Motor resistances

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Motor resistances

September 16, 2009, 11:18:05 PM

Has anyone measured the resistance of different (non-defective) motors around the car at some point? I'm talking about power window, locks, seat, etc. If anyone has, can you share what you found? I am trying to find some proper MOSFETs for a project but need to know these. I'll pull the center console apart this weekend if I can't find them sooner.

Motor resistances

Reply #1 – September 17, 2009, 07:45:11 AM

With an electric motor measuring the resistance at rest will not tell you much - they'll all measure close to zero ohms. You'd need to measure the current while it is running, then use ohm's law to figure out the resistance. Actually you'd need a "peak" ammeter to get a snapshot of the "starting" current the motor requires (and it'd be a good idea to know the "stall" load as well, especially with power window motors - you wouldn't want to pop your mosfets when the window reaches the end of its travel).

Are you planning on varying the speed of a motor? Remember, with electric motors, as voltage goes down current goes up, and it's very hard on the motor windings and brushes (this is why repeatedly trying to start a car with a low battery can fry the starter). The only good way to vary the speed is by using pulse width modulation.

Motor resistances

Reply #2 – September 17, 2009, 08:56:35 AM

PWM is the plan and we'll be cutting the power to the windows out when it reaches its max/min lift. The windows will move full speed but slow when they reach their desired position.

I understand we need more resistances than just at rest and this would be much easier using relays but I want to try something new. I haven't found a potentially better mosfet than http://www.datasheetcatalog.org/datasheet/irf/irfz44n.pdf but we have a gate threshold voltage issue when it comes to that one and the output of the microcontroller.

There are a few things I am still picking up and that's part of why I'm doing this from scratch but using powerful bitwhacker to begin. If you know of a mosfet that would work (12v/10A "should" be enough) while being sent a gate voltage of 3.3v, please let me know.

Motor resistances

Reply #3 – September 17, 2009, 08:43:45 PM

Not necessarily true about lower voltage = higher current, especially on permanent magnet motors, unless that lower voltage is due to excess loading. What will cause the worst heating is stalling the motor at a reduced voltage so it cannot provide the torque necessary to move the load. It takes less torque to keep a load moving than it does to start it moving. Dead right about the armature resistance being very low. That doesn't change, regardless of the voltage applied, or the speed it's turning. What does change, though, is the CEMF (Counter Electromagnet Force). The faster it turns, the higher the CEMF, subsequently less current draw, because the voltage developed across the armature starts getting close to the voltage supply. Slow it down, or stall it, and you've basically got the resistance of the brushes and the wire in the armature, which is not a whole lot. This is why they're equipped with thermal circuit breakers in places like door windows. Don't believe me? Take one apart and look at the brush rigging. You'll find a bimetal contact set in there that carries armature current. Get it hot, you're going to wait for it to cool again before that motor moves. Just hope it ain't raining. Bearing all this in mind, those little motors can actually become generators if there is an overhauling load situation, i.e., something trying to make the motor turn faster than rated speed at rated voltage.

Current is going to be proportional to the mechanical load or torque on the motor Speed is going to be proportional to the voltage applied to the motor, as well as the amount of mechanical load. The blower fan in your heater box works exactly that way. The load on that motor is fairly constant, but proportional to speed because of it being a fan. Uncoupled, that motor will scream at rated voltage. So, to slow it for low speed, throw some resistors in the armature circuit to reduce the voltage. Want it to turn faster, short out some of the resistors. This is exactly why the blower only works on high speed when those resistors burn out (open circuit). The window motor, however, does not have the same loading. It takes more current to lift that square yard of 1/4 inch thick glass (probably 50 pounds or more) than it does to lower it. That's why there's a great big honkin spring on the window regulator, to try to counter balance that glass. Now through in a few more variables, like alignment of the window channels, condition of the sers and guides, and you get a whole host of effects on the torque needed to move that glass. Going down with fresh dew wipes can possibly take more torque than it does to raise the window, because the dew-wipes tend to pinch on the glass as it's going down. Wet the glass, though, and it slides beautifully.

Close, though, on the theory of needing to know the current rating of the motor. That would be to prevent overheating your PWM drive. The ONLY way to control the speed of a permanent magnet motor is to vary armature voltage, either by means of resistance, or by chopping it and letting the motor average it out, which is wat a PWM drive does.

What you really need to know is what the locked rotor current is. Easy to find out if you have a spare motor, or even if you don't. You'll get better information if all you're working with is the motor and the short pigtail. You'll just have to devise a way to stall it. If you have a meter rated for at least 20 amps, put it in series with the motor. Direction doesn't matter, as long as the rotor is locked and cannot turn. Remember the bi-metal overload? It won't let you burn the motor up. When you get the meter set up, turn on the juice at full battery voltage, but don't let that armature turn. You won't hurt it if you do this quickly, but you should have plenty time to get a meaningful reading. You would probably see about 1 - 2 volts drop across the armature, maybe up to 5 volts if you could get the meter to the terminations inside the motor. I lean to the latter because smaller motors tend to have a higher brush drop and IR drop. When you stall it, though, keep in mind that the only impedance (resistance) you have across your battery is the brush drop, the IR of the motor, the wiring, and the contact resistance of the switch and circuit breaker in the fuse panel. Umm, probably a fuse link too. This is why these little bitty motors don't completely cave in the battery or roast the wiring when you put the windows up without the engine running.

Your max armature voltage is going to be (you guessed it) battery voltage less the brush drop and IR drop. The base speed is going to be determined by the number of poles in the rotor or armature, and the number of poles in the field. In the case of practically all automotive applications, that number is 2. Sometimes, you'll see 4 pole starters, but not often. Older starters were shunt field applications with spoogeulative compounding. spoogeulative compounding will strengthen the field and slow the motor, but provide much higher torque from the motor in stall or heavy load conditions. Ideal application for a starter. Most newer mini starters are permanent magnet too, though, and use high speed with a gear reduction to get the torque.

I could get into a lot longer diatribe about constant horsepower range and constant torque range, but the constant horsepower range doesn't apply to permanent magnet motors. ALL PM motors run in constant torque range because the magnetic field of the stator cannot be weakened. PM motors are notoriously strong motors for their size, and in fact, are used in a lot of applications that require high torque and precision rotor positioning.

End of class (Electric Machinery 101)

BTW, I'm an electrical engineer in industrial services and have been since May 1982. I've worked on some pretty big motors (AC up to 21000 HP and DC up to 8000 HP) in my day. Size doesn't matter, they still work the same way. If there's one thing I DO know about in a car, it's electric motor theory.

Motor resistances

Reply #4 – September 17, 2009, 11:02:45 PM

Yes, I am just looking for transistors and the heat they can tolerate at the moment. As far as I know, the above ones, at 50 cents a piece, should work considering both windows at max load at the same time cannot come close to causing circuit breaker problems. My needs are simple at first but I'd like to add complication, such as pwm, to it as simple calibrations and data tables are perfected. The window motors are a great place to start as they are easy to get, small enough to play with, and similar to the other motors in vehicles. Running all three motors in the power seat at their limits also doesn't blow the 20A circuit breaker so I should be perfectly fine with both power and thermal limits.

The biggest job is just building myself a decent GUI to do everything I want to be able to do from a pc in the car without needing to figure out how to get a xga or higher 8" panel that's also transflective. It seems the only readily available panels with controllers are 800x480 or 800x600 so standard Windows applications won't do. The rest is just to play with and will figure out as time continues on.

Motor resistances

Reply #5 – September 18, 2009, 01:36:40 AM

Seek, why do you do this to yourself? Don't you have enough little "opportunities to grow" without making it more complex? :D

Also, Old Paint...I never would have guessed...

Motor resistances

Reply #6 – September 18, 2009, 01:56:37 AM

sounds like over complicating simple stuff to me. Then again, im the guy who might say that a 60A dc ckt needs 6awg wire while on the next project that same 60A dc ckt requires no less than 1/0.

best clamp on at your battery cable with the key on and note your amp draw. next just raise the window and jot down the current draw subtracting the initial reading with just the key on.

you could repeat this step while holding down on the window while its going up to simulate more load. I would guess the current draw will increase. If it doesnt , it magically defies ohms law.

not sure what this "IR" reference is unless Isquared*R was the meaning then its more applicable to the inverse of siemens as stated already by old paint.

I know what i would do if all you want is the window to slow down when reaching either the fully closed or fully open position. Unless you have a mod done like auto window, i personally wouldnt want to have to hold the button any longer than i needed to.

Motor resistances

Reply #7 – September 18, 2009, 09:13:29 AM

Quote from: jrad235;291593

Seek, why do you do this to yourself

It's not really that difficult and it won't change the way the car behaves normally. These types of things I can reproduce indoors, partially with a plain motor and partially with a spare door. 14-15v on the other hand, that's not as easy to reproduce as a simple 12v power supply running the motors. Either way, the majority of it can be done inside where it's dry and warm and no part of it will leave me stranded on the side of/in the road.

Motor resistances

Reply #8 – September 18, 2009, 09:37:38 AM

I'll clamp on the batt cable and get some info.

Id rather clamp on the window motor wire at the hinge but it may be a bitch to get to. Actually, clamping on at the battery would be more appropriate for the application since you need to know the full loop length wiring voltage drop / loss as well.

Motor resistances

Reply #9 – September 18, 2009, 11:36:01 AM

To tell you the truth, I didn't even think about testing at the battery itself. I was going to pull the switches at the center console this weekend, although that is likely more accurate also. I do need a decent ammeter though - I don't like the ones in multimeters.

Motor resistances

Reply #10 – September 18, 2009, 09:21:28 PM

there is nothing wrong with an IDEAL 61-768. They are only a little over a couple hundered bux.
http://www.tequipment.net/Ideal61-768.asp

the best place to read the load is on the battery because,,,,,,,,drum roll.........

you probably are not going to replace all the wiring leading to the motors so the infrustructure will always need to be accounted for as the total load. So if your keeping the wiring, then its a true alternator/battery load.

I can get any ampacity reading on any function of the car by doing this up at the battery.

for intance, the fuel pump draws about 5 amps when it primes then cuts off. (meter goes from 16A with pump running to about 11.5 when it cuts off and the key still being on). You wouldnt believe what the hazards draw!

anyway,,,,,,,information.............

-With nothing on and no key in,, current is .2A (eec / clock ect)
-with key in ACCY= 2A (this amount we subtract from all future readings.

KOEO
-window up= @8A
-window down= @7.5A (my tracks , my wear, my window load)

-with window all the way up and pushing up switch= @16.3A
-with window all the way down and pushing dn sw= @16.5A

-while moving window up and holding presure keeping it from going up, amerage went from approx 8A to max reading of @16.5A.

Ohms law at work,, there's your I^R loss.
as you can see, its a very low resistance circuit even with the chassis harness voltage drop factor.

I know how id do the design your tinkering with.

Motor resistances

Reply #11 – September 18, 2009, 10:06:01 PM

Quote from: jcassity;291595

not sure what this "IR" reference is unless Isquared*R was the meaning then its more applicable to the inverse of siemens as stated already by old paint.

I squared R is power, in watts. IR is the voltage drop of the armature windings. Small amount, but not negligible if trying to build a sophisticated speed control with a CEMF regulator. I haven't figured out how to do sub-scripts in here, otherwise, that would be IaR, for armature current and resistance equating to the armature voltage drop. Normally, brush drop is considered part of that. IR Compensation is very necessary in CEMF regulators. Now if you really wanna get fancy, add a pulse tach to the motor, and build a true speed regulator as the outer loop. Then you can REALLY dial in the speed, and the current/voltage will do whatever is necessary (up to battery voltage) to go as fast or slow as you want. CEMF regulators are not very good low-speed regulators, but for what he's trying to do, probably good enough.

Quote from: jcassity;291595

KOEO
-window up= @8A
-window down= @7.5A (my tracks , my wear, my window load)

-with window all the way up and pushing up switch= @16.3A
-with window all the way down and pushing dn sw= @16.5A

There you have it then. Locked rotor current of the window motors is roughly 14.5 amps (accounting for the 2A you said was normal with key in ACC position).

I take it your window motors are not new. I just replaced both in the red-bird, and I wouldn't DARE stick my hands in that to try to hold it. Firstly, those windows move FAST. Secondly, I like my phalanges firmly attached at the metacarpals. Thirdly, I really wouldn't want my neighbor to come and find me with my fingers stuck in my car window. I've done some embarrassing and stupid things, but nothing I've had to call the neighbor over for. My window switches are on the console in both cars. You must have some REALLY long arms to be able to hold the window, push the switch, AND see a clamp-on under the hood.

To answer the first question, 14.5 amps locked rotor. Dunno how long it will maintain that before the overload in the motor trips. Might be another experiment for another day on a $130 window motor. So, size your transistors for 20 amps for up to 15 seconds (don't know of ANYONE that would hold the switch that long). You need only intermittent duty here. Something rated 10A continuous, or a whopping big heat sink should do the trick.

Now, if you were going to do something like a speed control for the heater fan ..............

Motor resistances

Reply #12 – September 18, 2009, 10:25:33 PM

I sat in the driver seat
hood open
driver door closed
meter clamped on
secondary bottom illuminated meter lense facing me

from viewing the meter while seated looking under the hood opening , i had the advantage to control the switches normally and jot down number.

you got too much stuff in your mind!! great read btw. I love magnets. Yes inrush might be a factor but in his design, im not sure it really matters. 1.5 x ampacity is way plenty like you said.

not sure what all was meant by the fingers and hands stuff, im not stupid.

I would place 2 momentary switches at 20% and 80% window track locations(bottom up orientation). from there I would parallel the proper size resistor to ground in order to split the current delivery. when you go up it will be normal speed until the window bumps the switch lever and the motor will be slowed down for the remaining span of a couple inches to complete close.

same way with going down. Motor will be full speed going down until the window bumps the 20% switch lever. It will then move slow until it seats.

either way , something tells me the resistor might need to be variable since the tracks are always the factor of resistance and then you end up with a window stuck at these two points due to crud ect.

Motor resistances

Reply #13 – September 18, 2009, 10:28:00 PM

wish my windows moved fast.

Motor resistances

Reply #14 – September 18, 2009, 10:34:40 PM

Wait, but I'm confused now on how I can hold both windows and there's not a single problem with the circuit breaker. It's 20A. I would have taken readings but my rearend noise has come back so I've been trying to figure out where it's coming from.