Sizing a Variable Frequency Drive (VFD) for a 3-Phase Motor

Drives can be set up as single units that can be programmed individually or, in the case of more complex and interconnected tasks drives can be linked together in a master/slave system that allows a single drive to control multiple other drives. Connecting drives together like this reduces the amount of time spent programming drives and allows for a reduction in hysteresis for feedback control applications which, in turn, allows for smooth process control.With hundreds of electric motor and motor control options on the market, it can be hard to decipher which motor control will be the right fit for your application. Numerous nameplate ratings, a few different source voltage options, and thousands of different application types can easily add to the confusion. However, when armed with the right knowledge, the process can go from daunting to simple.

Here are the top nine areas of consideration that will help you find a VFD for your three-phase input motor that will operate efficiently and for a long time.

Motor Nameplate Ratings

The majority of the information you need to collect to find the correct VFD for your application is found right on the motor nameplate.

Full Load Amps (FLA)

The Full Load Amps rating is the number of amps the motor will draw when it is at full speed and full load. This is the most important rating to take into consideration and compare between the VFD and your motor. Always give your application some leeway when matching the FLA ratings (especially in constant torque applications). A slightly higher FLA rating in the VFD allows for intermittent overload situations without overheating, increasing the reliability and efficiency of the drive.

Voltage

When selecting your motor and VFD, it is crucial to note the available source voltage as the equipment will not operate on the incorrect voltage. Common voltage options that you’ll find are 208V, 230/240V, and 460/480V.

RPM

While many applications require you to control the speed of your motor, there are a few limitations. You should not run your motor at any less than the manufacturer’s recommended turndown; this could be 4:1, 10:1, or 20:1. Without an additional cooling system, the motor will overheat and fail due to the damage caused. If you intend to use the VFD to consistently run your motor at lower speeds than the manufacturer’s recommendations, an auxiliary cooling system is recommended.

You should also avoid running your motor at more than 20% above its max-rated speed. As speed increases, torque decreases, inhibiting the motor’s ability to move the load. When the motor has to work harder to move the load, you risk an overload/overheating failure.

Overspeed should generally be reserved for no/small load situations for motors that are not explicitly rated beyond 60HZ. As for motors rated for over 60HZ, they will generally be rated for constant power performance which means the motors torque capacity drops linearly as speed increases.

Inverter Duty

Another important rating to note is if the motor is inverter duty rated. You’ll find this on the nameplate for most manufacturers; however, you may need to do some digging for others.

While a VFD is the best way to control a motor, it can put a lot of extra stress on different motor components. A VFD produces a digital output rather than a sinusoidal waveform (how power is delivered from source). This digital output causes high voltages that put stress on the motor’s windings and rotor, which can lead to overheating failures. To help account for these high voltages from the VFD, manufacturers have designed inverter duty motors.

An inverter duty motor comes from factory with additional protections like higher rating winding insulation, grounding rings, insulated bearings, and in some cases, additional cooling systems. If your application requires a constant torque or requires the motor to run at RPMs outside of the allowable ratings, an inverter duty motor may be the most reliable option.

Variable or Constant Torque

In your search for a VFD, you’ve likely come across both variable torque and constant torque drives. Selecting between the two is as simple as understanding your intended application. For reference,some manufactures use the terms "heavy duty" to describe a CT rated VFD and "normal or light duty" to describe VT rated VFDs.

Variable Torque (VT)

Variable torque drives are designed only to provide the amount of torque needed at each speed, giving significant energy savings for certain applications. Variable torque drives are often found in centrifugal applications like fans or pumps, applications that rarely exceed the rated current.

Constant Torque (CT)

Constant torque drives provide consistent torque across a range of speeds. As such, they have an overload current capacity of 150% or more for up to one minute. You’ll find constant torque drives applied to conveyors, punch presses, positive displacement pumps, and the like.

Control Method

Drives can be set up as single units that can be programmed individually or, in the case of more complex and interconnected tasks, drives can be linked together in a master/slave system that allows a single drive to control multiple other drives. Connecting drives together like this reduces the amount of time spent programming drives and allows for a reduction in hysteresis for feedback control applications which, in turn, allows for smooth process control.

2 Wire Control vs 3 Wire Control

VFDs have two control methods, each with their own intended applications: 2-wire control and 3-wire control.

2-Wire Control

2-wire control VFDs have one switch for on/off operation. They are common in HVAC applications as the VFD can remain “on” even in power loss situations, allowing the motor to automatically restart as soon as power is returned to the circuit. This feature also allows the VFD to operate the motor in “power loss ride-through” mode in power drops of two seconds or less.

3-Wire Control

3-wire control VFDs have two switches. One to start the VFD and one to stop the VFD. These 3-wire VFDs are most common in conveyor system applications or situations where the VFD is to be operated from multiple stations with one central safety circuit that applies power to the VFD.

Enclosures

Much like motors, VFDs also have different enclosure ratings based on ingress protection (IP). IP is the ability of an enclosure to protect the internals from a contaminant like water, dust, or fingers. It is critical that you understand the environment you intend to use the VFD so you can select the appropriate enclosure for your application. An adequately protected VFD will work more efficiently and last a lot longer.

Altitude

The further away from sea level you are, the thinner the air and thinner air has a more difficult time off-putting heat than air at or below 1000ft. If your intended application is at a higher elevation than 1000ft, you should consider an additional cooling system or select an oversized VFD capable of handling the heat.

Atmospheric Temperature

Particularly in this Canadian climate, it is essential to note the temperature of the operating location before starting your VFD. In the summer, when temperatures run high, the VFD will have a harder time off-putting heat. You may need to derate the VFD, select an oversized VFD, or install an additional cooling system. Alternatively, in the winter, you will want to warm up your VFD before starting it. The capacitors inside the VFD can’t handle below freezing temperatures and may fail.

Summary

Selecting your next VFD for your three-phase motor doesn’t need to be an overwhelming task. By focusing on a few critical nameplate ratings and understanding the atmosphere of the intended application, you’ll be able to select a VFD that will work effectively and efficiently.

Know what you need? Here is a guide to find your VFD.

Does your application have special circumstances or sizing? Our technical experts would love to hear about your project and provide support so you can do what you do best, run your business.