Installation manual

GEO Direct PWM Amplifiers – Preliminary Documentation
4 Overview
Motor Poles
Usually, the number of poles in the motor is not a concern to the actual application. However, it should be
noted that each pole-pair of the motor requires an electrical cycle. High-speed motors with high motor
pole counts can require high fundamental drive frequencies that a drive amplifier may or may not be able
to output. In general, drive manufacturers with PWM switching frequencies (16kHz or below) would like
to see commutation frequencies less than 400 Hz. The commutation frequency is directly related to the
number of poles in the motor.
Motor Inductance
Typically, motor inductance of servomotors is 1 to 15 mH. The GEO drive product series can drive this
range easily. On lower-inductance motors (below 1mH), problems occur due to PWM switching where
heating currents flow through the motor, causing excessive energy waste and heating. If an application
requires a motor of less than 1mH, external inductors are recommended to increase that inductance.
Motors with inductance in excess of 15mH can still be driven, but are slow to react and typically are out
of the range of high performance servomotors.
Motor Resistance
Motor resistance is not really a factor in determining the drive performance, but rather, comes into play
more with the achievable torque or output horsepower from the motor. The basic resistance shows up in
the manufacturer's motor horsepower curve.
Motor Back EMF
The back EMF of the motor is the voltage that it generates as it rotates. This voltage subtracts from the
bus voltage of the drive and reduces the ability to push current through the motor. Typical back EMF
ratings for servomotors are in the area of 8 to 200 volts-per-thousand rpm. The GEO drive product series
can drive any range of back EMF motor, but the back EMF is highly related to the other parameters of the
motor such as the motor inductance and the motor Kt. It is the back EMF of the motor that limits the
maximum achievable speed and the maximum horsepower capability of the motor.
Motor Torque Constant
Motor torque constant is referred to as Kt and usually it is specified in torque-per-amp. It is this number
that is most important for motor sizing. When the load that the motor will see and knowing the motor's
torque constant is known, the drive amplifier requirements can be calculated to effectively size a drive
amplifier for a given motor. Some motor designs allow Kt to be non-linear, in which Kt will actually
produce less torque per unit of current at higher output speeds. It is wise to derate the systems torque
producing capability by 20% to allow headroom for servo control.
Motor Inertia
Motor inertial comes into play with motor sizing because torque to accelerate the inertia of the motor is
effectively wasted energy. Low inertia motors allow for quicker acceleration. However, consider the
reflective inertia from the load back to the motor shaft when choosing the motor's inertial. A high ratio of
load-to-motor inertia can cause limited gains in an application if there is compliance in the transmission
system such as belt-drive systems or rubber-based couplings to the systems. The closer the rotor inertia
matches the load's reflected inertia to the motor shaft, the higher the achievable gains will be for a given
system. In general, the higher the motor inertia, the more stable the system will inherently be. Mechanical
gearing is often placed between the load and the motor simply to reduce the reflected inertia back to the
motor shaft.