Overloading a Chester Champion V20 Mill

[Peter de GrootParticipant @peterdegroot14065]

Thank you Mike. I may be missing something but there does seem to be something very daft about a machine whose virtue is variable speed but where the slow speed cannot be used because the motor overheats and its fan is ineffective at that speed! Sounds like mechanical speed reduction is a superior technology to its successor of electronic vairable speed. I also seems odd that the motor does not have any cooling ribs as many do, and that it is encased in a barely ventilated metal box. Some may say it's a poor design. I guess none of this matters for a 'hobby machine' which apparently is not expected to be used. Anyways some temp measurements first….

[Anonymous]

Posted by Peter de Groot on 03/09/2013 19:00:55:

I may be missing something but there does seem to be something very daft about a machine whose virtue is variable speed but where the slow speed cannot be used because the motor overheats and its fan is ineffective at that speed! Sounds like mechanical speed reduction is a superior technology to its successor of electronic vairable speed.

Peter: No, you're not missing anything, but people do seem to be strangely seduced by the 'ease' of changing speed by twiddling a knob. Of course from a manufacturing point of view it is much cheaper to provide a simple electronic speed controller, especially for a DC motor, as opposed to a belt or gear reduction system.

I much prefer a gear or belt speed change, so you get full power at low speeds, not just full torque. There are two exceptions in my workshop. My CNC mill uses an inverter and induction motor, but even that has a belt change for high and low ranges, and my cylindrical grinder uses a variable speed DC motor for the workhead.

If it's any consolation the same sort of woes befall some of the variable speed commercial CNC mills with regards to available HP as opposed to marketing numbers.

Regards,

Andrew

[Peter de GrootParticipant @peterdegroot14065]

Thank you Andrew, I have been thinking about variators, simple yet versatile, and if it works for a Bridgeport and scooters….

[Anonymous]

Peter: I have a varispeed head on my Bridgeport. It's a bit noisy at high speeds, but seems to work well. I'm happy with it, although I know some people don't like them, and prefer the earlier stepped belt heads.

Regards,

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Just a couple of notes :

(1) Don’t start thinking that good speed control by electronic means can’t be done .

There are direct drive motors on machine tools which will do anything from precision step indexing to running smoothly at any speed from 0 to 30000 rpm and just keep on working 24/7 without any problems .

Various motor technologies are available and they are not usually all that much more sophisticated than the ones used on cheap machines .

Favourites are :

(a)Basically high power stepper motors with more than two phases . Five phases is common .

(b) Almost commonplace motors but with two or more sets of windings that can be selectively switched in . This derives from the concept used for years of having two motors to drive machine spindle – one high speed and one low speed .

Electronic controls are sophisticated and usually digital but in terms of component count not that complex .

(2)Washing machine makers do quite well with variable speeds – they have electronic motor controls but real secret is the very large speed ratio between motor and drum – motor is always running adequately fast and is designed to go very fast indeed for spin . This would be a good way to go on cheap machines – high speed motors and large speed reductions .

MikeW

[David ColwillParticipant @davidcolwill19261]

I had always assumed that the reason industrial CNC machines used such seemingly huge motors and drives was so they could get usable torque at low speed without having any gearing. The CNC router at work has a 20kw spindle motor which presumably gives enough power at the lower speeds. I would be interested to know if this is the case.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi David ,

Just so – high power motors are used so that there is still useful power available at lower speeds .

The term ’20 KW ‘ only really applies to a fairly narrow band of running conditions .

Suppliers of motors for more exacting purposes supply graphs of motor characteristics showing available torque and output power against input current and running speed .

On the graphs are cut off lines dividing the characteristics into zones of acceptable and unacceptable running conditions .

Regards ,

MikeW

Edited By MICHAEL WILLIAMS on 05/09/2013 14:34:24

Edited By MICHAEL WILLIAMS on 05/09/2013 14:48:13

Edited By MICHAEL WILLIAMS on 05/09/2013 14:49:46

[MuzzerParticipant @muzzer]

Most motors intended for variable speed operation will generate their max torque up to "base speed" which is typically 1500rpm for motors designed for 50Hz. Beyond that, they are typically capable of constant power until the back emf approaches the available voltage, beyond which point you need "field weakened" operation.

In the constant power region, the torque falls off as the speed rises, given that power = torque x speed. The speed range for constant power is rather limited for fairly conventional machines. Once you start looking at wider ranges of constant power speed, you need more complex and expensive inverters and control schemes. For instance, for a hybrid electric vehicle, the engine speed may be 1000-6000rpm, so designing a motor and inverter for that application is more challenging than for a machine tool spindle motor that may have a more limited range of operating speeds.

To answer the original query, you can buy fairly standard industrial motors that have an independently powered fan on the back end. In this way you can actually operate the motor with high torques and low speeds without overheating the motor. And many industrial motors will have a temperature sensor which will allow overtemperature protection amongst other things.

My lathe and milling machines both have variable speed drives but I generally try to get the speed range roughly right with the belt or gearbox and then modulate the speed around that set point. Trying to operate the motor much above or below its base speed would result in significantly limited power. In both cases the original motor power rating is fairly well selected for the application (and assumes the presence of a gearbox) so unless you want to pussy foot around, this is the only sensible way to shift respectable metal.

Muzzer

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Another note :

A measure of the quality of an electric motor is simply the amount of metal used for a given power .

Better class motors use larger section copper in the windings giving less heating and better heat conduction together with more iron in the rotor and / or stator giving more efficient magnetic circuit and again better heat conduction .

Some motors of course have aluminium and pemanent magnets in their construction as well but similar principles still apply .

It’s a bit sad really but a quite small increase in metal content is all that is needed sometimes to change a poor motor design into a good one .

MikeW

[MuzzerParticipant @muzzer]

Other factors that affect performance, cost, quality, life etc:

• Quality of steel used in laminations
• Thickness of steel laminations
• Active cross section area of steel used in magnetic path
• Winding window area
• Fill factor of copper windings in the winding window
• Quality (purity) of copper
• Geometric tolerance of parts
• Size of air gap (tolerance etc)
• Balancing of rotor
• Rotor construction (partic induction motor)
• Bearing quality and life
• Cooling effectiveness
• etc

A lot of things to get right!

[Peter de GrootParticipant @peterdegroot14065]

Gentlemen thank you all for your contributions, I did some measurements today with interesting results which I will post in a new thread.

[Author]

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