starting methods of synchronous motors?

Question

how they are strted if it is not selfstart

Answer 1

Hope you've bothered to scroll this far down after the last answer that doesn't even answer the question.

There are a number of ways of starting a synchronous motor.

1. Use a pony motor to get it up to speed first. (This assumes the load can be removed for start)

2. The rotor has amortisesur windings on it. These act as a fairly crude squirrel cage just for starting. (Start currents tend to be high but motor can be rated to start "on load")

3. The rotor can have extra windings that act like a "wound rotor" motor for start. These can use start resistors to limit starting currents and will start "on load". One type of these is an "auto synchronous" that doesn't require the starting method mentioned later as it will "synchronize itself (the dc must still be applied after near synchronous speed is met). The dc winding and one of the start windings can be the same in wound rotor (synchronous induction)

For options 2 and 3, some form of control is needed. During start, the dc is not applied. When the motor reaches "near synchronous" speed, the dc needs to be applied. This has to be done at the correct time when the synchronous field is aligned with the rotor. This can be done manually by watching an analogue voltmeter connected to the rotor and applying the dc as the voltmeter reaches its maximum swing. There are relays that sense this and apply the dc automatically. Check out this site for more info
http://www.geindustrial.com/publibrary/checkout/38652.30055.22278.50388/PDF/spm-a7_3.pdf

Answer 2

Starting Methods Of Synchronous Motor

Answer 3

The large, low-speed synchronous motors I'm used to working with are designed with amortisseur windings that provide starting torque when power is applied to the stator. A relay connected to the rotor monitors the rotor frequency, and when the motor has reached near-synchronous speed the field is applied and the motor synchs. Starting torque is poor, however, so generally these motors are connected to their load with an air-operated clutch. The clutch is not engaged until the motor has synchronized.
The other way I'm aware of is through the use of variable frequency drives, either a Cycloconverter or a regular H-bridge type VFD designed to be used with a synchronous motor of either the permanent magnet or DC excited rotor design. With a variable frequency drive the rotor field is engaged before power is applied to the stator. However, when power is applied to the stator, it is done at very low frequency, so the synchronous speed is very close to 1RPM or less, and the motor can be accelerated - while synched - to operating speed. This is useful for large grinding mills in mines, on motors that may be in excess of 20,000Hp (15MW).

https://www.electrikals.com/products/rm-motors/havells?mid=28&cid=641&page=1&pagesize=20

Answer 4

Classically, synchronous motors (and even some synchronous generators) are started as induction machines.

If the field winding (on the rotor) is short-circuited, the induced currents will flow at the slip frequency, allowing the machine to develop torque and start turning. When the machine approaches synchronous speed, the field winding is un-shorted, and the DC field excitation is applied.

Note that the machine cannot reach synchronous speed without DC excitation. It can get close to synchronous speed, but will produce less and less torque as it approaches synchronous speed. Thus, the external DC excitation is applied to pull the machine into synchronism.

Answer 5

Start as a 3 phase(DOL Starter ) Induction motor ie wound rotor silipring, 3 terminals to be shorted by means of links with change over switch, note down the current speed ,power factor<0.8 (lagging) . keep ready DC excitation supply with rehostatic controller maximum position , with this condition release the change over switch to apply dc excitation slowly increase to rehostat to increase the d.c current up to the(near to over excitation) Motor attain synchronous speed ,P.F meter shows unity.Note down all the reading.

Answer 6

first shorted the armature winging as a result it will start as induction motor and then will be start as synchronous motor

Answer 7

some use starting capasitors, some are 3 phayse

Answer 8

Synchronous Motors :
Synchronous motors are inherently constant-speed motors and they operate in absolute synchronism with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and is always a ratio of the line frequency.

Synchronous motors are made in sizes ranging from subfractional self-excited units to large-horsepower, direct-current-excited motors for industrial drives. In the fractional-horsepower range, synchronous motors are used primarily where precise constant speed is required.

In large horsepower sizes applied to industrial loads, synchronous motors serve two important functions. First, it is a highly efficient means of converting ac energy to mechanical power. Second, it can operate at leading or unity power factor, thereby providing power-factor correction.

There are two major types of synchronous motors: nonexcited and direct-current excited.

Nonexcited motors are made in reluctance and hysteresis designs. These motors employ a self-starting circuit and require no external excitation supply.

Dc-excited motors come in sizes larger than 1 hp, and require direct current supplied through slip rings for excitation. Direct current may be supplied from a separate source or from a dc generator directly connected to the motor shaft.

Single-phase or polyphase synchronous motors can't start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the field is rotating at synchronous speed, the motor must be accelerated before it can pull into synchronism. Accelerating from zero speed requires slip until synchronism is reached. Therefore, separate starting means must be employed.

In self-starting designs, fhp sizes use starting methods common to induction motors (split-phase, capacitor-start, repulsion-start, and shaded-pole). The electrical characteristics of these motors cause them to automatically switch to synchronous operation.

Although the dc-excited motor has a squirrel cage for starting, called an amortisseur or damper winding, the inherent low starting torque and the need for a dc power source requires a starting system that provides full motor protection while starting, applies dc field excitation at the proper time, removes field excitation at rotor pull out (maximum torque), and protects the squirrel-cage winding against thermal damage under out-of-step conditions.

Pull-up torque is the minimum torque developed from standstill to the pull-in point. This torque must exceed load torque by a sufficient margin so that a satisfactory rate of acceleration is maintained under normal voltage conditions.

Reluctance torque results from the saliency (preferred direction of magnetization) of the rotor pole pieces and pulsates at speeds below synchronous. It also has an influence on motor pull-in and pull-out torques because the unexcited salient-pole rotor tends to align itself with the stator magnetic field to maintain minimum magnetic reluctance. This reluctance torque may be sufficient to pull into synchronism a lightly loaded, low-inertia system and to develop approximately a 30% pull-out torque.

Synchronous torque is torque developed after excitation is applied, and represents the total steady-state torque available to drive the load. It reaches maximum at approximately 70° lag of the rotor behind the rotating stator magnetic field. This maximum value is actually the pull-out torque.

Pull-out torque is the maximum sustained torque the motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor motors, and 175 to 200% for 0.8-leading-power-factor motors.

Pull-in torque of a synchronous motor is the torque that it develops when pulling its connected inertia load into synchronism upon application of excitation. Pull-in torque is developed during transition from slip speed to synchronous speed, as the motor changes from induction to synchronous operation. It is usually the most critical period in starting a synchronous motor. Torques developed by the amortisseur and field windings become zero at synchronous speed. At the pull-in point, therefore, only the reluctance torque and the synchronizing torque provided by exciting the field windings are effective.