Induction motors Torque

Motor stall torque Induction motors can tolerate 100% stall torque indefinitely because current is shared equally by all three phases at slip frequency. A synchronous motor will require a DC current at stall, and therefore thermal capacity is reduced in both motor and inverter at stall conditions. 

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. 

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

This is why an induction motor ceases to run at synchronous speed. The rotor, however, adjusts its speed, N such that the induced e.m.f. in the rotor conductors is Just enough to produce a torque that can counter-balance the mechanical load and the rotor losses, including frictional losses. The difference in the two speeds is known as slip. S, in r.p.m. and is expressed in terms of percentage of synchronous speed, i.e. 

NEMA, in its publication MG-1 for induction Motors, has prescribed four rotor designs. A, B, C, and D, covering almost all sizes of LT motors, to possess a prescribed minimum 77, Tp and pull-up torques. These torques are generally as drawn in Figure 2..3 to meet all normal industrial, agricultural or domestic needs. (Refer to the said publication or lEC 60034-12 for values of these torques. lEC 60034-12 has also provided similar stipulations.)... 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

Since the performance of an induction motor can be varied by altering the rotor parameters, a slip-ring motor, through its rotor circuit, can be made to suit any specific torque and speed requirement. 

Here we analyse the effect of variation in the incoming supply parameters (voltage and frequency) on the characteristics and performance of an induction motor (such as its flux density, speed, torque, h.p., etc). We also assess the effect of variation of one parameter on the other, and then choose the most appropriate solid-state scheme to achieve a required performance. We generally discuss the following schemes ... 

Induction motor mechanical considerations. If the motor is plaeed between the eompressor and the expander its shaft ends must be eapable of full torque transmission. Unit torque requirements are generally well above those required by the motor rating. The result is speeially designed motors. 

Induction motor electrical considerations. When motor starting is desired, the startup torque requirements generally dietate the motor design. The eleetrie eunent inrush at startup is signifieant. The plant eleetrieal grid must be analyzed for eompatibility. 

Pollard, Ernest L, Torsional Vibration Due to Induction Motor Transient Starling Torque, undated manuscript. 

The synchronous motor is a constant-speed machine. Unlike the induction motor which inherendy has slip from losses, the synchronous motor uses an excitation system to continually keep the rotor in synchronous speed with current flowing through the stator. Within its designed torque characteristics, it will operate at synchronous speed regardless of load variations. 

Figure 14-12. Typical torque curves for NEMA design B, C and D Induction motors having synchronous speeds below 1,800 rpm. These curves also apply to some 1,800 rpm design motors. (Used by permission E-M Synchronizer, Bui. 200-TEC-1120, p. 4, 1955. Dresser-Rand Co.

Torque, horsepower, and speed requirements demanded in drives for most machines can be met with one of four designs of squirrel-cage polyphase induction motors, Each design offers a different combination of torque, speed, and current characteristics to meet the operating requirements of various industrial applications. 

Figure 3-10 shows the typical torque-speed performance curves for various designs of polyphase squirrel-cage induction motors [9],... 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Power-Factor Correction. The induction motors used for oil-well pumping have high starting torques with relatively low power factors. Also, the average load on these motors is fairly low. Therefore, it is advisable to consider the installation of capacitors to avoid paying the penalty imposed by most power companies for low-power factor. They will be installed at the individual motors and switched with them, if voltage drop in the distribution system is to be corrected as well as power factor. Otherwise they may be installed in large banks at the distribution center, if it is more economical to do so. 

PrImary-Voltage-Control-AC Motor Driver. Induction motor torque at any slip s is proportional to primary V. Rotor-power dissipation is equal to s times the air-gap power. These two relationships define the boundary of operation of an induction motor with primary voltage control of speed. As the speed is reduced (s increased) at constant torque, the air-gap power remains fixed, but the power divides between rotor circuit dissipation and mechanical shaft power. 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

AC traction motor - Cage induction motor - Rated continuous power 45kW Speed, max. 2300 min 1 Frequency 0-78 cps Voltage 0-180 V Torque, max. ... 

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