Synchronous Speed

Motor nd Drive. The preferred prime mover for a fan is usually an electric motor. Eor fans of low to moderate power, V-belt drives are frequently employed. This permits selection of fans that can be operated over a wide range of speeds rather than being limited to motor synchronous speeds. Furthermore, change of speed is less expensive with V-belt drives. However, fans requiring powerful motors, 37—75 kW (50—100 hp) and higher, are generally directly connected to the motor and driven at synchronous speed. 

The typic medium-sized squirrel-cage motor is designed to operate at 2 to 3 percent shp (97 to 98 percent of synchronous speed). The synchronous speed is determined by the power-system frequency and the stator-winding configuration. If the stator is wound to produce one north and one south magnetic pole, it is a two-pole motor there is always an even number of poles (2, 4, 6, 8, etc.). The synchronous speed is... 

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. 

Synchronous speeds are calculated by Eq. (29-10). Speeds above the limits given are obtained through step-up gears large high-speed centrifugal compressors are examples. Two-pole (3600 r/min at 60 Hz) synchronous motors can be built but are uneconomical in comparison with geared drives. 

Reduced-speed operation reduces efficiency. Efficiency is approximately equal to speed expressed as a percentage of synchronous speed. Thus at 75 percent speed, about three-fourths of the motor... 

A constant field rotating at synchronous speed Ns Figure 1.4 Production ol magnetic field in a 3-0 winding... 

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. 

The last two parameters are maximum during start-up, diminish with speed and become zero at the synchronous speed (when S = 0). Therefore 7 = 0 when, 2 = 0. 

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. 

The power transferred by the stator to the rotor, P, also known as air gap power at synchronous speed, can be expressed in kW by ... 

Tj = rated or the full-load torque and should occur as near to the synchronous speed as possible to reduce slip losses. 

Note For simplicity, the synchronous speed of the motor is considered, which will make only a marginal difference in calculations. 

A motor can fall in a generator mode when the machine is energized and is run beyond its synchronous speed, such as when driving a load, travelling down hill or when its speed is reduced to perform a specific duty. The same conditions will appear when a running machine is reversed, whether it is an a.c. or a d.c. machine. 

Regenerative braking If the motor be run beyond synchronous speed by some external means it will work as a generator and feed back useful energy to the supply system. It will draw only the necessary excitation current, / , for the generator action from the source of supply. In such a condition, the motor... 

For the range of load for which the efficiency is determined, the measurement of slip is very important. To determine slip by subtracting from the synchronous speed the value of speed, obtained through a tachometer is not recommended. The slip must be directly measured by one of the following methods ... 

The spced-torqtie characteristic is the lehitionship between the torque and the speed, in the range from zero to synchronous speed. This relationship, when expressed as a eui ve. w ill include breakdown torque (pull-out torque), pull-up toi que and starting torque. The speed-current characteristic is the relationship of the current to (he speed. 

It is therefore necessary to take precautions during the test to avoid a excessive temperature rise and consequent damage to the windings. For wound rotor motors, speed-torque and speed-current tests may be taken between synchronous speed and the speed at which the maximum torque occurs. 

N = synchronous speed or speed of the engine at full load (r.p.m.)... 

The armature of the machine will normally have a residual voltage of around 8 V (for LT machines) across the terminals when running at the synchronous speed. If not, as when the generator is operated after a long shutdown, a d.c. voltage of 12 V can be applied through a battery for a few seconds to obtain the required residual voltage. 

As a synchronous motor The machine is run primarily to drive a mechanical load and is operated at the synchronous speed and at unity p.f. The efficiency is now better than that of an induction motor. Except in assisting the system by consuming power at unity p.f., it does not help the system to improve its p.f. 

If the field excitation is also lost, the generator will run as an induction motor again driving the primer mover as above. As an induction motor, it will now operate at less than the synchronous speed and cause slip frequency current and slip losses in the rotor circuit, which may overheat the rotor and damage it, see also Section. 1.3 and equation (1.9). A reverse power relay under such a condition will disconnect the generator from the mains and protect the machine. 

When an induction motor runs beyond the synchronous speed, it behaves like an induction generator and feeds power back to the supply system (Section 6.15). Below synchronous speed it behaves like an induction motor and draws power from the supply system. This protection trips the generator in such an eventuality and protects the machine. 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

Descending loads may overspeed the motor and iwerexcite the capacitor when connected across the motor due to motor generator action above the synchronous speed (Section 6.21). Such a situation may damage the motor as w ell as the capacitor and ntust be avoided. 

Synchronization Control. During startup, partial load is applied to the eddy current brake. The expander is then throttled to a speed below synchronous speed, and the brake voltage controller varied to lower the voltage of the coil, causing a small speed... 

Maximum continuous speed for generator drive duty is synchronous speed. 

Trip speed is the speed at which an independent emergency device is activated and sends a signal to close the expander inlet butterfly valve system. This is approximately 105% of maximum continuous speed or synchronous speed (whichever is higher). 

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