Torque versus speed

Figure 4-144. Typical torque versus speed curves for eddy current brakes.

Figure 5.4 Torque and current versus speed curves 22 kW two-pole motor, for different values of applied voltage. Also shown is a typical torque versus speed curve for a centrifugal pump or compressor.

Before thyristors and power transistors were introduced for AC to DC and AC to DC to AC converter systems, there were a number of special designs of AC motors that gave better performance than standard squirrel-cage motors. These motors required connections to the rotor windings. They had better speed control, superior torque versus speed characteristics and some methods were energy efficient. However, they were more complicated and hence more expensive. 

DC motors have a widely variable drooping torque versus speed characteristic and so, for any given torque and speed within its rating, the motor can be controlled to give a chosen speed and torque. Hence a DC motor can be controlled to accurately match and operate the characteristics of its driven machine from zero to beyond rated speed. 

DC drive (direct-current drive) n. A machine drive, particularly that of an extruder, powered by a direct-current motor. The availability of economical solid-state rectifiers, and the good torque-versus-speed characteristic and tight speed regulation of DC drives have made them the most popular choice today for variable-speed service. 

The average of the torque versus speed data at which the up and down curves coincide is the first step of the calculation procedure for obtaining the corrected flow curve. Care must be taken to ensure that the flow curve is not affected by the four error sources secondary/turbulent flow, viscous heating, end effect and wall slip. Further details of this procedure can be found in Alderman and Heywood (2004b) for the appropriate viscometric geometry used. 

Figure 3-3 Examples of (a) cup-and-bob viscosity (torque vs. speed) and (b) viscosity versus shear rate. (Lines are least-mean-square fit.)...

In this relation, M is the torque, K is the familiar consistency index from viscosity versus shear rate relationships, S is the rotor speed, and n is the power-law index, also familiar from viscosity versus shear rate plots. It can be seen from Equation 1 that a log-log plot of torque versus rotor speed should yield a straight line with intercept given by log (K C(n)) and slope given by n, the power-law index. 

The electric drive is selected among those commercially available for applications on electric mopeds. In particular, a brushless electric drive with a rated power of 3.5 kW and maximum speed of 6000 rpm is used. The technical spec-ihcations of this electrical drive are reported in Table 6.2, while the Fig. 6.3 shows its characteristic curves in terms of power and torque versus revolution speed. 

Run Speed is the motor speed for the YR-1 at which the material is tested. Choices range from 0.01-5.0 rpm. It is common for materials to appear stiffer when tested at higher speeds. That is, the slope of the torque-versus-time or stress-versus-strain curve increases with increasing speed. This is because the material structure has less time in which to react to dissipate the applied stress. Increasing the speed will, in most cases, increase the yield stress measured by the instrument. Most yield tests are conducted at relatively low speeds (<1 rpm) to minimize any inertial effects when using vane spindles. 

Figure 11 Plots of torque versus mixing time for 50 vol% and 50 vol% polyethylene as a function of speed. (After Ref. 10.)...

The damper bars or winding act in a manner very similar to an induction motor and provide a breaking torque against the transient disturbances in shaft speed. To be effective the damper needs to have a steep torque versus shp characteristic in the region near synchronous speed. The equivalent impedance of the damper requires a low resistance and a high reactance. High conductivity copper bars are embedded into the pole face to provide a low reluctance path for the leakage flux. 

Figure 5.6 Sensitivity of the torque and current versus speed curves to a 20% increase in the nominal value of the resistance or reactance for a 200 kW two-pole motor.

Starting Torque - The torque at the bottom of a speed (rpm) versus torque curve. The torque developed by the motor is a percentage of the full-load or rated torque. At this torque the speed, the rotational speed of the motor as a percentage of synchronous speed is zero. This torque is what is available to initially get the load moving and begin its acceleration. 

Figure 20-8 Torque versus impeller speed for water and a Cm = 0.10 bleached kraft suspension measured in the Couette test apparatus described in the caption to Figure 20-7. The insert diagrams show the observed pulp suspension motion at points along the flow curve.

The ODR measures the force (torque) required to oscillate a biconical disk (rotor) back and forth within a shallow cavity filled with rubber compound. As the compound cures, its viscosity increases and more torque is needed to move the rotor. The instrument plots a continuous curve of torque versus time. This curve is often called an ODR trace. From an ODR trace, the rubber compounder can estimate stock viscosity, scorch time, cure rate and cure state. Important test variables are the arc through which the disc oscillates, the speed of oscillation, and the test temperature. The usual arc is 1° (actually, this means 1 on either side of the starting point, or a total swing of 2°) some laboratories use 3 or 5 . Normal oscillation rate is 100 cycles per minute. Test temperature should be close to the expected cure temperature. 

If the load-torque requirement ever exceeded the maximum torque capability of the induction motor, the motor would not have enough torque to accelerate the load and would stall. For instance, if the load line required more torque than the motor could produce at the pullup torque point, i.e., 170 percent load torque versus 140 percent pullup torque, the motor would not increase in speed past the pullup torque speed and would not be able to accelerate the load. This would cause the motor to overheat. It is, therefore, important to ensure that the motor has adequate accelerating torque to reach full speed. 

FIG. 29-2 Typical speed versus torque curves for various NEMA-design squirrel-cage induction motors. (See Table 29-2 for an explanation of design types.)... 

FIG. 29-3 Typical speed versus torque curves for a wound-rotor induction motor with varying amounts of external secondary (rotor) resistance. Resistance values are based on resistance at 100 percent torque and zero speed = 100 percent. 

Figure 6.1 Approximate theoretical representation of speed versus torque for I///control of a motor at different values of /...

For a given cup and bob combination, the term n" represents the slope of a logarithmic plot of torque t versus rotational speed N at the particular value of N in question. Since the derivative in the last term of Eq. (48) is normally small, one may usually take as the final shear-rate equation ... 

In general, shear stress at one location (e.g., the bob surface in a concentric cylinder viscometer) is calculated from the dimensions of the sample gap and the measured or applied torque. Shear rate is calculated at the same location from sample gap dimensions and rotational speed. By making experimental measurements over a range of speeds or torques, the flow curve (shear stress versus shear rate) of the sample can be established. Suitable mathematical treatment of the flow curve data yields the sample s constitutive equation and rheological properties. 

---