Hello,

I am now making a motor driver for a 200W DC brushless motor from Maxon for a robot application. I would like to ask some details for the design, specifically the starting current.

I am following the driver presented by Microchip for the dsPIC with slight changes. The link is:
http://ww1.microchip.com/downloads/en/appnotes/00857a.pdf

The motor I have to use is:
The motor is the EC-4pole 30 Ø30 mm, brushless, 200 Watt, 24V.

I have implemented this same circuit before for a smaller motor (100Watt) and works almost perfect.

This motor has a starting current of 236A and the nominal current around 10A. I think that the mosfet that I should use should have a Pulsed Drain Current higher than the starting current of the motor, but I am not sure of this assumption. I am wrong?

The mosfet P (IRF5305) shown in the application note of Microchip basically is below this range (-110A). Is there a way to limit the current needed by the motor in order to use this same mosfet? or is it better just to find another one that can get higher pulsed drain currents.


Thanks for the help.

 

I have implemented this same circuit before for a smaller motor (100Watt) and works almost perfect.

You said you have implemented the circuit, then you should know what you should do.

But What I would do is use PWM to slow start the motor so that inrush current can be limited to safe values.
If you want to start the motor with full rated voltage than you have to use double the starting current rating for the mosfet. There are big module ones. But I have seen IGBT modules for this purpose. Still do not think they can survive long. I recommend to slow start the motor.

 

Thanks for the reply.

Well I am in doubt of what I should do for this particular case, since the other motor I used doesn't have so large starting current. The other motor only needs 78.8A. The new one need 236A... big difference I think. So I didn't pay much attention to this before.

By slow starting the motor do you mean using the PWM as a ramp? If so how I calculate how much current is needed for start up? I am slightly confused by this starting current thing now that I am paying attention to it.

Jose.

 

No 1 I think you have made an error with that starting current (5.6Kw)?
In any case these motors are almost never started with no control, IOW the current is what ever you decide is the maximum built in to the drive.
If you are talking the peak (destruction point) torque, then this should never be reached by design.
Max.

 

set the sped ramp in the control, jerkiing the motor and gear train with full output to motor from startup is hard on the system. there should be a speed ramp setting in the control.

 

MaxHeadRoom,

The current I am referring is the starting current, which is required to start the motor. The max. nominal current after is of 10A. You can see this values in the motor datasheet:

you can find it at http://www.maxonmotor.com/maxon/view/catalog/

The motor is the EC-4pole 30 Ø30 mm, brushless, 200 Watt, 24V.

What I am afraid is that if there is no current limit built-in the circuit, then the motor will try to get the 236A at start up and break the mosfet. On the other hand if I put a current limiter, then I am afraid that if in a given moment this current is required it will not handle it.

The application is for a robot biped controlled by torque. In normal circumstances the motor should not have to make sudden changes that will requiere large torque. Thus the ramp will work fine. However, if there is a sudden hard perturbation of the robot (e.g. the robot is pushed), then the motor may requiere a relatively fast response (around 100ms), which will translate in a high current peak. So my fear is that at this moment the motor will indeed requiere this 236A because the ramp would not be slow enough. The sample rate is set to 1Khz.

I don't think this situation will ever happen in our testing scenarios, since we are going to avoid this sudden hard pushes for now and focus on posture control during standing and walking in different terrains, but it is better to be prepared.

Again, this starting current value is really confusing me and I would like to know how is the best way to proceed in the design. My solution so far is to over design it and put Mosfets that can handle a Pulsed Drain Current of more than 240A and test it to see how it behaves. But I would really like to know how to make this kind of design in advance. This will avoid me some headaches later.

Thanks!

 

As others have said, a soft start is advisable. Have you considered using FETs in parallel?

 

This might be of interest for anyone following this thread:

http://www.apexanalog.com/wp-content/uploads/2013/06/PET-Final-Article-11.08.pdf

 

The Maxon line is a line of subminiature motors and I have never seen one that is capable of 5Kw.
If you are building a drive in the typical manner, the full voltage is NEVER placed across the motor when it is stationary.
Can you specify a certain model?
As all the ones I see around that specs, do not come anywhere near that maximum power rating.
What is its terminal resistance? as this will indicate what the maximum power could ever be for a supplied voltage value.
Max.

 

I'm guessing you have some kind of speed control using PWM like shown in the chip app note. If you don't you need to. Once you have that you have a couple of options:
You can use a slow ramp up to speed using the PWM duty cycle to limit the start current. This assumes the motor will not be stalled. If you can stall it ( a jam for example) you need to sense the current and limit the PWM to some safe value - say 5 or 10 amps.

 

I found a motor on the site which appears to describe the one you are using, this motor is just over an inch in diameter,.. 7.5hp ? I don't think so!
The no load current I see is in Ma, not amps.
Maybe this is where the confusion is?
Max.

 

Some of those little hobby motors have almost no resistance. I think they are made for electric planes.

 

Maxon motors are not really in the hobby class, they are precision miniature motors and run around the $700 mark.
The typical outrunner RC type run very high current but very short periods due to the heat produced.
These can be had for under $10.00.
Max.

 

This looks like his I think. Very low resistance.

http://www.maxonmotorusa.com/medias/sys_master/8807044087838/13-202-EN.pdf

 

Attached you can see the datasheet of the motor. The part number is: 305013, The terminal resistance is 0.102ohms.

I am using the PWM and will pay attention to all the advice so far! Thanks!

@Alec, I haven't considered using Mosfets in parallel, since I am able to find a Mosfet that has higher pulsed drained current. The thing is that I am not sure if this parameter is what I should be looking at in order to avoid problems at startup.

 

I would certainly think about the current limit and ramp up to speed. Lets say you get 300 amps worth of FETs and the motor stalls. What smokes next?

 

Attached you can see the datasheet of the motor. The part number is: 305013, The terminal resistance is 0.102ohms.

I am using the PWM and will pay attention to all the advice so far! Thanks!

You have just got to imagine what pushing 236 amps through that motor would do!?!

Maximum CONTINUOUS CURRENT 10amps.
Max.

 

I will limit the current and make a ramp and see how it goes.

@Max, I don't even want to imagine what would happen!!. it would be similar to pushing 236A through 600 Euros!!

Thanks guys for the help!

 

it would be similar to pushing 236A through 600 Euros!!

Either way, Painful!
Max.