Series DC Motor and Controller Selection???

brian_ said:

It looks like you're calculating an "electrical power to produce mechanical power" factor, in watts/horsepower. As for actual efficiency:

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Yes essentially Input Power so I could calculate how much input power input required for 10 HP

brian_ said:

input: 48 volts x 88 amps = 4224 watts (as above)
output: 4.14 hp = 4.14 x 746 = 3088 watts
efficiency = output / input = 3088 W / 4224 W = 73%

That's not good, but I don't know if any better can be expected.

The "Load Torque" and "Load Speed" do correspond to the listed "Continuous HP".

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Yes I had to use torque and rpms to extrapolate Continuous HP and it worked out to 4.14 HP. I also extrapolated 10 HP power by using the constant 1020 watts /HP x 10 = 10,200 watts, then factored out current of 222 amps peak.



So where have I gone wrong? Pretty sure I have made an incorrect assumption. I calculated current required based Input Power of 222 amps, so a 200 to 250 amp controller would work. However the manufacture recommends a 400 Amp controller with the motor. OK it occurred to me manufacture might be specifying Stall Current rather than based on 10 HP wattage.



I know peak power values of a motor is a Thermal Limit so max power might be debatable if more cooling factors in. Manufacture did not publish Peak Power watts for motor @ 10 hp.



You are throwing me off converting on mechanical power. In my minds eye Input Power is what a designer needs to know right?


Thx for your time.

Sunking said:

Yes essentially Input Power so I could calculate how much input power input required for 10 HP

Yes I had to use torque and rpms to extrapolate Continuous HP and it worked out to 4.14 HP. I also extrapolated 10 HP power by using the constant 1020 watts /HP x 10 = 10,200 watts, then factored out current of 222 amps peak.

So where have I gone wrong? Pretty sure I have made an incorrect assumption. I calculated current required based Input Power of 222 amps, so a 200 to 250 amp controller would work. However the manufacture recommends a 400 Amp controller with the motor. OK it occurred to me manufacture might be specifying Stall Current rather than based on 10 HP wattage.

I know peak power values of a motor is a Thermal Limit so max power might be debatable if more cooling factors in. Manufacture did not publish Peak Power watts for motor @ 10 hp.

You are throwing me off converting on mechanical power. In my minds eye Input Power is what a designer needs to know right?
...

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Hi Sunking, brian_,

Sun... You know units, right? Like inches and centimeters. Both in. & cm are units of length, or distance. So you have a wire 4 inches long, that same wire is also 10.16 cm long. No difference in the length, just a difference in units of measure. 4 inches = 10.16 cm. Or a conversion factor of one inch = 2.54 cm.

The same is true for power. Both watts and horsepower are units of measure for power. It does not matter in what form the power is, like electrical, or mechanical, or radiant, or average, or peak. Both hp & watts measure the same thing: Power. One horsepower = 746 watts. Here's a good review of power and its units of measure.
https://en.m.wikipedia.org/wiki/Power_(physics)

Efficiency is defined as the power output / power input * 100%. Of course you use the same units of measure for the numerator and denominator. So eff = watts out / watts in * 100%. Or eff = hp out / hp in * 100%.

Also, beware of ratings. Rarely will rated power translate to useful efficiency values. Manufacturers often use worst case tolerance limits on nameplates, or nominal values. And for a motor, peak power (output power) may not be a thermal limit at all.

And the controller current limit is just that. It does not relate to motor stall current or motor rated power. It is the same "5hp rated" motor if I use a controller having a 200, 400, or 1000 ampere current limit.

The peak power output for a motor and controller combination typically occurs at the point where at full throttle the controller transitions to current limit.

Regards,

major

major said:

Hi Sunking, brian_,

Sun... You know units, right?
The same is true for power. Both watts and horsepower are units of measure for power. It does not matter in what form the power is, like electrical, or mechanical, or radiant, or average, or peak. Both hp & watts measure the same thing: Power. One horsepower = 746 watts. Here's a good review of power and its units of measure.
https://en.m.wikipedia.org/wiki/Power_(physics)

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Now that is funny stuff Major, glad you chimed in. I am an EE so think I have units of measure down after 40 years.

major said:

Also, beware of ratings. Rarely will rated power translate to useful efficiency values. Manufacturers often use worst case tolerance limits on nameplates, or nominal values. And for a motor, peak power (output power) may not be a thermal limit at all.

And the controller current limit is just that. It does not relate to motor stall current or motor rated power. It is the same "5hp rated" motor if I use a controller having a 200, 400, or 1000 ampere current limit.

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This is where I am at Major, I fully understand if you use a 400 amp controller, is the Maximum Current the Controller cill Sink. Does not mean the motor will ever use that high of a current, just means it can if the Resistance is low enough for the applied voltage.

Riddle me this. Everything I know about Series Wound Motors Draw maximum current at 0 RPM's aka Stall Current. The Stall Current is simple Ohm's Law of Battery Voltage / Motors Resistance = Stall Current. Do you agree with that? There is no current higher than Stall Current.

Does not mean mean or imply you should size the controller to handle Stall Current, but is the only way to get maximum torque at very low RPM's. My thinking is select a Controller to some value less than Stall Current for protection. How much I do not know and the reason for this journey. Having said that I do not think any motor speed controllers would allow to apply Full Battery (100% PWM modulation) instantaneously to drive Stall Current. At least the Alltrax Controllers I am familiar with will not do that. They walk the voltage up over a period of time of say a few seconds, so by the time the Controller hits 100% modulation, the motor RPM's will have already spooled up to a few hundred RPM's to where Stall Current is not possible due to back EMF.

I know how to find Rm and have the equipment. Lock the Rotor down, apply 10 amps, and measure the voltage on the motor terminals. I even have a 4-point DRLO that does the same thing. But me thinks I will get a resistance so low on a 48 volt golf cart motor, would exceed any controller current limit out there. Just trying to figure out how to size correctly without a lot of over kill. Say 300 amp is more than sufficient and no reason to pay up for 400 amps. Part of me suspects Stall Current maybe higher than some of the controller current limit. That is what I am trying to nail down.


I work with AC motors all the time, and that is easy. Voltage specified is the most efficient operating voltage, and the other two numbers I want to know if LRA (which is the exact same thing as Stall Current on a DC motor), and FLA (full load current). Example for a 480 volt motor maybe specified at 470 volts, FLA = 40 amps. That tells me I need to size my wire so I incur a 10 volt drop with 40 amps, and coordinate breakers for LRA start up current if needed. LRA can be as low as 2 x FLA for soft start, and as high as 6 x FLA.

Hi Sunking

The resistance of a DC series motor is very low - as in 1/100th of an ohm (WAG) - so it only takes 10v to get 1000 amps

Anything that you use to provide that current will have it's own resistance - so a 12v car battery will probably deliver about 600 amps

You controller will control that right from the start and will do its PWM magic to feed your motor with it's 10v - by adjusting the PWM and thence effective voltage to get that current

The "stall current" is simply dependent on the voltage - and can be well above the Maximum safe current

As the revs rise the back EMF comes into play

Max power is where the controller is at 100% - battery voltage x set current
I think I hit that at about 60 kph
Then as the revs rise the current has to go down

Duncan said:

The resistance of a DC series motor is very low - as in 1/100th of an ohm (WAG) - so it only takes 10v to get 1000 amps

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I think you might be off by a magnitude 10. in my application. Size is going to have a lot to do with it. Most motors I am speaking of are 2 to 4 hp motors. Larger motors will have lower resistance. But I agree it will be low, and with the size of motor I am working with resistance will be 1/10th. I could be wrong.

Funny thing on another forum a member posted some specs from D&D Motors on a Controller / Motor Combo, a 48 volt golf cart motor and 400 amp Controller. It came with some dynamo test.

Rm was listed at 0.0891 Ohms.
Maximum Continuous Load current = 92 Amps
4.1 Kw Continuous, 12 Kw Peak.


Ignoring voltage sag Stall Current would be 48 / .09 = 533 Amps. Interesting enough Stall Current is specified @ 412 amps which is likely taking voltage sag in a 48 volt battery system into consideration. All the resistances in a golf cart is fairly uniform and known. So you could use fixed resistance constant into an equation that is realistic.

Duncan said:

The "stall current" is simply dependent on the voltage

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Voltage and Resistance.

Sunking said:

Now that is funny stuff Major, glad you chimed in. I am an EE so think I have units of measure down after 40 years.

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Then how can you write this?

Sunking said:

First thing it tells me I would be using 48 volts x 88 amps = 4224 watts. From that if I divide 4224 watts / 4.14 HP = 1020 w/hp efficiency.

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Sunking said:

If I assume a 1020 w/hp efficiency, a 10 HP would be roughly 10,200 watts.

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You clearly do not understand power units (hp & watt) and efficiency calculation.

Sunking said:

This is where I am at Major, I fully understand if you use a 400 amp controller, is the Maximum Current the Controller cill Sink. Does not mean the motor will ever use that high of a current, just means it can if the Resistance is low enough for the applied voltage.

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Disagree. The DC stall current using the equivalent series resistance of the motor is almost always, maybe always, higher, usually much higher, than the current limit of the controller. On any full throttle start, current limit is active. After maybe the first few pulse cycles (like 10 ms), there is no ramp function.

Sunking said:

Riddle me this. Everything I know about Series Wound Motors Draw maximum current at 0 RPM's aka Stall Current. The Stall Current is simple Ohm's Law of Battery Voltage / Motors Resistance = Stall Current. Do you agree with that? There is no current higher than Stall Current.

Does not mean mean or imply you should size the controller to handle Stall Current, but is the only way to get maximum torque at very low RPM's. My thinking is select a Controller to some value less than Stall Current for protection. How much I do not know and the reason for this journey. Having said that I do not think any motor speed controllers would allow to apply Full Battery (100% PWM modulation) instantaneously to drive Stall Current. At least the Alltrax Controllers I am familiar with will not do that. They walk the voltage up over a period of time of say a few seconds, so by the time the Controller hits 100% modulation, the motor RPM's will have already spooled up to a few hundred RPM's to where Stall Current is not possible due to back EMF.

I know how to find Rm and have the equipment. Lock the Rotor down, apply 10 amps, and measure the voltage on the motor terminals. I even have a 4-point DRLO that does the same thing. But me thinks I will get a resistance so low on a 48 volt golf cart motor, would exceed any controller current limit out there. Just trying to figure out how to size correctly without a lot of over kill. Say 300 amp is more than sufficient and no reason to pay up for 400 amps. Part of me suspects Stall Current maybe higher than some of the controller current limit. That is what I am trying to nail down.

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major said:

Hello Sunking,

Realize that the real resistance of a series wound golf cart motor is more on the order of 0.0167 ohms and that of a lift truck motor, half that. It requires a Wheatstone bridge to measure.

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Sunking said:

I work with AC motors all the time, and that is easy. Voltage specified is the most efficient operating voltage, and the other two numbers I want to know if LRA (which is the exact same thing as Stall Current on a DC motor), and FLA (full load current). Example for a 480 volt motor maybe specified at 470 volts, FLA = 40 amps. That tells me I need to size my wire so I incur a 10 volt drop with 40 amps, and coordinate breakers for LRA start up current if needed. LRA can be as low as 2 x FLA for soft start, and as high as 6 x FLA.

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Induction motor LRA is not "the exact same thing as Stall Current on a DC motor." Yes, both values are the respective maximum current draw for the motor across the mains. That's where the similarity ends, IMO.

Regards,

major

Watts per Horsepower also seemed a bizarre declaration of efficiency to me.

But... if we're being picky about units and equivalencies and such...

Major said:

Efficiency is defined as the power output / power input * 100%. Of course you use the same units of measure for the numerator and denominator. So eff = watts out / watts in * 100%. Or eff = hp out / hp in * 100%.

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100% is equal to 1. Yes?

In which case, why would you just multiply something by 1?

I'd amend it to: Efficiency is defined as the power output / power input * 100 ... as measured in percent... if you need that added

Myself I'm fine with saying/reading "Efficiency is 0.75" as it's an implied fraction in the concept of efficiency itself, but most people would want to read "Efficiency is 75%".

But that % should be stricken from the formula, no?

major said:

You clearly do not understand power units (hp & watt) and efficiency calculation.

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Don't be silly. The specs state both continuous mechanical hp (Torque and RPM) output = 4.14 hp and specifies electrical input power of 88 amps @ 48 volts. Efficiency = 3088 / 4224 x 100 = 73.1%. Now if I assume an efficiency of 73.1%, to get 1 mechanical hp out will cost me 746 watts / .731 = 1020 watts Electrical Input Power.



From the Specs Max Peak HP is 10 Hp. I assume that is mechanical HP and if I assume 73.1 Efficiency Input Power required is 10 x 1020 watts = 10,200 watts. My assumptions may be wrong, but my math is spot on.



At 73.1% efficiency requires 1020 watts for every HP mechanically out. So how is that wrong?

major said:

Disagree. The DC stall current using the equivalent series resistance of the motor is almost always, maybe always, higher, usually much higher, than the current limit of the controller. On any full throttle start, current limit is active. After maybe the first few pulse cycles (like 10 ms), there is no ramp function.

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Funny stuff because you just agreed with me. I said Stall Current = Battery Voltage / Motor Resistance. That is what I asked,and you cannot disagree with that, it is in all university subject matter on motors and boils down to simple Ohm's Law.


Now I agree with you DC Stall Current might, maybe, could, and/or will be higher than the Controller. Allow me to remind you the whole point of the thread was to pair a Controller/Motor. Question boils down to this.



Do you push a motor to DC Stall Current to obtain maximum start-up torque, or do you want to limit it and how much? Should not affect speed, just very low speed torque. I would think limiting Stall Current would be worth giving up some low end torque?

major said:

Induction motor LRA is not "the exact same thing as Stall Current on a DC motor." Yes, both values are the respective maximum current draw for the motor across the mains. That's where the similarity ends, IMO.

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Well let others decide for themselves. I can define to textbook definitions easily looked up


LRA = Locked Rotor Current aka Start-Up current is the amount of current the Rotor will draw when the Rotor is at 0 RPM or locked. As the RPM's spools up, back EMF will press back on the voltage applied to the Rotar and current will follow Ohm's Law for AC circuits. LRA = Applied Voltage / Rotor Impedance. Sound familar?


DC Stall Current occurs at 0 RPM when full battery is applied. As RPM's spools up Back EMF presses back against the battery, lowering voltage and current of Ohm's Law. DC Stall Current = Battery Voltage / Motor Resistance.


Sounds the same to me. I am just stupid. But you are right, they are different. One use Resistance to determine Current, and the other uses Impedance which will be a little higher than it would be for DC. What do I know?

Sunking said:

Don't be silly. The specs state both continuous mechanical hp (Torque and RPM) output = 4.14 hp and specifies electrical input power of 88 amps @ 48 volts. Efficiency = 3088 / 4224 x 100 = 73.1%. Now if I assume an efficiency of 73.1%, to get 1 mechanical hp out will cost me 746 watts / .731 = 1020 watts Electrical Input Power.

From the Specs Max Peak HP is 10 Hp. I assume that is mechanical HP and if I assume 73.1 Efficiency Input Power required is 10 x 1020 watts = 10,200 watts. My assumptions may be wrong, but my math is spot on.

At 73.1% efficiency requires 1020 watts for every HP mechanically out. So how is that wrong?

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Sunking said:

48 volts x 88 amps = 4224 watts. From that if I divide 4224 watts / 4.14 HP = 1020 w/hp efficiency.

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Sunking said:

If I assume a 1020 w/hp efficiency, a 10 HP would be roughly 10,200 watts.

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w/hp efficiency. O.K. I get it now. My bad. It is as brian_ says; an inverse efficiency and units conversion mixed into one convenient mess.

Sunking said:

Funny stuff because you just agreed with me. I said Stall Current = Battery Voltage / Motor Resistance. That is what I asked,and you cannot disagree with that, it is in all university subject matter on motors and boils down to simple Ohm's Law.


Now I agree with you DC Stall Current might, maybe, could, and/or will be higher than the Controller. Allow me to remind you the whole point of the thread was to pair a Controller/Motor. Question boils down to this.



Do you push a motor to DC Stall Current to obtain maximum start-up torque, or do you want to limit it and how much? Should not affect speed, just very low speed torque. I would think limiting Stall Current would be worth giving up some low end torque?

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It all depends on your design objectives. The drag racer may well put more value in the launch torque than the golfer.

I've seen controllers modified to 4000A current limits. I don't think I've ever come across a motor and controller combination where the natural motor stall current was less than the current limit.

Sunking said:

Well let others decide for themselves. I can define to textbook definitions easily looked up


LRA = Locked Rotor Current aka Start-Up current is the amount of current the Rotor will draw when the Rotor is at 0 RPM or locked. As the RPM's spools up, back EMF will press back on the voltage applied to the Rotar and current will follow Ohm's Law for AC circuits. LRA = Applied Voltage / Rotor Impedance. Sound familar?


DC Stall Current occurs at 0 RPM when full battery is applied. As RPM's spools up Back EMF presses back against the battery, lowering voltage and current of Ohm's Law. DC Stall Current = Battery Voltage / Motor Resistance.


Sounds the same to me. I am just stupid. But you are right, they are different. One use Resistance to determine Current, and the other uses Impedance which will be a little higher than it would be for DC. What do I know?

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I don't know why you bring it up. I didn't disagree. What I find vastly dissimilar between LRA on the induction motor and stall current on the series wound DC motor is the shape of the respective torque curves.

Regards,

major

major said:

w/hp efficiency. O.K. I get it now. My bad. It is as brian_ says; an inverse efficiency and units conversion mixed into one convenient mess.

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No problem, I just took a different road to get where I was going. I do understand the efficiency is going to be different at Peak HP and Continuous HP. Most likely Peak HP Efficiency will be lower than Continuous HP. Perhaps you can comment on that?


Whole point of the exercise is to determine how to size a Controller to a Motor. Example I know for fact the motor in my example would work with 100 amp controller. I would still have 4.22Kw Input Power @ 73.1% efficiency (4.14 HP). However Peak Input and Output would be severely limited with a 4.8 KW peak Input Power. Great for motor life, not so good for acceleration.


So this has me going to different directions. First direction I assume if a manufacture list maximum Peak Input Power is say 12 Kwh, (thermal limit) rated nominal Voltage = 48 volts, then one would need at least a 12,000 watts / 48 volts = 250 Amp Controller to get to 12 Kw.



Then when I see the motor manufacture says to use a 400 amp controller is telling me something else is at play. My educated guess from knowing quite a bit about batteries and wiring there is going to be significant voltage sag incurred on the batteries and wiring. Example say you are using Trojan T-105 batteries have an Internal Resistance of 3 milli-ohms x 8 in series is 24 milli-ohms, and 10 feet of 4 AWG resistance is 2.5 milli-ohms giving a total of 27 milli-ohms. With 400 amps flowing is 10.8 volts, just call it 11 volt sag dropping 48 to 37 volts. Just so happens 37 volts x 400 amps 14.8 Kw which is in the ball park of 12 Kw the thermal limit of the motor.


My last thought is select a controller capable of delivering Stall Current based on pure Ohms Law of the motor resistance.



Boils down to how big of a controller does it take.

major said:

It all depends on your design objectives. The drag racer may well put more value in the launch torque than the golfer.

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I agree 100%. A drag racer has no problem smoking a motor, that is what they are suppose to do, push the limit to the breaking point. That is why there are sponsors, someone to fill the trailer up with spare motors, controllers, and batteries. For me I want some quickness, balanced with motor/controller longevity.



So Major if you can help with that, I would appreciate it.

Hi Sunking

You can always do what I do - I have a controller set to 1200 amps

But I have three switches on the dash
Switch One - drops me to about 500 amps at full throttle
Switches Two and Three have smaller drops

So I can select different power modes
100% for the Track
40% for road use
20% for grass autocross

I have a zero to 5K Ohm throttle - so all I have is some resistors in parallel with switches

A Hall effect throttle could use something similar

I suggest you "overdo" your controller and add some switches

For starters, do like the Europeans; throw away the power unit of hp. All power stated in watts (or kilowatts). Next, keep it simple. Round off to the safe side.

For the sizing of the propulsion components to use with a chosen motor, namely the controller and battery, use current. The system voltage is assumed established, so power ratings are redundant.

In this case, or example, you have a 48V system and a 100A motor. That is the safe rounding of the motor's one hour rating with its design ventilation and normal ambient conditions.

So at minimum I'd use a 100A continuous rated controller. If your intended battery can deliver more than 100A for an hour, you may wish to increase the controller rating (125-150A). Motors are likely able to tolerate overloads of this type more so than controllers.

Also, any increase in controller rating will likely translate into extended life and cooler operation.

As for the current limit of the controller, I like to see at least 5x, that's five times the continuous rated current. So 500A in our example. If the cost was not unreasonable, I'd be tempted to 650 or 800A. Again, more is better and can come in handy if stuck in a pothole.

To me, it is pretty much useless to try to relate controller current limit to motor stall current. Motor stall (at battery voltage) should be like double or triple or more than the controller current limit.

My opinions,

major

Major and Duncan, thx.

Duncan you got me puzzled a bit. Add what switches where to control current limit? I understand how a current limiter works electrically, and I know many of the newer higher end controllers allow you to select what current limit you want with either software or dip switches in the controller. So do you mean move the dip switches outside the controller to the Dash? I am missing something.

Major thank you for taking the time to explain your design logic. I have no problem with it. It sounds reasonable.

What got me started thinking and grinding on this as I look at upgraded replacement 48 volt golf cart motors, I noticed the specs really do not say anything and the motor nameplate even gives less information. However a few motors like the one I used in the example had some numbers in the specs that could be crunched. Anyway all the after market motors I see suggest a controller size, and being the train driver I am wanted to know how to size the Controller.

Coming from the AC Motor side of the house we do not use speed controllers, just raw line voltage. (VFD not included in statement). Knowing AC motors and DC motors are virtually identically the same except for commutation, your 4x or more continuous current makes sense too me. Going back to LRA and FLA example in an AC Motor you will find LRA can be as high as 6 x FLA if no Soft Start is present. So if you use the motor example I used with a max continuous current rating = 88 amps, the 400-Amp Controller now makes sense, and is in the ballpark of 4 to 6 times normal load current. That I can accept and live with.


Again, thank you Major and Duncan.