How to dimension a power supply for an Ingenia drive

Choosing an appropriate power supply is an important step for a successful motion control system solution.  The choice of a power supply is mainly determined by voltage, current and power supply type.

Following are shown some guidelines to properly dimension the power supply for an Ingenia drive:

Voltage

The power supply voltage should be targeted for the motor requirements, without exceeding the nominal ratings of the Ingenia drive. For example, up to 48 V for the CAP-XCR-E. Make sure that the voltage rating of the power supply does not exceed the voltage rating of the motor, otherwise it could be damaged.

To calculate your power supply voltage needs use the motor specifications or curves and the following formula for a DC brushed:

For brushless motors when you know the phase to neutral values (for stator star connection):

• Trapezoidal commutation (BLDC):

• Sinusoidal commutation (BLAC, PMSM): 

For brushless motors when you know the phase to phase values (terminal to terminal):

• Trapezoidal commutation (BLAC):

• Sinusoidal commutation (BLAC, PMSM): 

Where:

V_DCbus is the power supply voltage (for a DC driver).

V_BEMF is the motor back EMF voltage at the maximum operating speed (in crest value, no RMS). This can be calculated from the motor datasheet. 

R_phase is the resistance of the motor coils.

I_Max is the maximum operating current of the motor in crest value (no RMS).

Take extra care with triangle - star connections and the datasheets provided by the motor manufacturers. It is a typical source of error when calculating the real BEMF.

It is not always necessary to operate the system at the maximum driver / motor voltage!

In many applications it is just not necessary to go to the maximum driver or motor voltage. Higher voltages mean:

• Higher EMI (Electromagnetic interference) problems

• More switching losses (heat)

• Higher current ripple to the motor (di/dt)

• And oversized power supply (cost)

• Lower power stage PWM resolution resulting to lower control performance. Since motor applied voltage is DC bus * PWMduty, the higher the voltage, the higher the steps.

There is no problem in operating a motor in a voltage lower than the nominal.

Nominal current

The nominal current of the power supply should be calculated from the maximum output power of the motor. Since a drive is a power converter, the input DC current does not have the same value as the output phase current.

Output power is calculated as the product between mechanical speed and power, or in electrical parameters, the product between the output voltage and current (RMS). In a three-phase system, the product of phase-to-neutral voltage and phase current has to be multiplied by the number of phases. More on how to calculate the output power of a servo drive.

The worst-case situation is when a maximum torque (and therefore maximum current) is required at maximum speed. In this situation .  However, in most cases maximum torque is only required during start-ups, so the requirements of the power supply can be reduced.

The required input current for a brushed DC motor can be calculated as:

For brushless motors, it can be calculated as:

• Trapezoidal commutation (BLDC):

• Sinusoidal commutation (BLAC, PMSM): 

Where:

I_{DC in} is the power supply nominal current (for a DC driver).

ω_max is the no-load maximum speed achievable with this power supply voltage.

ω_{max torque} is the speed where maximum torque is applied. 

I_{phase max} is the maximum phase current provided by the drive in crest value (no RMS).

For multi axis systems, share the DC supply between all the drivers

If you can, use a single power supply for as many axes as possible. This will reduce the cost and increase the efficiency of your system as the braking energy of one of the axis can be used for the others.

Power supply type 

The power supply type should be chosen according to the cost, efficiency, EMI, and feedback requirements of our system. 

• Non-regulated rectified power supplies, based on a rectifier and a passive filter are cheap and efficient. Although the voltage is not regulated, the low-frequency input voltage ripple is not a problem in closed-loop operation. If an input EMI filter is not included, low-frequency harmonics are injected into the grid. Always use an isolation transformer if not included in the power supply. 

• Switched power supplies are more expensive, but they offer high efficiency and regulated output voltage. Usually, they include a power factor corrector (PFC), which reduces the grid harmonic distortion and improves the power factor. Use good quality switched power supplies that will withstand the varying loads of driving a servo driver.

Isolated power supplies

For safety reasons, it is important to use isolated power supplies.

Overvoltage / overcurrent protection

There is a special consideration if the power supply will be working at or near the maximum voltage rating of the drive.

When the motor is commanded to rapidly decelerate a large inertial load from a high speed (i.e. to change direction suddenly), care has to be taken to absorb the returned energy, as the motor becomes a generator and is driven in the 2nd or 4th quadrant. In this mode, the power stage is essentially a boost converter that can elevate the DC bus voltage to potentially destructive levels.

In most cases, a Shunt braking resistor could keep the system safe. Learn more on how to dimension a Shunt power resistor for regenerative braking. However, even if the system includes a shunt braking resistor, its response can be too slow or insufficient in front of massive rotor inertia at high speed driven by a too aggressive PID controller. In extreme cases like this, the only way the drive can truly remain protected is by stopping the power stage extremely fast. If the reaction time is small enough, the power stage will stop being perceived as a braking load by the re-generating motor, which will keep turning as a freewheel until friction stops it. Whenever the motor is not receiving an external accelerating torque, its BEMF can never be greater than the DC bus, so the drive will remain safe. 

Some power supplies (mainly switched power supplies) include active protections. It is important to choose a power supply with wide overcurrent and overvoltage margins since current pulses can be common in high torque applications and overvoltages can appear during regenerative braking. If active protections are included, we highly recommend choosing a power supply with an auto-restart function in order to not stop the application during transients.

Ensure you do not exceed the OVP (overvoltage protection) during operation.

Power dissipation

Referring to power dissipation, we recommend power supplies with convection dissipation for cabinet mounted applications, where a fan can be added if required. For applications with limited space, conduction dissipated power supplies can be used. 

DC bus bulk capacity

As a simple rule of thumb, the bigger the capacity already included in the power supply, the better. However, specific applications may benefit from additional bulk capacity installed in the DC bus. The DC bus bulk capacity plays a crucial role in the EMI response, but also in the motion control by reducing DC bus ripple. Typically, having these capacitors will respond to 3 purposes:

• Reduce voltage ripple in the DC bus to improve conducted EMI. 

• Reduce voltage ripple in the DC bus to reduce the capacitor's self power losses or the power loss throughout the supply wires.

• Store excess energy during reinjection or regenerative braking.

The third of these goals is also the most potentially destructive. During a re-injection, the DC bus voltage can rise even to damage the drive permanently. In such an event, 2 phenomena are acting at the same time, but have different natures: the first is the power stage acting as a boost converter and the motor as a generator, as described previously and also here Dimensioning a Shunt Resistor for Regenerative Braking. The second is the discharge of the motor inductance.

When the power stage is disconnected, the motor inductance must discharge its energy, and can only do it through the power stage body diode rectifiers by increasing bus voltage. This effect is extremely fast (often faster than the shunt braking resistor reaction) and cannot be blocked by stopping the power stage; it can only be dealt by storing the discharged inductive energy into the DC bus capacitors.

In order to calculate the largest inductance for a certain capacitance or calculate the necessary bus capacitance for a certain motor inductance, please consider the following expressions: 

• Energy stored in a capacitor:

• Energy increase stored in a capacitor while bus voltage increases:

• Energy stored in an inductor: 

Then, one way to approximate the total energy stored in the motor (no parasitic effects considered) is to examine the scenario where the quadrature angle is multiple of 1/6. In this case, while . With this,

• Energy stored in the motor:

So, for a given motor inductance (L), maximum instantaneous phase current (I_Max), and DC bus capacity (C_Bus) the maximum DC bus voltage to be expected (V_{Bus max}) can be calculated as:

Or, for a known maximum DC bus voltage, and a given DC bus capacity, the maximum motor phase inductance (L_Max) that can be disconnected under safe conditions can be calculated as:

Where Vbus max is the minimum between the power supply and servo drive maximum absolute voltages, Vbus is the nominal voltage, Imax is the maximum current of the application and Cbus is the sum of the drive and power supply capacitance.

Finally, notice the following about these calculations:

• These calculations represent a worst-case. They work under the assumption that the whole energy stored in the motor inductors is transferred to the capacitors with no loss. Conductor resistance and losses, body diodes V_f drop, parasitic capacitors, or the motor BEMF are not considered.

• Once the required DC bus capacity is calculated, remember there is only a need for adding capacity to the DC bus if the output capacity already included in the selected power supply is insufficient, or if a diode is used as inverse polarity protection in the DC bus. 

• In this case, remember to consider derating criteria as a function of the capacitor technology selected. For instance, ceramic capacitors require to compensate for a strong DC Bias derating and a significant aging derating.