Installation manual

GEO Direct PWM Amplifiers – Preliminary Documentation
Overview 5
Motor Cabling
Motor cables are an integral part of a motor drive system. Several factors should be considered when
selecting motor cables. First, the PWM frequency of the drive emits electrical noise. Motor cables must
have a good-quality shield around them. The motor frame must also have a separate conductor to bring
back to the drive amplifier to help quench current flows from the motor due to the PWM switching noise.
Both motor drain wire and the cable shield should be tied at both ends to the motor and to the drive
amplifier.
Another consideration in selecting motor cables is the conductor-to-conductor capacitance rating of the
cable. Small capacitance is desirable. Longer runs of motor cable can add motor capacitance loading to
the drive amplifier causing undesired spikes of current. It can also cause couplings of the PWM noise into
the earth grounds, causing excessive noise as well. Typical motor cable ratings would be 50 pf per foot
maximum cable capacitance.
Another factor in picking motor cable is the actual conductor cross-sectional area. This refers to the
conductor's ability to carry the required current to and from the motor. When calculating the required
cable dimensions, consider the agency requirements, safety requirements, maximum temperature that the
cable will be exposed to, the continuous current flow through the motor, and the peak current flow
through the motor. Typically, it is not suggested that any motor cable be less than 14 AWG.
The motor cable’s length must be considered as part of the application. Motor cable length affects the
system in two ways. First, additional length results in additional capacitive loading to the drive. The
drive's capacitive loading should be kept to no more than 1000 pf. Additionally, the length sets up
standing waves in the cable, which can cause excessive voltage at the motor terminals. Typical motor
cable length runs of 200 feet for 230V systems and 50 feet for 480V systems are acceptable. Exceeding
these lengths may put other system requirements in place for either a snubber at the motor end or a series
inductor at the drive end. The series inductor at the drive end provides capacitance loading isolation from
the drive and slows the rise time of the PWM signal into the cable, resulting in less voltage overshoot at
the motor.