Delta Tau GEO PMAC Manual De Usuario

number of poles in the motor. 

PWM outputs require significant motor inductance to turn the on-off voltage signals into relatively 
smooth current flow with small ripple.  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 large ripple 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 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. 

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 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 de-rate the systems torque 
producing capability by 20% to allow headroom for servo control. 

Motor inertia 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 
reflected inertia from the load back to the motor shaft when choosing the motor’s inertia.  A high ratio of 
load-to-motor inertia can limit the achievable 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 be inherently.  
Mechanical gearing is often placed between the load and the motor simply to reduce the reflected inertia 
back to the motor shaft. 

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