Alternating current motor control system with emergency control responsive to failure of power supply

A control system for controlling an induction machine which is selectively driven in power mode in which alternating current (AC) power is supplied from an AC source thereto and a regeneration mode in which AC power is regenerated therein, comprising:

• a converter connected to the source of AC power for converting the AC power into DC power;
• an inverter connected through relatively positive and negative conductors to the converter for inverting the DC power into AC output power having adjustable frequency, voltage magnitude and current magnitude to drive the machine;
• the inverter including a rectifier for providing DC in response to the AC power from the machine for regeneration mode;
• the converter including regeneration means controlled to permit flow of DC from the rectifier to the AC source during regenerative mode;
• an electric resistor connected to a common output of the converter and rectifier, responsive to a dynamic brake signal, for dissipating power caused by a flow of DC therethrough;
• a mechanical brake, responsive to a mechanical brake signal, for mechanically braking the machine;
• a control circuit for controlling the inverter to drive the machine at an adjustable speed, the control circuit including a first emergency control means responsive to a power failure signal for disabling the regeneration means to prevent flow of AC from the regeneration means to the AC source and reducing the current magnitude of the inverter output AC of a predetermined minimum limit low enough to maintain a synchronous relationship between the inverter and the machine, said first emergency control means for providing the dynamic brake signal for dynamically braking the machine when the power failure signal continues for a predetermined first period and a second emergency control means responsive to at least one of current level and voltage level in the converter exceeding the predetermined level during regeneration mode operation of the control for the machine, for providing the mechanical brake signal for activating the mechanical brake for decelerating the machine, said second emergency control means for monitoring the condition of the power source while the mechanical brake is active, the second emergency control means activating the first emergency control means when a power failure signal is detected while the mechanical brake is held active, and for otherwise providing the braking signal for controlling braking torque of the mechanical brake for decelerating the machine to stop.

Apparatus for controlling a rotating machine powered by a fixed frequency alternating current (AC) source, comprising:

• first rectifier means, responsive to power provided by said AC source, for providing direct current (DC) to a DC bus;
• inverter means, responsive to said DC from said first rectifier means and to a variable frequency drive control signal, for providing variable frequency power for driving said machine as a motor;
• second rectifier means, responsive to AC from said machine operating as a generator for providing DC to sad DC bus;
• means responsive to said AC source for sensing a power failure of said AC source or of said DC bus for providing an AC power status signal (NV) indicative thereof;
• means responsive to said DC bus for providing a DC power status signal (UV);
• a capacitor, responsive to said DC bus for smoothing said DC from said first rectifier means and responsive to said DC from said second rectifier means for storing energy provided by said machine operating as a generator;
• means responsive to said DC bus for sensing a voltage greater than a selected voltage for providing a DC overvoltage signal (OV);
• power supply means, responsive to said DC bus, for providing a regulated power supply signal;
• control means, responsive to said regulated supply signal for providing said variable frequency drive control signal for controlling said machine at a selected electrical current magnitude, responsive to both of said power status signals (NV, UV) and having means responsive to a persistence of said DC overvoltage signal for a selected period, for providing a dynamic brake control signal thereafter;
• and
• dynamic brake means, responsive to said dynamic brake control signal, for absorbing energy provided by said machine operating as a generator for slowing its rate of rotation.

An AC machine drive control having a first rectifier for supply by a fixed frequency AC source for providing DC to a first inverter controlled by first switching signals for providing variable frequency AC for operating the machine as a motor for hoisting an elevator at a variable speed and having a second rectifier responsive to AC from the machine during operation thereof as a generator for providing DC to a second inverter controlled by second switching signals for providing the AC source with AC at a fixed voltage and frequency, the control further comprising:

• first sensor means, responsive to a failure of said AC source, for providing a first failure signal;
• first failure mode means responsive to said first failure signal for preventing said second switching signals from being provided to prevent said second inverter from providing said AC source with fixed voltage and frequency AC from said second inverter for controlling said first switching signals, for limiting the magnitude of the current in said machine to a selected minimum sufficient to maintain synchronism between said limited current and said machine and for providing a first dynamic braking signal after said first failure signal has persisted for a selected period for thereafter initiating dynamic braking;
• dynamic braking means, responsive to said first dynamic braking signal and a second dynamic braking signal for absorption of DC from said second rectifier for slowing said machine;
• second sensor means, responsive to one of said second inverter providing in excess of a selected AC current or said second rectifier providing in excess of a selected DC voltage, for providing a second failure signal;
• a mechanical brake, responsive to a mechanical brake signal for providing a brake force for said elevator or machine;
• second failure mode means responsive to said second failure signal for providing said mechanical brake signal for initiating mechanical braking forces on said elevator or machine of insufficient magnitude and for a selected period of insufficient duration to prevent synchronism between AC provided by said first inverter and said machine and subsequently responsive to said first failure signal for providing a second mechanical brake signal for increasing the magnitude or period of duration of said braking force for stopping said elevator or machine but only in the absence of said first failure signal during said selected period and otherwise preventing said second inverter from providing said AC source with fixed voltage and frequency AC from said second inverter.

A failure management method for a variable speed control supplied by a fixed frequency AC source for controlling an AC machine capable of operating in a motoring mode and a regenerative mode, comprising the steps of:

• sensing an overvoltage or overcurrent condition and providing an excess power signal;
• providing a braking signal in response to the excess power signal;
• decelerating the machine in response to the braking signal;
• timing the duration of the excess power or braking signal and providing a stop signal after a selected duration;
• and
• decelerating the machine with increased force to a stop in response to the stop signal.

A variable speed control supplied by a fixed frequency AC source for controlling an AC machine capable of operating in a motoring mode and a regenerative mode, comprising:

• means for sensing an overvoltage or overcurrent condition for providing an excess power signal;
• means for providing a braking signal in response to the excess power signal;
• means for decelerating the machine in response to the braking signal;
• means responsive to the excess power or braking signal for timing the duration thereof and for providing a stop signal after a selected duration;
• and
• means for decelerating the machine with increased force to a stop in response to the stop signal.