# high-power voltage to current converter

• posted

I've been told the torque produced by a D.C. motor is quite proportional to the current drawn. I have a low-voltage, low-power signal voltage (say 0-10 V at 1 mA) that I want to use to produce a proportional torque in a large D.C. motor (say about 1 H.P. and 100 V, hence around 15 A). So what I want is a circuit that adjusts the voltage fed to the motor in such a way that the current drawn is proportional to the input signal. Does any one have an idea how to do that? Robert

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In case it makes things easier, my signal source is actually a potentiometer, so if it is more convenient to have resistance rather than voltage as the input, it is available. However, the potentiometer cannot handle high currents. I was thinking of simply attaching the potentiometer to the base of a large transistor, and have the collector/emitter in series with the motor. But transistors aren't linear near cutoff, so this won't work.

Robert

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You should repost this question on sci.electronics.basics

The basic answer to your question is an operational amplifier - Apex make parts that can deliver a few amperes, or you can add one or more high current boosters to a regular operational amplifier.

In either case you use negative feedback around an amplifier with a relatively high open loop voltage gain to make the booster transistors look linear even near cutoff.

-- Bill Sloman, Nijmegen

• posted

Depending on the range where you want to operate this will produce a HUGE amount of heat. 1HP is ~750Watts. That 7.5A @ 100V If you run at low RPM or in a stall condition, that would put 750W on the transistor and surely blow it.

Your best approach is a PWM method.

You're looking for PWM torque control of a DC motor.

• posted

Yeah....driving any uncoupled inductive load produces the same loss in a linear driver as occurs when its driving a short circuit.

RL

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That's a good point that I overlooked. Even if I get sufficiently large transistors that can handle the load, the expense, cooling demand, and power waste seem ridiculous. The PWM idea seems much more efficient, if more complicated to implement.

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Hey, here's an idea that might bring back memories if they're any old- timers out there. A motor-generator set. The motor runs constantly and turns the generator. The control voltage adjusts the field of the generator, which in turn controls the generator's electrical output. That in turn runs another motor, whose torque is then proportional (?) to the control voltage!

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This is a very common requirement that I've seen in quite a few "off the shelf" servo amlifiers. Westamp comes to mind as one that did have a 1 or 2 HP 100vdc PWM amplifier. I don't know if westamp is still around, but you could do a search for servo amplifiers to see what's available. Bill Sloman's answere is dead on, but I'll add to it. All you need is a low valued resistor in series with the motor. The current thru the resistor is equal to the current thru the motor , which develops a voltage proportional to motor current. You now have a voltage representing motor current, and it is a simple matter of using op amps to compare the feedback voltage to the input voltage, process it as you desire. BTW a .667 ohm resistor would give you a 0 to 10 volt feedback for a 0 to 15 amp motor current.

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A few obvious considerations: You probably can't put the sense resistor and the PWM drive both on the low side. One of them has to go on the high side -- so choose your poison. You need a freewheel diode connected so the freewheel current goes through the sense resistor. And I think you need to low- pass filter the voltage from the sense resistor before it goes to the op amp.

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That would also generate 150W Watts of heat!!! (15^2 * .667 ohms) Your sense resistor would likely be something like a 0.005 ohm run to a differential amp with a gain of 133. That would produce a 10 V signal at full current and would dissipate ~1 Watt. Make sure the resistor is low inductance (prefereably surface mount)

You might want to look at something like a UCC1800 from TI configured as a forward converter. Just replace your transformer with a motor with a fast diode in parallel. by fast I mean a diode with a reverse recovery time less than 50nS. the old 1n4000 series won't get it. For motor use, you should look at 20KHz operation so the PWM wine i nout of the audible range (if that is a concern). Don't go much higher than this or you're only heating up the mosfet more with no gain in performance.

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