Save energy using a shredder driven by a frequency converter with field-oriented vector control. VFD vs LRS.

The most common configuration of the shredder drive - is the slip-ring induction motor with a liquid resistor that allows it to start smoother and prevent motor breakdown in the case of partial rotor lock by a hard-to-crash material. This configuration became incredibly popular a decade ago due to its simplicity, price, and ease of maintenance and still be the main option.

But times are changing and we all should go green, so even the best solutions should be reconsidered focusing on energy efficiency, caring about the planet, and your electricity bill.

So how is the efficiency of shredding with the LRS system going? Let's go deep

A shredder rotor is rotated by a slip-ring induction motor. Motor connected to mains with the circuit breaker. A liquid resistor system (LRS) is connected to the rotor windings via slip rings.

Before starting the rotation of the motor, the LRS moves electrodes to achieve the maximal resistance ~100*Short circuit rotor resistance. Then, the circuit breaker becomes closed and the motor rotation accelerates. During the acceleration, the resistance of LRS decreases until achieving the minimal value and the rotor accelerates to rated speed. During the start, the instant current increased to the value ~2 x full load current and then normalized to idle current value ~0.2-0.4 x full load current. When the feed begins, the motor operates with its full load value.

In the case of the shedder rotor lock by hard-to-crashing material, the resistor activates and adds resistance to the rotor circuit to prevent the motor breakdown. The speed of the motor decreases to ~70 % and consumed power jumps to ~180 %, the motor torque decreases to ~95-98%. The feed speed decreases until the normalization of the material structure, then the resistor recovers the motor stator circuit.

So as we can see there are two factors of energy loss and cost efficiency:

• In the moments of current surges, the excessive energy dissipates in the rotor resistor;
• The current peaks significantly increase the demand charges in the electricity bill.

Let's see how to improve the efficiency by using VFD:

The shredder rotor rotates the squirrel cage induction motor (or short-circuited slip ring wound rotor motor). Motor connected to the output of variable frequency drive. VFD connected to mains via the circuit breaker.

Variable frequency drive applies to the motor stator the AC sinusoidal voltage with predefined magnitude, phase, and frequency. The VFD using feedback of instant values of current controls the stator flux vector to maintain the maximal instant torque on the required speed.

During the start of motor rotation VFD linearly increases RPM from 0 to motor-rated speed, applying full torque at any speed and limiting the current by an exact value.

In the case of a shedder rotor lock by hard-to-crushing material, the VFD drops the speed increasing the torque without consuming the power peak. According to the equation of motor active power, if decreasing the speed there is the ability to increase the torque with constant consumed power. That provides to VFD system the ability to overcome hard-to-crashing material without significant feed reduction.

Due to motor magnetic flux control, VFD provides the possibility to maintain 100% of rated motor torque consuming 100 % of motor rated power in drop and recovery mode of shredder operation.

This means that all energy consumed by the motor with the LRS system would be saved with VFD operation as shown on geometry interpretation below: all energy consumed above the power limit does not dissipate to heat in the resistor and respectively not spent.

According to the calculations, the VFD with flux vector control allows to save around 19% of electricity costs according to Britain's industrial electricity prices. 40 % of this cost is related to the demand charge, the rest of it - is the savings of the active power.

This is how we can improve the efficiency of the shredder system precisely controlling torque and speed and if you are looking for the VFD that can do it - Triol AT27 link https://triolcorp.eu/products/triol-at27? and AT24? https://triolcorp.eu/products/triol-at24 drives are done exactly for this!

More? information about the application of VFD for Shredders can be found here: https://triolcorp.eu/solutions/improving-frequency-speed-and-torque-control-electric-motor-technological-breakthrough-can-increase-shredders-profitability?