Electric motor loads; regulating voltage AND current

Discussion in 'General Electronics Chat' started by Tenacious Techhunter, Apr 21, 2008.

  1. Tenacious Techhunter

    Thread Starter New Member

    Apr 21, 2008
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    Hello folks. First time posting, but I’ve got an interesting question. I hope I’ve done enough searching to avoid duplication, but you never know with these things.

    If you were to put power across a single resistor, a specific wattage, for instance, the result would be a specific voltage and current. You could not, for instance, use a higher voltage with less current, but use the same amount of total watts.

    Electric motors, however, are very different. Their behavior varies depending on voltage AND current; a specific wattage across a motor tells you nothing about how it will behave. You have to know the current and the voltage, or equivalent information, to know what speed and torque it will have at that moment. Even at the same wattage, but different voltage and current, it will have different speed and torque characteristics.

    But suppose you want to choose what voltage AND what current will run through your motor, in order to more tightly control its behavior? How do you regulate a circuit in that way? Can it be done precisely even if your voltage and current are not constant, but still sufficient?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Your statement:

    "If you were to put power across a single resistor, a specific wattage, for instance, the result would be a specific voltage and current. You could not, for instance, use a higher voltage with less current, but use the same amount of total watts."

    is not quite correct. Power is the product of voltage times current. For any product P there is no limit (except practical) to the values of voltage and current whose product equals that P.

    You would need to be more specific about the motor you refer to. AC motors, especially, are designed to be run at a constant voltage. They will vary current draw under load, however.

    As to your last paragraph, you might want to look into the subject of pulse width modulation for electric motors. It is a technique for DC motors, but allows you to control the average amount of current going to the motor. PWM is done with a constant voltage.
     
  3. Tenacious Techhunter

    Thread Starter New Member

    Apr 21, 2008
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    You misunderstand my point. For a given resistor, and a specific amount of power you are feeding it, there is only one voltage value and one corresponding current value that the power across the resistor can have. You cannot put a specific amount of power across the resistor in any form you like.

    V=IR
    P=VI
    P=R*I^2
    I^2=P/R
    V=IR

    DC electric motors are, however, completely different. You will find charts for them where they behave differently at one voltage than another, even where the total wattage is the same across both charts. Thus, to truly benefit from an electric motor’s performance range, you need to control both the voltage and the current to the motor.

    You are right, I did fail to mention I meant DC motors. But as you’ve already made clear, you don’t vary the voltage on an AC motor. So I couldn’t really be talking about it, could I? I’ve looked into Pulse Width Modulation. It is fine if you want to settle for a specific voltage (usually) or a specific current (rarely). But if you want full control over your electric motor, you need to be able to vary the voltage AND the current. I’ve already stated I want to vary both voltage and current across the load. So, folks, how do we construct a circuit that varies the load’s voltage AND its current?
     
  4. Caveman

    Active Member

    Apr 15, 2008
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    The amount of power that the motor takes is related to the mechanical load. Basically, the
    Power Out = Power In*efficiency.
    So if you know the load well enough and the efficiency well enough, you can drive the appropriate voltage to get the appropriate current. However, if the mechanical load increases, the current will go up. Or you can drive the motor with a constant current source, and the voltage will go down on increasing load.

    Now, you want to know if you can drive a known power output. One way to do this would be to create a constant voltage output with a current sense that feeds back to reduce the voltage output when the current rises. I haven't thought much deeper about it though.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    Sure you could! If you read through some of the posts here at AAC, you'll find folk will "really be talking about" all manner of unexpected things. These unexpected things do indeed include varying the voltage on an ac motor. It is impossible to know what someone is thinking about unless they actually type their words into a post.

    Current draw at a given voltage will depend on motor mechanical load, as has already been noted. For example, inrush current and stall current will be higher than running current under normal conditions. A motor running under load will draw less current than it will when the shaft is unconnected.

    In other words, your R depends on your mechanical load. Vary your E, and your I will follow in accordance with Ohm's Law.
     
  6. Tenacious Techhunter

    Thread Starter New Member

    Apr 21, 2008
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    You’re assuming a given voltage. I don’t want it to be “given”, I want it to be variable. I want BOTH Voltage AND Current to be variable, so I can respond to the mechanical load with better control.

    Let’s assume that you don’t just want to control the torque of a motor, or just its speed, but it’s torque AND its speed. The only way to have enough control over a motor that you can have any torque at any speed is to control both the current and the voltage.

    So, getting back to the key question, how do we make a circuit that can drive a load with any reasonable combination of Voltage and Current?
     
  7. Caveman

    Active Member

    Apr 15, 2008
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    If you want to control Torque and Speed, you sense those values and drive either a voltage or current to correct them to what you want. It's a control system.

    You cannot as everyone has said directly control both current and voltage without changing the load. Because it is a motor, that means changing the mechanical load.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Hey just a cotton picking minute,

    What do you mean by the voltage through a motor?

    or the Wattage across a motor?

    Surely you mean the voltage across and the wattage drawn by or supplied to or some such.

    Now the curent through the motor is easy enough to define and can only be taken one way, but the voltage acrtoss a motor is different.

    This is because all motors generate a back EMF and the actual voltage across them is (simplified) the difference between the supplied and the back EMF.

    So are you referring to the supply voltage or the actual voltage across the motor?
     
  9. Tenacious Techhunter

    Thread Starter New Member

    Apr 21, 2008
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    You don’t understand. Just because a motor is under a specific mechanical load, that doesn’t mean there aren’t different ways to respond to that load. Sure, if you have a circuit designed for constant voltage, you increase the speed by increasing the current. But if you have a circuit designed for constant current, you increase the speed by increasing the voltage. It doesn’t matter if the motor is at the same speed, same voltage, and same current in both cases. What matters is, you could change the voltage OR the current, or even both, and the motor will respond. Why do you guys keep insisting that I shouldn’t have a circuit that gives me the choice of either one, or some combination of both, at any given moment??? There are advantages to being able to choose HOW you respond to the load at any moment.

    Motors have unique torque curves and speed curves across different constant voltages (also different constant currents). They are not the same values, even at the same total power. The result of which is that, unlike a resistor, even though there may be a specific power across it, that will not tell you anything about the voltage or the current, much less the speed or the torque involved. In addition to the speed and torque curves (although across two variables, it’s really a surface), is efficiency. Why wouldn’t you want to be able to follow that efficiency curve wherever it goes, even if that means you have to control voltage AND current?

    Furthermore, a motor doesn’t just START with load on it. It has to build up speed. And until it does, it’s VERY torquey, and uses lots of current. In particular, wouldn’t you want to efficiently control torque and current here, and speed up efficiently rather than wastefully? But once you get up to speed, it’s mostly about controlling speed with voltage.

    I’m not interested in constant voltage or constant current solutions here. I want to know how I can precisely control both in order to have the motor control I’d like to have.
     
  10. Caveman

    Active Member

    Apr 15, 2008
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    I understand quite well, actually. I'm not saying constant voltage or current. I'm saying that you have a variable voltage or current output.

    Just take a motor and put a voltage, Vx, across it at any instant in time. It will pull a specific amount of current, Ix. You can't change that. That is what that motor will pull at that instant.

    If you take that same motor and try to make the same current, Ix, flow through it, no matter how you drive it, you will end up with a specific voltage, Vx, across the motor. That voltage is equal to what you had before.

    You can specifically drive voltage or current. It doesn't have to be constant, but no matter what the motor will define the relationship with the other.

    However, you can always sense them both and decide what you are doing based on what you want.
    Let's say you are driving with a variable voltage source. It will pull a certain amount of current. Let's say you want to change the current. Just increase the voltage, and the current will increase. If you had a current source that you were increasing, the voltage would increase the same anyway.

    What's the difference? There really isn't.

    ------
    As far as a motor starting, it does pull a lot of current, if you drive it with full voltage. The problem is that you don't have the back emf to cancel the voltage. Almost all of your voltage is being used to increase the coils' magnetic field which increases the torque and therefore speed. As the motor spins up, most of the driving voltage is being cancelled by back emf, so only some of it is used for driving additional torque. So if you want to back off a bit so you have a more constant current, just drive a smaller voltage that ramps as the motor spins up. OR, you could drive a constant current, which will generate a voltage ramp as the motor spins up.
     
  11. Tenacious Techhunter

    Thread Starter New Member

    Apr 21, 2008
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    No, that’s if a motor was a resistor. A motor is not a resistor! If you give it a voltage, you won’t know anything about what it is doing until you know what current it’s drawing! Look at a motor specification sheet (particularly one rated under different voltages). A motor will behave differently at 72 volts and 50 amps than it will at 72 volts and 100 amps! Furthermore, if you run that motor at 36 volts and 100 amps, it will behave completely differently than if it had 72 volts and 50 amps, despite being the same power! Now granted, these are different load conditions. But why shouldn’t a motor be used under wildly different load conditions?
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
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    I'm sorry, but the universe just doesn't work that way.

    You plot your speed/torque curve from your no-load speed and your stall torque. You plot your current/torque curve from your no-load current and your stall current. Impedance is inversely proportional to torque.

    Now observe: The no-load speed will always be proportional to the voltage. Always. Plots of speed/torque curves at different voltages are parallel lines.

    Ohms Law shows the rest.

    If you want a motor to provide more torque at a given speed & voltage, or more speed at a given torque & voltage, you must add more windings.
     
  13. Caveman

    Active Member

    Apr 15, 2008
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    Yep, sure will. But that is because the loads and/or speeds are different. Not because the driving circuit *forced* current into the motor.

    But the load conditions are what drives the the current to be different. If you take that 36 volt situation, and you have the load that produces 100Amps, and you don't change the load or voltage, there is no way to cause more current to flow.
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Did I post or did I not post here?

    In order to help you understand the working of a motor and therefore the clear and correct guidance Caveman and thingmaker have offered start with this statement about the relationship between current and voltage in an electric motor.

    In a perfect motor when the rotator is rotating it will generate a voltage opposing the applied voltage. This called the back EMF. The Back EMF depends upon the speed of the motor.
    If no mechanical load is applied the two voltages are equal an no current is drawn.
    If a mechanical load is applied it will try to slow down the motor, reducing the Back EMF.
    The resultant voltage difference causes curent to flow consuming power.

    You cannot avoid the relationship between mechanical output (speed, torque,power) and electrical input (current voltage).

    The point of graphs between is to determine unknowns when you have some fixed related variables.

    You cannot fix all youreself.

    Or are you actually saying you want a module/instument where you can set either current or voltage and see what the resultant voltage or current is for various loads. A bit like with a laboratory bench power supply which offers current mode or voltage mode?
     
  15. Caveman

    Active Member

    Apr 15, 2008
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    If you want to really control everything, you can attach a second motor to the shaft of the first, and use it as both a generator and motor.

    For example, let's say that you have the "load" motor completely disconnected so it theoretically is providing no load. You put 50V on your main motor which has some other unknown mechanical load. Let's say it pulls 50A, but you want it to pull 75A. All you have to do is load the second motor electrically and it will act as a mechanical load for the first motor. Just create a loop that adjusts the loading of the second motor until you get to your set point.

    If you wanted to reduce current you would drive the second motor.


    Makes no real sense to do this, but it would work.
     
  16. brightspark07

    New Member

    Dec 20, 2008
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    I think we need to clarify what is being asked here in order to answer the question accurately, and then both sides can see where they are both coming from.
    when you say 'Their behaviour varies depending on voltage AND current', do you mean voltage and current working with each other at the same time, or do you mean voltage and current working separately?
    are you saying you want to control voltage and current, separately and individually, for or against each other?
     
  17. beenthere

    Retired Moderator

    Apr 20, 2004
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    You may notice that this is an abandoned thread - the last entry was made 8 months ago.
     
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