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

Oh do give over. We are talking generally here. More volts = faster speed =more current. Forget the non-linearities for the moment. We all know it's more complicated than that but it just muddies the waters. If you are worried about overheating it's current you need to watch. More current means more dissipation. Increase the speed and you get more current. Increase the loading and you get more current. Increase both and you are even worse. Low speeds create less cooling if the fan runs slower. Low speeds usually involve larger cutters and consequently higher loads. There will be an optimum load and speed. If the controller can supply more power than the motor can handle you will burn it out,bad design. If there is a motor fault change the motor because thats not the really the issue. If they are manufacturing a machine that can be burnt out by overloading a temperature guage is what is called for so you get some warning when you a re pushing the machine too hard.

Martin

Martin: Clearly I'm not understanding DC motors properly. It would be useful to do some thought experiments, and you can tell me where my theory is going wrong.

Let's take a permanent magnet DC motor and assume that there are no losses of any kind, electrical or mechanical, and it is under no load. If we apply a voltage, a current will flow which will create a magnetic field. This will interact with the field from the permanent magnets and the motor will start to turn. But as the motor turns the coils are rotating in a magnetic field so a voltage will be induced. This tends to counteract the applied voltage, so the current is reduced. In the limit the induced voltage will balance the applied voltage and no current will flow. Now if we increase the applied voltage a current will again flow. This will cause the motor to speed up until once again the induced voltage balances the applied voltage. Again the current will be zero, but the motor speed will have increased. So it seems to me that the speed of the motor is proportional to applied voltage, but not to current, as the current is zero in this idealised case?

Once we get this sorted out then we can introduce loads and real life losses to see what happens in reality.

Regards,

Andrew

PS: I'm happy for anybody else to chip in and point out my errors too!

Well put Andrew. Anyone should be able to understand that.

Russell.

I also totaly agree with Andrew's understanding. The current will not increase under no load or constant torque as the speed is increased other than that caused by the fan loading (Which means the motor is providing more torque.) on the motor. One other point is the heating effect will be proportional to the square of the torque (As the current is proportional to the torque.) The power loss due to the resistance of the windings will be I^2 x R. This power will be in the form of heat.

Les.

Andrew,

I know AC motors are a different kettle of fish, but I was fascinated to see how my inverter puts out slightly more current to run the lathe, off load, at lower speeds, even though the voltage drops.

A DC motor experiment I did the other day was to run a big 12V motor, again no load, of 12V and 24V. The current went up from 0.8A at 12V to 0.9A at 24V, much in line with your suggestion. Obviously the stall current would be double (resistive load only).

I agree that main issue for motors, of any type, run slowly is lack of cooling.

Neil

Wow thank you for all the feed-back. The general consensus appears to be that there is something wrong with the motor - funnily enough I had not considered that option - just thought it's a poor design! I will contact Chester with the measurements tomorrow and ask them.

I will try and move the motor spindle while disengaged from the gears though I think the spindle may not be accessible. As you can see from the picture the spindle sits inside the housing. Will also measure amps later this week if Chester thinks the motor is OK.

Here is the motor spec:

To answer one of the questions: the reason I started measuring at 32 C was simple: the gauges run from 32 to 49! So I ran the machine until it reached 32 then did the first test, let it cool down, did the second test etc.

I know these tests are not realistic - they are 'best case' scenarios. So if the machine doesn't work properly under NO LOAD conditions then it will definately not work properly when it's loaded. And I dont want to test the machine to breaking point - I need it.

If any of you have a Chester Champion V20 I'd be interested to hear if you can feel the motor fan working at high refs - mine is barely generating any airflow. Again I thought this was a design 'feature'.

In the mean time I did some measurements today with the motor cover off. I just did the test on the HIGH gear at 2500 rpm on the milling spindle, as this was the most demanding of the 'no load' scenarios. Here's the result - it takes 20mins to get from 32 to 40 C, compared to 12 mins with the cover on. Better but still pretty poor, with no sign that the temperature settles.

0 mins - 32 C

2 mins - 33 C

4 mins - 35 C

6 mins - 36 C

8 mins - 39 C

10 mins - 40 C

12 mins - 42 C

14 mins - 44 C

16 mins - 47 C

18 mins - 48 C

20 mins - 49 C

As the insulation system is most likely Class F or Class H, the wire will be rated and designed to run up near 150C or so. Given that that's a permissible rise of 120C or so, your results (<30 rise?) are pretty modest. The pain threshold for most people is only 50C or so, so is it clear there is actually any overheating problem? A little knowledge is dangerous - or scary?

Industrial motors are rated for duty cycle. This may be 100% for some applications but most equipment isn't likely to be used that way so it's sensible and reasonable to specify a duty cycle. Have a look at this **LINK** for instance.

Put the cover back on, use it as you expect to and contact the supplier if it fails?

Muzzer

Peter,

I am not familiar with the machine[s] in question, but this HK20 Vario uses the same motor as yours [and may, for all I know, be another "badge-engineered" version]

The linked document has some useful assembly diagrams, and a brief warning about motor overheating.

MichaelG.

.

P.S.   It may be worth translating this 

Edited By Michael Gilligan on 10/09/2013 06:49:58

Nice rating plate - 240 V DC at 50 Hz???

Russell.

Hi Andrew

I wouldn't disagree with that either. In real systems the load on the motor is going to increase with the speed so speed will increase with voltage and current will increase too. Perhapse I was less than precise in my wording.

I think a simple temperature monitor readout would make a usefull project to write up for ME or MEW as there are so many of these machines about.

regards Martin

.

Mmm ... I wondered about that

Could it be that all the numbers are maxima ?

[i.e. 50 Hz is the Maximum Pulse Repetition Frequency]

MichaelG.

That's right, however that high temperature rise would only be reached under maximum rated load conditions. With no load the temperature rise should be minimal. The temperature rise of the motor casing will be much less than that of the windings.

My mill, a different mill but with a similar motor power, can run continuously on no load with no noticeable temperature rise. With heavy cuts at low speed the temperature rises fairly quickly but the casing of the motor never gets more than hand hot.

Russell.

No doubt the 240VDC and 50Hz is just covering all options.

Neil: On your inverter my guess would be that some of the parameters are set slightly differently to reality on the motor. The simpler, no feedback, inverters use a V-f curve when the motor is operating below base speed. If the frequency is lower the motor spins more slowly, so the back emf, and the winding impedances are lower, so you need less volts to drive the same current. This curve can go a bit awry at very low speeds, where the resistance of the windings dominate the impedance. This can be corrected for, but since there are tolerances on the winding resistance, and other motor parameters, the compensation isn't going to be perfect. So I wouldn't expect the current to be absolutely steady as the speed decreases.

I Googled the part number of the motor. It came up with some pretty weird links in langauges I don't speak, but one went to the manual for a different mill, but same motor. There was a hint in the manual to make sure that both wires to the motor were connected. A possible consequence was listed as the motor overheating. Might be worth checking?

Regards,

Andrew

Edited By Andrew Johnston on 10/09/2013 11:53:10

I hope Peter doesn't mind me partially hijacking his thread in an attempt to improve my understanding of DC motors.

Now that we seem to have reached a consensus on the behaviour of the DC motor with no losses or loads, let us consider the case where there are mechanical losses. This could be due to bearing friction, wind resistance etc. If there is no current flowing in the motor there is no torque being produced. But the mechanical losses produce a torque that tends to slow the motor down. If the motor slows down, but the applied voltage remains the same, then the back emf will drop and a current will flow. This current will produce a torque that will accelerate the motor thus decreasing the current, and torque, but only until the torque produced equals the torque needed to overcome the mechanical losses. So, if I have understood it, mechanical losses in the motor require a current to flow to produce a torque to balance those mechanical losses. The speed of the motor will be slightly less than for the lossless theoretical motor. One would hope that the mechanical losses are small, and therefore the offload current will also be small.

Now that we have a current flowing there will also be electrical losses due to the resistance of the windings, imperfections in the commutator, eddy currents etc. These losses will heat the motor, while not producing any useful work.

Does this seem reasonable?

Regards,

Andrew

Yes, absolutely, but only for a permanent magnet motor or a separately excited motor with a constant voltage on the field windings. It gets more complicated for a series wound motor.

Speed regulation (ie;, the drop in speed when a load is applied) of a PM motor is typically 10 to 15 % at full load.

Typically the efficiency will be of the order of 80% at full load so for this motor it will be dissipating about 170 W and will need a fair bit of cooling. At low loads the efficiency will be low but the dissipation will also be low.

Peter,

If you are going to measure the motor current it would be best to do it with an old analogue meter which will measure the average current with the pulsed control.

Russell.

Quite so, but being a bear of little brain I like to take things in small steps.

Andrew

Hi Andrew,

I think the Jaguar Cub is a 'upper-mid range' inverter. It has various options for modifying the torque curve, for example to give increased torque at low speeds. I am wary of trying to be too clever with this, but if it's short of power at low speeds but doesn't overheat I will try using this feature. The inverter assumes various parameters and estimates motor te,perature from the usage. The brave can change these parameters if they can get the data! It's a damn clever bit of kit, I hate to think what the top end ones are capable of - playing the stars and stripes by changing eth PWM frequency?

Neil.

Neil: I had a look at the manual for the Cub. It claims to use a simplified version of vector control, which is in a different league to the old V-f curve inverters. The unit seems to be pretty good. It's not clear whether the unit actually measures phase currents and is therefore capable of operating closed loop.

Vector control operates in 'dq' space. In essence the method uses the Park-Clarke transform to go from the three phase rotating vectors in the electrical machine to a two dimensional, usually but not always, stationary two dimensional frame of reference. The 'd' and 'q' refer to direct, ie, in-phase, and quadrature currents. I'm not sure if it's a good analogy but I like to think of the transform as removing all the rotating stuff for the purposes of calculations, which simplifies the maths, and then back to rotating vectors to drive the motor. Use of vector control helps with features like full torque at zero speed. Possibly not much use for a machine tool, but for an electric vehicle it's the equivalent of slipping the clutch on a hill. Electric vehicles is where most of my inverter design knowledge has been gained, mostly in the 100-200kW range. We always measured all three phase currents, even though you only need two. The third one can be calculated from the other two. However, redundancy is good; when dealing with hundreds of volts and hundreds of amps loss of control can be spectacular.

I think it is fairly common to allow for changes in PWM frequency, if only to remove annoying audible effects.

Regards,

Andrew

PS: I'll duck now in case I've mis-remembered the technical details.

Well I emailed the results to Chester Machine Tools, and their view is that 1) my tests are unrepresentative, 2) that the motor is working at it should be, and 3) that I should just use it.

So I did, lightly milling (the cutter did not heat up significantly) some aluminium at speeds of around 2000 rpm, with the motor cover off. Once the machine is warmed up, I can use it for about 30 mins (including the occasional stopping to brush off swarf) before it gets to 50 C, and it then takes about an hour to cool back down to 30 C or so. So a dutyr cycle of 33 % - at least when it at high refs. Perhaps this is what Chester Machine Tools means by the phrase 'Hobby rated' ?

I understand that the motor 'fan' on the Champion V20 is some bent sheet metal at the bottom end of the motor, and that indeed it doesn't produce significant airflow. Clearly a poor design, all for the sake of a few pence spent on a properly moulded fan and ventilated motor cover.

As this is how the machine is designed to work and since it is still in the warranty period I will try and keep the motor temperature down with a desk fan for the moment. Chester have been unable/unwilling to tell me what the maximum operating temperature for the motor is, so based on advice here I will stick to 50 C.

Thank you all for your contributions. I am now in the market for a robust and reliable bench mill by a supplier that understands and stands behind their product - any suggestions ?

You could look at a Wabeco but you do get what you pay for as they cost about £4000 for the high speed version