Installation manual

Geo Direct PWM Amplifier
Connector PinOuts and Wiring 27
Motor Torque
Torque requirements in an application can be viewed as both instantaneous and average
Typically, the instantaneous or peak torque is the sum of machining, and frictional forces required to
accelerate the inertial load. The energy required to accelerate a load follows the equation T=JA where T is
the torque, J is the inertia, and A is the acceleration. The required instantaneous torque is then divided by
the motor torque constant (Kt) to determine the necessary peak current of the Geo Direct PWM Drive.
Headroom of about 10% is always desirable to account for miscellaneous losses (aging, wear and tear,
calculation roundups).
The continuous torque rating of the motor is bound by thermal limitation. If the motor applies more
torque than the specified threshold, it will overheat. Typically, the continuous torque ceiling is the RMS
current rating of the motor, also known as torque output per ampere of input current.
Required Bus Voltage for Speed and Torque
For a required motor Speed, and continuous Torque, the minimum DC Bus Voltage (V
DC
) can be
estimated by looking at the equivalent single phase circuit:
The vector sum of back EMF, voltage across resistor and inductor should be less than
6/V
DC
.
For a Rotary Motor:
 
6
V
Mπ2
3
K
60
R
R
K
T
K
T
Lπ2N
60
R
VVV
DC
derate
2
t
RPM
p
t
M
2
t
M
pp
RPM
2
BEMFR
2
L



















Where:
V
L
: Voltage Across equivalent inductor
V
R
: Voltage Across equivalent resistor
V
BEMF
: Back electromotive force voltage
R
RPM
: Required Motor Speed [rpm]
N
P
: Number of pole pairs
L
P
: Phase Inductance [H]
R
P
: Phase Resistance [Ω]
T
M
: Required Continuous Torque [N.M]
K
T
: Motor Torque Constant RMS [N.M/A]
M
derate
: De-rate parameter (typically 0.8)
For a Linear Motor:
 
6
V
M
3
K
D
V
R
K
F
K
F
L
D
V
VVV
DC
derate
2
t
pitch
motor
p
t
M
2
t
M
p
pitch
motor
2
BEMFR
2
L



















Where:
V
L
: Voltage across equivalent inductor
V
R
: Voltage across equivalent resistor
V
BEMF
: Back electromotive Force voltage
V
motor
: Required Motor Speed [m/s]
M
derate
: De-rate parameter (typically 0.8)
L
P
: Phase Inductance [H]
R
P
: Phase Resistance [Ω]
F
M
: Required Motor Force RMS [N]
K
t
: Motor Force Constant RMS [N/A]
D
Pitch
: Magnetic Pitch [m]
Example:
An application requires running a motor at 500 RPM with a continuous torque of 30 N.M. The motor
specs are as follow:
10mHL
p

,
2OhmR
p

,
16N
p

,
ps2.187Nm/AmK
t

Using the equation above, a minimum bus of 233 VDC (~165VAC) is necessary to achieve the speed and
torque requirements.