Motor drive
Adjustable and variable speed drives may be purely mechanical (termed variators), electromechanical, hydraulic, or electronic.
Direct-current motors allow for changes of speed by adjusting the shunt field current.
Power electronics-based variable frequency drives are rapidly making older technologies redundant.
Historically, adjustable-speed drives were developed for process control, but energy conservation has emerged as an equally important objective.
Cycling the pumps on and off results in frequent high surges of electric current to start the motors that results in electromagnetic and thermal stresses in the motors and power control equipment, the pumps and pipes are subjected to mechanical and hydraulic stresses, and the sewage treatment process is forced to accommodate surges in the flow of sewage through the process.
Fans and pumps consume a large part of the energy used by industrial electrical motors.
Where fans and pumps serve a varying process load, a simple way to vary the delivered quantity of fluid is with a damper or valve in the outlet of the fan or pump, which by its increased pressure drop, reduces the flow in the process.
With a variable-speed drive on the pump or fan, the supply can be adjusted to match demand and no extra loss is introduced.
For example, when a fan is driven directly by a fixed-speed motor, the airflow is designed for the maximum demand of the system, and so will usually be higher than it needs to be.
Following the affinity laws, for 50% of the airflow, the variable-speed motor consumes about 20% of the input power (amps).
The input-output speed ratio is adjusted by moving the rollers to change the diameters of the contact path.
The transmitted torque is proportional to the pressure exerted by a hydraulic cylinder that presses the discs together.
Control can mean either manually adjustable - by means of a potentiometer or linear hall effect device, (which is more resistant to dust and grease) or it can also be automatically controlled, for example, by using a rotational detector such as a Gray code optical encoder.
Since the speed of a DC motor is directly proportional to armature voltage and inversely proportional to motor flux (which is a function of field current), either armature voltage or field current can be used to control speed.
Eddy current drives are slip-controlled systems the slip energy of which is necessarily all dissipated as heat.
Power proportional to the slip speed times operating torque is dissipated as heat in the clutch.
While it has been surpassed by the variable-frequency drive in most variable-speed applications, the eddy current clutch is still often used to couple motors to high-inertia loads that are frequently stopped and started, such as stamping presses, conveyors, hoisting machinery, and some larger machine tools, allowing gradual starting, with less maintenance than a mechanical clutch or hydraulic transmission.
Slip energy recovery variable-speed drives are used in such applications as large pumps and fans, wind turbines, shipboard propulsion systems, large hydro-pumps andgenerators and utility energy storage flywheels.
Regenerative AC drives are a type of AC drive which have the capacity to recover the braking energy of a load moving faster than the motor speed (an overhauling load) and return it to the power system.
