4. Wiring Considerations

The CC control is self-protected against transients and over-voltage pulses normal AC line. Perhaps additional external protection is required if high-energy transients present in the source of incoming power supply. These transients can be caused by sharing a power supply with arc welding equipment for direct start (through the line) of large motors or other industrial equipment requiring high transient over-currents.

To avoid damage caused by disturbances in the power source, you must consider:

a) Connect the control to a separate supply line for supplying large inductive loads.

b) Power to control power through a properly sized isolation transformer. By using an isolation transformer to power the control, you should always disconnect and connect (switch to "off" and "on") the power between the transformer secondary and the control input to avoid impulses (spikes) in the power control by removing the side

primary.

All wiring external to the control signals CC should be installed in conduit separate from other wiring. It is recommended to use shielded cables (shielded) twisted pair (twisted) for all signal wiring. The display control wiring must be connected only to ground control analog CC. The other end of the screen should be secured with tape to the cable jacket to prevent electrical short circuits.

The wires from the motor field and armature can be installed together in the same conduit, complying with NEC and local electrical codes and procedures.

5.-Effect of IGBTs

The introduction of IGBTs in the inverter frequency to increase the concern of the impact of these semiconductors in engine life, especially with regard to their isolation.

Any two wires have some capacitance and inductance. When current flows through the wire, it produces a magnetic field close to the drivers and when the current changes, the change in the magnetic field produces an electromotive force against the cable return.

The capacitance and inductance are proportional to the length of cable.

Where Z is the line characteristic impedance (Z * (alpha) * root (L / C)), and where R is the resistance of the load, except that R and Z are equal, there will be a reflected wave on the line and something of incident power reflected in R will return to the source. The resulting magnitude of the reflected wave can be twice the peak voltage amplitude. In a frequency, peak voltage magnitude is equal to the DC bus voltage.

The speed of the reflected wave is also dependent on the impedance of the cable. The rate of change dv / dt of the voltage source is used to determine distance from the source to the reflected wave will be fully developed. See Figure 4-2.

Some expressed concerns about the use of inverters with IGBTs are:

a) The IGBT switching characteristics have dv / dt (less time on), larger than standard bipolar devices, increasing the speed of the reflected wave.

b) Most switching speed enables higher carrier frequencies which increases the capacitance of the cable.

c) Both involve a shorter distance at which the reflected wave has its maximum amplitude and, therefore, a shorter length of cable to investors based on IGBTs. The result is a dual voltage DC bus can be applied across the motor windings, causing premature failure.

d) Assuming that in a typical installation, the impedances of the motor and cable are not equal, then reflected voltage will occur. Then there will always be reflected voltage and investors are used with IGBT, BJT and GTO.

Engine manufacturers have experience in implementing investor, have the following considerations:

1 .- The engine failures or reduced life expectancy in the engines are associated with its use in investors.

2 .- The required insulation requirements for the engines is twice the nominal voltage 1 000 volts. This is to protect against the reflected waves.

3 .- The smaller, cheaper engines have lower safety factors and therefore are more likely to harm.

The design criteria for the motors have higher expectations of life are:

A. Winding.

B. Isolation.

Conclusions:

1 .- There are indicators that support the introduction of IGBT investors have increased engine failures.

2 .- Any early failure can occur with various designs including BJT and GTO inverter.

3 .- engines must use quality (Inverter Duty Motors) for all applications and limit the length of the power cord whenever possible.

4 .- Whenever possible, all motor drivers, including the ground wire must be contained in a metal conductor.

6 .- overload current

The level (150% of nominal) control which automatically reduces the engine speed due to an overload condition by reducing voltage and frequency until the condition is lifted.

The relationship between overload and time is an inverse function. If the over ¬ load is continuous, the limit is approximately 105%.

In many applications where one or more of the motor is started across the line, the controller must be sized for the worst: up + startup condition, which takes into account the locked rotor current of the motor to be started. The control must be sized so it does not indicate failure on the current limit.

7. Low-impedance lines

Figure 4-3 shows a scenario that can be given occasionally. A computer drive is installed near the main power supply or a bank of power factor correction.

Due to proximity, the drive may experience failures diode bridges as being linked to the low-impedance. Power semiconductors can fail from excessive di / dt of the current.

The working characteristic of the drive allows them to operate under normal conditions of food and distance from the main bar and provided enough power and impedance for safe operation.

One way of resolving the problem shown in Figure 4-3, is to increase the line impedance to the inverter.

This objective is met with adding an isolation transformer, as shown in Figure 4-4.

8. High impedance lines

Figure 4-5 presents a case contrary to earlier. Here the power supply is remote or poor contacts on your joints that work when the team have voltage drops that do not support the operation of the drive.

9. Distance between drive and AC motor

Manufacturers specify the maximum distance you must have the drive and AC motor. Sometimes due to the application, it is possible to respect this distance and see the need to work at longer distances. Is there a solution to the problem at hand?. The answer is yes. These manufacturers provide us with the necessary devices to achieve greater distances. One of the solutions are presented in Figure 4-6.