amps ------------144v

In order for the rpm to rise the current must go down - the controller will be on 100% trying it's best to meet your desired current

In practice the dropping torque will intercept the rising wind resistance before them


I did this with my car - changed the controller from a 500 amp and 150v model to a 1000 amp with the capability of 400v
And at the same time reconfigured my batter to get more kwh - but at a reduced voltage - 130v

It took off like a scalded rat - then the acceleration dropped and at about 3500 rpm and 100 kph the current had dropped to 200 amps - which balanced the increased wind drag - Maximum speed!
😂😂😂 Would have loved to have seen that take off!..
 

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Discussion Starter · #75 ·
I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
It's a direct drive - no gearbox.
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
Major has answered this - but I would like to answer it a little differently

Your controller will control the Current - so if you have a 600 amp controller then full throttle = 600 amps and half throttle = 300 amps

In the forklift the controller will set to a certain maximum current - with my motor it was 200 amps

Rpm ------Motor Current ----Voltage ----Battery current ----- Battery voltage
0 -----------600 amps-------------10v------------42 amps-------------144v
This is well above the sustainable current - and will melt in a minute or so
1700--------600 amps------------144 v-----------600 amps-------------144v

NOT 5100 rpm - because You are using three times the current and will have three times the Back EMF


Strewth!.. I didn't realise that back emf rises that way. I thought it falls as a fraction of input voltage as the input voltage rises..
 

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Discussion Starter · #76 ·
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
I think you might be confusing max ratings with an expected result. This is a fictional motor I presume, so it probably wouldn't be rated this way.

For almost any other motor type, you can easily predict the unloaded RPM of a motor in wide circumstances. For example, synchronous AC motors will rotate based on however fast the coils get pulsed, asynchronous will always be behind a bit but fairly close. Permanent Magnet DC motors will turn a certain speed based on the voltage you give them, and you can pretty much linearly plot that (double the voltage, double the RPM). You might sometimes hear the term "KV" applied to a motor which means how many RPMs it will spin to, per volt you feed it (so a KV of 100 means if you give it 1 volt it will spin 100 RPM, 12 volts it will spin 1200 RPM, etc. They're telling you the ratio of RPM:Volts). SepEx and Shunt (parallel) wound DC motors will have similar, predictable speeds.

But DC Series motors you can't tell. With any voltage and no load, they accelerate forever, very quickly. The only thing that slows them down is having a load on them, and their speed/current curve isn't linear. So for example, they're not a great choice for any drive system that has a chain, because when the chain inevitably snaps or hops off a sprocket, the motor will just about instantly accelerate until it rips itself apart.

So in your example, if the motor is "rated" for 1700 RPM, that probably means that is its mechanical ceiling before its physical structural integrity can't be relied on anymore (as per whatever conservative degree of its rating and circumstances). Or, perhaps that in that specific application with that load, at 48V it would be expected to spin 1700 RPM. But it would have to be a very specifically known load.

For example, if you give an unloaded DC Series motor 5v, it will reach 12,000 RPM and then maybe explode. If you give it 50v, it will reach 12,000 RPM in much less time, but then maybe explode. It doesn't really matter what voltage you give it, without some drag, it spins up and then explodes. 😛

By tripling the voltage, you don't physically change anything about the motor. It becomes no more strong at holding itself together. Much like if a chain is rated for 1700 lbs, you can't make it be rated for 5100 lbs. It just is what it is.

Bigger motors will tend to be rated for a lower max RPM than smaller motors because the outside edge of the rotor is moving faster on a larger diameter.

TL;DR -
1 - Your (fictional?) motor is probably not rated for a max of only 1700 RPM. That's pretty slow. But if it was, max means mechanically the max before it blows up, nothing to do with voltage, and,
2 - Your (fictional) motor is probably not rated for 1700 RPM @ 48volts, because you say it's a DC Series motor and that's nonsensical since there isn't a "no load" speed for that type of motor.

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?
In short yes. Demand too much of anything and it will fail. Almost all electronics have a thermal limit by which they fail (in addition to the RPM limit mentioned earlier which you probably won't reach).

That's probably not the easiest way to think about it, for any motor type.

For example, think about going down a steep downhill. You might barely have to push at all to make the motor spin at 5100 RPM. So it's not how fast you spin the motor, it's how hard it is to spin the motor that fast.

I would kind of abstract the whole "target speed" thing and not think about it that way.

Think about a normal gas car driving down a highway. You have the accelerator pushed down some amount. Let's say it's flat and not windy so nothing is changing. Because wind resistance increases as a cube of speed (it's increasingly hard to go faster), you will eventually come to a steady speed based on how far you're pushing the pedal.

You don't have to know the math or quantities behind how much gas is pouring into the engine every second, exploding and propelling the car. You know that if you back off the gas, the car will slow down and reach a slower steady speed. If you step on the gas, the car will speed up and reach a faster steady speed. You don't have a pedal position that dictates "speed" and half way down is half speed, all the way down is full speed, etc. It depends on wind, whether you're climbing, whether you're towing, whether you're accelerating, etc. More is more and less is less.

Same thing for electric motors. Tell the throttle to give you "more", it will demand more. Tell the throttle for "less", it will give less.

If you redline a gas engine, suppose you have a small engine and you're trying to tow a heavy trailer up a hill, or, suppose you're trying to go 200mph on a crappy car... you will be demanding More and More and More. At some point you will only be able to put so much gas into the engine and it can't give you move (pedal to the metal), but more troublesome is that with you redlining the engine it's going to be getting hotter than you can keep cool, temperatures will rise, and something will blow up.

Ditto for electric motors. If you have too small of a motor and you're trying to do too much to it... accelerate too fast, reach too high a speed, drag too much weight up a hill... it's going to overheat and something will blow up. Unlike with gas engines though, motors have a bigger swing in what you can demand from them. You can only shove so much fuel and oxygen into a piston, and chemistry can only happen so quick. But electric motors you could abuse to a wider rate, several whole multiples of their rated steady power for very short periods, hitting only magnetic limits in the core I suppose.

I would think more in terms of power.

How much power is required from your motor, and can it sustain that?

You can know about the power it takes to travel a certain speed, or accelerate a weight at a certain rate, or travel up a hill a certain speed from that calculator I linked earlier. Those are physical facts. It will take X amount of power to get a certain car of a certain weight and shape, to travel a certain speed up a certain incline in certain weather. That amount of power has to come from the motor.

When you press the accelerator, you're in the end asking for more power. The actual sequence will be something like:

1 - Pedal position tells a circuit what duty cycle to pulse the motor on/off at since changing the actual voltage is impractical.
2 - The power transistors do as instructed, and pulse the voltage to the motor with those proportions of on
ff time.
3 - This averages a net voltage on the motor (i.e. 50% on/off time for 100 volts, appears like 50 volts and the motor would act the same if you gave it 50v steady).
4 - Voltage causes current (amps) to flow.
5 - Current makes the motor spin.
6 - The harder it is to spin, (not speed directly, but all the things that make a car difficult to move like wind, hills, and accelerating) the more current flows for a given voltage.
7 - The more current the more heat.
8 - Too much heat and something melts, either power transistors or motor brushes or motor wires or, something.

So you're not really telling it "Go this fast", you're telling it "Push this much harder". And pushing hard is what makes heat.

Think of perhaps, a toy electric car that you put your foot on. What happens? The motor can't pull you, it tries as hard as it can, and it overheats and melts. Ditto for an undersized motor in a car. It tries, it overheats, and melts.

By whatever method you're asking for "more", if it's more than the motor can provide, it dies.
Thanks Matt,
There's a lot of information here. The complexities of using AC or Sepex motors & controllers is my challenge. So far, I'm still likely to use a repurposed forklift motor.
 

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DrGee said:
Thanks Matt,
Just FYI,

On forums it's generally good etiquette to exercise selective quoting, especially for larger posts. Selective quoting is where you only quote the part of the previous reply that is relevant to what you're going to refer to.

If it's not specific, and just refers to the whole post, then you can skip quoting entirely and just say what you want to say if it's obvious.

That way the content is a bit more streamlined and easier to follow.
 

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Discussion Starter · #78 ·
Thanks Matt, /QUOTE]

Just FYI,

On forums it's generally good etiquette to exercise selective quoting, especially for larger posts. Selective quoting is where you only quote the part of the previous reply that is relevant to what you're going to refer to.

If it's not specific, and just refers to the whole post, then you can skip quoting entirely and just say what you want to say if it's obvious.

That way the content is a bit more streamlined and easier to follow.
No probs, thanks for the tip Matt.
 

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Discussion Starter · #79 ·
This is a 13" forklift motor. It's similar to the imaginary motor in my question earlier. It says it's rated at 1380 rpm. This looks like a good one to repurpose doesn't it?
 

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Hi - looks good

You will need to find something that matches that spline - probably not difficult

How heavy is it?

What top speed/rpm do you need? - as a bigger motor it will have a lower "burst speed" - I have been told my Hitachi 11 inch will be OK at 6500 rpm - a 13 inch - maybe 5000 rpm??- we need Major to tell us about this one
 
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