A Step-by-Step Guide to UPS Sizing Based on Motor Load Calculation

Orson Zhong

2 months ago

Uninterruptible power supplies (UPSs) are critical components in safeguarding sensitive electronic equipment against power disruptions. When it comes to motors, which can draw significant amounts of power, especially during startup, correctly your UPS sizing is essential. When it comes to motors, the process of selecting and calculating the right UPS sizing can be slightly more complex. This is due to the higher power requirements and specific inrush currents, which often vary depending on the motor type.

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Understanding Motor Loads and Startup Currents

Before delving into the specifics of UPS calculations, it’s important to understand the nature of motor loads. Motors, particularly those used in industrial settings, often require a higher current during startup than they do when running at steady state. This surge in current demand, known as “inrush current,” can be several times the motor’s rated full-load current.

Considerations Before Calculating UPS for a Motor

Before jumping into the calculations, it’s essential to identify several factors that influence UPS selection:

Motor Power Requirements

Motors often require higher power during startup, which should be factored into the UPS capacity.

Motor Type

AC motors and DC motors may have different startup and running characteristics, affecting UPS sizing.

Runtime Requirements

Determine how long the motor needs to run on UPS power during an outage.

Efficiency

Motors and UPS units both have efficiency ratings, which impact the total power demand and UPS sizing.

Battery Capacity

The battery needs to be robust enough to handle the initial inrush current as well as sustained runtime.

Calculate UPS Sizing for a Motor

Step 1: Calculate the Motor’s Full Load Current

The first step in calculating the UPS sizing for a motor is to determine the motor’s full load current (FLC). This is the maximum current the motor draws under full load conditions, which is typically mentioned on the motor’s nameplate.

For example, if you have a 3-phase motor with a power rating of 10 kW and it operates at a voltage of 400V, you can calculate the FLC using the formula:

FLC = Power (kW) / （√3 * Voltage(V) * Power Factor）

Step 2: Account for Starting Current or Inrush Current

Motors typically draw 5-7 times their FLC during startup, known as inrush current. This inrush current places a high demand on the UPS, especially when selecting an inverter or battery capable of handling this peak load.

Calculate the starting or inrush current by multiplying the FLC by the inrush multiplier (usually between 5 and 7):

Starting Current = FLC * Inrush Multiplier

Step 3: Determine the Required UPS Power Rating

The UPS power rating should be able to handle the inrush current and sustain the motor during its operation. A general rule of thumb is to choose a UPS with a power rating that is 1.5 to 2 times the motor’s rated power to ensure stability.

For example:

If the motor power rating is 10 kW, the UPS power rating should ideally be between 15 kW to 20 kW.

Step 4: Calculate Battery Capacity

To calculate the battery capacity needed for the UPS, follow these steps:

Determine the Motor Runtime Requirement: How long should the motor run on battery power?
Calculate Power Consumption: Multiply the motor’s rated power by the runtime in hours to get the total energy requirement.
Consider Efficiency Losses: UPS systems typically have an efficiency of around 85-90%. Adjust the battery capacity by dividing by the efficiency.

For instance:

A 10 kW motor running for 1 hour with 90% efficiency would require a battery capacity of approximately:

Battery Capacity = (Motor Power * Runtime) / Efficiency

Step 5: Select the Appropriate UPS Type

For motors, online UPS systems are generally recommended due to their ability to handle high inrush currents and provide clean power with minimal switching time. Additionally, look for pure sine wave inverters, as they are better suited for motor applications.

We highly recommend that end users choose a transformer-based online UPS for motor loads. This type of UPS serves as an effective buffer between the power source and the load, significantly reducing the impact of sudden voltage spikes or surges.

The transformer’s surge protection safeguards connected equipment from damage due to transient voltages. In conditions such as brief power interruptions or substantial voltage fluctuations, the transformer stabilizes the output voltage, ensuring a consistent power supply to the equipment. This steady regulation minimizes the risk of malfunctions or shutdowns in connected devices.