HELP! -- Motor amps vs Ohm's laws????

[JohnnieB]

How can a golf cart motor draw more Amps than Ohm's law allow?

The DC resistance of the armature in my Sepex motor (Listed below) is roughly 0.3Ω. I used three DMMs with new batteries installed and they all agreed within 0.1Ω, 0.3 being the lowest value.

I've recorded numerous data logs from my Alltrax XCT controller and previously from a DCX controller that show both applied voltage and armature amps. Picking the max armature amps and voltage applied to the armature from one of the XCT data logs, it shows 362.0A with 34.1V applied, yet Ohm's law says only 113.7A can be drawn through 0.3Ω.

In other words, the data in the log says the armature has 0.094Ω of resistance while the DMMs say it has 0.3Ω, so one of the two Ohmic values is wrong, or there are other factors involved.

Note: The armature is stationary when measured with the DMMs and spinning at about 1700RPM in the data log.

Granted, DMMs are notoriously inaccurate when measuring low ohm values, but the value I get is fairly consistent with Ohmic values for stock sepex motors listed on check sheets from a reputable controller supplier/re-builder.

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I have posed this question to D&D, but no answer as of yet. I'll post it here if and when I get a reply.

[chuckinnc]

Strange, with motor running it should be heating alittle which would increase resistance/ohms, I could see where a brush would connect to two communicator strips at one time when turning but that would only cut ohms in half. Hope someone has the correct answer.

[Sergio]

I think the answer is Your 0.3Ω reading is not accurate.

One would either have to use a 4-wire ohmmeter or perhaps add a resistor in series with the armature and use current/voltage to determine the low Armature resistance.

The Motor amps can certainly be higher than the battery amps during operation, but the maximum Motor amps should not be higher than the stall current which is determined by the Armature resistance.

Hopefully D&D will answer your question or at least provide the Motor data sheet for your motor.

[JohnnieB]

Originally Posted by Sergio

I think the answer is Your 0.3Ω reading is not accurate.

I'm about 99.9% in agreement and I'd be at 100% except for the armature resistance values given for stock PDS (0.3Ω to 1.0Ω), DCS (0.2Ω to 1.0Ω) and Series (0.2Ω to 2.0Ω) motors in the checklists FSIP sends with the controllers they sell.

Of course, those values are most likely what is to be expected when measured with the typical ohmmeter that most cart owners have rather than a special purpose one the does measure low Ohms accurately, but there is a whisper of doubt left behind.

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DC motors, Ohmmeters and Ohm's laws have been around for ages, so I was wondering if the question had been asked before and was hoping someone who knew for sure would reply.

[WalterM6]

The simple basic answer is Ohm's law is for resistive circuits. When you get into inductive circuits you don't exactly use Ohm's Law. You got to take into consideration the inductance of the motor and the freq of the PWM you have a new ball game. So you got to use a new formula which is Inductive Reactance (Xl) = 2 * Pi * F * L . The impedance now replaces resistance. Maybe somebody could add on but that's as much as I remember from electronics school. One other thing not only the current can get really high but the voltage also. A good example is the flyback circuit (diode) added to a relay. That voltage can increase to over three times applied voltage in a inductive circuit

[Sergio]

That is not the issue we were discussing, the impedance will always be more than the plain wire resistance since impedance = resistance + reactance (inductive in this case).

The reason you see a high voltage when de-energizing a relay coil is also the reason why when the Controller PWM is less than 100% duty cycle You can have a higher current circulating through the motor than the current coming out of the batteries.

That is because inductors react against change in current similar to how capacitors react against change in voltage.

What prompt this question is can an Armature have a DC static resistance of 1Ω but consume considerably more current when running?

[pattyandrick]

It's impedance of coils in a circuit probably
Impedance is measured in ohms, too

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[pattyandrick]

The faster a motor turns the less amps and more impedance

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[Sergio]

The current goes down on a running motor due to the effect of the Back-EMF produced which reduces the voltage across the Armature.

but that was not the question posed on the first post.