No matter what the level of sophistication, the primary measurements used to design and test motor controllers are those of voltage and current. A digital oscilloscope with appropriate probes is the best method of making these measurements.

Start-Up Testing

Small motors such as those in power tools and some appliances have a switch that directly connects the motor to the power source. The switch might have multiple positions. For example a ceiling fan might have a forward/reverse switch and a variable speed control. The same might be true of a home drill press. These simple motor controllers often have no overload or overcurrent protection. They rely on this function being provided by a circuit breaker or other device. More complex and expensive motors used on factory floors usually have overload/overcurrent protection and also may have thermal sensing, metering and variable frequency drives. Such devices can range from lathes to conveyor belts. Several of these types of motor controllers may be contained in a single Motor Control Center (MCC) which makes it easier to monitor and maintain the motor controls.

One of the most important functions of the motor control is to properly start the motor. Start-up currents can be 7 to 10 times larger than the steady state current used by a motor. Inrush currents last for brief amounts of time and therefore the components in the controller and in the motor do not have to be rated to function continuously at those current levels. Nonetheless, proper start up is crucial to the safe operation and longevity of the motor.

Measuring Inrush Current and Power Dissipation

Measuring in rush current and the power dissipation during start up is an interesting measurement for both motor controllers and for power supplies. In both cases it is important to know the amount of inrush current and the power dissipation. Figure 1 shows an example of testing a device that has a substantial current spike. This example is a power supply, but the technique is the same for testing start up for any type of device. On the upper grids of the oscilloscope the yellow trace shows voltage vs time (100 volts/div) and the red trace shows current vs time (2 amps/div).

Figure 1:

The yellow trace on the upper grid of the oscilloscope is voltage and the red trace is current. The third trace is a zoomed detail of voltage and current (the highlighted portions of the upper two traces). Note the spike in current prior to the ramp up. The trace on the lower grid is voltage times current. The dashed lines are used to define portions of the trace used to compute parameter values

Although the general shape of the current trace is a well-controlled ramp, there is a clear current spike immediately at turn-on time. A detailed zoom of a portion of the voltage and current traces is shown on the third grid. The orange trace in the lower grid is instantaneous power (current x voltage) at each point in time. It is on the same time scale as the zoom traces. The two sets of vertical dashed cursors define “measure gates.” These are areas of interest for which the engineer would like the oscilloscope to compute specific measurements. In this example parameter 3, (P3), and parameter 4, (P4), are integrating the area within two important portions of the instantaneous power trace. Using this technique the oscilloscope measures the amount of power dissipation.

The values of voltage, current, instantaneous power and total power will vary greatly for different types of motors or power supplies. Motor control centers used on factory floors typically use 208 to 600 VAC but larger motors may require 2300 VAC or more. Large motors may have servo controllers or stepper motor controls which carefully limit the surge of current as the motor ramps up.

Probing Large Voltages and Currents

It is crucial when measuring large voltages and currents to use the proper probes. This is important for physical safety and for measurement accuracy. In addition to having proper rating of the probes to ensure they can handle the signal, the timing of the current and voltage signals is also crucial. In Figure 1, if the current pulse at start up is shifted slightly earlier or later in time compared to the voltage trace the measured power dissipation changes dramatically. However the time delay for signals to pass through the current and voltage probes may be different. Due to the Hall Effect and transformer technology used in most current probes the delay time for a current signal to pass through them and be received by the scope is usually longer than for a voltage probe. A delay time mismatch can be removed using an accessory such as LeCroy’s Deskew Calibration Source, the DCS015. This item is included as part of the Power Measurement Analysis (PMA) package. It produces simultaneous voltage and current signals. An engineer can connect one or more voltage and current probes to the DCS015 and look at the signals on the screen of the scope. Any delay mismatch between the probes can then be compensated.

Recommended Test Equipment

There are many oscilloscopes that have enough bandwidth to capture the signals from motors and motor controllers. The WaveSurfer and WaveRunner lines of oscilloscopes from LeCroy are a good match to this application. These oscilloscopes also have enough memory to capture the relatively long periods of time it may take to ramp up a motor. If digital signals are used to control the motor an MSO (Mixed Signal Oscilloscope) option is available for both the WaveSurfer and WaveRunner. Equally important are the accessories. A deskew calibration source (mentioned above) is crucial as are a variety of current and voltage probes. Because start-up of a motor may draw up to 10 times more current than steady state operation the engineer is likely to need more than one type of current probe to monitor both start up and steady state. Figure 2 shows an example of two current probes.

Figure 2:

The smaller current probe is rated for 30 amps while the larger one can handle 150 amps and still make accurate measurements. LeCroy offers a full line of current and voltage probes rated up to 500 amps and 20kv. High voltage differential probes and differential amplifiers are also available

In some situations the engineer may want to make high voltage differential measurements that are not referenced to ground. The LeCroy ADP300 (20 MHz) and ADP305 (100 MHz) are active differential probes - both rated at 1400 volts - that are well matched to this type of measurement. Finally, there are cases in which it is necessary to measure small signals riding on top of much larger voltages. For those measurements a differential amplifier such as the LeCroy DA1855A is required. This instrument has unequaled CMRR (Common Mode Rejection Ratio) of 100,000:1 and excellent overdrive recovery.


Motor controllers vary widely in design. Some are simple and easy to operate and test. Others are much more complex and, because they typically are used on more expensive motors, need very careful testing. A wide range of oscilloscope accessories that can handle a variety of currents, voltages and differential measurements are required for this application.