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Brushless motor question

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Keith Matheson19/12/2020 14:52:49
41 forum posts
16 photos

I have just discovered after about 3 years of use that I can get the Chuck rotation down to zero in a fraction of a second when I just crank the potentiometer anti clockwise. In the past I would hit the stop button and wait for the Chuck to slow down over a number of (wasted/waiting) seconds. I must say I prefer the almost instant stop so I can then go onto what ever is the next stage. My question is, does this aggressive reduction to zero rpm do any harm ( short term/long term) to the motor or electronics? Thank you in advance.

Martin Connelly19/12/2020 15:00:48
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1999 forum posts
214 photos

It would be a poor design that didn't cater for that. It would only take a couple of clamping diodes worth very little to dump any overvoltage from the motor. You can buy safety systems for machine tools that either short across motor windings or inject a DC current into the motor to act as an emergency stop. It may be that the electronics for the brushless motor does something similar to slow it down if it is running faster than the set speed.

Martin C

not done it yet19/12/2020 15:15:17
6504 forum posts
20 photos

down to zero in a fraction of a second

Guessing this is a DC drive? It might help if machine details were included. Some chucks must not be stopped that quickly depending on the machine design. I am guessing this motor is of OEM?

Howard Lewis19/12/2020 15:24:31
5734 forum posts
13 photos

Control boards tend to be none too keen on the motor being stopped or started with the speed control set to anything other than Zero. Manuals often warn against this practice, for just that reason.

Blowing control boards is an expensive hobby!

Usually, machines with such a speed control facility have flange mounted chucks. A screw on chuck, unless provided with some form of retention could unscrew under fierce deceleration. Being chased round the shop by a "wild" chuck can be damaging to the machine, and to one's health.

My advice would be to slow the machine, gradually, using the speed control. A rapid stop after cutting hard, gives little time for the motor to dissipate the heat built up by hard work.

Howard

JasonB19/12/2020 16:07:27
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Well I start and stop my lathe with it in whatever speed and still going OK after 12yrs, ditto the X3 mill after about 14yrs. Brushless SX2.7 in tapping mode stops and reverses the spindle back upto speed in less than a second.

Howard Lewis19/12/2020 16:12:18
5734 forum posts
13 photos

I was repeating the advice in the manual for a S3 mini lathe with a brushed motor.

Based on Jason's experience, the control boards for brushless motor powered machines are more robust and tolerant than the earlier types..

Howard

JasonB19/12/2020 16:35:40
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The lathe and my main mill are the earlier type with brushed motorssmiley

Keith Matheson20/12/2020 08:49:21
41 forum posts
16 photos

Thank you for your replies. In answer to one post it’s a Toolco 1130 which is very similar to a warco 280. I think it’s made by Weiss In China as the wooden box it came in had a Weiss stencilled logo on it in a couple of places. It has a bolted on Chuck. Probably 3-4 years since I purchased it. So the view seems to be the lathe is simultaneously in both states. (It’s OK, it’s not OK) when I use the lathe doing this/ not doing this, the lathes wave function will collapse into either- many happy years of usage OR a bust control board. I think I will flick a coin! LOL On balance I think I will let it slow down under its own steam just in case. Many thanks

Anthony Knights20/12/2020 10:34:43
583 forum posts
235 photos

My CL300 mini lathe has a relay interlock system and a switch on the speed control pot which makes it impossible to start the machine if the speed control is not at zero.

Edited By Anthony Knights on 20/12/2020 10:35:13

Robert Atkinson 220/12/2020 10:51:35
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1145 forum posts
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When you slow or stop a free running lathe (or any electric motor driven machine) by reducing the power input to motor below that required to maintain speed, the motor becomes a generator powered by the inertia of the rotating parts. This effect is mos noticable in permanrnt magnet, brushed AC/DC and brushless DC motors. If you just disconnect the mottor windings from the power source there is no elecrical load and the machine slows through friction. In this case ath voltage across the windings can be much higher than the normal running voltage. If a variable speed drive is involved what happens depends on th design of the drive and the motor switching.
If the windings are not disconnected (this could be electronically e.g. diodes not just mechanical switching) the voltage and power from the machine inertia has to be stored or dissipated by the drive circuit. Storage is typically in the capactors used to smooth the rectified AC supply to steady DC. If these are not large enough for the load the voltage may rise too high. High voltage causes protection circuits to operate in good designs or failure in poor or cost constrained designs. Some of the energy can also be dissipated in the switching devices, diodes and resistors. Again how much can be absorbed depends on the design AND the mechanical load. You are much more likely to cause damage trying to stop a heavy flywheel from hgh speed than long bar.
Some motor drives have provision for external braking resistors if the machine has to requently slow load with lots of inertia.
The more complex drives allow you to set controlled ramp up, ramp down and on some even E-Stop speeds to suit the application. Simple ones require more care frm the operator.
You can gett even faster stopping by shorting out the motor windings, but this puts a lot of mechanical and electrical stress on parts and cetainly should not be done with screw on chucks. As a demonstration of this try turns a small brushed DC motor and then connect the leads torgether.

For simple drives on low cost lathes, reducing the speed with the speed contol is the safest option. For drives that allow programmed ramps use that setting and the stop (not E-stop) and start control inputs.

Robert G8RPI.

Tim Stevens20/12/2020 11:15:40
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1516 forum posts

My understanding of the VFD systems used on many lathe mods is that the same principles apply as are used in 'brushless' drives. After all, an induction motor is brushless, but the main difference with model aero motors and some cordless kit is the use of permanent magnets in the rotor (so you cannot turn off the magnetism when stopping).

All I can add is that suddenly stopping the motor on my Colchester Student (whether by applying the brake or overloading the system) causes the flat belt to come off the pulley. I guess that this is because the slacker side of the drive goes completely slack in such cases, and now I have tightened the belt it certainly happens less.

It seems that the degree of crowning needed on the pulley is related to the speed of the motor, so reducing the speed (using VFD controls) requires a greater degree of crowning - difficult to arrange, really.

Regards, Tim

SillyOldDuffer20/12/2020 11:54:37
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7883 forum posts
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Posted by Keith Matheson on 20/12/2020 08:49:21:

... the view seems to be the lathe is simultaneously in both states. (It’s OK, it’s not OK) when I use the lathe doing this/ not doing this, the lathes wave function will collapse into either- many happy years of usage OR a bust control board. I think I will flick a coin! LOL On balance I think I will let it slow down under its own steam just in case. ...

Don't flick a coin - understand why the answers conflict! There are a multitude of motor combinations fitted to lathes, all with different characteristics.

JasonB and I own WM280 lathes and Keith has a ToolCo1130. Although the iron is much the same, our motors and control electronics are completely different:

  • Jason's WM280 is fitted with a DC Brushed Motor and an electronic speed control that converts 240vac into pulsed DC,. Speed is controlled by altering the ratio between pulse on and off times.
  • My WM280 is fitted with a 3-phase AC motor and a Variable Frequency Drive (VFD). 240vac is converted electronically to a kind of synthesised 3-phase, and motor speed is varied by altering the frequency. VFDs often apply advanced features like regenerative breaking, wave-shaping to put more current into the motor at slow speeds, and optimising efficiency and performance across a desired range of loads and speeds.
  • Keith's ToolCo1130 is fitted with a so-called 'Brushless DC' motor. Despite the name these are more closely related to 3-phase motors than brushed DC motors, in that their coils are phase energised electronically. They might be said to combine the advantages of 3-phase and brushed DC motors - excellent torque and speed control, coupled with high-efficiency.

Three very different set-ups all worked by a speed-control knob and on/off switch. Look similar, but how it all works under the bonnet is disparate.

The PWM controllers on a brushed motor are stressed by power-cut stops and high-speed starts. Plenty of evidence of mini-lathe and small mill owners popping their control boards by smacking power on & off rather than taking speed up and down gently on the pot.

A VFD controller might be smart enough to manage both snap power-on & off operation. For example regenerative braking is often applied to take energy out of a potential accident when the operator does a hard-stop. Unlike most inexpensive PWM controllers, a VFD is likely intelligent enough to handle it. Or not!

Brushless controllers have even more options that VFDs, but whether or not they're used on a particular lathe is unknown. They might be happy.

While it's possible to give general advice about old-fashioned motors and controllers, modern set-ups are less predictable because they're more advanced. Without understanding the design and configuration used on a particular box of tricks the operator doesn't know! And nor can anyone else on the forum.

My view is Hobby Machines are unlikely to be designed for snap on/off operation. Their electronics are designed to cope with the occasional emergency stop, not to speed-up stop-starts as an operator convenience. Unwise to set the speed and then just switch on and off! It's equivalent to pulling away in a car by red-lining the engine and dropping the clutch with a bang, and then stopping from 70mph by shifting gear from 5th to reverse.

My advice is to always Start and Stop with the speed control in the lowest position because it minimises the risk of blowing expensive electronics we don't understand. Unless the manual says different! Professional machines may be OK with Mr Bean at the controls - they're designed to work fast and hard, and might well include several hundred pounds worth of bulletproof electronics.

Dave

Bob Worsley20/12/2020 12:30:45
104 forum posts

A brushless motor is one where electronics does the current switching in the stator, not a commutator in the rotor, or armature. The electronics senses where the armature is and energizes the relevant windings. Saves lots of money.

There is a disadvantage to this that isn't often mentioned. If the controller is too energetic in putting current through the stator windings then it will noticeably demagnetise the permanent magnet rotor. With an old commutator motor any amount of current can be put through the armature and field because the only disadvantage is magnetic saturation. When you stop the saturation stops. So if your brushless controller stops the rotor instantly then by definition a lot of magnetic flux has been created to counter the rotating inertia and magnetic flux, and this will demagnetise the rotor magnet. This is probably why they say turn speed to zero before starting or stopping, they know if they don't then there will be endless complaints about the motor lost its power.

All permanent magnets have a remanence. When magnetised part of the procedure is to demagnetise it back to a known point, this is the remanence. By putting more and more reverse magnetic flux through the permanent magnet and the remanence point will slowly move down the curve closer and closer to zero.

Exactly the same occurs with hand tools. If drilling say and it snatches and stops then the motor current will peak, and demagnetise the rotor magnet. A decent make will hopefully catch this current rise in time. Similarly if running a cordless drill hard for long periods then this too will demagnetise the rotor. I don't have a brushless drill but wonder what happens if you connect one with a brushed drill and turn both on, which one wins, which one heats up?

I would have thought that all these type of controllers have a soft start and ramp up and down time? Obviously not.

Andrew Johnston20/12/2020 14:40:44
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6390 forum posts
682 photos

Induction motors and brushless DC motors are not the same.

An induction motor is an asynchronous motor, ie, the motor does not run at a speed equivalent to the applied frequency, but slightly slower. The speed difference increases slightly with increased load, and it is the speed difference that generates the torque. A VFD drives an induction motor with a PWM signal where the PWM width varies from pulse to pulse to simulate a sine wave.

A brushless DC motor is simply a DC motor without the commutator. The mechanical commutator is replaced by electronic switches. Like a brushed DC motor the brushless motor is a synchronous motor, ie, the motor turns at a speed commensurate with the applied frequency - no speed difference. The applied waveform is either on or off for a period each cycle. There is no need for PWM as with an induction motor.

The electronics within a VFD could drive either an induction motor or a brushless DC motor, but the control software is very different, with the induction motor being significantly more involved mathematically.

Things like braking resistors and DC injection braking can be applied to both types of motors, although there are are differences. The quickest way to brake a DC motor is to reverse the applied voltage for a short period. The reverse voltage needs to be removed before the motor starts turning the other way. The same can be done with a induction motor. When I change direction on my repetition lathe I simply flick an electrical switch, the motor grunts, and changes direction all in less than a second.

It's a moot point as to whether the DC motor controller on a hobby level lathe would have these advanced features. In theory turning the speed control down quickly should be fine. Effectively it reduces the frequency of the switching waveforms leaving the motor out of sync. The out of sync magnetic fields have the effect of braking the motor until it's back in sync. But it depends upon ones faith in the designer of the original controller. Reading the various tales of woe on this forum I wouldn't trust the designer further than I could throw him. smile

Andrew

Ed Dinning 120/12/2020 21:29:03
29 forum posts

In a brushless machine the speed is set by the frequency of the driving waveform and the rotor locks to this frequency with a pretty high torque. On commercial machines this is often exploited for tight control of axis positioning. This causes motor heating and on those machines the motor is sized for it.

On a hobby machine there will not be so much "meat". Some of the energy will go into regen or freewheel diodes and some will still be lost in the motor windings.

Provided the speed is not taken from max to min with great rapidity there should be no problem, keep an eye on motor temps.

I always find it easier to turn the speed control up and down at starting and stopping, perhaps over about a second as it gives a smoother feel to things.

On older induction motors fitted with emergency stops it was the practice to inject DC into windings. This would give a PDQ stop and cause quite a lot of motor heating. A BLDC motor is a quite different beast

Ed

Martin Kyte20/12/2020 22:26:06
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2634 forum posts
46 photos

Does it have a screwed chuck? If so have you managed to spin the chuck off doing that?

I would have thought there was a chance of the spindle stopping (as you say in sub second range) and the chuck carrying on rotating.

regards Martin

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