An electric asynchronic engine with a wound rotor, liquid rheostat
and dynamometer was used as the engine brake on the mounted universal stand.
Quad Plus Inc., Joliet, Ill., is offering the SPR (slip power recovery) drive as an alternative to liquid rheostat models used in auto shredder systems.
When the motor current reaches a pre-defined current limit (such as 190 percent or 250 percent), the speed drops, but it recovers more quickly than with the liquid rheostat, according to the company.
As a solid-state piece of equipment, the SPR drive does not require the same maintenance associated with a liquid rheostat device, and no cooling tower is needed, according to Quad Plus.
A variable speed drive may be obtained by 1) using a wound rotor or slip-ring motor directly connected to the pump; 2) using a constant speed motor connected to the pump through a variable speed hydraulic coupling, magnetic drive, or eddy current coupling; 3) steam turbines with reducing gears; 4) a wound rotor induction motor with a secondary speed control by mechanical or liquid rheostat
; 5) an adjustable frequency power supply provided by a synchronous generator; and 6) a variable frequency converter, such as an adjustable frequency drive, for energy savings.
* Liquid rheostat
maintenance and cooling water requirements.
For AC wound-rotor drives, shredder manufacturers buy new motors from Germany and may hire Mueller or another company to install and maintain the liquid rheostat. This has proven to be a reliable solution for manufacturers to offer their customers.
AC wound-rotor drives with liquid rheostats (resistors for regulating a current by means of variable resistances) have primarily been installed and maintained by companies such as Mueller Engineering.
"People think AC drives and liquid rheostats are maintenance free, but they require just as much service as a DC System."
Speed control is provided by two liquid rheostats
(adjustable resistors with separable electrodes in a water/sodium-carbonate electrolyte).
The new drives and motors will replace a pair of 35,000-hp induction motors and their liquid rheostats
which are now used to control speed.