Hi Guys,
Nope, it's not a motor for hi-fi it's for the spindle of my CNC machine!
I blew up the PWM motor speed controller by accident, but I didn't like it anyway as it interfered with the computer working reliably, and also didn't provide enough power. So... I'd like to build a new speed controlled DC motor drive.
What I'd love is a speed controller that still enables the motor to use full torque even at slow speed. Is this possible without PWM?
Specs of the motor are 100-200watts power handling. 180 Ohms resistance when working (calculated from 0.2A draw with 38V). 5 Ohms resistance when stationary if it matters.
Thanks for your suggestions!
Nope, it's not a motor for hi-fi it's for the spindle of my CNC machine!
I blew up the PWM motor speed controller by accident, but I didn't like it anyway as it interfered with the computer working reliably, and also didn't provide enough power. So... I'd like to build a new speed controlled DC motor drive.
What I'd love is a speed controller that still enables the motor to use full torque even at slow speed. Is this possible without PWM?
Specs of the motor are 100-200watts power handling. 180 Ohms resistance when working (calculated from 0.2A draw with 38V). 5 Ohms resistance when stationary if it matters.
Thanks for your suggestions!
For a small motor like this you could drive it with a linear amplifier, but the efficiency would be very low and you would need a big power supplyand a huge heat sink. You also need overload protection
Hi,
Am I right in thinking that controlling the speed of the motor by changing voltage will reduce motor torque at slower speed, while a PWM driver will keep full torque as it uses full voltage but just controls how frequently it applies the power?
Am I right in thinking that controlling the speed of the motor by changing voltage will reduce motor torque at slower speed, while a PWM driver will keep full torque as it uses full voltage but just controls how frequently it applies the power?
Sounds about right, at least the low DC voltage causing low torque-it would need different windings to work with lower DC and maintain power.
No, in a PWM drive the inductance of the motor causes the current to remain (more or less) constant at steady state. (The current will change for dynamic conditions). In the steady state condition the effective motor voltage is the PWM duty cycle times the DC supply voltage. So either a linear amp of a PWM amp can proved the same effective voltage.
Hi,
So just to be clear you are saying that PWM also provides less torque at lower speeds, so it has little advantage other than efficiency of the circuit?
Is there any way to maintain high torque with slow speed on a standard DC motor other than gearing?
So just to be clear you are saying that PWM also provides less torque at lower speeds, so it has little advantage other than efficiency of the circuit?
Is there any way to maintain high torque with slow speed on a standard DC motor other than gearing?
That is the same thing I experienced using PWM for my pcb drill. My workaround was a linear controller but with a slight twist...clicky here.
Torque is directly proportional to current. Both drives should be designed to provided rated current and voltage to the motor, so both can drive the motor over its entire speed - torque envelope.
Yes I guess what I'm really looking for is only possible with a feedback system monitoring spindle speed and increasing voltage if it drops below the set speed.
Half way to that is like djQUAN suggests which will put out the same voltage regardless of load impedance. An easy way to do that would be with something like an LT1083 regulator (7.5A capable) but the problem is the limit of 30V. Does anyone who if this is simply an input to output difference of 30V or if it really is limited to that maximum?
Half way to that is like djQUAN suggests which will put out the same voltage regardless of load impedance. An easy way to do that would be with something like an LT1083 regulator (7.5A capable) but the problem is the limit of 30V. Does anyone who if this is simply an input to output difference of 30V or if it really is limited to that maximum?
Hi,
My simplistic understanding is a DC motor is voltage controlled to set the speed,
back emf = applied voltage, the load determines the current if its not limited.
However the DC resistance of the windings mucks all that up, dropping
some of the back emf, slowing the moter. Consequently a simple drive
schema is an amplifier with negative output resistance (see ESP project)
equal to the restance of the motor windings with some current limiting
to keep the motor and amplifier* both within their SOA's.
Within the current limits for the above the motor should maintain constant
speed related to applied voltage, current drawn depending on the load.
To be more accurate I guess you'd need to temperature compensate
winding restance variation, going up as the windings get hotter.
rgds, sreten.
* A DC variable voltage regulator is a more accurate term here,
but i've only seen the current feedback used for negative output
impedance implemented for amplifiers. Same thing though I'm sure.
My simplistic understanding is a DC motor is voltage controlled to set the speed,
back emf = applied voltage, the load determines the current if its not limited.
However the DC resistance of the windings mucks all that up, dropping
some of the back emf, slowing the moter. Consequently a simple drive
schema is an amplifier with negative output resistance (see ESP project)
equal to the restance of the motor windings with some current limiting
to keep the motor and amplifier* both within their SOA's.
Within the current limits for the above the motor should maintain constant
speed related to applied voltage, current drawn depending on the load.
To be more accurate I guess you'd need to temperature compensate
winding restance variation, going up as the windings get hotter.
rgds, sreten.
* A DC variable voltage regulator is a more accurate term here,
but i've only seen the current feedback used for negative output
impedance implemented for amplifiers. Same thing though I'm sure.
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Thanks for all the help!
I found something interesting. I've just measured the voltage and current of the motor with and without a load. When I put it under load the voltage drops as expected, but also the current draw goes up. I guess the impedance of the motor goes down when it spins slower, and not just enough to keep constant current but it actually draws more.
This is on an unregulated linear DC supply (beefy TX and rectifier). So I guess with this sort of behaviour a normal voltage regulator will give me the most torque from the motor as it will maintain voltage under load and even more current will then draw into the lower impedance if the motor slows.
Does that sound right?!
I found something interesting. I've just measured the voltage and current of the motor with and without a load. When I put it under load the voltage drops as expected, but also the current draw goes up. I guess the impedance of the motor goes down when it spins slower, and not just enough to keep constant current but it actually draws more.
This is on an unregulated linear DC supply (beefy TX and rectifier). So I guess with this sort of behaviour a normal voltage regulator will give me the most torque from the motor as it will maintain voltage under load and even more current will then draw into the lower impedance if the motor slows.
Does that sound right?!
My suggestion is to buy a 36V electric scooter motor controller. Dirt cheap. Takes 0-5 V DC as control voltage. May have to adjust supply as such controllers are very finicky about operating range (Low battery shutdown etc.)
(31 =/- .5V min) A 500W controller will run as little as $18 US. I'm certain that there are some equivalent British sites but here is the one I deal with:
36 Volt Controllers
Doc
Thyristor bridge with a standard transformer can be a good option if anyone doesn't want PWM for any reason.
Hi,
Its as stated in my earlier post, the only thing stopping the motor
maintaining constant speed with increased load is the voltage
drop across the winding resistance caused by the extra current.
(With no winding resistance back emf proportional to speed
has to equal the applied emf, applied voltage = speed.
Current drawn will depend on the load applied.)
You need a variable voltage regulator with negative output
resistance equal to the winding resistance for no speed drop.
rgds, sreten.
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sreten,
You don't regulate the output of a voltage regulator by having a regulator with a negative output resistance. This is nonsense and could easily lead to oscillations in a real system. The only reasonable way to do this is to use a feedback loop that senses the output and applies a correction signal to adjust for any changes in the output.
You don't regulate the output of a voltage regulator by having a regulator with a negative output resistance. This is nonsense and could easily lead to oscillations in a real system. The only reasonable way to do this is to use a feedback loop that senses the output and applies a correction signal to adjust for any changes in the output.
I used to be into model railways and had to do a similar thing with 12VDC motors.
I used a constant current source to limit maximum current into a PWM output.
I used a PIC to control the pwm from a pot.
I used a constant current source to limit maximum current into a PWM output.
I used a PIC to control the pwm from a pot.
That's how it's done at the controller in the link I posted earlier. Worked well.
Mine did oscillate. But I planned ahead for that and made it with adjustable gain. You just dial in enough gain to counter the connection and winding resistance. After that initial alignment, it works like a charm. Although once adjusted, it will only work for that particular motor. if you change motors or cables going to the motor, you'll just need to tweak it a bit again to make it stable.
djQuan,
Your design looks very nice but it can't operate at the ~150V my motor needs. Can you tell me how it might be adjusted to do that?
Your design looks very nice but it can't operate at the ~150V my motor needs. Can you tell me how it might be adjusted to do that?
It might have to be redesigned entirely as the regulator could only handle 35V max. but I think the concept could be used in a scaled up version.
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