DOL Motor Connection - Hypothetical Question

[Scooterman]

Hi, this is just a hypothetical question and I'm not intending on trying it!

But what would happen if once having achieved top speed you connected the batteries directly to the motor? I.E. Bypassing the controller with a large automotive relay. Would you achieve much more top speed?

The reason that I ask is that I don't know if powerchairs and scooters use DC series motors? But I seem to remember from a college course I once attended that a DC series motor will accelerate to destruction under no-load conditions. I know AC motors are different in that max speed is limited by the number of poles and mains frequency i.e. 50Hz ~ 3000rpm.

A DC scooter/powerchair motor obviously wouldn't be under no-load condition so would the top speed limiting factor be the amount of power the batteries can deliver vs the rolling resistance and wind resistance of the scooter/powerchair plus rider?

I was just curious and as they say, "If you don't ask you never know!"

[Burgerman]

On a scooter nothing would happen. Pulsewidth is already at 100% on. So the batteries ARE connected straight to the motor.

On a powerchair you would gain around 7 percent extra speed on average, as some headroom is kept on most systems to help steer the chair. So pulse width is slightly less than 100 percent at fullspeed.

[LROBBINS]

Assuming the max power from the controller is 100% PWM, you will have negligible increase in top speed, perhaps none at all. The resistance of the controller's MOSFETS is quite low, perhaps even lower than the contacts of an automotive relay and the extra connectors needed.

Except for rare older chairs that used field-wound rather than permanent magnet motors, there's no chance of overspeeding. Even if a field wound motor was used, they were generally parallel connected with perhaps a small series coil to help starting torque, so even they can't keep speeding up indefinitely (to infinite except for friction) at no load. These motors need a different kind of controller, these days often referred to as SEPEX (separately excited). There can be advantages to these, but I don't know of any being used in today's wheelchairs.

[Scooterman]

On a scooter nothing would happen. Pulsewidth is already at 100% on. So the batteries ARE connected straight to the motor.
On a powerchair you would gain around 7 percent extra speed on average, as some headroom is kept on most systems to help steer the chair. So pulse width is slightly less than 100 percent at fullspeed.

Thankyou for that, at least by asking I now know otherwise I would have always have wondered.

Assuming the max power from the controller is 100% PWM, you will have negligible increase in top speed, perhaps none at all. The resistance of the controller's MOSFETS is quite low, perhaps even lower than the contacts of an automotive relay and the extra connectors needed.

Thankyou. I hadn't heard of the acronym MOSFET before but have just googled it and discovered it's a type of transistor. Are MOSFETs whay are sometimes referred to as 'power transistors"?

[LROBBINS]

A large MOSFET is one kind of power transistor. They are most often used these days for motor controller because they have very low resistance in the ON state, so lose little power as heat. The physics and circuit design for a MOSFET is quite different from that for a bipolar transistor and there are advantages and disadvantages to each e.g if you want to make an amplifier, a bipolar is easier use, if you want just ON/OFF a mosfet is more efficient.

[Burgerman]

I think a bit of clarification of how the controller controls the motor is in order first so that it can be undrstood what on / off means.

The mosfet is a on off switch. There is a bunch of them but for the sake of explanation we only need consider one. The battery power is fed in at one side. And taken out of the other to the motor. The mosfet is turned on and off at a high speed (just above audio frequency so you cant hear its affect on the motor). This is a fixed frequency and happens all the time the motor controller is turned on.

The length of time the Mosfet is turned on depends on how much power you want. So when stationary, you have it turned on 0% of the time.

At half speed, you have it switched on for 50 percent of the time. Thats a 50% pulsewidth. So its the FULL 24V battery voltage for half of the time, and NO battery voltage for half of the time, continually at high frequency. The AVERAGE voltage is then half battery voltage. An occiloscope will show a 24v square wave pulse.

Like this:

[Scooterman]

A large MOSFET is one kind of power transistor. They are most often used these days for motor controller because they have very low resistance in the ON state, so lose little power as heat. The physics and circuit design for a MOSFET is quite different from that for a bipolar transistor and there are advantages and disadvantages to each e.g if you want to make an amplifier, a bipolar is easier use, if you want just ON/OFF a mosfet is more efficient.

Thankyou for that. Is a bipolar transistor what is commonly thought of a conventional 'tansistor'? Aka one of these: (see attached sketch).

I think a bit of clarification of how the controller controls the motor is in order first so that it can be undrstood what on / off means.

The mosfet is a on off switch. There is a bunch of them but for the sake of explanation we only need consider one. The battery power is fed in at one side. And taken out of the other to the motor. The mosfet is turned on and off at a high speed (just above audio frequency so you cant hear its affect on the motor). This is a fixed frequency and happens all the time the motor controller is turned on.

The length of time the Mosfet is turned on depends on how much power you want. So when stationary, you have it turned on 0% of the time.

At half speed, you have it switched on for 50 percent of the time. Thats a 50% pulsewidth. So its the FULL 24V battery voltage for half of the time, and NO battery voltage for half of the time, continually at high frequency. The AVERAGE voltage is then half battery voltage. An occiloscope will show a 24v square wave pulse.

Like this:

Thankyou I understand now. The large 415v ac 3ph squirrel cage motors where I used to work were controlled in a similar way. They used diodes to convert the ac to dc, then chopped the dc up into a square ware using thyristors. I assume thyristors can cope with alternating current? But I don't know very much about electronics.

Back to my scooter, would you be able to explain to me how the braking and downhill speed regualting works? For what I've learnt is there's an electro/mech brake on the side of the motor which acts like a handbrake. This is what I hear clicking on powering up/off and when coming to a halt when riding. It acts like a handbrake or auto hold system in an automatic car?
But regarding say speed regulating when going downhill. There's a long downhill section I sometimes go down, cylists reach about 40mph+. Anyway while trundling down the hill I've often thought of stopping at the top, disengaging the drive, and freewheeling down the hill while controlling the speed on the single rear mechanical disc brake the scooter has. see attached photos. (brake lever is top left of tiller). But I don't believe it'd work because I seem to remember reading somewhere that the scooter still regulated the top speed even in freewheel motion?
So this brings me back to the question, please could you explain how the downhill speed is regulated both when in drive, and (out of drive) if possible? It's not that I want to disengage the drive and freewheel down the hill becuase I'm not too sure how efficient the disc brake is and I'm not in any fit physical state to bail out into the bushes!
Also why is there more than one MOSFET? Is it to do with needing additional switching to be able to put the motor into reverse? and also deliver different speed commands to the LH and RH drive wheels on powerchairs, and also the tilt mechanism?

Sorry for all the questions but I like learing new things as long as they're not too complicated!

[Burgerman]

At full speed, the mosfet behaves the same as a bit of cable. So to understand what regulates max speed on your scooter/hil forget about the controller for now.

If you were to connect the battery to an 8 mph scooter motor directly, the wheels rotate at 8mph. Thats the free rotation speed of its motor/gearbox. DC motors are linear. As in you can know that they rotate at x rpm per volt. So double the volts = double the speed.

A DC motor is also a generator. So if you were to push the scooter at 8 mph without a battery, it would measure 24v at the motor wires.

So you are now on the scooter, and traveling on the flat at 8mph with batteries directly connected. Because thats the RPM 24 volts from the battery gives you. Now you go down a hill. Speed increases SLIGHTLY to about 8.5 mph, and the motor now tries to make more volts than the battery... So instead it charges the batteries. You cannot go much faster even on a very steep hill because to do this you would have to push the battery voltage still higher. And that takes many Amps... The EMF energy needed, keeps your speed in check and charges the battery.

[LROBBINS]

Yes, that's a bipolar transistor. The symbol for a "field-effect transistor" (that's the FET part in MOSFET) is different reflecting the different physics of the way it works. The MO part is "metal oxide" which is a microscopic insulating layer between the parts of this particular kind of FET. There are also fets for lower current uses that are not MO.

John (Burgerman)'s description of regenerative braking is exactly what you were looking for. That's also essentially how you get normal braking action - the motor's energy is dissipated by pumping electrons into the batteries or just "throwing it away" through a low resistance. The clicking brakes are just parking brakes; they keep you from "drifting" if, for example, you are stopped on a hill, or if you're on a bus when it makes a sudden stop. Those brakes aren't doing anything when you're moving, whether on the level, going up or down hill. Putting the scooter in freewheel will not change the way it behaves except after the motors have brought you to a stop (and, will let you push the chair if the motors/controller aren't working).

You need at least 4 MOSFETs in what is called an H-bridge configuration to control a motor going both forward and reverse - one in each motor wire for forward, and one (of opposite sign) in each motor lead for reverse. Then, to get more current handling capacity, each mosfet is replaced by 2 or more in parallel, so you end up with multiples of 4. There will also be smaller transistors, or an integrated circuit chip, to control turning the mosfets on and off in a coordinated fashion, for example making sure that opposite-sign pairs are never conducting at the same time - that short circuit the battery. Some of these driver circuits can get pretty sophisticated, for example, especially useful on smaller motors without solenoid parking brakes, electronically shorting the two motor leads (NOT the battery) when you are stopped to get as strong as possible dynamic braking.

[Scooterman]

Thankyou I am starting to understand now.
Regarding this:

If you were to connect the battery to an 8 mph scooter motor directly, the wheels rotate at 8mph. Thats the free rotation speed of its motor/gearbox. DC motors are linear. As in you can know that they rotate at x rpm per volt. So double the volts = double the speed.

What would happen if a third battery was added in series giving 36v? I can't imagine it would be a good result as everything is rated for 24volts but I'd be interested in your opinion?

Yes, that's a bipolar transistor. The symbol for a "field-effect transistor" (that's the FET part in MOSFET) is different reflecting the different physics of the way it works. The MO part is "metal oxide" which is a microscopic insulating layer between the parts of this particular kind of FET. There are also fets for lower current uses that are not MO.

John (Burgerman)'s description of regenerative braking is exactly what you were looking for. That's also essentially how you get normal braking action - the motor's energy is dissipated by pumping electrons into the batteries or just "throwing it away" through a low resistance. The clicking brakes are just parking brakes; they keep you from "drifting" if, for example, you are stopped on a hill, or if you're on a bus when it makes a sudden stop. Those brakes aren't doing anything when you're moving, whether on the level, going up or down hill. Putting the scooter in freewheel will not change the way it behaves except after the motors have brought you to a stop (and, will let you push the chair if the motors/controller aren't working).

You need at least 4 MOSFETs in what is called an H-bridge configuration to control a motor going both forward and reverse - one in each motor wire for forward, and one (of opposite sign) in each motor lead for reverse. Then, to get more current handling capacity, each mosfet is replaced by 2 or more in parallel, so you end up with multiples of 4. There will also be smaller transistors, or an integrated circuit chip, to control turning the mosfets on and off in a coordinated fashion, for example making sure that opposite-sign pairs are never conducting at the same time - that short circuit the battery. Some of these driver circuits can get pretty sophisticated, for example, especially useful on smaller motors without solenoid parking brakes, electronically shorting the two motor leads (NOT the battery) when you are stopped to get as strong as possible dynamic braking.

Thankyou that's really interesting. I didn't realise the controller was so sophisticated. Do you know if powerchairs and scooters were around before the advent of solid state electronics?

[Burgerman]

What would happen if a third battery was added in series giving 36v? I can't imagine it would be a good result as everything is rated for 24volts but I'd be interested in your opinion?

My BM3 uses about 43V in place of 24. It goes much faster!

Butyou cant do that with 24v controller. Hence the Roboteq.

[rlnguy]

Scooterman,
"Do you know if powerchairs and scooters were around before the advent of solid state electronics?"
according to this:
https://www.reference.com/history/elect ... 9745bb4a8#
power driven wheelchairs go back to about 1916.
that was well before solid state electronics.
Power chairs, with switch controls (no proportional control), were made until the 80's, that I am aware.
Typically they used wound field motors, and switches to control the direction.
Switching the windings and batteries, into different configurations, gave different speeds.
In all honesty, they could barely move themselves, on a level floor, but they were all we had, at the time.
I found this in my files.

[Burgerman]

Marvelous! Non proportional pudding stirrer controls are STILL what stock chairs use!

Only now its because they have abysmal programming! Probably trying to copy the old ones...

[Scooterman]

Scooterman,
"Do you know if powerchairs and scooters were around before the advent of solid state electronics?"
according to this:
https://www.reference.com/history/elect ... 9745bb4a8#
power driven wheelchairs go back to about 1916.
that was well before solid state electronics.
Power chairs, with switch controls (no proportional control), were made until the 80's, that I am aware.
Typically they used wound field motors, and switches to control the direction.
Switching the windings and batteries, into different configurations, gave different speeds.
In all honesty, they could barely move themselves, on a level floor, but they were all we had, at the time.
I found this in my files.

No I didn't know that, thankyou. The wiring diagram you attached reminds me of the wiring diagrams we had at work for the pumping station electrical panels. The speed of some of the motors was controlled by switching motor winding arrangements, and also large resistance banks connected via slip rings.

My BM3 uses about 43V in place of 24. It goes much faster!

Butyou cant do that with 24v controller. Hence the Roboteq.

Thanks I understand now. Because I'm so used to working with AC motors it's hard to get my head round the fact that DC motors speed/volts curve is linear. I've been reading about your BM3 powerchair but not in depth yet, but I will. Considering I'm a light rider circa 8 stone, in the future would it be possible to upgrade the batteries and controller on my scooter to your BM3 powerchair spec thus achieving similar speeds? I was pleased the help members of this forum gave me in upping the top speed to 15kph, but in the future if more speed is possible, more speed is desired!

[Burgerman]

Yes. But its easier. 36v scooter controllers already exist..

[Scooterman]

Yes. But its easier. 36v scooter controllers already exist..

So would an extra battery and a 36v controller give me 33% extra speed? Out of curiosity I googled "36v mobility scooter controller", it came back with lots of hits but they all seemed to be for the small hobby scooters? You wouldn't be able to point me in the direction of a 36v mobility scooter controller so I could check one out? Ta! :thumbs:

[Burgerman]

No. 50 percent more speed.

If 24v gives 8mph, then 12 will give 4mph, and 36 will give 12 mph.

Not sure about which scooter controllers as I don't have much to do with them.

[Scooterman]

No. 50 percent more speed.

If 24v gives 8mph, then 12 will give 4mph, and 36 will give 12 mph.

Yes of course, silly me

[ex-Gooserider]

Typical mobility scooters only have one motor (often driving through a transaxle / differential, just like in a car) which makes it much easier to soup them up than the dual motor / tank steer powerchairs....

If your scooter has a single motor, any controller out of a higher volt scooter or other single motor device would work in theory. You would need to make sure any other parts of the scooter would work at the higher voltages (Might be easier to use the control stuff that went with the new controller, and fixing voltages for things like lights) Obviously the existing motor would also need to be up to the task or upgraded, but that is a separate issue...

ex-Gooserider

[Scooterman]

Typical mobility scooters only have one motor (often driving through a transaxle / differential, just like in a car) which makes it much easier to soup them up than the dual motor / tank steer powerchairs....

If your scooter has a single motor, any controller out of a higher volt scooter or other single motor device would work in theory. You would need to make sure any other parts of the scooter would work at the higher voltages (Might be easier to use the control stuff that went with the new controller, and fixing voltages for things like lights) Obviously the existing motor would also need to be up to the task or upgraded, but that is a separate issue...

ex-Gooserider

Hi, yes my scooter has a single 24v 800w motor, single transaxle, 2 x 75Ah batteries, and a 120A S-Drive controller. I'm happy with the scooter at the moment as WD members recently helped me up the speed from 12kph to 15kph via the S-Drive programming. But out of curiosity I did some googling and found this on eBay http://www.ebay.co.uk/itm/Electric-Scoo ... SwI-BWGArI
In the future if I wanted to increase the speed would this sort of controller be suitable, It seems very cheap so I'm a bit dubious?

Btw, I don't really know what a transaxle is? Is it two transverse drive shafts with a differential in the middle?

[Burgerman]

Trans-axle is a car term. It means Transmission and Axle. In one part.

Basically a drive gear ad a differential on a scooter. The rear axle...

There's no such thing as a '24v' DC electric motor. And the 800W part only tells you the maximum power it can provide/take in continuous amount over time. It does not give an indication of actual or peak motor power or torque for e.g. So really that 800w is pretty meaningless unless you exceed it for long periods in use. Its entirely possible for a quality motor of 300watts to have far more power than one of 800 watts. Its just that the 300 watt motor would suffer damage more easily if you keep doing that for a long time.

As an example. Your 120A controller allows 120A to your motor at times of high load. Say accelerating from a standstill, climbing a curb etc. Thats 120A x 24V = 2240 Watts. From your nominal 800 Watt motor... But you don't KEEP doing that.

So if you change to 36V and set the Amps on your controller to the same 120A then at stall, or low speed, there's no change. Your motor sees no difference. It simply spins faster as you go past 24V point. It means though that you CAN use more watts on average over time on say a hill, or while accelerating to speed. As long as you are aware of this, and treat the motors with a little sympathy then running them at 36v isn't a problem.

[Scooterman]

Trans-axle is a car term. It means Transmission and Axle. In one part.

Basically a drive gear ad a differential on a scooter. The rear axle...

Understood thankyou.

There's no such thing as a '24v' DC electric motor. And the 800W part only tells you the maximum power it can provide/take in continuous amount over time. It does not give an indication of actual or peak motor power or torque for e.g. So really that 800w is pretty meaningless unless you exceed it for long periods in use. Its entirely possible for a quality motor of 300watts to have far more power than one of 800 watts. Its just that the 300 watt motor would suffer damage more easily if you keep doing that for a long time.

As an example. Your 120A controller allows 120A to your motor at times of high load. Say accelerating from a standstill, climbing a curb etc. Thats 120A x 24V = 2240 Watts. From your nominal 800 Watt motor... But you don't KEEP doing that.

So if you change to 36V and set the Amps on your controller to the same 120A then at stall, or low speed, there's no change. Your motor sees no difference. It simply spins faster as you go past 24V point. It means though that you CAN use more watts on average over time on say a hill, or while accelerating to speed. As long as you are aware of this, and treat the motors with a little sympathy then running them at 36v isn't a problem.

Ah, I think I'm starting to understand now! When a manufacturer rates a motor at say 24v do they mean that when 24v is applied it will spin the rated rpm under no load? And as more load is applied the motor will try and sustain that rpm by drawing more current from the batteries. And volts x amps = watts = horsepower.
So for example when going up a hill with 24v batteries the maximum HP is 120A x 24V = 2240W/746W = 3HP and for 36v batteries maximum HP is 120A x 36V = 4320W/746W = 5.8HP? But only for a short period of time because of possible overheating issues. And also if you only need say 2.5HP to get the hill the heat factor wouldn't be any difference whether you were using 24V or 36V batteries as you wouldn't be need to apply maximum power in either case and overheating wouldn't be a problem unless it was a really long hill. And also the motor could well be drawing less current at 36V full speed on the flat, than at 24V and half speed going uphill.

So when my scooter manufacturer states the peak motor watts is 2700W, they should state how long it can run at that load before overheating, say room temperature i.e. 20 Celsius. And the voltage rating just means that it spins at a certain rpm at them volts.

Btw please feel free to correct me if I'm wrong, I'm EXTREMELY fallible!

[Burgerman]

Correct. Running full speed shouldn't be a problem. On the flat. As load is low. It isn't on my BM3 and that's doing double the speed at 16mph instead of 8.5. Where is may be an issue is when climbing a hill at speed because load increases and watts exceed the rated level for long enough to melt something. As long as you know this, and think before torturing it, no worry. RPM alone isn't an issue. You never burn out a power drill free running. Only if overloaded!

[Scooterman]

Correct. Running full speed shouldn't be a problem. On the flat. As load is low. It isn't on my BM3 and that's doing double the speed at 16mph instead of 8.5. Where is may be an issue is when climbing a hill at speed because load increases and watts exceed the rated level for long enough to melt something. As long as you know this, and think before torturing it, no worry. RPM alone isn't an issue. You never burn out a power drill free running. Only if overloaded!

Thankyou.
With the S-Drive controller can you not just feed 36V to the S-Drive's battery input terminals (marked) and just 24V to the S-Drive's control circuitry and scooter ancillaries? Would it not work?

NB: The M should be S-Drive power battery input

[Scooterman]

This isn't a powerchair or mobility scooter but it is relevant and the guy don't half go fast!
https://www.youtube.com/watch?v=z6Rlb4mE2lo

[Burgerman]

36V into a 24V controller can work. Depending on what you want to achieve. I suspect it might achieve smoke and expense! At very least it will achieve an over voltage error and it wont go anywhere!

But some of the curtis instruments and curtiss wright and other controllers do specify 36 volts.

[Scooterman]

36V into a 24V controller can work. Depending on what you want to achieve. I suspect it might achieve smoke and expense! At very least it will achieve an over voltage error and it wont go anywhere!

That was funny, made me larf!

[Burgerman]

As a POWERCHAIR user I get asked all the time why I would ever need to go faster than 6mph or 8mph.

I come across endless obstacles on footpaths. On major roads or junctions and small quiet ones, where maybe some retard has parked a car or truck onto the footpath, while delivering, where there's road works and pavements dug up, or sometimes there simply isn't a footpath.

For eg I was watching the news just now, and saw a very common example. How would I stay on the footpath, and get past that bin? And there's a lot of bins on that road. So it makes sense to take to the road. 15 mph is a hell of a lot safer! Especially in winter in the dark. You may also notice the next corner has no curb cuts so going past the cars will be needed... At least.

If you read the caption, you will see that the religion of peace has murdered another female in an honor killing. But that wasn't the purpose of the pic!

[ex-Gooserider]

Quite aside from the smoke remarks, you can't wire it up as you suggest, at least not without major re-engineering of the entire controller (and remember you can't get any schematics to help) The final stage of the controller is the MOSFET H-bridge that sends power to the motor, but for safety reasons (particularly on anything marketed as a mobility device) this is seriously monitored by the earlier stages, and they will have all sorts of errors if they see something that doesn't match what they have been designed to 'expect' - some of this is good (a shorted MOSFET would mean a run-away for instance) some of it is over-protective crap...

Much better to start with a unit designed for 36V input. The e-Bay unit you linked to earlier might work but I'd be suspicious. Chinese parts are notoriously optimistic in their ratings, and that unit may not have enough output power to drive your motor at speed... OTOH, it is cheap enough to play with....

There are controllers made that are 36V from better known companies (P&G, Curtis, etc) that come with English documentation beyond the scanty Chinglish web page, and probably more robust construction (I didn't see a lot of weather sealing for instance) but they will probably cost more, so pick one...

ex-Gooserider

Correct. Running full speed shouldn't be a problem. On the flat. As load is low. It isn't on my BM3 and that's doing double the speed at 16mph instead of 8.5. Where is may be an issue is when climbing a hill at speed because load increases and watts exceed the rated level for long enough to melt something. As long as you know this, and think before torturing it, no worry. RPM alone isn't an issue. You never burn out a power drill free running. Only if overloaded!

Thankyou.
With the S-Drive controller can you not just feed 36V to the S-Drive's battery input terminals (marked) and just 24V to the S-Drive's control circuitry and scooter ancillaries? Would it not work?

NB: The M should be S-Drive power battery input

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I

[Scooterman]

As a POWERCHAIR user I get asked all the time why I would ever need to go faster than 6mph or 8mph.

You're so right in your reasons. Although I can't match your BM3 speeds without major mods, but even then I don't think I'd be able to match your speed. Plus because my wheels are smaller the ride quality becomes too rough at faster speeds cos of our lousy roads in the UK, I nearly get bounced off the scooter. But I do plan to increase the acceleration on my scooter. I've upped the Max fwd speed from 84%-100% and now want to up the scooters pickup. There's a bit of lag before it picks up speed which I don't like it when crossing roads. Especially when I've pulled the wig-wag throttle (I love that word! ) and commited myself to cross and suddenly a car appears bearing down on me!
I was thinking of upping the values ringed in red, especially the Fwd Acc Fast Prof, is that one I want?

.There are controllers made that are 36V from better known companies (P&G, Curtis, etc) that come with English documentation

Thankyou for your advice I do appreciate and understand what you say. I'd not heard of Curtis Instruments but found them and this one looks suitable if I decide to do the mod in the future? http://curtisinstruments.com/Uploads/Da ... _RevG2.pdf
But the P&G S-Drives only seem to rate the controllers by current, they don't mention the voltage. Is that because they are all 24V do you think?