Thread: Milling Machine problem - spindle slow to start

My mistake. I thought he had replaced the run cap. Yeah, I suppose it will be OK.

Originally Posted by Stustoys

My 2 cents(which are worth some what less than that)
It will be fine. Its speced by an au supplier as a starting cap, its starting a mill so wont be used often or for long when it is(unlike say the starter cap on a lathe).
With any luck someone that knows what they are talking about will come along.

I think I paid $35 for mine at the local motor rewinder.

Still. You'll know what it is next time

Stuart

Thanks Stuart this is helpful. It certainly seems to run better than ever with this capacitor. The motor has two capacitors (mentioned previously - one 220v 150uF and one 400v 20uF) and they are both connected together by one lead from each capacitor to a single terminal on the terminal block.

It would however be good if we can get a definitive consensus on this. Last thing I want is to lose an eardrum (or worse) thanks to this thing going off.

Originally Posted by variant22

It would however be good if we can get a definitive consensus on this. Last thing I want is to lose an eardrum (or worse) thanks to this thing going off.

The definitive answer is you should never use a capacitor with a voltage rating lower than the operating voltage. In fact, depending on the application, you should add a margin to the voltage rating. When you exceed the rated voltage of an electrolytic capacitor you can get arcing between the foil layers. This damages the foil which eventually leads to failure of the cap. It can also cause them to explode. In fact, I'm retracting my statement in my previous post about it probably being OK for a start cap. The truth is, I don't know so I wouldn't do it.

When I was a stupid teenager my mate and I used to blow up salvaged electrolytic caps by attaching them to his dad's welder then running away. They used to go off with one hell of a bang and fill the garage with acrid smoke.

Sorry to be the bearer of bad tidings.

Motor start and run capacitors are not actually electrolytic, they're polyester.
Electrolytic capacitors and AC voltage (at power supply currents) do not mix at all and will go bang fairly quickly regardles of the voltage rating if you reverse their polarity (which occurs half the time with AC).
The start & run capacitors need to be physically big because you need a lot of surface area to create the capacitance levels needed and the insulation between the rolled up layers needs to be thicker to survive the voltage levels. This method of constuction makes them look similar to electrolytics.
When motor caps fail catastophicly you generally don't get a bang rather a loud hissing and lots of smoke and some oily goop running out of a hole that wasn't there before the failure.

I would order a 400v rated capacitor then you know for sure you're within the range, of course it might be bigger which doesn't solve your where will I put it problem.
Cheers,
Greg.

Originally Posted by kwijibo99

Motor start and run capacitors are not actually electrolytic, they're polyester.

Greg,
the capacitor he has replaced his start cap with is an aluminium electrolytic motor start cap:
Buy Aluminium Capacitors Capacitor,electrolytic,aluminium,motor start,220/275Vac,150uF Kemet 150MS22ACMA1RSC online from RS for next day delivery.

I did some more digging on RS and found that under the "Range Overview" tab (on the capacitor page) it has some more information on the voltage tolerances. It states specifically "Surge Voltage: 1.25 x rated voltage". So this is where they get their 275 volts from. This is also listed on the data sheet: http://docs-asia.electrocomponents.c...6b80fe2bfb.pdf

START capacitors are only under power for a few seconds when the motor accellerates and until the centrifugal switch cuts them out. To keep start capacitors reasonably small and cheap, these are made as bipolar electrolytic capacitors. If left in circuit for more than seconds, they get hot and explode after a few minutes. There is also a limitation on how many motor starts per hour will such a capacitor survive. A larger and more expensive capacitor (for any given capacity and voltage rating) will of course support more (or longer) starts per hours.

RUN capacitors are under power for as long as the motor is turned on. They are designed to run indefinitely long, several 10,000 to 100,000 hours (as opposed to seconds/minutes for a start capacitor!!!). Run capacitors are large and expensive. The best run capacitors are MKP type (metallized polypropylene film). MKP caps are "self healing". Meaning that if a spike (lightning sriking a power line) causes them to spark through in one spot, the metallized film evaporates only around this one spot, and the capacitor remains fully functional.

Hope this helps understanding why start caps are so small despite the high capacity. Chris

PS: you definitely get what you pay for with capacitors!!!

My mistake, I guess I should have read the links a bit more closely.

All I know is that when I had to replace the start capacitor on my air compressor motor I bought a cheap one from ebay and it lasted about a week
I had a chat to a motor rewinder who said to use a good quality polyester start capacitor.
This must be to handle the high number of starts required due to the motor switching in and out all the time to maintain pressure.

When I was building a split phase setup to run my old three phase mill which required both a start and run capacitor both were polyester.

I certainly agree with Chris that you get what you pay for, the polyester start capacitor for the compressor was around $60.00 but five years later it's still going.
Cheers,
Greg.

Regarding bipolar electrolytic start caps, I have found this Italian maker with a 4-page .pdf data sheet: www.xenonitaly.it/italcap.pdf On the last page we get some useful information. Look at table 3. T is the time the motor needs for starting. This can be a fraction of a second for a motor in a machine with little inertia. It may one or two seconds in a lathe with lots of inertia like a heavy chuck with heavy workpiece in it. It may be tens of seconds in a conveyor belt feeding crushed rock to a cement mixer. t is the time between motor starts. Also note the temperature rating. Start capacitors for household appliances like refrigerators are only rated at an ambient temperature up to 40C.

In general, the makers of electrolytic start capacitors will recommend something like "no more than 20 motor starts per hour taking no more than 3 seconds each". If your demands are higher, you are better off to substitute a real run capacitor for your start capacitor. It will be much larger in size, and will cost several times more, but it will last.

Also, I would personally only use a motor with separate start capacitor if I really needed the high starting torque. A high starting torque is needed in an air compressor or fridge, or to drive a conveyor belt with large friction and inertia. But here we were talking a mill. I know nobody that has the requirement to start a mill with the tool already engaged in the workpiece and powerfeed on. I think it is cost saving that small Chinese mills and lathes so often use high starting torque capacitor start motors, when a simple and more reliable single capacitor run motor would do just fine. Chris

Hi Chris,
I have trouble understanding the table in your link.
The second page of variant22's link http://docs-asia.electrocomponents.c...6b80fe2bfb.pdf much easier to follow.(thought I'm pretty sure to someone that knows what they are talking about they are saying the same thing)

Interesting the the same cap at 330V rms has a duty cycle of 0.55% and at 260V rms its 1.67%

I dont follow your point about starter caps, would it really be cheaper to fit two caps and the centrifugal switch rather than one better cap?
Would a motor with a smaller run cap use less* power while ruming? or maybe they have to meet a pf number?
Could you take a two cap motor, throw out the starter cap etc and fit a larger run cap? or would that be to much for the coils?
Last question... can single cap motors be reversed easily?


Stuart

Stuart, I guess it much depends on motor design too. It is all about finding the least bad compromises. Unfortunately single phase induction motors are always compromises. Single phase induction motors are by nature about 2 to 4 times less efficient than 3-phase motors. And all single phase induction motors suffer from quite bad torque vibration, which can cause surface finish patterns when used for example on a lathe. For small machine tools, I feel that today a 3 phase motor and VFD is far far superior to any single phase motor, and not that much expensive anymore. There is no excuse not to use them.

As far as reversing a single cap motor, this is very easy: just reverse the start winding polarity. Chris

Hi Chris
Thanks for that

Originally Posted by cba_melbourne

For small machine tools, I feel that today a 3 phase motor and VFD is far far superior to any single phase motor, and not that much expensive anymore. There is no excuse not to use them.

Thats already in the "to do" pile. Got the motor and VSD now I just need an "aroundtoit"

Stuart