Motors, Drives

Selecting a scope for a variable frequency drive

Select the right oscilloscope to fit a variable frequency drive (VFD) application. Consider scope types, probe, voltage, software, and harmonics.
By Robert Bonczar February 9, 2019
Courtesy: Yaskawa America Inc.

There are many brands and styles of oscilloscopes to consider and it’s critical to understand how to select one based on certain criteria. Many of them look very similar leaving it difficult to select one that will suit the user’s needs. There are precautions to take to avoid pitfalls during the selection process.

An oscilloscope, sometimes referred to as a “o-scope,” is an electronic piece of test equipment used to measure and monitor and provide a visual representation of signals on a screen.

First, the user measures the vertical height of the signal (amplitude) in voltage and the horizontal cycle (period). This is done by connecting probes to the scope and connecting them to the test point of a circuit.

A probe is a cable with a banana connection on one end that affixes it to the oscilloscope and the other is the end connect to the point of a circuit to measure so the signal appears on the screen. The probe also will have a connection for the common or ground.

Examples of these signals can be voltage, current, or frequency, to name a few. The displayed signal can show any anomalies or problems that might not otherwise be seen with other test equipment such as a digital multi-meter (DMM).

Selecting an oscilloscope

Different types of oscilloscope models are desktop, USB, and portable (handheld). A desktop model is typically used in a facility or lab environment. A handheld model would be suitable for travel because of its portability and built-in features such as ground isolation and rugged design. A USB model would be used in lab or the field for its small portable size, as well as being able to interface with a laptop or PC thus removing the need for a separate screen. This would require also purchasing software to use it on a computer interface. All these scopes do the same thing. So why buy one over the other? What are the differences? See table.

Scope type   Low voltage probes High-voltage differential probes Software Current probes
Handheld Standard * * Option
Desktop Standard Option * Option
USB Standard Option Not option, but extra charge Option
* -not a necessary option

Although many of the desktops look similar and pricing of the oscilloscope is comparable, they cannot check high voltage by themselves. Oscilloscopes come with a lower voltage passive probe. These probes connect to the oscilloscope to measure the point of a circuit to see the signal.

The voltage measured on the probe is the intended signal and the other is the ground or common, which is referenced with 0 V. This common can also be tied to ground internally in the oscilloscope and the plug. The voltages are smaller also for example 5 V, or 24 Vdc with low current (see Figure 1).

Figure 1: Example of a low-voltage passive probe that comes standard with an oscilloscope. This shows how using the ground clip of the probe is internally tide to ground of the scope. Courtesy: Yaskawa America Inc.

Figure 1: Example of a low-voltage passive probe that comes standard with an oscilloscope. This shows how using the ground clip of the probe is internally tide to ground of the scope. Courtesy: Yaskawa America Inc.

An optional probe is required when checking the higher line voltage like 230 Vac or a variable frequency drive’s (VFD’s) pulse width modulation (PWM) output for the higher power checks. It’s critical to read the side of the probe for the intended voltage levels. They can be damaged if they’re undersized.

The ground may also be tied to other probe connections. It is necessary to isolate these connections so the scope is not damaged, shorting the power to the ground, or interfering with the other signal connections.

The ground clip is now a voltage potential and the signal is compared to the voltage the probe is connected to.

When used with a scope that doesn’t have a ground tied to the earth of the scope, a high-voltage differential probe should be used so it isolates the grounds. If someone cuts the ground off the plug and lets it float, this is dangerous because now if someone comes into contact with the scope and touches any metal the person may get shocked or killed.

This high-voltage differential probe is not a standard feature, but an option. Depending on the manufacturer then this adds an additional $600 to $1,300 on average per probe. This is another reason why it’s important to review all options before making a decision.

A handheld model is an exception to this because it is designed to be able to use the probes that come standard to check high and low voltages. The ground is isolated and the unit is designed against shocks unlike desktops.

Eight factors to consider for an oscilloscope

The key is to understand what is needed for each application. The following eight factors need to be considered when selecting an oscilloscope:

Signal displays: Oscilloscopes typically come with two or four and some with more connections or channels to connect the probes to. This also allows the user to have more signals on the screen to compare.

Current probes: These are used to measure current by the field around a wire that would go to a motor or line on the side of the voltage supply. This can show the current strength and any imbalance when comparing to another line or phase (see Figure 2). Some of the higher-end scopes will include one in the kit while others require this as a separate add-on option.

Figure 2: A coil probe is shown around the wire its measuring. Courtesy: Yaskawa America Inc.

Figure 2: A coil probe is shown around the wire its measuring. Courtesy: Yaskawa America Inc.

Harmonics measurement: Some of the more economical scopes will have this as an add-on option or not at all. Looking at the circuit’s harmonics could be important because they might play a part in the performance of a device such as a VFD.

Manufacturer: Users might go with a manufacturer they are familiar with rather than a new one. Do some research. A new manufacturer might be a better fit and more cost-effective for a particular application.

Service: Talk to someone in the tech support group. See how much they know about their product. Ask about the product and its specific features.

Testing: Request a demo of the product. Typically, companies will be open to a trial period before purchase. They also are often willing to meet to answer questions. This also a good opportunity to get a sense of supplier representation and support.

Check the warranty: On average, the typical warranty is between 1 to 3 years.

Flexibility: Is this product upgradeable or is this an older model that will be phased out? This might explain why it is such a good price if it’s the latter.

An easy setup and economical solution that fits the user’s needs also leads to selecting the right type of oscilloscope.

Robert Bonczar, is a product training engineer at Yaskawa America Inc. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media,


KEYWORDS: Oscillosopes, variable frequency drives, VFDs

Factors to consider when selecting oscilloscopes

Different types of scopes are desktop, USB, and portable.

Choose an oscilloscope that is easy to set up and fits your application.

Consider this: What criteria do you need to consider to select the right scope for your needs?

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Robert Bonczar
Author Bio: Robert Bonczar is a product training engineer at Yaskawa America Inc