Motors 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 motor synchronous speed is determined by the frequency of the rotating field in the stator (60 cycles in the U.S.) and the number of poles (coil connections in the stator) such that motor synchronous speed... 

Figure 9.1 Two-pole motors, synchronous speed 3000min 1 for 50 cps. Price ratio ex-protected to non-ex-protected cage induction motors versus rated power. Types of protection (E)ExellT3 (E) Exd IICT4 (E) Exd IIBT4.

Motor Synchronous Speed, rpm Motor Type Horsepower, hot cold... 

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... 

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. 

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... 

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. 

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. 

Semiconductor control modules gate the thyristors, which switch cm rent to the motor field at the optimum motor speed and precise phase angle. This assures synchronizing with minimum system disturbance. On pull-out, the discharge resistor is reapplied and excitation is removed k> provide protection to the rotor winding, shaft, and external electrical system. The control resynchronizes the motor after the cause of pull-out i.n removed, if sufficient torque is available. The field is automatically applied if the motor synchronizes on reluctance torque. The control is calibrated at the factory and no field adjustment is required. The opti-... 

Examination of the drive ratios shows that if both motors have ISOOrpm synchronous speeds that the pony motor is made to operate near 3600rpm when the big motor is operating. This is typically not a problem as to rotor balance or bearing duty because manufacturers make 3600 rpm versions of these same motors. Nevertheless, the duty should be checked and if this is a problem, the small motor can be changed to a 900 rpm model and the synchronous speeds of each motor will not be exceeded. 

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. 

The rotating magnetic field in the stator travels around the stator at what is called synchronous speed. By grouping stator coils together in what is called poles, the motor rotor can be designed to turn at a certain speed (revolutions per minute/rpm). On an induction motor the number of poles cannot be seen or counted without the drawings. 

The speeds of induction motors are slighdy less than for the synchronous motors due to slip. For example, a synchronous speed of 1,800 rpm will actually he 1,750-1,734 rpm in an induction motor 1,200 rpm hecomes 1,150 rpm and 3,600 rpm hecomes 3,450 rpm. 

Synchronous Speed, rpm Motors hp Generators, Kilowatt at 0.8 Power Factor... 

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.

Pull-out torque For a synchronous motor, this is the maximum sustained torque that the motor will develop at synchronous speed for one minute with rated voltage applied at rated frequency and with normal excitation. 

Fixed speed. Advise synchronous speed desired (i.e., 3,600, 1,800, etc.) for AC motors. For DC motors, advise desired basic speed at full, load, and maximum speed by field control. 

---