DC motor RPM question

[John HaineParticipant @johnhaine32865]

Time in seconds, inductance in Henries. Apples and oranges? If you are not precise you get wrong answers.  To get seconds from Henries you need to divide by a resistance. The input/output voltage differential also comes into the equation.

[Robert Atkinson 2Participant @robertatkinson2]

Hi John,
If you understand that why did you ask the question?
I was trying to keep it simple and correct.
The resistance is not a significant part of this, it the difference in current over time as the voltage changes relatively rapidly.
While I would not say I’m an expert on the subject but I have designed PWM motor controllers at component level for professional equipment as part of my day job.

Robert.

[SillyOldDufferModerator @sillyoldduffer]

Re Robert and John’s discussion of pulses and inductance, I plead guilty to starting the debate by over-simplifying!  Didn’t want to confuse the principle that speed control is achieved by altering the power available within a time slot, not by altering the voltage.

In mitigation, I was trying to answer Henry’s follow-up question:  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?

I thought it sufficient to outline how Pulse Width Modulation works in principle.  What I said is correct for a resistive load, but capacitive and inductive loads are more complicated, because they store energy, introducing another time factor. Quite difficult, and maths heavy.

From his questions I believe Henry’s taking an intelligent interest in how controllers and motors work in the context of choosing a motor & controller for his lathe.  If so, a crash course in impedance is probably over the top!  Maybe later, it’s an interesting subject.

Dave

 

 

[John HaineParticipant @johnhaine32865]

Robert, I don’t think I did ask a question, just made an observation.  With a PWM motor driver one is a bit stuck with the motor inductance, rather than being able to choose an optimum. But a flywheel diode comes to the rescue to keep the current flowing while the PWM voltage is “off”.  If the driver is in a closed control loop the motor inductance may introduce a significant time constant as it will limit how fast the current can increase or decrease in response to a change in duty cycle.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

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.

On Peter Cook 6 Said:

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!!

I realize I don’t need 209 watts and don’t want the motor to get red hot. So, if i use a speed controller and set the RPMS to 2430 RPMs and only take light cuts wouldn’t the amp draw be

On Robert Atkinson 2 Said:

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.

If using a motor controller, I set the speed at 2834 RPMs and took light cuts, would that reduce the current draw well below the 8 amps specified in the spec sheet, and the motor wouldn’t get red hot?

[Henry RancourtParticipant @henryrancourt22682]

I did a search for the differences between brushed DC servo motors and basic brushed DC motors and found a key difference. Servo motors have three wires, one is for position control, and basic motors have two wires. So. in an earlier post I was told “The Torkmotion motor is a servomotor. While it will work it’s not optimal.” So is the Torkmotion motor actually a servo motor, and does “not optimal” apply to this two wire motor?

[Robert Atkinson 2Participant @robertatkinson2]

Hi,
Not all servo motors have an intergrated feedback signal.
By not optimal I mean that the inevitable compromises in motor design are biased to servo use rather than constant speed. Servo rated motors generally carry a price premium too.
If you have a servorated motor already it will work for this application, but if you are buying a new one it may not be the best choice.

Robert.

[Henry RancourtParticipant @henryrancourt22682]

It appears that that motor has internal brushes and I’ve read they are very difficult to replace, so internally brushed motors are off my buy list.

[Henry RancourtParticipant @henryrancourt22682]

I’m considering buying a torkmotion brushed DC motor but I concerned with the 115C temp. rise and was told “Your motor “can cope” with a continous (ie all day long) current of 2.6A but it will get hot (115° above ambient), the speed at which you make it pull those 2.6A doesn’t really matter, be it 435 or 4350rpm. However, at 4350rpm and 2.6A (0.21Nm) your motor will put 96W of mechanical power and produce (2.6*2.6*2.7=18W) of heat. At 435 rpm and 2.6A your motor will put out 9.6W of mechanical power but STILL produce (2.6*2.6*2.7=18W) of heat.

I like all the motor parameters except for the high heat, and I don’t understand the above formula showing 18W of heat. But wanted to know the heat generated by 18W. So I found an online watts to temp. calculator (https://calculator.academy/watts-to-temperature-calculator/) and entered thermal resistance 4C/W, from the spec sheet, and 18W from your calculation. The result was a temp. rise of 70C. Is the calculator appropriate for this application and is the result valid?

 

 

[Robert Atkinson 2Participant @robertatkinson2]

Duplicate thread. See : https://www.model-engineer.co.uk/forums/topic/motor-temperature-question/#post-835867

[Henry RancourtParticipant @henryrancourt22682]

Thanks for catching that. Is there a way to delete that post?

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