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Thread: Constant vs Variable Torque

  1. #1
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    Constant vs Variable Torque

    Can someone please explain the difference between constant and variable torque.

    I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

    I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?

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    AC Motors - Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.

    AC Motors - Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.

    JJ
    that nothing's so sacred as honor and nothing's so loyal as love Wyatt Earp

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    Quote Originally Posted by philly View Post
    I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?
    Because variable torque loads vary with the square or cube of the speed (as JJ said), attempting to drive these loads faster than base speed can significantly impact the amount of power required.
    Just because you can, doesn't mean you should.

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    Horsepower = (Torque in foot-pounds x speed in rpm)/5250

    On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.

    Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)

    In general:

    pumps and fans = variable torque loads (Positive displacement pumps not included.)


    conveyors, elevators, machines = constant torque load.
    Bob Wilson

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    Quote Originally Posted by rcwilson View Post
    Horsepower = (Torque in foot-pounds x speed in rpm)/5250

    On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.
    I'm assuming that with a conveyor if you add more material or weight then the torque will indeed increase. However if weight is kept the same and belt is increased in speed than torque still stays the same

    Does current increase as a function of speeding up a conveyor. If the horsepower increases then I"m guessing the current would too.

    Quote Originally Posted by rcwilson View Post
    Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)
    What if in addition to the fan being operated faster a damper opens allowing more air to flow. Would the fan then increase its torque per the cube of the speed plus due to the additional air loading?

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    Quote Originally Posted by philly View Post
    Can someone please explain the difference between constant and variable torque.

    I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

    I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?
    I assume that this refers to variable frequency inverters?
    The constant torque/variable torque classification is one that VFD manufacturers often use.
    The difference is usually the overload rating of the VFD
    Typically, the constant torque rating is based on a 50% overload capability and the variable torque rating on a 10% overload capability.
    Based on manufacturer's data, the same VFD can do 37kW VT and 30kW CT.

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    Quote Originally Posted by rcwilson View Post
    Horsepower = (Torque in foot-pounds x speed in rpm)/5250

    On a conveyor, it takes the same torque to move the load whether the belt is moving fast or slow. The weight that the conveyor is lifting doesn't change. Torque is constant. Double the conveyor speed and you double the HP needed because torque is constant.

    Torque needed to spin a centrifugal fan or pump varies with the speed. It takes more torque to spin the pump fast. At faster speeds the fan builds more pressure. Think of the pressure pushing back against the fan blades increasing the torque needed. Since HP= torque x speed, the HP increases rapidly with speed increase. (Flow varies directly with speed. Pressure varies with the square of the speed. Power needed = pressure x flow. So fan HP varies with the cube of the speed.)
    In general:
    pumps and fans = variable torque loads (Positive displacement pumps not included.)
    conveyors, elevators, machines = constant torque load.
    I agree with all of that.
    Your initial formula again reminded me on how much simpler SI (metric) is.

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    Quote Originally Posted by philly View Post
    Can someone please explain the difference between constant and variable torque.

    I have read that conveyors are considered constant torque loads. Does this mean that as you increase the speed of the conveyor the torqure requirement stays the same?

    I have also read that fans are considered variable torque loads. Does this mean that as you increase the speed the torque requirement increases?
    Centrifugal loads, such as pumps, blowers, fans and agitators are subject to the following laws. The laws explain the power, capacity and speed relationship to each other. These are called variable torque loads.

    easy reference: http://www.gouldspumps.com/cpf_0010.html

    Constant torque loads are where the load does not change with the speed. The easiest example to understand is the conveyor belt example. It's length is fixed, so is the material on it, therefore the mass to be moved is unchanging weather it is moving at 1fps or 10fps.

    Torque is defined as the force needed to move a certain mass.

    Power will define how fast that certain mass can be moved.



    1HP=550 pound-feet per second. In other words; 1 HP is needed to move 550# one foot in one second.

  9. #9
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    Quote Originally Posted by weressl View Post

    Constant torque loads are where the load does not change with the speed. The easiest example to understand is the conveyor belt example. It's length is fixed, so is the material on it, therefore the mass to be moved is unchanging weather it is moving at 1fps or 10fps.
    Wouldn't constant torque loads still see an increase in current as a result of running faster since the HP would increase?

    Is current more a direct relationship of required torque or horsepower? It looks like current will increase in both CT and VT applications when speed is increase except it will rise at a much faster rate in an VT application and not linearly.

    Why do VFD's have a CT and VT rating if current is current. Both a VT load and a CT load can draw excess current for any given amount of time.

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    Quote Originally Posted by philly View Post
    Wouldn't constant torque loads still see an increase in current as a result of running faster since the HP would increase?
    Yes, it would, because current is more directly related to HP, which is defined by torque AND speed. But the torque required to move that load would remain the same because torque is more directly related to load.

    Is current more a direct relationship of required torque or horsepower?
    HP, see above.
    It looks like current will increase in both CT and VT applications when speed is increase except it will rise at a much faster rate in an VT application and not linearly.
    Essentially, yes, because in a VT load, the LOAD is increasing along with speed, so the effect on HP and thus current is exponentially increased. Another term for a Variable Torque load in Europe is a "Quadratic Load", because the equation for power use is quadratic.

    Why do VFD's have a CT and VT rating if current is current. Both a VT load and a CT load can draw excess current for any given amount of time.
    Because the "excess" current is only seen when you over speed the motor. But in a VT load, as you turn the speed DOWN, the load decreases at the same quadratic rate, meaning that at 1/2 speed, you need only 1/2 x 1/2 x 1/2 the HP, or 1/8th. Therefore, a VT drive is, by definition, never going to need to deliver high power, and therefore high current, when running at lower speeds.

    In VFD design, the total circuit efficiency drops with speed because in order to get lower speed, the PWM pattern has the transistors on for longer periods of time in each cycle (because the cycles are slower). That is why when you see a VFD mfr quote that their drive is "97% efficient" they will always qualify that statement by saying that is a full speed. At 1/2 speed, the best are only 95% efficient, most are less. So that means that the heat rejection by the transistors (switching losses) goes up as speed goes down. In a CT drive then, you must size the transistors for the worst case scenario, which is below 1/2 speed with full load. That then means you either have to have a massive heat sink and cooling system, or you have to oversize the transistors compared to their loading at full speed. Most manufacturers are doing a combination of both factors.

    In a VT load however, the drive would never see that scenario! So the VFD mfrs can "get away with" using smaller transistors for larger loads, or in actuality, rating a VFD for a higher HP if used on a VT load. The VFD in either case is exactly the same, it is just the max. HP rating that is changing because of the expected load profile.
    Last edited by Jraef; 11-12-08 at 09:20 AM.

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