Grinder Capacitor Specs

[Owen Lowe]

In the CBN wheel discussion Bill Boehme wrote the following about a very slow to ramp up grinder:

My guess is that the start capacitor is on its way out. <snip> It should have the capacitance (in microfarads) and working voltage marked on it.

How does installing a different microfarad rating cap affect the motor?

I have an old(er) Delta 8” slow speed grinder that has been discontinued. It’s a beast of a machine but it seems to need more than its share of capacitors over the last 16 years. I’m now looking at #3. The grinder is once again hesitating on which direction to rotate when switched on. I have only replaced capacitors with the same rating as the previous. Any advice?

 

[Bill Boehme]

In the CBN wheel discussion Bill Boehme wrote the following about a very slow to ramp up grinder:



How does installing a different microfarad rating cap affect the motor?

I have an old(er) Delta 8” slow speed grinder that has been discontinued. It’s a beast of a machine but it seems to need more than its share of capacitors over the last 16 years. I’m now looking at #3. The grinder is once again hesitating on which direction to rotate when switched on. I have only replaced capacitors with the same rating as the previous. Any advice?

Here's my short answer:

The tolerance on the capacitance of AC electrolytic capacitors is pretty loose, but it is best to use the specified value even though other values would probably still work.

Here's my TMI answer:

The motor start windings are physically rotated 90° from the main windings and the purpose is to create a rotating magnetic field that "pulls" the rotor around. However, for this to work there needs to be a second phase connected to the start winding that is electrically 90° out of phase with the primary power source. This is accomplished by putting a capacitor in series with the start winding. The value of the capacitor is calculated based on the start winding inductance to produce a 90° phase shift. A smaller or larger capacitance value will result in less starting torque.

If the motor requires 6 μF (μF and MFD are both abbreviations of microfarad) and instead you install an 8 μF capacitor, that's not going to kill the motor, but stick to the right value if possible.

 

[Owen Lowe]

Are capacitors a commodity item or are there high quality sources/companies that may give a longer life?

On Practical Machinist forum, one guy replied to a capacitor question:

If you do replace the start cap., be sure to spend the extra dollar and buy one rated for 330v. They have better insulation and usually last a very long time.

Do you think he’s right about 330v?

 

[Doug Rasmussen]

In my experience a good many motor capacitors, especially on older motors, don't have the specs visible on their capacitors.

Google on "formula for motor starting capacitor sizing". I've used a formula several times although, it's worth noting the formula value didn't always exactly agree with the specs on a brand new motor's capacitor. But, as Bob said the specs are pretty loose.

 

[Mike Johnson]

All capacitors have an average life expectancy depending on the type and number of starts and ambient temperature it is subjected to.

 

[Bill Boehme]

Are capacitors a commodity item or are there high quality sources/companies that may give a longer life?

On Practical Machinist forum, one guy replied to a capacitor question:


Do you think he’s right about 330v?

I think that is a pretty typical working voltage. Start capacitors are fairly generic off the shelf items. Only as a last resort go to Rikon because we all know how you get gouged on appliance and electric tool repair parts.

I've always been able to read the value of a motor capacitor, but I wouldn't doubt that there are cases where it has faded or never was there. If the motor has a proper nameplate then the capacitor specs will appear on it. But, all bets are off on imported motors. There are some repair technician tricks of the trade to read invisible markings. One is to stick the capacitor in a freezer and let it thoroughly cold soak. Take it out and quickly use a bright light at a grazing angle to see if the markings show up. They will often show up as faint differences in the frost pattern. Sometimes gently wiping the side of a pencil lead across faint markings will make them more visible.

 

[John Turpin]

Take it out and quickly use a bright light at a grazing angle to see if the markings show up. They will often show up as faint differences in the frost pattern.

Brilliant.