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Old 2nd February 2012, 05:10 PM   #1
Tenson is offline Tenson  United Kingdom
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Default DC Motor Drive?

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!
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Old 5th February 2012, 02:14 AM   #2
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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
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Old 5th February 2012, 12:08 PM   #3
Tenson is offline Tenson  United Kingdom
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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?
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Old 5th February 2012, 02:06 PM   #4
GloBug is offline GloBug  Canada
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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.
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Old 5th February 2012, 02:56 PM   #5
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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.
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Old 5th February 2012, 03:23 PM   #6
Tenson is offline Tenson  United Kingdom
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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?
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Old 5th February 2012, 03:32 PM   #7
djQUAN is offline djQUAN  Philippines
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Quote:
Originally Posted by Tenson View Post
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?
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.
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Old 5th February 2012, 04:10 PM   #8
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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.
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Old 7th February 2012, 08:03 PM   #9
Tenson is offline Tenson  United Kingdom
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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?
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Old 7th February 2012, 08:19 PM   #10
sreten is offline sreten  United Kingdom
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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.
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Last edited by sreten; 7th February 2012 at 08:24 PM.
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