Installation manual

GEO Direct PWM Amplifiers – Preliminary Documentation
18 System Wiring
WARNING
Installation of electrical control equipment is subject to many regulations including
national, state, local, and industry guidelines and rules. General recommendations
can be stated but it is important that the installation be carried out in accordance
with all regulations pertaining to the installation.
Wire Sizes
Drive electronics create a DC bus by rectifying the incoming AC electricity. The current flow into the
drive is not sinusoidal but rather a series of narrow, high-peak pulses. Keep the incoming impedance
small to not hinder these current pulses. Conductor size, transformer size, and fuse size recommendations
may seem larger than normally expected. All ground conductors should be 8AWG minimum using wires
constructed of many strands of small gauge wire. This provides the lowest impedance to high-frequency
noises.
Fuse and Circuit Breaker Selection
In general, fusing must be designed to protect the weakest link from fire hazard. Each Geo drive is
designed to accept more than the recommended fuse ratings. External wiring to the drive may be the
weakest link as its routing is less controlled than the drive’s internal electronics. Therefore, external
circuit protection, be it fuses or circuit breakers, must be designed to protect the lesser of the drive or
external wiring.
High peak currents and high inrush currents demand the use of slow blow type fuses and hamper the use
of circuit breakers with magnetic trip mechanisms. Generally, fuses are recommended to be larger than
what the rms current ratings require. Remember that some drives allow three times the continuous rated
current on up to two axis of motion. Time delay and overload rating of protection devices must consider
this operation.
Use of GFI Breakers
Ground Fault Interrupter circuit breakers are designed to break the power circuit in the event that not all
outgoing currents can be accounted for by the returning currents. These breakers assume that if outgoing
currents are not returning then there is a short circuit at the load. Most circuit breakers of this type account
for currents as low as 10mA. This operation is problematic when used with PWM drive technology. The
PWM switching allows parasitic leakage currents to flow into the ground system in the motor cable and
the motor frame. Careful installation practices must be followed. The use of inductor chokes in the output
leads of the drive is often required to keep these leakage currents below a few hundred milliamps.
Normally the use of GFI breakers is not recommended because of these problems. Adjustment of a GFI
breaker may be necessary.
Transformer and Filter Sizing
Incoming power design considerations for use with Geo Drives require some over rating. In general, it is
recommended that all 3-phase systems using transformers and incoming filter chokes be allotted a 25%
over size to keep the impedances of these inserted devices from affecting stated system performance. In
general, it is recommended that all single-phase systems up to 1kW be designed for a 50% overload. All
single-phase systems over 1kW should be designed for a 200% overload capacity.