# No torque on dc motor



## dougingraham (Jul 26, 2011)

Something is limiting the current. Torque is proportional to current. Motor controllers can't directly limit the current, they can only lower the voltage which will have the side effect of limiting the current. An 18khz pwm would not cause this issue. The only downside of increasing the pwm frequency is that you increase the switching losses. The necessary inductance goes down as switching frequency goes up. More likely there is a setting in the motor controller that is too restrictive and you are bumping into a limit. There could be insufficient current on the field winding as this would also kill the torque but unloaded it would spin up fine. If this was a PM motor I would also posit that the magnets had been partially demagnetized. 

Good luck!


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## Drpink70 (Apr 24, 2014)

thanks for your reply!

I also thought it was the controller settings. We tried three different field map curves, in the end I put on 5V continuously on the field coils, to be sure it worked, thngs changed a little, but no torque. I measured currents and found 100A at the armature and 7A at the fieldcoils. Also tried all kinds of different settings, even took out all restrictions, but no change. very annoying...

A friend who repaired elevators has had a problem with frequency also. The scooter is about 15 years old, so I also think it might be logical, since fets were not as good then maybe.. I was just wondering if people have ad the same problem. 

I will build my own controller, making the frequency a factor I can test myself, to see if this really the problem, only one way to find out 

if you have more suggesstions, or I am overlooking something, please let me know! I am glad I can mirror ideas, hope to learn a lot more, also hope to contribute..


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## onegreenev (May 18, 2012)

Sounds like you need more amperage to the field windings. You need the amperage to get the torque and then reduce the amperage to gain speed once your up and running. A series motor runs full amperage through the fields as well as the armature. That gives tremendous torque. SepEx motors don't give nut busting torque like the series motors. But looking at the spec sheet you can dump in as much as 30amps. Might crank it up to see what happens.


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## major (Apr 4, 2008)

Sounds like you need higher field current for starting. That is what the field map is supposed to do. Try to set the map for max field current at max armature current and minimum field current too heavy just for starters, like 10 or 15 Amps. I doubt the modulation frequency has anything to do with it.


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## Drpink70 (Apr 24, 2014)

Hello,

thanks for your suggestions. The problem with Alltrax is that they have to provide the field maps, they supplied three different, but also seem not to know anymore what to do. All didn't work. When I apply 5V DC (so 7A measured) directly on the field coils, and the controller on the armature (field coil output loaded with a power resistor to fool the controller), motor kind of shakes, but does not turn, armature gets 100A current according to the Alltrax monitor. When I apply 12V (and the 5V to the FC) directly to the armature, I get a lot of torque.. 

So I think it somehow it has to do something with the controller, and frequency is a clear suspect.. The motor has a very low inductance, 11.8uH, which I also think is strange, combined with the 0.2R dc, in principle not enough for 18kHz, frequency should be much higher..

I also read something about mechanical inductance, it feels also like the current is not strong enough, or the length of the pulse too short to get the motor to the next phase, it pushes but falls back, needs a bit more time to get there..

Extra current will not do the job, since it works fine with 5V normally.. But to be sure I will make my own controller, also trying 18kHz (16MHz processor, I write the software myself). see what happens... I now have to find otu


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## onegreenev (May 18, 2012)

Just an afterthought, do you have your cables connected to the correct terminals on your motor?


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## Drpink70 (Apr 24, 2014)

Always a good question, but I am sure they are connected right. Even though it should not matter I even tried reversing both armature and field coil polarities and indeed didn't change anything. 

Building my own controller really appeals to me, so I did already started that, also gives the possibility to experiment with frequency, so I can find out if it is the case.. Will follow up on that, thanks for your answers and I will be back in a few (??) weeks when it (hopefully) runs...


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## PStechPaul (May 1, 2012)

The inductance of the motor windings and the PWM frequency should have little if any effect on the performance based on the DC level supplied. But if you do not have commutating diodes, or a true half-bridge of full bridge drive circuit, you may get some strange results. However, if you are actually measuring DC current and voltage, (and you are using the right sort of meter), then this is a mystery.


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## gaminde (Sep 18, 2011)

Well wonder if it could be because that controller is rated at 24 to 48 volts and your feeding it 12 ???


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## gunnarhs (Apr 24, 2012)

Drpink70 said:


> Hello,
> 
> In my journey towards finally a real car, I rebuilded an electric step and now am in progress of rebuilding a Peugeot scooter. I use a DCX300 controller. The motor runs fine when the wheel is in the air, lot of speed, but there is no torque when the wheel is on the ground, motor just vibrates..
> 
> ...


Hi, based on my experience with 4 Pole Leroy-Somer the problem has nothing to do with the (modulation) frequency applied but only with the voltage/current- field/armature-mapping. 
I have been using controllers providing 5 kHz up to 16 kHz on these motors, the smaller being in anLeroy-Somer testbench and the bigger beeing in a Peugeot car.
The limiting factor is the power of the motor, meaning in mechanical terms the product of speed and torque and in electrical terms the product of the field current and the armature voltage (current). 
In other words , you can not have both much speed and much torque at once in these kind of motors.
If you are over-speeding the motor you have to lower the field current a lot, and that provides problems for most controllers I know (which limit field current when getting under 2A). 
I have not over-speeded this motors more than 10% and not exceeded nominal armature voltage for more than 10% (you seem to exceed by 50% by using 24V for a 17V motor, this will destroy the motor quickly).
The lowest field current I have used is 0,5 A.
So when using this in a vehicle you do (as mentioned by other members here) apply a current map as followed
Start the vehicle:
Apply at least 70% of the rated MOTOR field current in your case at least 5A (3,5V) on field winding BEFORE driving 
(armature voltage/current = 0 at start)

During increased pedal slowly lower the field voltage/current and increase the voltage/current applied to the armature, you should end with field-current of 1A and armature-voltage current of > 100A for high speed /full load
The field map of the controller should do this automatically, there could be an additional constant which you set to apply a ratio between battery and motor current depending on controller, start with ratio of not more than 2.
(I recomend a voltage control setting, not current control setting, especially when fooling the controller with a additional serial/parallel resistor in the field area during low currents).

The advantage of using own controller is that you could adapt the optimal voltage/current to the motor (frequency setting can also help for smoother behavior but has nothing to do with the fundamental torque problem.
You still will have no torque at high speeds because of the motor structure.
The only thing to fix this is rewinding of motor or change mechanical gearing.


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## Drpink70 (Apr 24, 2014)

thank for the comment, great to hear you know these motors. Because this will be my new hobby, I already decided to also build the controller myself, already did, just have to fix everything together. I can limit voltage (duty cycle) for the motor, so it will not be overrated too much, but since I have a spare motor, just will try and see how long it holds. Very good to know I should lower teh field current with higher motor current, upto now I just did otherwise. 

I can measure voltages (ADCs of uP) and have installed Hall sensors for both field and motor current, also installed temperature measuring on the motor power FETs (4 ps of 200A types parallel), software is totally upto myself.. I will start with 5kHz (I hope it will make sound , and field curves 7A (rated) at zero motor voltage, to 1A at 120A motor current, will limit the motor current to 120A for now, since it is rated, after that maybe strech it a little.. I can very much regulate all voltages and currents. When I go higher in power, I can see if I can lower filed current, I now use a PWM directly out of 24V, so low duty cycles, contra against motor current, so motor PWM goes on and off, and field current goes off and on, to have lowest battery current average, less peaks. But if I read your comments I better use a dc-dc 24V to 5V (I have some 20A types), and then use that voltage for PWM to the field coil, so I can regulate much better..

thanks for the very usefull information!


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