Ohm's law doesn't seem to be working for this electric motor

Question

I'm a beginner in this field so please forgive me if I'm confusing with my question.

There is a component that I can't understand with Ohm's law which is a washing machine drain pump. Washing machine drain pumps from most manufacturers have similar specifications. Their winding resistance is usually between 10-20 Ω and it operates under 120 VAC.

However the specifications written on the label are quite different. 120 VAC, 1.1 A, and 80 W.

The actual current draw, 0.9 A, is close to the specification value which is 1.1 A.

I really don't understand that according to Ohm's law the resistance value calculated per the specification should be (R = U/I) 133.33 Ω where U is 120 V and I is 1.1 A.

But why is the winding giving me 14.8 Ω?

Shouldn't it draw 8.11 A as I = U / R = 120 V / 14.8 Ω = 8.11 A?

Answer 1

Have you ever played around with an electric motor connected to something like a light bulb or another motor? If you spin the motor, the motor acts like a generator and spins the other motor or lights the light bulb. The same thing happens when the motor is spinning under electrical power, the motor will behave like a generator, looking something like this:

^{simulate this circuit – Schematic created using CircuitLab}

Notice how although you see 12V across the motor, the motor resistance only sees 1V, making the current through the motor 100mA instead of 1.2A. This phenomenon is called Back-EMF, and is the reason why motors will draw a huge current on startup, but not much when running normally (when you turn on your vacuum the lights dim for an instant).

Answer 2

You're missing the reactance, which is the AC resistance (EDIT: And back-EMF - see comments). When you measure the resistance with a meter you're only measuring DC resistance and you're missing a significant part of the system.

Reactance comes from either capacitance, inductance, or a combination of the two. In the case of a motor most of the reactance will be inductive due to the inductor-like nature of the windings.

When using Ohm's Law in AC systems you use impedance instead of just resistance. Impedance, usually denoted Z, is a combination of the DC resistance and the AC reactance.

Answer 3

Aside from the excellent answers on the differences with AC motors, the thing you need to understand is that what they were wanting from having you check the DC resistance would be to see if it was too LOW, which would indicate it was shorted out, or WAY TOO HIGH, as in an Open Circuit because of a broken conductor. Anything in between just meant it was NOT one of those obvious forms of failure.

Answer 4

The DC resistance of the winding conforms to Ohm's law perfectly, and if you actually and directly (without eg a commutator) fed that winding 120V DC, it would perfectly dissipate 80 watts of heat and perfectly go up in smoke, perfectly in accordance with Ohm's law.

The actual power draw is dominated by inductance - any power dissipated in the DC winding resistance is actually LOST, all it does is heat up the motor (there is a magnetic field built, but you would get just the same field from a lower voltage if the winding resistance was lower).

The inductance of the windings alters with motor load (energy conservation law has something do with it) - an idling motor (if the motor design is safe to idle - some are not!) might actually draw even LESS current than the nameplate says, while a heavily overloaded motor (say if you pumped molasses with that pump) will get closer to the above scenario - very little inductance will be in effect, and DC losses will dominate and eventually overheat the motor.

Answer 5

Ohm's law is not a fundamental law of nature.

It's just a law that some very specific components observe; we call those resistors.

Now, it so happens that quite a bunch of components that aren't specifically designed as resistors still behave as if they were resistors – but only under specific circumstances. In particular, simple homogeneous metal parts obey a local Ohm's law. That includes also the wire with which the coils of an electric motor are wound, which is the reason you can some sort of reading when using an Ohmmeter with the motor.

Nevertheless, the motor as a whole does not actually obey Ohm's law, because the wire is electromagnetically coupled to other stuff: in operation, there's a constantly changing magnetic field inside the motor, and such a field induces voltages in the coils. It is these voltages that dominate the electrical behaviour of the motor in any real use situation, not the voltage from Ohmic resistance.

Only if you let a small DC current flow through the coils, nothing actually moves in the motor, the magnetic field is everywhere constant, and since induction only depends on the time-variation of the magnetic field, you then get a much voltage reading that corresponds to the ohmic resistance of the wire alone. That's why your Ohmmeter shows such a small value.

Answer 6

There are two effects at work here: one is that the DC resistance may be \$15\Omega\$, but that does not tell you the impedance of the coil.

At 1.1A, 120V, and 80W, the \$\cos\phi\$ is 0.6 which does not actually correspond to \$15\Omega\$, so what gives? It turns out that the DC resistance as measured corresponds to a blocked motor. If the motor is allowed to turn (and to pump water), additional countervoltage is induced significantly lowering the amount of current that is actually flowing.

So you have both the difference between DC and AC impedances here, and the difference between blocked and rotating (though loaded) motor.

Answer 7

The manufacturer is stating the coil resistance so that you as the technician can determine the "health" of the motor winding/s. Each winding should be the same as the others (if 3 phase) and the same as the manufacturers specification. This as well as an insulation resistance test between each phase and earth and between phases should form part of any motor inspection regime to determine the servicability of the motor windings.