When using the TI-3000JX, a common question is why a particular motor will not run up to full rated
speed. Of course, running speed is one of the parameters you want to evaluate when test running
a motor after a repair, so it is an important question. Most people who have used the TI-3000JX for
very long have noticed that some motors can easily be run up to speed while others cannot. This
document will describe some reasons for this speed variation and provide some guidance as to
whether a motor failing to run at rated speed is a problem or not. In general, the TI-3000JX should be able to run even the largest PM brushless motors in an unloaded setup. The main concern then turns into figuring out what the max run speed will be. The answer is a combination of how much voltage you are supplying to your drive and how much voltage the motor generates while turning (KE).
Voltage and Current when Running a PM Brushless Motor
It is tempting to think of a permanent magnet brushless motor in terms of an AC synchronous
motor. That, basically, is what it is. But, AC synchronous motors are typically connected to a
constant frequency, 3 phase AC drive voltage and run at synchronous speed (which depends on
the voltage frequency and number of motor poles).
When a PM brushless motor is connected to a servo motor drive, it actually behaves much more
like a brush type DC motor. An unloaded motor will run at a speed that is basically determined by
the voltage that is applied to it. The frequency of the drive voltage is determined by the feedback
signal which is determined by the motor speed. So, the drive voltage sets the speed, and the
motor feedback tells the drive when to switch phases (thus setting the frequency) based on the
motor position (and speed).
The current has little to do with the speed. The motor will draw current from the drive based on
the load torque that it has to overcome. An unloaded motor will draw very little current. When an
unloaded motor is not running up to speed, it almost never has anything to do with the current
capacity of the drive.
The major reason that voltage is responsible for setting the motor speed is due to the generated
voltage. Servo technicians are familiar with the generated voltage that is created by back driving
a motor. Back driving is the technique of mechanically coupling a drive motor to the shaft of the
motor under test and turning that motor so that it acts as a generator and generates a voltage at
its armature leads. Technicians do this to check whether the magnets and windings are doing
their job and sometimes to use dynamic feedback alignment techniques. As you know from
experience, the faster you drive the motor, the higher the generated voltage. Normally it will be
proportional in that, if you double the speed, the voltage will double as well.
The important thing to remember is that this same generated voltage is created when the motor is
connected to a drive and run normally. The generated voltage is in opposition to the drive voltage.
The drive voltage must be greater than the generated voltage or the drive will not be able to drive
current into the motor and overcome the torque. It is similar to the fact that the voltage from a
battery charger must be larger than the battery voltage in order to drive current into the battery
and charge it.
In the case of the unloaded motor, the torque is small (bearing and seal friction mostly), which
makes the required current small, and this means that the drive voltage will only be slightly higher
than the generated voltage.
An important concept is probably becoming obvious. A drive cannot run a motor at an RPM that
generates a voltage higher than the maximum drive voltage. In fact, the RPM has to be
somewhat less than that.
KE Constant for PM Brushless Motors
Most servo technicians are familiar with the KE constant for servo motors. This constant specifies
the voltage that will be generated when the motor is driven at 1,000 RPM. This constant is what
really determines how fast you can run a motor on a drive. Since there is a wide variation in
values for this constant, there will be a wide variation in maximum speeds for motors on the TI-3000JX.
Theoretically you should be able to use the KE number to figure out the TI-3000JX max speed.
However, sometimes this number is in RMS voltage, sometimes peak voltage, and some
nameplates do not even list it. It is probably just as simple to back drive a motor at 1,000 RPM
and read the U-V peak voltage on a scope. That way you know what the number means.
IMPORTANT: When measuring generated voltage while back driving a motor, the
motor that is being measure should never be connected to a drive. The armature
leads from the motor under test should be connected only to a meter or
Typical Motor Example
An Allen Bradley Model 1326AB-B410G-M2L motor was used to provide some example numbers.
The following back generated voltages were recorded for the corresponding speeds in RPM:
The generated voltage was read in RMS with a calibrated Fluke meter and multiplied by 1.414 to
get the peak voltage. This motor produced a very good sine wave generated voltage, so this
calculation should be accurate. Some motors do not produce a good sine wave and using the
peak voltage from a scope would be more accurate.
The power line voltage was about 117VAC RMS, so the drive would be able to rectify that to
about 117 X 1.414 = 165 VDC at the very best. It would most likely be somewhat less.
The generated voltage at 1,430 RPM was about 161 volts peak, and this is about the maximum
RPM at which the drive would run this motor. Now we see why it would not run any faster. The
motor’s generated voltage was near the maximum voltage that the drive was capable of
In this case the AMC B25B20AC drive was used. Similar results would be expected with the
B30A40AC drive connected to the same AC line voltage of 117VAC. However, the B30A40AC
drive can be run from 240VAC and this would produce 240 X 1.414 = 340 VDC. We would expect
to be able to run the motor at nearly 3,000 RPM in that situation.