# Dumb DC Controller Question



## Duncan (Dec 8, 2008)

I am not ready to buy my controller yet but I have been doing my planning with certain assumptions

The controller works by allowing the battery voltage to the motor switching the motor on and off in a Mark Space manner

This means
The motor (and com) *sees* battery voltage
The batteries *see* motor current

So my motor has got to be able to take battery voltage
And my batteries have got to be able to take (give) motor current

I now see that the Soliton (that I lust after) can control motor voltage

Does this mean I can up the battery voltage ? (340v ????)
What does this mean for the battery current ??

Can one of the controller guys help this Mechanical Engineer


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## major (Apr 4, 2008)

Duncan said:


> This means
> The motor (and com) *sees* battery voltage
> The batteries *see* motor current


Not quite the case Duncan. The motor controller is a buck converter which uses the load itself as the output inductor. Meaning that the motor is actually part of the controller. And with the series wound motor, the series field plays a major role as the output inductor. This actually lowers the voltage seen by the commutator during PWM. The motor inductance also provides freewheeling current during the off period of the PWM making current multiplication possible.

So the net result is that motor voltage can be lower than battery voltage and motor current can be higher than battery current. The comm and brushes in the series motor will see slightly higher than average motor voltage during PWM, but the series field will prevent full battery voltage from reaching there during the on pulses.

Regards,

major


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## Duncan (Dec 8, 2008)

Thanks Major,

So - I have a Hitachi MT4100 48v 10Kw motor, it has nice big brushes advanced by 10 degrees

What should I plan for as limits?
- I have been thinking of 150v max with 600 amps for acceleration and translating this into a 150v and 200Ah battery pack (3C)

Should I be thinking 200v and 160Ah ??


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## EVfun (Mar 14, 2010)

I have a way to get a rough estimate of the voltage a DC motor can take. Count the spaces between commutator bars between 2 brushes. Each gap between comm bars can hold back around 20 volts. That can often be pushed to 30 volts, but that's asking for reliability issues (let the racers do it


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## Tesseract (Sep 27, 2008)

Duncan said:


> The controller works by allowing the battery voltage to the motor switching the motor on and off in a Mark Space manner
> 
> This means
> The motor (and com) *sees* battery voltage
> The batteries *see* motor current


As ol' maj pointed out, the controller is a buck converter but with some twists; namely the motor is both the current-smoothing inductor and the load, and while there is an input voltage-smoothing capacitor (can't do without it, actually), there is no output voltage-smoothing capacitor.

So, the motor does see the full battery voltage, but because the switch chops the voltage so rapidly - interrupting it dozens of times as each commutator segment passes by the brushes - the net effect is that the commutator acts as if it sees the average voltage, and this is what the Soliton1 controls. A capacitor could be placed across the motor to convert this chopped, rectangular voltage waveform into a continuous, triangular one, and this might improve commutation (it would certainly improve noise emissions) but this capacitor would experience very brutal conditions so it would have to be very large and expensive. The inductance of the motor does smooth out the current waveform substantially, however, resulting in a small amount of triangular ripple riding on top of mostly dc current (1-10% ripple is typical depending on the motor's inductance and the degree to which it is saturated (and that is essentially proportional to loading)).

Unlike the motor side of the controller, however, there must be a smoothing capacitor present on the battery side to supply the pulses of current demanded every time the switch turns on. So the current drawn from the battery is averaged out, and the triangular voltage ripple that results is proportional to the current draw, capacitance, switching frequency and, most importantly, the impedance of the battery/controller loop (including the internal resistance of the capacitor ("ESR")).

As an aside, the Soliton1 - and most of the motors out there - will be happiest with a pack voltage somewhere between 192 and 288 volts. Too high a pack voltage forces the controller to spend most of the time at low duty cycles; too low results in the opposite situation, of course.


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## major (Apr 4, 2008)

Duncan said:


> What should I plan for as limits?


Sorry, there are no hard and fast rules. All these motors and specific applications differ. I think it best if you can design your system such that you can alter the V, I, RPM (ratio) and such and not be tied into buying different components each time you want to make a change.



> - I have been thinking of 150v max with 600 amps for acceleration and translating this into a 150v and 200Ah battery pack (3C)
> 
> Should I be thinking 200v and 160Ah??


Pros and cons here. Similar discussion on this thread http://www.diyelectriccar.com/forums/showthread.php/144v-288v-47910.html If you elect a higher voltage battery, it does give you some of that versatility I spoke about. But you should have a controller with the means to limit motor voltage. And this adds cost to the front end of your project.

No one size fits all 

major


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## Duncan (Dec 8, 2008)

Thanks for your help guys,

From EVfun - 20v/com bar - I have 52 bars = 13 bars/quadrant = 260v probably not going to be my limit!!

The Soliton will handle lots!

So the decision is lots of volts!

Battery current, - I am assuming that cost will limit me to Thundersky with a 3C max 

How much difference does the capacitor on the battery end of the controller make?
If I was to compare a 150v 200Ah pack and a 300v 100Ah pack would the capacitor drop the current by half?

Or more likely 150v -200Ah and a 200v 100Ah

The higher voltage will give more available power at higher revs 
(I have decided 5000rpm is probably my max safe rpm)

Will I have more available torque at low revs with the lower voltage pack?

Hi Major, - that thread was what got me thinking about this,

I am making a fairly low budget machine - 
The car including motor - $3000
the expensive parts are
The controller - a Soliton or Zilla $3000

The batteries $12,000

As the batteries are the lions share of the cost I don't want to have to get another set,

I had been simply looking at motor current (guessed!) and specifying the batteries,
600 amps = 3C therefore Ah = 200

This gave a 30Kwhrs pack = 100 miles range

Where I live and the way I intend to use the car this is probably excessive, a 15Kwhr pack would be overkill

But then I wouldn't get the current - or I would kill the batteries
I think I will re-visit my UltraCap calculations


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## major (Apr 4, 2008)

Hi Dunc,

Here's my take.



Duncan said:


> How much difference does the capacitor on the battery end of the controller make?


Having the capacitor makes it work. Without it, unlikely it would turn off successfully the first cycle 



> If I was to compare a 150v 200Ah pack and a 300v 100Ah pack would the capacitor drop the current by half?


The controller has to be designed with the proper bus capacitor. Tess can tell you why. But once you have the proper capacitor design, the input current is set by the duty cycle of the PWM. These controllers are very efficient, so it is not far off to consider them 100%. Then the input power is equal to the output power. So if you have a 50% PWM and half voltage to the motor, then the battery current will be half of the motor current. Vb * Ib = Vm * Im. 



> Will I have more available torque at low revs with the lower voltage pack?


Low revs meaning low RPM on the motor requires a low voltage to the motor. For example, a low RPM condition during acceleration may require only 20 volts to the motor. If you have a 200 volt battery, then at 600 amps motor current, the battery will see only 60 amps. Of course as the motor speed increases, you have to increase the motor voltage to keep accelerating, so the battery current will increase as well. Use that equation from above.



> I had been simply looking at motor current (guessed!) and specifying the batteries,
> 600 amps = 3C therefore Ah = 200
> 
> This gave a 30Kwhrs pack = 100 miles range


Obviously figured on a 150 V battery. So at 3C (600A), you maximum power would be 90kW. Do you need that high? 



> Where I live and the way I intend to use the car this is probably excessive, a 15Kwhr pack would be overkill.


With the same type of battery, that would drop you to a max power of 45 kW. But that would cut your RPM range for the 600A in half (I think). You'd still have the low speed torque as before, but up to a lower RPM.

My take on it, but it's late. Let's see if you know who chimes in.

major


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## EVfun (Mar 14, 2010)

Duncan said:


> Thanks for your help guys,
> 
> From EVfun - 20v/com bar - I have 52 bars = 13 bars/quadrant = 260v probably not going to be my limit!!
> 
> The Soliton will handle lots!


You are welcome. I do want to point out that you probably don't have 13 slots between 2 adjacent brushes. The brushes may cover 2 or 3 depending on comm position. That could leave as little as 10 open slots or about 200 volts max. 

My Prestolite MTC-4001 has 33 comm bars I think (I will have to count some day.) That gives me 6 open slots between bars minimum, and they don't like run much over 120 actual volts though racers have pushed them higher.


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## major (Apr 4, 2008)

EVfun said:


> My Prestolite MTC-4001 has 33 comm bars I think (I will have to count some day.)


Don't bother. It is a 33 bar commutator. Prestolite MTC-4001. One of my favorite motors. Likely manufactured in the late 1970's or early 80's. So how many miles you got on that motor? Any rebuilds or brush replacements?

Yeah, off topic, but I couldn't resist.

major


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## Tesseract (Sep 27, 2008)

Duncan said:


> From EVfun - 20v/com bar - I have 52 bars = 13 bars/quadrant = 260v probably not going to be my limit!!


EVfun already pointed this out, I see, but it bears repeating that only the exposed gaps between comm bars are counted for this rule of thumb. Also keep in mind that the likelihood of commutation failure goes up with armature current. A motor with that many comm bars but only rated for 10kW continuous probably shouldn't be hammered with 1000A for very long, if ever (not in a daily driver, anyway). 



Duncan said:


> How much difference does the capacitor on the battery end of the controller make?


It's impossible to answer that question without getting very technical, but suffice it to say that the amount of actual capacitance required for a controller to work properly is surprisingly low. Some of the parameters are totally under the control of the engineer - like ripple current, switching frequency and equivalent series resistance -while others can only be estimated, such as battery pack voltage and impedance. The impedance can be reduced by using larger Ah batteries, lower resistance interconnects and running the + and - cables next to other over as much of the battery pack as possible, all of which generally suggests going with a lower pack voltage



Duncan said:


> If I was to compare a 150v 200Ah pack and a 300v 100Ah pack would the capacitor drop the current by half?


That's also not quite so simple to predict, unfortunately. While it is true that a pack with twice the voltage will only need to deliver half the current for a given output power, the twice-voltage battery pack might have 4x the total resistance because there are 2x as many batteries each with half the Ah capacity, and internal resistance is usually inversely proportional to capacity. The controller will also be happiest if it is supplied with a voltage close to the maximum of what the motor itself can take as controller losses not only increase with current, they also increase - and this seems to be less widely known - with voltage. If you go with a 300V pack but your motor can only take 170V then until the pack is nearly dead the controller will almost never exceed 50% duty cycle, and even then you are playing with fire because the Soliton1, at least, will happily dish out 1000A at 170V and only the 11" and larger diameter motors can take that level of power for more than a second or two!


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## Duncan (Dec 8, 2008)

Thanks guys

I was planning on using 46 off (150v) 200Ah cells - costing $11,500 weighing 276 - 335 Kg (depending on which numbers I use) 

I am now planning on 62 off (200v) 100Ah cells - costing $7,750 weighing 217Kg 
Or possibly 56 off (180v) 100Ah

I will be reducing my range from 80 miles (at 80%) to 50 miles (at 80%)
but that should be more than enough for me!

The reason for the larger cells (200Ah) was to allow me to keep below the 3C limit, By looking at power rather than current I can use smaller cells 

This saves money and weight! - and the shorter cells will fractionally reduce the height of my center of mass

To Tesseract's point about increased internal resistance - the current Thundersky's are not reported as heating up so I think I will be OK

Are there any other legal/safety boundaries that I will cross moving from 150v to 200v?


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