Motor and gear settings

Motor Configuration

The servo drive needs several motor parameters to operate correctly. They can usually be found in the motor datasheet and are entered in the corresponding objects via the “OBLAC Drives” configuration wizard.

Pole pairs

In a three-phase PM motor, the number of poles depends on the rotor design and shows how many “south” and “north” magnetic poles exist. These magnetic poles are created by permanent magnets which are built into the rotor structure.

This number can be found in the motor’s datasheet.

Attention

If the number given in the datasheet is not designated as Pole pairs but Poles, it must be converted:

Number of pole pairs = \frac {1} {2} \cdot Number of poles

Torque constant

The torque constant is the ratio between generated torque and the phase current amplitudes. The torque constant unit is in [µNm / A rms]. The motor torque constant can be found in the motor’s datasheet.

If in the motor datasheet, the unit of torque constant is in [Nm / A peak], the following equation can be used to convert the unit of torque constant to [µNm / A rms]:

K_t \bigg[ \frac {Nm} {A_{rms}} \bigg] = \sqrt 2 \cdot K_t \bigg[\frac {Nm} {A_{peak}} \bigg]

Phase resistance

For a three-phase motor with star winding configuration, the phase resistance represents the resistance between motor terminals and the motor neutral point. If only phase to phase resistance is available in the motor datasheet, the value for phase resistance is half of the value phase to phase resistance in the motor datasheet:

Attention

Phase resistance = \frac {1} {2} \cdot Phase to phase resistance

Phase resistance of delta winding motors

The software assumes that the motor windings are in a star configuration. For a “delta” winding configuration, the values which should be entered in the configuration must be converted to the equivalent star winding configuration.

If the value for phase resistance is available in the motor datasheet, the motor phase resistance for the equivalent star configuration equals to:

Phase resistance equivalent star configuration = \frac {2} {3} \cdot Phase resistance motor datasheet

Phase to phase resistance of delta winding motors

If the value for phase to phase resistance (also designated as terminal to terminal resistance or line to line resistance) is available in the motor datasheet, the motor phase resistance for the equivalent star configuration equals to:

Phase resistance equivalent star configuration = \frac {1} {2} \cdot Phase to phase resistance motor datasheet

Phase inductance

For a three-phase motor with a star winding configuration, the phase inductance represents the inductance between the motor terminals and the motor’s neutral point. If only phase to phase inductance is available in the motor datasheet, the value for phase inductance is half of the phase to phase inductance value in the motor datasheet.

Attention

Phase inductance = \frac {1} {2} \cdot Phase to phase inductance

Phase to phase inductance of delta winding motors

If the value for phase to phase inductance (also designated as terminal to terminal inductance or line to line inductance) is available in the motor datasheet, the motor phase inductance for the equivalent star configuration equals to:

Phase inductance equivalent star configuration = \frac {1} {2} \cdot Phase to phase inductance motor datasheet

Note

Sometimes motor datasheets do not provide the phase inductance values but instead they provide the inductance values in direct and quadrature directions (Ld and Lq). In this case, the value of phase inductance can be calculated from the following equation:

Phase inductance = \frac {1} {3} \cdot (Ld + Lq)

Errors in phase resistance and inductance

Wrong values of the parameters phase resistance or phase inductance will affect the quality of the current loop controllers:

  • narrower bandwidth

  • less stability margins

Note

Internal algorithms prevent degradation of the steady state operation when the error is less than 20% of the exact value.

Motor phases inverted

This parameter depends on the order in which the motor terminals are connected to the inverter terminals. It can be either “normal” or “inverted”. Details can be found in Commutation Offset Detection. This parameter is detected automatically when the sensor offset measurement procedure is triggered.

If Motor phases inverted is set wrong (manually), the following problems might appear:

  • Wrong motion polarity (positive “torque target” resulting in negative “motor generated torque”).

  • Low efficiency of motor

Selecting the PWM frequency

The PWM frequency can be changed in object 0x2010:9 Torque controller: Switching frequency.

  • Enter the value 0 for 16 kHz

  • Enter the value 1 for 32 kHz

  • Enter the value 2 for 64 kHz

By default, the switching frequency is set to 16 kHz. Higher switching frequency values will result in higher switching losses in the inverter and less copper and iron losses in the motor. Moreover, higher switching frequencies will result in less ripples in currents and voltages of the DC link which results in less ripple amplitudes in the currents and voltages of the power supply.

Note

If the motor time constant is bigger than 100 ms, it’s recommended to use 16 kHz. If the motor time constant is less than 100 ms, or the ripple amplitudes of the DC-Link is too high, it’s recommended to select higher switching frequencies.

Gear Ratio

A gear ratio between motor and driving shafts can be configured. This ratio defines the relationship between the motor shaft and driving output shaft. The position feedback and command will always be relative to the driving shaft.

gear~ ratio = \frac {motor~ shaft~ revolutions} {driving~ shaft~ revolutions}

The feature dual-loop cascaded position controly can be used in such setups.

Advanced motor configurations

Attention

It is strongly recommended configuring the I2t protection before using the drive in regular operation.

  • Cogging Torque Compensation Removes ripples caused by magnetic interaction between motor rotor and stator.

  • Field Weakening The velocity range of electric motors can be extended by weakening the magnetic field of the rotor linearly over the speed range.