Reverse motor in fwd gear

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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 04:30

Hey I was just wondering can I reverse the direction of an electric motor where the gearbox is still in a forward gear?
ie have the gearbox in 3rd then run the motor controller backwards for reverse.

Or will the helical cuts gears unmesh or something?

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So the short answer is NO but the long answer is YES.
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Reverse motor in fwd gear

Post by acmotor » Wed, 29 Oct 2008, 04:36

Interesting you should ask !
Gearbox will be quite happy. Remember that engine braking is the same as reverse thrust through gearbox.
Helically cut gears do have a normal thrust direction and will often make a different noise when this is reversed, but it all still works OK.
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 05:02

I was wondering how a front wheel drive car would go with an industrial AC conversion.
For ease/low cost the gearbox stays.
But for the RPM/torque loading on the gearbox a 2-pole motor is better.
This way you get cont torque upto 3000RPM which is more than any ICE would do.
I had a quick look at a Lancer CE and upto a 180 frame fits in - barely.
I did some maths through the box with a 15kW and was quite respectable (better than original).
But the same size I could get a 22kW in and just lock it in 3rd and have really good perfromance.

So use the controller to go in reverse rather than the box.

If someone was keen they could take out the other gears to reduce losses from running the unused gears.

So it becomes similar to a RWD direct drive setup except on a cheap newer car.

So the short answer is NO but the long answer is YES.
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Reverse motor in fwd gear

Post by acmotor » Wed, 29 Oct 2008, 05:19

Sound fine to me.

Problem is, when I excelled the available 2, 4 and 6 pole motors in the 5 to 30kW range, I found that 4 poles were the best power to weight by as much as 3 x over some 2 and 6 pole units. Also that 11 and 22kW 4 pole units were the best kW sizes for power to weight. Pull out torques were also best with 4 pole motors.
Thus I chose 11kW 4 pole for the suzi.

Be certain to do your homework on the actual motor your looking at. It varies from brand to brand.

A 5kW 4 pole wound for a lower voltage and running at 3000RPM and producing 10kW can be four times as good as a 10kW 2 pole in kW per kg.

A 3000RPM emotor can be usefull with a gearbox though. Image
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 05:58

I have found 2-poles with BDT just as good if not better than 4-pole.
As you say depends on the manufacturer.
Also the Power to weight ratio is more determined by the frame size.
So if you find an 11,15 and 18kW in a 160 frame then the best power to weight ration will be the 18kW.
But once again 11kW comes in 132 frame so will have a higher power to weight ratio than the 15 or 18kW.

Sustainable kW is the problem.
If you are happy for a car that is city driving then rewinding and overpowering a smaller kW motor would be good.
But if you want continuous highspeed I would think a stock higher kW motor would be better.
So the short answer is NO but the long answer is YES.
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Reverse motor in fwd gear

Post by acmotor » Wed, 29 Oct 2008, 06:10

But Richo, the point is that a 4 pole motor running at 3000RPM (with correct v/f that is) is under no more stress than at 1500 RPM. i.e. the current is the same, just twice the power as revs are twice. That is the point I make all along.
That is how Asure and acpropulsion and siemens get the power !
It is sustainable kW and there is still 3 x peak to be had.

Be aware that a 132 frame 4 pole 11kW may have less pull out torque or earlier magnetic saturation and also very likely a reduced heat capacity for overload.

Plot up some Nm/kg vs poles graphs for us to see.
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 06:18

Just for comparison.
11kW on the CE lancer is about 85kph.
22kW is 110kph.

11kW 4-pole 0-100Hz=0-3000RPM 200V@50Hz 71Nm nom 3x max
22kW 2-pole 0-50Hz=0-3000RPM 400V@50Hz 71Nm nom 3x max
So both should be the same except continuous running power.
Oh and the 11kW in a 132 frame much lighter than 22kW in 180 frame.

So I guess when planning an executive decision needs to be made on the max continuous kph to determine the continuous running kW.

So the short answer is NO but the long answer is YES.
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 06:24

acmotor wrote:It is sustainable kW


So running a 11kW 4-pole 200V@50Hz out to 22kW(400V@100Hz) can maintain this?
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Reverse motor in fwd gear

Post by acmotor » Wed, 29 Oct 2008, 06:57

Absolutely. Think about it.

Current is the limiting factor for a motor not voltage or frequency as long as you can maintain the v/f ratio. (until insulation has a problem)

Same torque(current) at twice the revs means twice the power(kW).
That is why they make 400Hz motors etc and why the up market AC emotors are high revving. You get a better power to weight.

Yes it is continuous, other than a few extra iron losses due to the higher frequency, the emotor is dumb, fat and happy to run at twice the kW at twice the revs. It is not being overloaded.

I am running a 5kW 2 pole at 6000RPM off an 11kW Danfoss on a circulating pump project at UWA at present. Controller shows it producing around 9.5kW continuous even though the v/f has fallen short. (as a result, current is higher than normal, but it is still at just less than pull out even with the reduced voltage and 100Hz)
It would be better though if it were 230V delta so v/f (and pull out ratio) could be maintained.
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 07:12

Image Took me a while Image
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Reverse motor in fwd gear

Post by woody » Wed, 29 Oct 2008, 15:38

so the efficiency is better at higher frequency? Or is the cooling better? At higher speed, same torque, the reactive (magnetising) current should be higher (same magnetism and demagnetism, but doing it twice as fast) but the active (rotor) current should be the same. So higher I2R losses, but not twice as high. Hmmm.
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Reverse motor in fwd gear

Post by acmotor » Wed, 29 Oct 2008, 17:52

Intuitively, efficiency will be less and cooling will be considerably more at higher Hz.

The rotor is unaffected by the higher RPM (until it goes ballistic). Only the stator iron feels the increase in Hz. Net effect is increased impedance and some increased iron and copper losses with Hz. Thus the need to increase voltage to maintain v/f and magnetising current.

Magnetising current only increases to meet the increased iron losses. (and would not increase if there were no iron - viz. CSIRO 'solar car' motor and other ironless designs.) Having no (AC field) iron changes the whole design. (Magnetic field strength is less for the same current and the surface area of rotor to stator interface must increase thus the axial flux design is often used. Magnetic fields are kept DC - usually PM and motor design is that of a completed magnetic path being cut by copper wires. Power to weight can be high, but so will be revs unless considerable multipole is chosen, 24 or so. Needless to say, the future of EV propulsion motors lays in this direction)

But back to the industrial induction motor....
Stator is typically chosen laminated iron, for all the right AC magnetic field reasons, particularly in modern motors. Copper and iron losses are typically only 2 or 3 % at design load and frequency with overall efficiencies pushing 95%. (fan and bearings being the other losses)
Even if you double the losses, you will still be over 90% overall efficiency.

These are efficiencies a brushed series DC motor can only dream about !
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Reverse motor in fwd gear

Post by woody » Wed, 29 Oct 2008, 19:20

ABB Technical Guide #7 backs you up there in equation 4.3.

In my spreadsheet I had been going with "Reactive Current is proportional to Frequency", so that is clearly wrong. This means my approximately horizontal drooping lines are bogus :-(

Eqn 4.6 says "current approx. proportional to torque" when Torque is between 0.8 nominal and 0.7 breakdown. Above that current really picks up (e.g. 11kW 4pole @ nominal is 20.5 amps, at double torque/nominal speed it's 41 amps, at triple torque/nominal speed, it's 78 amps), so I2R losses will be about 14.5x nominal.

I'll figure out new graphs, but for example a 5042 will only be able to get 190Nm out of an 11kW motor rewound for 230V before the current limit becomes an issue. I had previously predicted 215Nm, so it's about 10% down.
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Reverse motor in fwd gear

Post by Richo » Wed, 29 Oct 2008, 20:40

Yeah so an 11kW 4-pole in FWD car looks good.
Approx 60kg weight and Peak torque of around 200Nm.
A CE lancer is around 174Nm but this is a peak at 4500RPM.
As apposed to emotor which is flat 200Nm 0-3000RPM.
132 frame fits easy.
Tn=72Nm so Tm = 2.9 x Tn (208Nm) so doesn't even need to be a good motor

Like I said I could just leave it in 3rd permanently and get better performance than the original car.

Pitty the fins on the motor run the wrong way for sideways mount engine Image

So the short answer is NO but the long answer is YES.
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Reverse motor in fwd gear

Post by Gow864 » Wed, 29 Oct 2008, 22:29

you could really freak people out too. imagine 80kmh silently in REVERSE!
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Reverse motor in fwd gear

Post by acmotor » Thu, 30 Oct 2008, 04:19

Woody,

Just to put the copper losses in perspective...

Looking at red suzi, the winding resistance INCLUDING cabling as measured by Danfoss AMA is = 0.3579 ohm (per phase)

Consider 415V 1500RPM...

At 22.17A nominal, total copper losses = 1.73 x (22.17)^2 x .3579 = 304W when motor is producing 11kW
.304/11 = 2.76% loss Image

At 44.34A at 2 x Tn, total copper losses = 1.73 x (44.34)^2 x .3579 = 1217W when motor is producing 22kW
1.217/22 = 5.5% loss

At say 70A at 3 x Tn (before Tmax), total copper losses = 1.73 x (70)^2 x .3579 = 3033W when motor is producing 33kW
3.033/33 = 9.2% loss (serious loss ! Image)

All this in an Ozi made 24 y.o. emotor !

Iron losses will remain the same as will bearing and fan losses.
Forget the fan losses. I have separate 6.8W fans (2).

Percentage losses will work out to be greater at lower revs, but you don't spend much time there, or with motor at 3 x Tn.

Basically I don't fear the copper or iron losses in a 3PIM !Image

BTW, try the same calcs on series DC motors ! Image     
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Reverse motor in fwd gear

Post by woody » Thu, 30 Oct 2008, 05:05

Richo wrote: Pitty the fins on the motor run the wrong way for sideways mount engine Image
They're the right way if you're sideways :-)
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Reverse motor in fwd gear

Post by Richo » Thu, 30 Oct 2008, 05:50

Yeah remove the fan and cap.
Shortens the motor an inch or two.

I was thinking just a fibreglass scoop from where the radiator was and directing it down across the motor.
Even at 40kph that's over 2000cfm with a 1' x 1' (30cmx30cm) opening.

Even 7.5kW would be ok.
But they are in a 132 frame as well so no weight/size advantage.
Price 7.5kW $1100 vs 11kW $1900.
Rewind around $800.

Questions is if I just leave it in 3rd should I take out the gear stick and control leavers?

Sideways is good Image
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Reverse motor in fwd gear

Post by woody » Thu, 30 Oct 2008, 05:59

My (circa 1995) algebra is failing me. Can someone rejigger equations 4.3, 4.4 + 4.5 from ABB Technical Guide #7 to give Load torque as a function of motor current, cos theta, nominal current, nominal torque and breakdown torque ?
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Reverse motor in fwd gear

Post by acmotor » Thu, 30 Oct 2008, 06:24

Do you want the equations to still be valid ?
I believe the answer is 42 in any case.Image

East-west still sends shudders down my spine when I recall the morris 1100 !

Re fan.... I have 2 x 120mm 6.8W computer fans on the 11kW.
After driving 10km to the Perth open day, I flipped the bonnet and the emotor was at least 5 deg C above ambient !
So don't get too concerned about cooling.

7.5kW is getting small.
Don't disable gearbox as you may need first !
Suzi uses 20kW to come up the hill from dog swamp at 60kmph and around 7kW cont. on the flat.

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Reverse motor in fwd gear

Post by Richo » Thu, 30 Oct 2008, 07:00

Unfortunately with the lack of RWD cars it's getting to the point of convert FWD or die.

Were the fans on?
I was thinking 2000cfm with NO fan.

Yeah since the frame is the same for 7.5kW/11kW it's worth spending the extra $800 for more power.

It's more work anyway to remove the stick and control leavers Image

The same setup should basically suit most small FWD cars.
Even the Honda's where the engine rotates backwards!
No problem for the ACIM.
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Reverse motor in fwd gear

Post by bga » Thu, 30 Oct 2008, 18:41

Richo wrote: Yeah remove the fan and cap.
Shortens the motor an inch or two.

I was thinking just a fibreglass scoop from where the radiator was and directing it down across the motor.
Even at 40kph that's over 2000cfm with a 1' x 1' (30cmx30cm) opening.
The rod mount version of the motors already have the fan removed and have 4 rows of holes along the length of the sides for mounting. I don't know how useful the output end of the rod mount motors are for a gearbox adaptor.
Richo wrote:Unfortunately with the lack of RWD cars it's getting to the point of convert FWD or die.
I was thinking this myself and measured up a 1990 Lancer a few weeks ago for a 160 frame 11KW 4pole motor and thought that the shaft sensor might be a problem against the chassis rail.

4 Cyl motors are rather short. I haven't investigated the drive end of the induction motor, it may be able to modify the shaft to get a more compact coupling to the gearbox.

It's not all bad though. I think that FWDs are a bit more efficient in a straight line than a RWD because of hypoid gear is replaced with a straight mesh or a chain.


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Reverse motor in fwd gear

Post by Johny » Thu, 30 Oct 2008, 19:24

When you do the gearbox coupling you will probably have enough room to put an inductive pick up inside the housing where the clutch used to be (or use the clutch for pulses) rather than use a shaft encoder. Include the requirement of 'teeth' on the shaft coupling.
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Reverse motor in fwd gear

Post by Richo » Thu, 30 Oct 2008, 20:22

There is about 505mm from bell housing to end of standard engine.
So a 180 frame (not long "L" versions) should fit in.
A 132 frame defn fits in.
when the fan is removed put the shaft sensor there.

A standard flange mount should be fine.

Just a coupling between motor and gearbox shaft and a motor adpator plate to the bell housing.

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Reverse motor in fwd gear

Post by woody » Sun, 02 Nov 2008, 06:47

woody wrote: My (circa 1995) algebra is failing me. Can someone rejigger equations 4.3, 4.4 + 4.5 from ABB Technical Guide #7 to give Load torque as a function of motor current, cos theta, nominal current, nominal torque and breakdown torque ?
It took me a while, attempt number 5 worked it out. Attempts 1-4 kept giving me the wrong answers :-(

Re-arranging those three equations gives the maximum load torque under sync speed as:

T[load] = sqrt(x^2-((t^2+2*c^2*(x^2-t^2)+2*c*t*sqrt(1-c^2)*sqrt(x^2-t^2)-(m*t/i)^2)/(2*c*(t*sqrt(1-c^2)+c*sqrt(x^2-t^2))^2)

where
c = cos(theta) AKA nominal power factor
x = nominal max torque = pullout torque = nominal torque * breakdown ratio
t = nominal torque
i = nominal current
m = max motor current

So: a danfoss 5042 (97.6 amps) will only get:
190Nm from a 230V wound 11kW motor (Johny) - instead of 218Nm
344Nm from a 400V wound 22kW motor (acmotor rodeo) - instead of 486Nm (178Amps)
289Nm from a 400V wound 15kW motor (a4x4kiwi) - instead of 297Nm (8Nm is finger tight :-)

So maybe no room for torque boost on those motors.

Above sync speed, the formula is more complex, I won't do that now, it's past my bedtime :-)

cheers,
Woody
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