Method for changing speed of motor group

A method for changing the speed of a motor group, in particular for starting or stopping it, the motor group comprising a plurality of squirrel-cage induction motors or synchronous motors and network converters arranged for their control, when the nominal supply power, the acceleration power at the final speed of the motors and the losses of the used power are known, on starting, the method comprising:

• determining a first time instant, when the motor input power corresponds to the nominal supply power, acceleration taking place at a predetermined acceleration rate;
• accelerating the motors up to the first time instant substantially at the predetermined acceleration rate:
• calculating a base value for the acceleration rate as a function of the speed of the motors, at which acceleration rate the motor input power corresponds to the nominal supply power;
• accelerating the motors with a varying acceleration rate after the first time instant so as to achieve the final speed;
• and wherein the base value for the acceleration rate as a function of speed is determined by:
• a
• 2
• =
• (
• P
• n
• ⁢
• ⁢
• s
• ⁢
• V
• max
• 2
• )
• ⁢
• (
• V
• 1
• ⁢
• P
• acct
• -
• P
• loss
• ⁢
• v
• )
• T
• acct
• ⁢
• (
• P
• acct
• -
• P
• loss
• )
• ⁢
• (
• v
• ⁢
• ⁢
• V
• 1
• ⁢
• P
• acct
• )
• ,
• where V1 is the speed at the fist time instant;
• Pns is the nominal or maximum power;
• Vmax max is the final speed;
• V1 is the velocity at time T1:
• Pacct is the acceleration power to reach the final speed;
• Ploss is the power loss in the machine;
• v is the velocity.