Motor current

An important, and often underutilized, extruder measurement is the motor current. This is a measure of the electrical load required to turn the motor that drives the screw, displayed in either amps or percent of maximum load. At a specified rotational speed, current depends primarily on the mechanical resistance exerted by the polymer around the screw. Since the required current depends on the resistance provided by the polymer, it is an indication of the energy, or torque, being transmitted through the screw to the polymer. Further, it provides information on certain inherent properties of the polymer and on the stability of the processing conditions. 

Simulator Exercise The ammeter will show changes in motor current when adjustments are made to the mechanical energy input (screw speed) or the polymer viscosity (melt temperature). 

---

Overload Protection Overload relays lor protecting motor insulation against excessive temperature are located either in the motor control or in the motor itsell. The most common method is to use thermal overciirrent relays in the starter. These relays have heating characteristics similar to those ol the motor which they are intended to protect. Either motor current or a current proportional to motorline current passes through the relays so that relay heating is comparable to motor heating. 

Starter type Motor voltage Motor current Line or source current Motor torque Source voltage dip... 

The residual temperature fall in terms of lime, after the motor current is reduced to zero, can be expressed exponentially by... 

Motoring current drawn from the source Generating or braking current fed to the source... 

This will also decrease in the same proportion as the increase in slip. For a rough estimate, we may also ignore the higher slip losses for an equal reduction in the required kW. However, due to the lower voltage the motor current will increase proportionately and will be /,/0.8 or 1.25/,. The motor will thus run overloaded by 25% for 20 minutes which is likely, and not more than once an hour. The frequency of occurrence must be known. A higher derating may be necessary if such a condition is frequent. 

Such relays are in the form of a small tube inside which is a loosely fitted rotatable shaft, held by a very thin film of this alloy. The alloy senses the motor temperature through a heater connected in series with the motor terminals and surrounding this tube. When the motor current exceeds the predetermined value, the alloy melts and enables the shaft to rotate and actuate the lever of the tripping mechanism. 

In position 2 the capacitor is connected on the line side, although switched by the same switching device. The relay setting is not affected since only the full motor current will flow through the starter. In position 3 the capacitor is connected to the circuit, through an additional switch fuse unit. 

Shaft coupling Coupling slipping, sheared pin or key Cannot open valve Motor current when in operation, inspection... 

Reduce motor current pulsations and power costs. 

Primary resistance A resistance is inserted into the supply to the motor. This reduces the voltage available at the motor terminals on starting, and this voltage increases gradually as the motor current falls when the motor speeds up. Once the motor has reacted at a predetermined speed the resistance banks are short-circuited. 

The electrical current used by the motor is an important indicator of the performance of the extruder. The current as measured in amperage is directly proportional to the torque applied to the motor shaft and also to the screw. The motor current can be used to estimate the power and specific energy that are dissipated by the screw. The variation in the motor current with time is also an indicator of the stability of the extruder. Numerous examples of the motor current stability and process stabilities are presented in Chapter 12. 

Figure 10.12 Pow/er and torque as a function of motor speed for a 100 kW DC motor w/ith a maximum speed of 1750 rpm w/ith a full voltage on the field and at the maximum motor current. The motor can be operated at screw speeds up to 2100 rpm by weakening the field voltage...

Figure 10.13 Extrusion rate and motor current for an HDPE resin at a screw speed of 50 rpm. The highest rate was achieved at a Zone 1 temperature of about 230 °C...

Figure 12.5 An extrusion process with a downstream gear pump with the screw operating in inlet pressure control and followed by the screw in manual operation (constant screw speed). The large level of variation in the motor current during constant screw speed control suggests that the extruder process is unstable, and the control algorithm is not the root cause for the variation in the motor current...

---