Thread: Which Drill Press Capacitor?

Mick,
two things:

The capacitance value is the important one to match so that the start winding works properly.
The capacitance values are measured in Farads (F) and the symbol in front of F should tell what proportion of a Farad is used. Capacitors are generally in the range of micro Farads and the correct symbol is actually the greek letter mu which is like a u with a tail. (Micro = one millionTH, so quite small) The slack manufacturers of these capacitors have used a capital M (which really stands for Mega = a million). For your case anything that has 150 uF is correct.

Providing the voltage rating is OK.

The rating on the capacitor is a working voltage (WV) going higher will blow up the capacitor. Your original capacitor was 125V as it was most likely in series with the start coil and so the normal 240V is shared across the coil and capacitor (each gets less than 240V, but I can't tell what the ratio is without knowing more about the start coil). Also if the capacity of the cap is wrong this sharing ratio wont be right and the motor may not start or the cap may go bang(!)
So IF that was the correct capacitor from the manufacturer it should be OK to use the 125Vac rated one. If you use the 240Vac one instead, it will be running at a lower voltage than designed for so not stressed and quite OK. The down side is the 240Vac cap will be physically bigger (as it has more insulation inside) and might not fit where the 125Vac capacitor came from.

So
You MUST have the correct capacity rating (150uF in your case if the cap is original)
The voltage rating can be the original or higher. (The 125V cap should be OK, once again, if that is what the motor originally had. But going higher in the voltage is OK too, if it physically fits).

Regards
SWK

Just to clarify, dead cap _could_ be the correct original capacitor.

Does it look like it was meant to fit (in the housing?) attached to the motor? If so it probably was correct. Has there been any obvious wiring mods done?

The 125Vac marking by itself doesn't mean it couldn't run on a 240Vac motor. The important thing is what voltage the capacitor is subject to from inside the motor.

Regards
SWK

SWK, many thanks for your multiple replies, it may be the original, although I cannot be certain as the unit has had some fiddles done to it. I don't know what it did in a former life, but it must have been some kind of work environment because the plug lead has been tagged, with the last tag 4 years ago.

It was sitting under an eave in an industrial estate not far from my place when I found out about it, some water ingress happened and although there was a layer of surface rust over much of the unprotected parts of the press, I don't believe it was too bad.

That said, it would seem that the capacitor housing did receive a fair bit and obviously had a water level at times inside the housing, as per the pictures.

The accompanying pictures show the capacitor still attached, the black almost Bakelite stuff that fell out of the housing cover when I was opening can been seen attached to one of the wires, which I presume is the live feed.

I will go ahead and order the 125 Vac capacitor.

Once again many thanks.

Mick.

Hi all,

the 125 Volts working voltage bugs me. As a rule, motor capacitors are always rated at the motor's own operating voltage, to minimise damage in case of a coil set failure (partial or total short-circuit), in which case the full mains voltage would be across the cap's terminals. The energy of overvoltage let loose on a cap, vaporises parts of its contents, which results in bulging, bursting open or exploding of the housing.

Could it be so that your particular drill press motor can be used on two voltages; 115/120 and 230/240 Volts? In that case the auxiliary winding array is probably a series connected setup, in which case 125 Volts would suffice. But if the motor was built for use on 240 Volts only, i have doubts about the cap being original. If i were to be given the task, i would choose the 250 Volts option (275 Volts is even better; 250 Volts used to be typical for appliances in the era of the pre-1993 European 220 volts mains voltage level). Indeed the cap would no longer fit the original enclosure, but i would build an alternative for that.

The rather high value of 150 uF is another hint at a possible dual voltage motor design. If a startup coil set inside a 240 Volts motor would need eg. 75 uF to generate a predesigned startup torque, the cap value would need to be 150 uF for a 120 Volts motor, to reach the same amount of startup torque. Since power is expressed in Watts (being the product of voltage x current), the cap would need to draw the double amount of current, to yield the same amount of startup watts at half the mains voltage.

150 uF is a very high value for a true 240 volts motor design. Your drill press motor is in the typical "fractional horsepower" league, meaning its output is less than 1 hp. I never saw starting caps bigger than 100 uF in that league, and that was in a professional barrista style coffee bean grinder (780 Watts input, 440 Watts output). Coffee grinders are often switched off and started up again with coffee beans and their ground remnants still being present in the grinding works, meaning that starting friction and the necessary torque to overcome it, are both very high. This explains the rather high value of 100 uF, as opposed to eg. a vintage fractional-horse dual rpm washing machine motor. Such motors also have to accelerate heavy loads ( 8 kilogram drums with wet laundry, up to 700 rpm or more in spin cycle), but they are fitted with caps no larger than 30 uF. In some cases these motors have additional boost caps the generate extra spin-up torque. These caps are switched off when full rpm is reached, and are usually in the 40 top 50 uF league.

So your story is an interesting one. Motors on drill presses are often of a "multi-purpose design" meaning that manufacturers may also sell the type to other clients, who put them to other use (fans, pumps, grinders, shredders, kneaders, mills, a.s.o.). In that case the design must be up to light and heavy startup conditions alike. Which means that the high cap value is possibly chosen for multi-purpose circumstances, but need not necessarily be that high for your particular drill press application. It would merely need to spin up the pulley and spindle assembly mass and would only need to overcome high startup when you press down the drill bit into the material prior to switching the machine on.

Sticking to exactly the same uF value is important when the cap is used during the full motor run time instead of the startup cycle. In that case the cap is part of the motor's running efficiency and total output power, so its exact value is paramount. But when your motor uses a centrifugal switch, which switches off the cap after motor spin up, you may get away with a somewhat smaller (and cheaper) cap value, since the cap plays no role once the motor is spun up and spinning up a spindle is a light load. My advice would be: have an electrician or an AC motor expert look at the whole situation for a second opinion, and be sure that choosing a 125 Volts cap is a safe option.

success and greetings

gerhard

Hi Mick,
I had to change mine a while back, & never cost me a cracker.
I went to Jaycar, but they were sq. & wood not fit in the required spot, & at $22.00.
So I told my Electrician Friend, " no worries, said he ", & he took apart a 4 x 20W Fluro Light, took Cap. out, put a sleeve over the wires & crimped them together, goes like a beaut.
Is 0.56Kw, = ½ HP. as that is what my motor was + it was round & it fitted back in the place where it was to go.
Hope that might help.

Thought an update would be in order, the new capacitor arrived from Hong Kong, interestingly, the description of the goods in the custom declaration is, "Cell Phone Accessories (China)".

Apart from the fact that the capacitor is probably a bit bigger than many current mobile phones, it works very well.

I now have a pedestal drill, for approximately 30.70% of the current new price of what appears to be a today version of this drill from the same company (Carbatech).

It should see me out.

Many thanks for all of the advice.

Mick.

Ps: This is an identical post to coincide with the thread on the same drill press but regarding the stepped cone pulley system.