Controllers for electric travelling cranes or electric vehicles may be mounted on the mobile equipment. Industrial motors may be more complex controllers connected to automation systems a factory may have a large number of motor controllers grouped in a motor control center. Power tools and household appliances may have a trigger switch that only turns the motor on and off. A wall-mounted toggle switch with suitable ratings may be all that is needed for a household ventilation fan. The physical design and packaging of motor controllers is about as varied as that of electric motors themselves. Alternating current motors may have little or no speed response to adjusting terminal voltage, so controllers for alternating current instead adjust rotor circuit resistance (for wound rotor motors) or change the frequency of the AC applied to the motor for speed control using power electronic devices or electromechanical frequency changers. For direct-current motors, the controller may adjust the voltage applied to the motor, or adjust the current flowing in the motor's field winding. Some types of motor controllers also allow adjustment of the speed of the electric motor. Motor controllers may be manual, requiring an operator to sequence a starting switch through steps to accelerate the load, or may be fully automatic, using internal timers or current sensors to accelerate the motor. Some motor controllers limit the inrush starting current, allowing the motor to accelerate itself and connected mechanical load more slowly than a direct connection. A motor controller may also supervise the motor's field circuit, or detect conditions such as low supply voltage, incorrect polarity or incorrect phase sequence, or high motor temperature. A controller includes means to connect the motor to the electrical power supply, and may also include overload protection for the motor, and over-current protection for the motor and wiring. Motor controllers are used with both direct current and alternating current motors.
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