DC Motor Search

[badvibes]

I am in the process of designing a high speed rotary surgical hand-piece (drill). The hand-piece must be a DC motor (brushed or brushless). The specifications of the hand-piece are as follows:
1)0 - 45,000 rpm (variable speed)
2) ozf-in torque through-out entire speed range
3)battery source to be disposable or rechargeable can be thrown away if cost is low enough
4)Duty cycle is 5 one minute cycles (1 minute on, 1 minuteoff)
5)DC motor must be apporximately 3/4" in diameter
6)Battery source to accomodate the current drain and supply proper voltage for speed

My question are:
1) Can anyone refer me to a good website for the motors?
2) Is this possible to do with a brushed motor or do I need to go brushless with an electronic control
3) Can anyone recommend a method of benchtop testing for current drain while a torque is applied to the motor?

Thanks in advance for your help.

~Bob

 

[blacksea]

Bob,
of course a motor must be DC brushless (with Hall or sensorless) for such high speed. I hope that Maxon motor (

http://www.maxonmotor.com)

EC 16 or EC 22 would be suit for your application.

 

[blacksea]

Hi Bob,
of course that must be DC brushless (with Halls or sensorless) motor. Review Maxon website (

http://www.maxonmotor.com)

- EC 16 or EC 22 would be suit for your application.

 

[badvibes]

Hi Blacksea,

Thank you for your reply!

I have looked into using a Maxon motor but they don't seem to have the an EC16 or EC22 motor with a power rating that fits my reqquirements. I did the following rough power calculations that do not include losses:

5 ozf-in = .026 lbf-ft
(45,000rpm * .026lbf-ft) / 5252 = .220 horse power
.220 hp * 746 watts/hp = 165 watts

The highest output power for Maxon EC16 and EC22 does not come close to my requirements.

Perhaps I am not approaching this calculation correctly.....

If anyone can offer some advice, it would be greatly appreciated.

Thanks again blacksea.

~Bob

 

[israelkk]

The motor works only one minute out of a two minute cycle therefore, the maximum power consumption is half your calculations. I assume that it will not operate all the time at 45,000rpm. You should consult the surgents how long it will be used in a session and how the speed will change and what duration for each speed. Then calculate the highest temperature the motor will reach during a surgury session if it is lower than the maximum Maxon allow then you are OK. You will find the formulation for temperature rise calcutation in Maxon literature.

Other similar manufacturers are Minimotor and Escap.

 

[badvibes]

israelkk,

Thank you for your reply.

I am not sure if you misunderstood the duty cycle of the instrument but it is for a total of 5 cycles - 1 minute on and then 1 minute off - for a total of 5 minutes of operation.

I took your advice and spoke to the surgeon. He told me that the profile of the duty cycle can have a pattern such as this:

1) 2 ozf-in for 15 seconds for 3 cycles
2) 3 ozf-in for 45 seconds for 3 cycles

The 2 ozf-in = .0104 lbf-ft - multiply this torque by 45,000rpm and convert to watts you get 67 watts.
The 3 ozf-in = .0156 lbf-ft - multiply this torque by 45,000rpm and convert to watts you get 100 watts.

I then multiply the 67 watts by the .25 minute (15 second) duration and multiply that by the number of cycles to get 50 watt-minutes.
I then multiply the 100 watts by the .75 minute (45 second) duration and multiply that by the number of cycles to get 225 watt-minutes.

When you add these watt-minutes together you get 275 watt-minutes. I guess this value would be useful for selecting my battery source. But it still seems as if I need a motor that can output at least 100 watts, which is the higher power output between the two cycles.
I am assuming this approach might be correct, but I still don't seee any motors that have that high of an rpm and/or that high of a power output.

Any comments?

~Bob

 

[israelkk]

The Watt output in the motor data is for continuous use. In your case the working session is very short and the motor own heat capacitance take place meaning it is a transient case and you may find that even after the 5 cycles the final motor temperature is lower than the maximum allowed. You should calculate the final temperature as the result of the working profile instead of looking just at the power figure which is for a steady state case.

 

[badvibes]

israelkk,

The maximum temperature allowed seems to be misleading on the specification sheets. One motor gives the maximum coil temperature as 125 degrees celcius. Then they give a thermal resistance value of 13 degrees celcius per watt. If I still require 100 watts then this would be 13 * 100 = 1300 degrees celcius???? I was trying to avoid buying a motor and doing benchtop testing, but it seems as if this might be the best method to test for torque, speed and temperature.

Now my next question............
Have you ever set two dc motors up where one motor has electrical power to it and its spindle is mounted to the spindle of another dc motor? The "free" dc motor gets an adjustable resistive load across its poles and measurements of current draw, voltage drop and the resulting torque and rpm drops are recorded.
Does this sound like an accurate method of testing dc motors? The "free" motor becomes a generator of sorts but will this work with a brushless ec motor or do they have to be brushed?

Please advise?

Thanks!

~Bob

 

[blacksea]

Bob,
pay attention that thermal resistance noted by manufacturers isn't take into account heatsink ot cooling method used with practical application.
Yes, using other motor for loading of tested motor for such high speed is simply only possible solution.

Pls reply your email and we'll continue our disscussion more effectively.

 

[israelkk]

To your first question you should take also the time constant of the motor. It takes time for the motor to heat. A test is a good approach too, but to my opinion as a measure to validate your calculations which should not be too complcated.

To your second question, yes but not exactly! We measured the tested motor parameters from it's speed (using a stroboscope) and the voltage output at its terminals as the result of speed and torque from the driving motor. The voltage output of the tested motor is Voltage=-Ke*omega.

Where:
omega=motor speed
Ke=Motor electrical constant

Knowing omega and the voltage we can calculate Ke. From Ke we know Km - the motor mechanical constant. However, for the driving motor you should have the exact and known parameters (Ke,Km Etc.) not just the catalog typical values.

In our case where the motors were used for critical application we received them with a testing report for each motor for the first batch. Then we used motors from the first batch as the driving motors for follow orders of motors. We used brushed permanent magnet DC motors.

I am not sure about brushless motor I think it is much more comlicated because of the way they are electrically driven (PWN etc.).

 

[blacksea]

israelkk: do you suggest DC brushed motor for high speed (up to 40000RPM) application really? Even is as loaded motor?

 

[israelkk]

blacksea

I did not refered to the speed limit I just responded to the heat calculations and motor test possiblily. I will go with the manufacturer data as to the maximum speed allowed.

 

[israelkk]

blacksea

Just wanted to add that a speed increase gear can be used if the motor speed is lower than the desired speed.

 

[blacksea]

israelkk:
I don't think that geared load motor is good idea. Also I never heard about mechanical multiplexer (gear with increased ratio) with output speed 40000 RPM.
Please pay attention that we talk about specific application.

 

[sreid]

Take a look at Aveox. Their R/C motors have specs.

http://www.aveox.com/