Capacitors: Bottle Cap Challenges
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By Michael Teahan
A Bunch of Magnets
As brilliant as many Italian engineers may be—and many are—my favorite complaint is how they don’t understand how electricity works. At least how it works here in this hemisphere. The truth is that most of us don’t. Most of us have the basics: checking for voltage, resistance, continuity and even amperage. But we have to dive a little deeper to understand how motors and capacitors work in grinders and espresso machines.
The primary components of our most common motor are the stator, rotor and capacitor. The stator (think stationary) is the body of the motor through which wires are wound. These winding create a magnetic field when current is applied. The working part of the motor is the rotor, comprised of ferrous elements that react to the magnetic field of the stator.
The capacitor is the secret sauce that makes the motor spin.
Fundamentals
For the magnetic field to create rotational movement, the field has to change. It’s the rapid push/pull effect of the stator on the rotor that drives the rotation as the polarity changes from north to south (positive to negative). If it didn’t change, the rotor would just lock in one place like a solenoid coil.
Even though the fields change, the force is perpendicular to the center of the rotor. Its’ like trying to get a ball bearing to spin by hitting it the top of it with a hammer. Even when you are banging both sides, one after the other, it’s has no reason to spin. The rotor will just sit there and hum, sounding a low 60hz bass note (50hz in Europe). If you can get the rotor to even start turning, the alternating current will pull the rotor up to speed. This is why on motors with a bad capacitor, you can sometimes give a little spin with you thumb and finger and get it started.
Spin
The motors we work with use a second winding to get the motor started. This creates an additional field to pull on the rotor and an opportunity to determine direction and provide additional starting torque. These two windings are separate, with one tasked for running the motor and the other to get it started.
Merely having an additional magnetic field, however, isn’t enough. The alternating current that creates the magnetic field still flip back and forth in unison. With both fields firing at the same time, they simply fight each other to pull against the rotor. Without some way to keep them fighting each other, essentially to force them to take turns, the motor still won’t move. It just hums a little louder.
It is difficult to wrap one’s head around how the alternating poles create a rotating field. Because it is simply two poles flipping back and forth, it doesn’t on its face seem to be rotating at all—which is how the rotor sees it. A decent analogy could be the chasing lights on a sign over a store. They appear to be chasing each other, one after another in one direction. In reality, its just lights two lights alternating back and forth. Once you see it in action, it’s hard not to see it.
Enter the Start Capacitor
The capacitor has often been compared to a battery, rapidly charging and releasing current. In motors, though, it does something more important. When the capacitor charges it creates resistance (more like impedance) as it reaches capacity, creating a slight delay in the current applied to the start winding field. This delay means that the fields are no longer fighting each other for dominance. As run field transitions from positive to negative and the attraction weakens, the start circuit applies enough of a magnetic field draw the rotor away from its position to start the rotation. Once started, the motor runs normally.
Single phase motors with capacitors come in two varieties:
Motors that require significant starting torque have start windings that are substantial, powered by large capacitors to overcome whatever resistance the motor has getting started. The power required to start is so great that if the capacitor were allowed to run continuously they would overheat and burn out in a matter of seconds. For these applications, the start circuit and capacitor has to be taken offline once the motor starts to come up to speed. Larger motors will actually have a centrifugal switch that removes power when it reaches about 75% of the rated running speed. Refrigeration compressors have a different kind of relay that removes power from the start circuit. This relay is wired to the run windings of the motor and uses the EMF current to pull it open so long as the motor is running.
Motors used in espresso machines and grinders are a little different. The capacitor that powers the start circuit never drops out, it is always in the circuit and both start and run windings are under constant power. The impedance of the windings are different—usually a little closer to the rotor—and the rating and type of capacitor is different. The microfarad ratings are lower, so it doesn’t pull quite as aggressively as the first type.
These are permanent split capacitor start motors. They provide enough of a delay in the start winding to get the motors turning with good torque and have the added benefit of acting like a run capacitor while under load.
What does a run capacitor do?
Capacitors have the effect of conditioning the power, smoothing out the spikes and changing the angle of attack the current has on the rotor. If the start and run windings are pulsing in unison, the rotor will fight itself at 90-degree angles. The rotor relies on inertia alone to keep spinning with the magnetic field, but the motor will vibrate and draw more current as it fights with itself.
The run capacitor delays the current in the start windings just enough that it assists the inertial force of the rotor without fighting it. The two windings create a magnetic field without getting in each other’s way. Not only does it create the pull necessary to get the rotor started, but it allows the motor to run more smoothly and actually draw less amperage to create the same power. It behaves more like a two-phase motor than a single and is more efficient.
These permanent split capacitor motors are what we find in our equipment. Unlike conventional start capacitor motors, the capacitor stays in the circuit and provides power the whole time the motor is running. They are high voltage, low capacitance designs so that they handle the start load without overheating when they run. We often refer to the capacitors on our motors as start capacitors, but the reality is that they do both.
The Bottle Cap Part
All of this happens because of the nature of alternating current. It doesn’t just flip on and off like a light; it behaves like a wave. Voltage smoothly builds to positive and declines to negative 50 or 60 times a second. As the wave transitions between the two extremes it passes through a midpoint where there is no voltage at all: a neutral. This wave is how voltage builds in the capacitor and also shapes the magnetic fields in the stator. This rise and fall in voltage moves around the stator to create a rotating magnetic field that pulls the rotor around.
The capacitor, when it creates the phase shift in the start winding, effectively changes the angle at which the wave attacks the rotor. This was always the hardest part to wrap my head around, therefore the bottle cap challenge analogy. In order to get the bottle cap off, you can’t kick it dead on. You have to attack it on an angle. The capacitor shifts the phase of the field to effectively kick the rotor on an angle to get it spinning.
The same thing applies to 220vac motors here in the North America. While 220vac power doesn’t have a neutral, it combines two lightly out of phase 110vac lines to mimic a power line and neutral. Where the two waves overlap, they compound each other to create a 208-240v peak. Because the waves still change polarity in sync, they work together to create a magnetic field that flips north to south but with a higher effective voltage.
Practical Stuff
Diagnosis is pretty straightforward but easier with a capacitor tester. Not many meters have them and there really isn’t an easy way to get around it. The tester charges the cap and calculates the microfarad rating of the capacitor. There is a little room for error but it’s usually pretty close. Capacitors are matched to the impedance of the start windings and there isn’t a lot of wiggle room. Unlike straight start capacitor motors that drop the current while running, you can’t just oversize the cap to solve a hard-starting problem.
If the motor hums with no load but spins up to speed with a little push (carefully spin it you’re your fingers, for example), the capacitor is likely bad. Swapping the cap with one that’s at least close to the same rating should indicate whether it was the problem.
If a new capacitor doesn’t solve the problem, it’s probably the start winding of the motor.
Because the capacitor only affects the start windings, the motor will try to do something when power is applied, just not very well. You can even get it to spin in either direction with the start circuit offline. If it doesn’t want to do anything or doesn’t manage to spin up with an assist, the run windings may be shorted. The start windings will try to get it going but won’t have enough pull to make it run.
The rating of the capacitor will often tell you what kind of motor you have. A microfarad rating for most Permanent Split Capacitor motors are relatively low: 6.3 and 12.5 are common but can be as high as 40 for motors designed for a more aggressive field rotation on startup. If the capacitor is north of 100, you are likely working with a start only capacitor.
We often think of capacitors as a booster, something that provides a jump start of sorts. This leads to a temptation to mess with the size of the capacitor to try and fix a problem. But it doesn’t work that way. The capacitor doesn’t increase voltage or current, it merely relocates the peak of the wave and sends it to a separate set of windings.
Dead capacitors are common and should be a normally stocked item. So long as the voltage rating is sufficiently greater than the supply and the microfarad ratings are close, you should not have to stock a truck load of them.