DC motor RPM question

[Henry RancourtParticipant @henryrancourt22682]

I’m looking for a DC motor for my watchmakers and found this Torkmotion motor and spec. list. It lists RPM nl = 5000 RPM,  followed by RPM 1= 4350 and RPM 2 = 2834 RPM and the torque and power is different for each RPM. If I wanted to use it at RPM 2, so it will provide 209.6W, does the RPM have to be set at that speed using motor controller?

And, I’ve tried attaching a jpeg of the spec sheet by dragging the file into the attachment area and it shows up below, but when I do a preview it’s not visible. What am I doing wrong?

[Peter Cook 6Participant @petercook6]

Not sure what you will be turning on a watchmakers lathe that will need 209 watts of motor power.

You are misreading the spec sheet. At 48v with no load that motor will run at 5000 rpm. As the torque load on the motor increases from zero the speed will fall, and the current drawn increase. RPM 1 and RPM 2 are simply data points on the manufacturers (theoretical and ideal) load speed curve at 48V. Without more detail it’s impossible to say how the torque speed cure will shift as you reduce the voltage.

For a watchmakers lathe motor that you want to run at say 3000rpm max, a 100w 3000rpm motor will be more than adequate. Something like this .

PS the seller of that motor on Ebay rates it at 96W not 209. 209 Watts is the power consumption when the torque drags its speed down to RPM2. Given that at T1 the spec sheet says the temperature will rise to 115C above ambient, at RPM2 its probably going to glow red hot!!

[Robert Atkinson 2Participant @robertatkinson2]

The Torkmotion motor is a servomotor. While it will work it’s not optimal. The one linked by Peter is a better choice.
See my post on this topic for some speed controller ideas

dc Voltage step down converters

Note you will want ta 24V supply with the motor suggested by Peter.

Robert.

[Henry Rancourt]

On 26 January 2026 at 17:08 Henry Rancourt Said:

I’m looking for a DC motor for my watchmakers and found this Torkmotion motor and spec. list. It lists RPM nl = 5000 RPM,  followed by RPM 1= 4350 and RPM 2 = 2834 RPM and the torque and power is different for each RPM.

True of all electric motors.  Though related torque and power aren’t the same thing.   For a given motor and load, the relationship is best expressed as a graph.  Your table gives a partial view.

The figure that matters is Typical Output Power, with the sweet spot occurring at 4350rpm, and a load of 0.1Nm, when the power is 96.5W.   This is a 100W motor, intended to run at Ambient + 115°C.

However, the motor has a peak output of 209.6W with an exceptionally heavy load of 0.71Nm.   This the power output if you hammer it, which should only be done briefly.  The motor draws 8A in, way over the typical 2.6A, and will soon overheat.   If you really want to burn it out, the table says a load of 1.63Nm will stall the motor, drawing 17.8A at zero RPM, with most the power turning into heat.  Don’t stall the motor and leave it cooking.

The numbers mean the motor is flexible enough to run a small watchmakers lathe.  It will:

• take light high speed cuts at 5000rpm.  Not deep heavy cuts.
• perform best at 4350rpm, 96.5W, perhaps a little underpowered, but not unreasonably so for watchmaking.  The Cowells 90 lathes have a 125W motor @ 4000rpm – similar.    Sherline smaller from memory.
• Cope with brief overloads, able to take heavy deep cuts of short duration.  Don’t overdo it because the motor will get hot, and it might stress  the speed controller and PSU,

Much depends on the pattern of work.   My lathe, not watchmaking, tends to work not too hard in short bursts.  I take fairly heavy cuts, deliverately not pushing the machine to the limit, and then the motor cools down whilst I have a think, reset the job or change the cutter.  The average power is low compared with the peak.  Others take long protracted deep cuts at high feed rates in order to shape a large lump of metal quickly. That’s much more stressful on the motor because the average power is persistently high, and a heavy handed operator in a hurry risks burning out a hobby motor and controller.

I perceive watchmakers doing genteel precision work, not hacking metal for hours on end?

 

If I wanted to use it at RPM 2, so it will provide 209.6W, does the RPM have to be set at that speed using motor controller?

Yes, but don’t for the reasons above.   Set the RPM to whatever you like, and take sensible cuts that don’t overload the motor.  I listen to mine, the cut should be applied such that the motor is heard to take up the load, without sounding unhappy.  If 210W is persistently required, buy a bigger motor!  Or a bigger lathe.

And, I’ve tried attaching a jpeg of the spec sheet by dragging the file into the attachment area and it shows up below, but when I do a preview it’s not visible. What am I doing wrong?

Probably nothing. A forum quirk sometimes causes images, and posts, to appear later rather than sooner, and perhaps disappear temporarily.  I can see what you posted, so it did work, even if it glitched for you.  Something to do with buffering, but I can’t pin it down.

Dave

[Andrew TinsleyParticipant @andrewtinsley63637]

Not sure I would want to run my Boley watchmakers lathe at some of the talked of speeds.

Andrew.

[Henry RancourtParticipant @henryrancourt22682]

Do PWM dc motor controllers actually change the voltage to the motor. Using my fluke meter, would i see the voltage change as the speed is changes?

[Michael GilliganParticipant @michaelgilligan61133]

No … they do Pulse Width Modulation

The peak voltage is constant, but the mark/space ratio is varied, resulting in a lower average voltage.

MichaelG.

[John HaineParticipant @johnhaine32865]

An old fashioned moving coil meter probably would show a varying voltage as it will effectively average it.  The same is probably true of a true rms digital meter, the m in rms standing for “mean”. Ordinary digital multimeters can give fluctuating readings on a pulsed signal but if you add a resistor and capacitor to do the averaging you would see a steady voltage that varied with the mark/space ratio.

[Julie AnnParticipant @julieann]

Ordinary digital multimeters (DVM) use dual slope ADCs, which are essentially integrators. The integration time will normally be significantly longer than the pulse repetition rate of the PWM waveform. So the DVM will show a varying DC voltage as the PWM ratio is varying.

Julie

[Henry RancourtParticipant @henryrancourt22682]

I’m currently using a 48V PSU and a DC 6-90V 20A controller with a 24V brushed motor. When I measure output from my DC 6-90V 20A controller, while changing the speed, the voltage is always 48V. I want to power the motor with 24V so I’m planning to buy a different controller. Do PWM dc motor controllers, or some other type of controller actually change the voltage to the motor. So, using my fluke meter I would see the output voltage change as the speed is changed?

[Andrew Tinsley]

On Andrew Tinsley Said:

Not sure I would want to run my Boley watchmakers lathe at some of the talked of speeds.

Andrew.

On Henry Rancourt Said:

Do PWM dc motor controllers actually change the voltage to the motor. Using my fluke meter, would i see the voltage change as the speed is changes?

It depends on the specification of the meter. If it is a True RMS model it will read the voltage correctly. A standerd meter will give you an idea but will not be accurate. This is one case when an old fashioned analogue meter can be better. It will average the pulses in a similar way to the motor.

A “switch-mode” power supply uses PWM and an inductor to store energy and provide a constant voltage output. They also have capacitors to futher smooth the output to a constant DC voltage”. But a lot of loads do need a constant output. Examples are motors (thay inherntly have inductance and also mechanical inertia, heaters (thermal inertia) and lights. Our eyes are not fast enough to see the pulses with LEDs and the old filament lamps also have thermal inertia. Additionally our eyes do not have a linear response and a pulsed LED light will appear brighter than the same LED on constantly at the same average power. e.g. 5W pulses on for 20% of the time (1W average) looks brighter than 1W on constantly.

Robert.

[Henry RancourtParticipant @henryrancourt22682]

The Fluke 117 is a true RMS meter.

[Henry Rancourt]

On Henry Rancourt Said:

The Fluke 117 is a true RMS meter.

Then it should read the RMS value of the PW modulated wave

MichaelG.

[Michael Gilligan]

On Michael Gilligan Said:

Then it should read the RMS value of the PW modulated wave

MichaelG.

Correct, and the rms value will be lower than the average value as there is a square root in the equation.

Julie

[Henry RancourtParticipant @henryrancourt22682]

My question in my earlier post was meant to be…what type of controller, using my fluke meter, would I see the output voltage change as the speed is changed? For clarification, I meant when it is changed via the DC controller.

[Julie Ann]

On Julie Ann Said:

Correct, and the rms value will be lower than the average value as there is a square root in the equation.

Julie

The RMS value will be higher than the average value.

[Michael GilliganParticipant @michaelgilligan61133]

For comprehension of RMS

.

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[many other tutorials are available, but this pretty good]

MichaelG.

 

[Henry Rancourt]

On Henry Rancourt Said:

My question in my earlier post was meant to be…what type of controller, using my fluke meter, would I see the output voltage change as the speed is changed? For clarification, I meant when it is changed via the DC controller.

Any.
The motor will respond to the voltage at its terminals so any change in the speed controller resulting in a change of speed of a brushed DC motor will also result  in a change in the voltage measured at the motor terminals.
In simple terms a DC motor will also act as a generator. Ignoring lossed, when running as a motor the  speed of a DC motor idetermined by the voltage generated at that speed. The internal voltage balances the incoming voltage and the motor speed is stable. The current provides the torque and thus power. A electrically lossless motor has no resistance so the voltage differencerequied to produce the current is also zero. For a paractical motor the resistance and thus voltage difference is also low. You can use this to stabilise the speed of a DC motor aginst changing load by increasing the voltage as a the current (load) increases. If the resistance of a motor is 1 ohm and we arrange the control control circuit to measure the current and increase the voltage supplied to the motor at a proportion of 1V per amp  the motor speed will remain constant regardless of load change. In practice you would set the increase to slightly below unity to ensure stability. Note that if you connected this circuit to a resistor or similar load it would immediately run to full output.
Some more sophisticated controllers measure the motor speed by measuring the voltage generated by the motor when the PWM switch is off.

Robert.

[peter1972]

On peter1972 Said:

The RMS value will be higher than the average value.

Ooops, you’re correct, my mistake. Lesson learnt, don’t post after having a glass of wine. 🙁

Julie

[Henry Rancourt]

On Henry Rancourt Said:

My question in my earlier post was meant to be…what type of controller, using my fluke meter, would I see the output voltage change as the speed is changed? For clarification, I meant when it is changed via the DC controller.

A linear controller, such as a rheostat, changes the voltage in a way a multimeter can read.  The meter, and operator, doesn’t have to cope with complications like RMS (sine waves), or PWM (Pulse Width Modulation).

Apart from being easy to understand and measure, linear controllers have few advantages.   The motor develops power proportional to the amps it gets, and the amps are proportional to the volts, which are constrained by the controller.   Unfortunately, dropping the voltage very wastefully converts electricity into unwanted heat, and dropping the voltage slugs the motor, reducing torque.  Not good.

Pulse Width Modulation is much better:

• Speed is controlled by altering the time amps are available to the motor rather than throttling the amplitude of the current by altering the voltage. Instead,  DC is chopped electronically into ‘n’ slices per second, so the motor gets the average.  The duty cycle is anything between 0% (OFF) and 100% (Flat Out).
• As each slice is either fully ON, or fully OFF, the controller wastes very little energy.
• The performance of the motor is improved considerably at low speeds because all the amps from the power supply are available to the motor for the duration of each pulse.  More torque.

This graph shows an example PWM waveform.

Not many multimeters can measure RMS, and few have the electronics needed to measure frequency and duty cycle, essential to make sense of PWM.   A Fluke 77 is the wrong tool.  An oscilloscope is needed; it displays the waveform allowing the peak voltage, pulse width and duty cycle to be measured.

Although interesting, rarely necessary to understand the detail if the requirement is simply to select a motor and controller for a small lathe.

Most watchmaking lathes are driven by small motors, between 50 and 200W, with enough RPM to spin the mandrel at the lathe’s top-speed, usually via a belt.   Widely available, new and second-hand. The Universal Motor from grannies sewing machine will do.  Top-speed depends on the lathe, and can be surprisingly high because watch parts are tiny!  Fastest I’ve seen claimed 7000rpm, though I believe most watchmaking lathes are considerably slower.  The top speed of a Cowells ME90 is 2100rpm.  Over-speeding and overloading bearings is a good way of wearing them out quickly, so don’t rush to drive them with a big fast motor.

PWM controllers for DC motors are bog-standard, and very affordable.  Buy one to match the motor’s power input and voltage.

The PSU is anything that provides the right voltage and current.   In recent years I’ve taken to LED lighting PSUs. Not ideal, but delightfully cheap because they’re mass-produced. So far no failures.

I wouldn’t get wrapped round the quality/reliability axle.   If anything doesn’t work on arrival, send it back, and if a component fails in the future, replace it. Unlikely better is needed unless something special is planned.  How much work is the lathe expected to do?

Dave

 

[John HaineParticipant @johnhaine32865]

I believe that the watchmaking variant of the Cowells, 90CW, can be run much faster as it has much better bearings.

[Robert Atkinson 2Participant @robertatkinson2]

Being pedantic, which I am, I have to take issue with the statement fronm Dave SOD:
“Speed is controlled by altering the time amps are available to the motor rather than throttling the amplitude of the current by altering the voltage.”
That would only be true if the speed at which the PWM switches is low in comparison to the inductance of the motor.  This would be akin to powering the motor, letting it coast and powering again and relying on inertia to smooth out the variation.
In practice the PWM switching speed (frequency) is high compared to the inductance of the windings. The inductance stores energy when PWM switch is on and releases it when the switch is off. Thus it avaerages the available power. The torque advantage only comes if the supply voltage is higher than required and the motor resistance is high. This allows full current to be developed more quickly.

Robert.

 

[John HaineParticipant @johnhaine32865]

Not sure you can compare a pwm switching time with an inductance! But if the stored energy in the inductance is to be released when the switch is off the circuit will also need a commutation diode across the motor.  The little MFA speed control module I have somewhere certainly has one.

[John HaineParticipant @johnhaine32865]

<p style=”text-align: left;”>The motor back emf also has an important affect.  The winding resistance should be very low.</p>

[John Haine]

On John Haine Said:

Not sure you can compare a pwm switching time with an inductance! But if the stored energy in the inductance is to be released when the switch is off the circuit will also need a commutation diode across the motor.  The little MFA speed control module I have somewhere certainly has one.

Of course you can. It’s a major consideration in switched mode power supply design. The larger the inductance the longer time it takes to reach maximum current / saturation. High frequency PWM means smaller inductance. It’s also why a lot of cheap ebay switchmode regulator modules don’t meet their sales specifications. They tend to quote the full voltage range and maximum current from the integrated circuit datasheet but use the inductor value from the “typical” or “test” circuit. You need to select the correct inductance value for the operating conditions (Vin Vout and Iout) for optimum performance. Most chips won’t operate properly over their full range of voltage input and output with a single value of inductor.

Robert.

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