Speed control for induction motor having improved sensing of motor operative conditions

A method for controlling the speed of the rotor of an AC induction motor and avoiding regenerative operation of the motor;

• said motor having a stator winding electromagnetically coupled to the rotor, the stator winding having input terminals coupled to an AC power means having a DC bus means along which power for the motor is provided, the polarity of the voltage on said DC bus means remaining the same during motoring or regenerative operation;
• the operating frequency and the current magnitude of the AC power means being determinable by frequency command and current command signals, respectively;
• there being a synchronous speed of the motor corresponding to the operating frequency of the AC power means;
• said method comprising the steps of:
• establishing a reference signal proportional to the desired speed of the rotor;
• determining the magnitude of the CEMF of the motor and providing a signal proportional to same;
• applying the CEMF signal, as a feedback signal, in opposition to the speed reference signal to form an error signal;
• amplifying the error signal to provide a current command signal to the AC power means;
• supplying the CEMF signal to the AC power means as the frequency command signal for varying the operating frequency of the AC power means in accordance with the magnitude of the CEMF of the motor so that the magnetizing current component of the stator current may be maintained at a constant value at all operating frequencies;
• determining the magnitude of a load current component of the stator winding current and providing a signal proportional to the load current component;
• applying the load current component signal to the speed reference signal in a manner that is additive to the speed reference signal;
• comparing the magnitude of the unipolarity voltage of the DC bus means of the AC power means with the magnitude of a reference signal to determine whether the motor is motoring or regenerating;
• and
• allowing the operating frequency of the AC power means to be determined by the magnitude of the CEMF signal when the magnitude of the voltage of the DC bus means is less than the magnitude of the reference signal and increasing the frequency command signal to the AC power means when the magnitude of the voltage of the DC bus means is greater than the magnitude of the reference signal thereby increasing the operating frequency of the AC power means and avoiding regenerative operation of the motor.

A method for determining the speed of the rotor of an AC induction motor, said motor having a stator winding electromagnetically coupled to the rotor and forming a magnetizing inductance for the motor, the stator winding having terminals coupled to an AC power means having a DC bus means along which power for the motor is provided, the polarity of the voltage on the DC bus means remaining the same during motoring and regenerative operation;

• the operating frequency and the current magnitude of the AC power means being variable, there being a synchronous speed of the motor corresponding to the operating frequency of the AC power means, the rotor speed differing from the synchronous speed by the slip speed of the rotor;
• said method comprising the steps of:
• determining the magnitude of the stator winding current at the terminals of the stator winding;
• determining the magnitude of the magnetizing current component of the stator winding current, the flow of the magnetizing current component through the magnetizing inductance of the motor inducing a CEMF across the magnetizing inductance which is out of phase with the magnetizing current component;
• determining the magnitude of the load current component of the stator winding current from the magnitudes of the stator winding current and the magnetizing current component, the magnitude of the load current component being proportional to rotor slip speed;
• comparing the magnitude of the unipolarity voltage of the DC bus means of the AC power means with the magnitude of a reference signal to determine whether the motor is motoring or regenerating;
• and
• algebraically summing the rotor slip speed, as determined by the load current component, and the synchronous speed, corresponding to the operating frequency of the AC power means, in accordance with said comparison to determine the speed of the rotor by subtracting the slip speed from the synchronous speed when the magnitude of the voltage of said DC bus means is less than the magnitude of said reference signal and adding the slip speed to the synchronous speed when the magnitude of the voltage of said DC bus means is greater than the magnitude of the reference signal.

A method for energizing the stator winding of an AC induction motor from an AC power means at a frequency providing a synchronous speed equal to the existing speed of the rotor, said stator winding being electromagnetically coupled to the rotor;

• the stator winding being connected to the AC power means, the operating frequency and the current magnitude of the AC power means being variable, the AC power means having a DC bus means along which power for the motor is provided, the polarity of the voltage of the DC bus means remaining the same during motoring and regenerative operation;
• the synchronous speed of the motor corresponding to the operating frequency of the AC power means, the rotor speed differing from the synchronous speed by the slip of the rotor;
• said method comprising the steps of:
• applying AC energization of an arbitrarily selected magnitude and frequency as excitation to the stator winding;
• determining the CEMF generated by the excitation of the stator winding for providing a current magnitude controlling signal to the AC power means;
• comparing the magnitude of the unipolarity voltage of the DC bus means of the AC power means with a reference signal to determine whether the DC bus means voltage is greater or less than the reference signal, the reference signal having a magnitude corresponding to the voltage of the DC bus means when the motor is operating at synchronous speed;
• and
• altering the operating frequency of the AC power means in accordance with the foregoing comparison, from the selected frequency to a frequency that causes the synchronous speed of the motor corresponding to the operating frequency of the AC power means to be equal to the rotor speed by decreasing the operating frequency of the AC power means when the magnitude of the unipolarity voltage of the DC bus means is less than the magnitude of the reference signal and increasing the operating frequency when the magnitude of the unipolarity voltage of the DC bus means is greater than the magnitude of the reference signal.

A control for energizing an AC induction motor from an AC power means, the rotor of the motor rotating at the time the energization of the motor is commenced;

• the motor having a stator winding electromagnetically coupled to the rotor, the stator winding having terminals couplable to the AC power means, the AC power means having a DC bus means along which power for the motor is provided, the polarity of the voltage of the DC bus means remaining the same during motoring and regenerative operation;
• the operating frequency and the current magnitude of the AC power means being determinable by current command and frequency command signals, respectively;
• there being a synchronous speed of the motor corresponding to the operating frequency of the AC power means, the rotor speed differing from the synchronous speed by the slip of the rotor;
• said control comprising:
• means for providing a signal corresponding to an arbitrarily selected speed of said motor;
• means for providing a signal corresponding to the magnitude of CEMF developed in the stator winding as a result of the flow of the magnetizing current component of the stator current through the stator winding, said CEMF signal being appliable to the AC power source as the frequency command signal to vary the frequency of the AC power means in accordance with the magnitude of the CEMF signal, so that the magnetizing current component of the stator current may be maintained at a constant value at all frequencies;
• regulating means having input means receiving said speed signal as a reference signal and said CEMF signal as a feedback signal, said regulating means having an output providing a signal appliable to the AC power source as the current command signal for establishing the magnitude of the stator winding current in the stator winding;
• means for providing a signal proportional to the magnitude of the load current component of the stator current in the stator winding, said load current component signal being applied to said input means of said regulating means in a manner additive to said speed signal;
• means for comparing the magnitude of the unipolarity voltage of the DC bus means of the AC power means with a reference signal to determine whether the DC bus means voltage is greater or less than the reference signal, the reference signal having a magnitude corresponding to the voltage of the DC bus means when the motor is operating at synchronous speed;
• and
• means for allowing the frequency of the AC power means to be determined by the magnitude of the CEMF signal when the magnitude of the unipolarity voltage on the DC bus means is less than the magnitude of the reference signal and for increasing the frequency command signal to the AC power means when the magnitude of the unipolarity voltage in the DC bus means is greater than the reference signal.