In synchronous motor - Why the frequency should be made constant?

Replies

• 

  lal

  In simple words it is because frequency and speed are directly proportional.
  
  Speed, N = 120f/p
  N in rpm
  f= frequency (Hz)
  p= no. of poles
  
  So clearly speed is directly related to frequency. If frequency increases, speed also increases and vice versa. To have to motor run at constant speed, supply frequency should be kept constant.
  
  Infact, frequency control is a method used to control the speed of synchronous motors in various traction and industrial applications. The supply frequency is adjusted by power electronic devices to make the motor run at desired speed in such applications.
  
  In the case of a synchronous generator, the speed of the rotor should be kept constant in order to make the supply frequency a constant too. That is where governers are used to control the power input to the prime mover and keep the rotor run at constant speed.
• 

  sree_23

  thank you for your reply sir,i agree with your point in case of synchronous generator that is speed have to be controlled in order to controll the prime mover,but in case of synchronous motor why we have to maintain frequency..,is harmonics related to this?
• 

  Abhishek Rawal

  sree_23

  but in case of synchronous motor why we have to maintain frequency..,is harmonics related to this?

  It's one line answer : Because Synchronous motor works at constant speed only.
  Mathematically,Ns = 120f/p,Keeping poles constant then Ns is proportional to f.
  If frequency is fluctuating at every instantaneous of time then Synchronous motor will not work.
  In short,constant speed is needed to keep Synchronous motor working hence frequency is kept constant.
• 

  lal

  Well, see the relation between speed and frequency. Generally the supply frequency is not at all varied. For instance, in India it is 50Hz. So at 50Hz, the motor will be running at a constant speed by the relation
  N=120f/p
  Here f is 50Hz and the number of poles 'p' is a constant.
  
  If a variable frequency source (like a speed controller) is used, the speed will change in accordance with frequency. So the frequency has to be kept constant to obtain a constant speed at the output. Harmonics doesn't have any role in keeping the frequency constant.
• 

  Abhishek Rawal

  Adding to #-Link-Snipped-# post.
  As frequency is constant,poles are constant - hence the speed of synchronous motor mainly depends on "type of starter".
  There are different type of starters for different purposes & to gain different speed.
• 

  lal

  Well, the final speed of the motor will be the 'synchronous speed' corresponding to the Supply frequency, no matter which starter is used. Different starters give a different acceleration, but in the end the rotor speed locks with synchronus speed corresponding to the supply frequency and only depends on it.
  
  If the rotor rotates at a lower speed than the synchronous speed, it cannot produce the necessary torque to keep rotating and slowly comes to a halt.
• 

  Abhishek Rawal

  Interesting !
  I wanna know more.
  So,basically all synchronous motor has same speed ?
  If yes,then why we need different starters ?
  Isn't it that synchronous speed mainly depends upon rotor speed & stator speed ?
  If in India frequency used is 50Hz & I wanna increase speed of synchronous motor,then what should i do to achieve it ? 😀
• 

  pinank

  Troll_So_Hard

  Interesting !
  I wanna know more.
  So,basically all synchronous motor has same speed ?
  If yes,then why we need different starters ?
  Isn't it that synchronous speed mainly depends upon rotor speed & stator speed ?
  If in India frequency used is 50Hz & I wanna increase speed of synchronous motor,then what should i do to achieve it ? 😀

  
  Synchronous motor has constant speed.. We can not change the speed of the synchronous motor unless the frequency is varied or the number of poles are varied(that is not possible) as Ns=f/P.
  In all the starting methods the rotor is made to run at Synchronous speed as this will lead to interlocking of poles of the stator and rotor. As stator poles run at Synchronous speed the rotor will also run at synchronous speed.. So we can not change the speed of Synchronous motor. The basic advantage of this type of motor is that we can run it on different power factors i.e the stator current can be lagging, unity or leading wrt the stator voltage(the power supply)... 😉
• 

  lal

  Not all synchronous motors have the same speed. For the same frequency, it depends on the number of poles the motor is constructed with.
  
  Small synchronous motors do not need starters. But as the size increases, the inertia of rotor doesn't allow the machine to auto start. A starter is used to bring the rotor to synchronous speed or close to it before energising the rotor field.
  
  Synchronous speed depends on supply and the number of poles. In fact the rotor speed depends on synchronous speed and stator speed equals it.
  
  You have to use a variable frequency drive(VFD) to control the speed of a synchronous motor. With a VFD, the supply frequency can be adjusted which infact is an AC-AC converter. AC supply is first converted to DC which is then fed to an inverter to produce a variable frequency/voltage output to feed the synchronous motor.
• 

  pinank

  lal

  Not all synchronous motors have the same speed. For the same frequency, it depends on the number of poles the motor is constructed with.
  
  Small synchronous motors do not need starters. But as the size increases, the inertia of rotor doesn't allow the machine to auto start. A starter is used to bring the rotor to synchronous speed or close to it before energising the rotor field.
  
  Synchronous speed depends on supply and the number of poles. In fact the rotor speed depends on synchronous speed and stator speed equals it.
  
  You have to use a variable frequency drive(VFD) to control the speed of a synchronous motor. With a VFD, the supply frequency can be adjusted which infact is an AC-AC converter. AC supply is first converted to DC which is then fed to an inverter to produce a variable frequency/voltage output to feed the synchronous motor.

  using a VFD wont make the system a lot more expensive and what about the efficiency of the system?
• 

  lal

  Yeah, a VFD makes the total system much expensive. But this method of speed control doesn't have a large impact on the over all efficiency of the system. Yes, it decreases as nothing is ideal. Total efficiency becomes the product of VFD and motor efficiencies. In fact the use of vfd, practically, helps reduce the energy usage in the long run by limiting the power according to what is just required.