Equipment manufacturers and engine speed drives AC are best placed to give guidance on installation methods to use with their teams. This is why it is of fundamental importance to use the installation and maintenance manuals provided by such suppliers.

1. Line Impedance

Equipment manufacturers variable speed drives are very picky about the quality of energy supply that will receive their equipment.

The line impedance is a parameter that provides information on the quality of any power source.

The input impedance of the power lines can be determined two ways:

a). Measure the voltage between phases (line to line) in the motor without load (no load) and full rated load. Use these measured values ​​to calculate impedance as follows:

b). Calculate the short circuit current capacity of the power line. If such capacity exceeds the published values ​​of maximum short circuit current for control, should be installed in a line reactor. Following are two methods for calculating short-circuit capacity:

Method 1

Calculate the short circuit current:

Example: 50KVA Transformer with Impedance @ 460VAC 2.75%

Method 2

Step 1: Calculate short-circuit KVA:

Step 2: Calculate short-circuit current:

2. Motor Selection

Engine performance is affected by the drive that will be used as its power source. Therefore, it is important to select the drive and motor together. Engine performance is based on the capacity of the drive to be used and the ability to drive is based on the current consumption of the engine.

Analyze the profile of the machine and process. Determine where to place higher torque requirements. The magnitude of the torque, is used to define the minimum required motor size. See Figure 4-1, which shows the relationship between power and torque of a set of AC motors vs. the frequency of the wave control.

It is observed that for frequency above 60 Hz, the motors lose torque. Also for frequencies below 7 Hz motor manufacturer has not specified if the torque is maintained, being more likely to have a decline in value.

At the end of this unit in the section examples, it has a practical application to determine the size of the drive and motor in an application on conveyor (conveyor).

3. Thermal Considerations

Both electrical machines and variable speed drives are computers that control a large amount of energy and therefore must be sufficiently protected against overheating. For example, take for granted the use of forced ventilation systems for both the motor to the drive, so the installation location must be in a cool, dry place.

As a security measure is recommended to install a mechanism to prevent operation of the drive if the ventilation system has stopped working.

Currently working with motors have thermal protection drives among their windings, which is a thermostat with normally closed contacts.

The thermostat wires are called P1 and P2. Typically these cables are installed in the control terminal block corresponding to the "Reset" the drive. When there is an overload or over-temperature, the thermostat opens its contacts, which will force the drive to stop working.

Likewise, the drives have a thermostat located in contact with your heatsink, which when it detects over-temperature will inhibit its operation.

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 off the power to control the primary side.

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.