Duty Cycle for a Chester Champion V20 Mill ? - part 1

This is a follow up to a previous posting about overloading a Chester Champion V20 Mill: http://www.model-engineer.co.uk/forums/postings.asp?th=86758&p=1

In the absence of any information from the trading company (Chester Machine Tools) about a duty rating for the Champion V20 Mill ("It is hobby rated and not for extended use", I did some temperature measurements today, both on the control box and the motor. Stick-on temp gauges were affixed to the outside of the control box, and directly to the motor.

The first interesting result is that the control box did not reach 32 C during any of the tests ( ambient was about 20 C I guess), so any previous overloading of the control board was probably not temperature related. So for the moment I won't worry about cooling the control box and assume the previous control board failure was a one-off.

The measurments on the motor were even more interesting and contrary to the experience of some people on this forum. In essence the problem is with high motor speeds and apparently NOT low motor speeds.

Temperature was measured in the range 32 to 40 C ( in one case to 49 C) under NO LOAD condition, with various gearing and spindle rpm settings. All tests were made with the motor cover in place.

Interestingly this shows that at slow motor speeds the motor temperature goes up slowly and then stays at the same temperature.

However at high motor speeds, the temperature goes up rapidly and keeps going up - I stopped measuring at 49 C in one case! In the case of the HIGH gear setting at 2500 rpm on the milling spindle, the temperature went up 17 degrees C in 12 minutes. All of this without any load. And it then takes a whole hour before the motor temperature comes back down to 32 C - and that is with the motor cover taken off.

So at higher speeds the amount of heat generated cannot be dissipated so the machine doesn't settle down to a stable tmperature. No wonder the trader didn't want to publish a duty cycle, it would be something like 'Use for a couple of minutes, then leave to cool down for an hour'.

Passive cooling would be the preferred first solution (the Mill is still under warranty) and clearly the current motor cover prevent this, so I will take some more measurements this week with the motor cover removed.

Here are the measurements:

LOW gear - Milling spindle at 500 rpm

0 mins - 32 C

2 mins - 33 C

4 mins - 33.5 C

6 mins - 34 C

8 mins - 35 C

10 mins - 36 C

12 mins - 36 C

14 mins - 36 C

16 mins - 36 C

LOW gear - spindle at 1500 rmp

0 mins - 32 C

2 mins - 34 C

4 mins - 35 C

6 min - 36 C

8 mins - 38 C

10 mins -40 C

HIGH gear, Milling spindle 500 rmp

0 mins - 32 C

2 mins - 32.5 C

4 mins - 33 C

6 mins - 33 C

8 mins - 33.5 C

10 mins - 33.5 C

12 mins - 33.5 C

14 min - 34 C

16 mins - 34 C

18 mins - 35 C

20 mins - 35 C

22 mins - 35 C

24 mins - 36 C

26 mins - 36 C

28 mins - 36 C

30 mins - 36 C

HIGH GEAR - Milling spindle speed 1500 rpm

0 mins - 32 C

2 mins - 33.5 C

4 mins - 35 C

6 mins - 37 C

8 mins - 39 C

10 mins - 40 C

12 mins - 42 C

HIGH GEAR - Milling spindle 2500 rmp

0 mins - 32 C

2 mins - 34 C

4 mins - 36 C

6 mins - 39 C

8 mins - 41 C

10 mins - 47 C

12 mins - 49 C

Hi Peter. I have owned a Chester Champion 20V for over two years now. The mill gets lots of use and often for quite long periods, but i have NEVER had cause to run at over 1000 RPM, and have never used the high range, so perhaps that is why i have had no problems !!!!

Cheers Derek.

Derek,

I mill mostly aluminium so usually at high motor refs. As my measurements show, in LOW gear it goes from 32 C to 40 C in 10 mins, it would be interesting if yours did the same ? If not then there is something wrong with my motor!. Certainly the inbuilt fan does not seem to generate much airflow....

Sorry Derek ignore that - that test was at 1500 rpm sorry. Must engage brains before fingers...

Hi Peter,

interesting results you got from your mill. My mill is similar to yours (different paint at least), but I have never done tests similar tou yours. Like Derek I rarely use high speeds and have never experienced the motor getting very hot.

Thor

"Temperature was measured in the range 32 to 40 C ( in one case to 49 C) under NO LOAD condition, with various gearing and spindle rpm settings. All tests were made with the motor cover in place."

Peter, if all your measurements were taken under NO LOAD conditions then they are rather unrealistic. Motors draw only enough current to keep rotating with the applied load. More load needs more current, until the load becomes too much and the motor stalls. Other than friction, motors get hot because of the electrical current in the windings. You need to measure under typical loads.

Does the mill not go any slower than 500rpm, the problems of overheating talked about were when running at 50-100rpm due to the motor fan turning slowly. Also the motor would be under load as the reason for running slowly would be a large tool dia ( work dia on a lathe)

Unless a set of tests are done with known loads I don't think this test is of much use as none of us run our machines with no load where current draw will be significantly less.

J

The mill uses a DC motor doesn't it? The heat generated by a DC motor is going to be proportional to the current. Under no load conditions ignoring the friction losses current is proportional to speed and thus to heat generated. Hence it gets hot at high speeds. At low speed under load the controller will up the applied voltage to the motor in order to maintain torque and thus regulate the speed.

At top speed under no load conditions the motor is probably working at somewhere near max input voltage (in order to acheive the speed) and hence max current which be equivalent to lowest speed under max load conditions. The no load test does provide some information but it would be better to do a test and measure motor current.

regards Martin

Hi Martin,
The comments I am about to make assume that the motor is a permanent magnet brush motor. (Not a brushless motor) I do not think Peter has stated the type of motor used.
I do not agree with your statement "current is proportional to speed". The current is proportional to torque. Under no external load conditions the torque required to drive the fan and due to the drag caused by the lubricant in the bearings will increase with speed. I think there may also be more eddy current losses as the speed increases. I think these effects will be small compared with the current increase when the motor is under load. The permanent magnet motor on my X3 mill does not heat up much rotating at higher speed under no load or light load. I think your suggestion of measuring the motor current is a good one. The fact that the motor gets so hot at higher speeds under no load makes me suspect a problem with the motor. I think it would be worth Peter doing the tests with the motor not driving the mill drive chain. ( Remove the belt or gear on the motor etc.)

Les.

More rpm ::More horsepower

More torque:: More horsepower

More horsepower requires more current. More current genererates more heat . Most hobby motors are made to smallest possible size for lowest possible cost so have the least possible heat dissipating metal in them ; therefore they are likely to overheat. There is little room for heavier motors. Colin

Hi Les

Totally agree re torque comment. However I said that heat is going to be proportional to the current. Under constant torque conditions speed is proportional to current. I was making the approximation that frictional losses were contant over the speed range (I know they are not) and could be ignored as the effect is going to be small. If you consider friction the load is going to be slightly higher at high speed.

I was merely trying to extract some clues from the data provided.

You could be right when you say there could be a motor problem but I would look to the bearings (motor bearings or spindle bearings or gearbox?) for the cause. I suspect these machines are going to vary somewhat in the build standard. You probably have got a good-un. What we realy need as you say is current measurement and a value for max continuous current for the motor. The last parameter should be obtainable by test as it is esentially that current which can be maintained to give an acceptable constant motor temperature.

On a slightly alternative note a digital temperature readout would be a good idea if not a resettable thermal fuse. A readout would be handy to judge how heave a cut you could get away with.

regards Martin

I also wonder why all the tests started at 32deg with no running time when the ambient temp was 20deg? Surely the motor would be at room temp before being switched on.

J

I was under the impression that the speed of a DC motor, off load, was proportional to voltage not current?

Peter: At higher rpm the motor may have a reasonable load, even when not cutting. The fan will present a fair (non-linear with speed) load of it's own. The suggestion of measuring the current is a good one, albeit I supect the motor is driven with a PWM waveform, so a simple current probe might not be adequate. I would not expect the motor to have reached thermal equilibrium after 12 minutes. It's quite a big lump of metal. I'd be inclined to leave the motor running at high speed and no load and see what happens over, say, an hour. Motors are often designed to run quite hot, enough to burn you. It would be interesting to see whether Chester have any comment on normal motor operating temperature, or whether there is a clue on the motor rating plate; an insulation rating may be?

Regards,

Andrew

The motor should certainly not heat up like that under no load conditions. Check that the motor can be spun freely by hand when disconnected from the drive. Is the spindle stiff to turn when disconnected from the motor?

If both turn freely you probably have a shorted winding in the motor. As has been said, check the motor current when running under no load. It should be very low but must be quite high for it to be heating up like that!

Russell.

I agree with MikeW, and Russell, but you will often find a modern motor will run a bit hotter than older ones of 40+ years ago, they have a higher temperature rating for the insulation, and a bit less metal. Ian S C

Andrew

It is proportional to voltage and current.

Martin

We don't know what the motor is running at as the quoted rpm relates to spindle speed, the motor could well be geared up as well as down

Edited By JasonB on 09/09/2013 12:50:01

Reminds me that the motor on my Maximat V10P would get very very hot, too hot in fact to touch, just doing 'normal' light work.

Made me very concerned, but it never produced any whisps of smoke.

Found out that the company was penny-pinching with regard to the motor frame design by using aluminum in their construction if memory serves, and something that that model suffered from.

Geoff - Lathe mods continue.

Not so. Assuming that it is a permanent magnet motor the no load speed is proportional to the voltage. When a load is applied the speed drops slightly and there is a difference between the back emf generated by the motor's rotation and the applied voltage. This difference divided by the winding resistance gives the motor current which thus increases (approximately) in proportion to the applied load.

A motor with a separate field winding fed at a constant voltage has the same characteristics. Shunt wound and series wound motors get a bit more complicated.

Russell.

P.S. We really need to know a bit more about this motor before making an accurate diagnosis.  However, one thing is clear. It is faulty and should be exchanged by the supplier.

Edited By Russell Eberhardt on 09/09/2013 13:54:39

Another point I have not seen mentioned- It's not unknown for the cooling fan to be loose on the shaft. Some cheap ones are just pushed on to " splines "