Motor slip

For squirrel-cage motors, slip ranges to 5% of the synchronous or constant speed. For variahle-frequency motor drives, see Reference 89. 

Motor slip is the difference between the rotational speed of an idle motor and the motor under load. Motor slip measurements, although relatively inexpensive, do not offer advantages over the power consumption measurements. The method did not gain popularity, probably because the slip is not linearly related to the load, despite some claims to the contrary. 

Timko, R.J. Johnson, J.L. Skinner, G.W. Chen, S.T. Rosenberg, H.A. Instrumentation of a vertical high shear mixer with a motor slip monitoring device. Drug Dev. Ind. Pharm. 1986,12 (10), 1375-1393. 

Pressurization p EN 60079-2 The occurrence of an explosible atmosphere inside a casing is prevented by maintaining a protective gas overpressure with respect to the surrounding atmosphere. If necessary the interior of the casing is permanently supplied with a protective gas so that the flammable mixture is diluted. Switchgear and control cabinets, analysis equipment, big motors, slip ring and collector motors... 

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

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. 

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

The performance of a motor is greatly influenced by a voltage unbalance in the supply system. It reduces its output and torque and results in a higher slip and rotor loss. This subject is covered in more detail in Section 12.2(v). For likely deratings, refer to Figure 12.1. Asystem with an unbalance of up to 1 % or so calls for no derating, whereas one having an unbalance of more than 5% is not recommended for an industrial application, because of a... 

During a run, if the supply voltage to a motor terminal drops to 85% of its rated value, then the full load torque of the motor will decrease to 72.25%. Since the load and its torque requirement will remain the same, the motor will star to drop speed until the torque available on its speed-torque curve has a value as high as 100/0.7225 or 138.4% of T to sustain this situation. The motor will now operate at a higher slip, increasing the rotor slip losses also in the same proportion. See equation (1.9) and Figure 1.7. 

Since the motor now operates at a higher slip, the slip losses as well as the stator losses will increase. A circle diagram (Figure 1.16) illustrates this. 

Judicious electrical design will ensure a pull-out torque slip as close to the full-load slip as possible and minimize the additional slip losses in such a condition. See Figure 1.8. A motor with a pull-out torque as close to full load slip as possible would also be able to meet a momentarily enhanced load torque during a contingency without any injurious heat or a stalling condition. 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

The maximum value of the output and torque of the motor can be obtained by dropping perpendiculars CC and CC3 on the output and torque lines respectively from the centre C.CjCi and C C4 indicate the magnitude of the maximum output and torque, respectively, that the motor can develop. This torque is the pull-out torque Tpp. In slip-ring motors it can be obtained at any speed on the normal speed-torque curve by inserting a suitable resistance into the rotor circuit to vary the slip. 

The higher the full load slip, the higher will be the rotor losses and rotor heat. This is clear from the circle diagram and also from equation (1.9). An attempt to limit the start-up current by increasing the slip and the rotor resistance in a squirrel cage motor may thus jeopardize the motor s performance. The selection of starling current and rotor resistance is thus a compromise to achieve optimum performance. 

Because of heavy start-up inrush eurrents, the use of LT motors should be preferred up to a medium sized ratings, say, up to 160 kW, in squirrel eage motors and up to 750 kW in slip-ring motors. For still higher ratings, HT motors should be used. 

Figure 1.18(b) Screen protected drip proof slip ring motor (Cooling system ICOA1)... 

The nearest standard rating to this is 7.5 kW, and a motor of this rating will suit the duty cycle. To ensure that it can also meet the torque requirement of 10.5 kW, it should have a minimum pull-out torque of 10.5/7.5 or 140% with the slip at this point as low as possible so that when operating at 140%... 

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