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TMDSHVMTRPFCKIT: TMDSHVMTRPFCKIT Voltage Measurement

Ron Stewart
Intellectual 320 points
Part Number: TMDSHVMTRPFCKIT


I have three (3) of the HV Motor Drive Kits that we use on our servo motor test bed fixtures. We installed Murata DMS-20RM-3-AC1-R-C Panel Meters (PM) on our test fixtures, the inputs which are connected to a three phase multi switch to selectively monitor the L1-L2, L2-L3 and L3-L1 voltage outputs from the TI HV Drive. The Murata PM voltage measurements are steady, but radically higher than the actual HV Drive output voltages. A Murata engineer told me the PM measured voltage would not be accurate due to the Pulse Width Modulated signal (PWM) generated by a Variable Frequency Drive (VFD). He said he was not aware of a panel meter that had a built-in Low Pass Filter (LPF) to eliminate the inaccurate PM measurement.

My question is; can you help or guide me with putting together a LPF that will correct the PM display measurement? The measurements below hopefully will illustrate the voltage spreads between the Murata PM and the voltages and frequencies I measured with a Fluke 289 with an internal LPF. As you know, the Fluke 289 is designed to accurately measure the parameters of PWM VFDs.

The following are actual measurements taken while running our servo motor with the TI HV Drive with the Murata PM and a Fluke 289 with the LPF engaged.

1000 RPM - Murata PM: 184 Volts. Fluke 289: 50.7 Volts @ 67.1 Hz.

2000 RPM - Murata PM: 329 Volts. Fluke 289: 101.3 Volts @ 134.2 Hz.

3000 RPM - Murata PM 360 Volts. Fluke 289: 151.3 Volts @ 201.4 Hz.

If someone could help me with a LPF (or other device) to correct the measurement problem, I would very much appreciate your help.

Thank you,

Ron Stewart             

  • 0
    TI__Mastermind 27240 points

    If the PWM frequency is 10KHz, you may choose the LPF corner frequency to be a tenth of it, say 1000Hz. 

    Wc = 2*pi* 1000 = 1/Rf.Cf.

    Pick a 400v AC cap of whatever value you find and then calc Rf accordingly. 

    Hope it helps.

  • 0 in reply to Ramesh Ramamoorthy
    Intellectual 320 points

    Thank you for your help; I do appreciate it very much!

    Ron

  • 0 in reply to Ron Stewart
    TI__Mastermind 27240 points

    Forgot to mention that to reduce power losses in the resistor, you are better off picking a high value resistance and a corresponding cap. Estimate the power loss in resistor and pick the right wattage.

  • 0 in reply to Ramesh Ramamoorthy
    Intellectual 320 points

    Ramesh,

    Thank you for the additional information.

    I have an additional question; Since I am a novice when it comes to configuring a Low Pass Filter (LPF), my project involves measuring two phases of the TI HV Motor Drive such as phases, U-V, V-W and W-U. I know in a single phase application, the LPF is interconnected to the Line In and the Neutral. In my application, I need to measure the combined voltage of two phases (not one phase to neutral) from the two outputs of my three phase multi-switch. When measuring the combined voltage of two phases, is the configuration of the LPF and the interconnection of the LPF the same or, does it require a LPF for each of the two phases? 

    Thank you,

    Ron       

  • 0 in reply to Ron Stewart
    Intellectual 320 points

    Ramesh,

    Thank you for the additional information.

    I have an additional question; Since I am a novice when it comes to configuring a Low Pass Filter (LPF), my project involves measuring two phases of the TI HV Motor Drive such as phases, U-V, V-W and W-U. I know in a single phase application, the LPF is interconnected to the Line In and the Neutral. In my application, I need to measure the combined voltage of two phases (not one phase to neutral) from the two outputs of my three phase multi-switch. When measuring the combined voltage of two phases, is the configuration of the LPF and the interconnection of the LPF the same or, does it require a LPF for each of the two phases? 

    Thank you,

    Ron       

  • 0 in reply to Ron Stewart
    TI__Mastermind 27240 points

    You can do either ways. If you do it line to line, you need a higher voltage rated cap. If you use a fluke hand held meter to measure, then it is like this --> you need 2 LPFs such that the resistors of the two LPFs connect to A and B while their caps connect to C. The voltages across the individual caps wrt C will give Vac and Vbc, while the voltage between the two caps give Vab.

    Simple one is to use LPF between phase and DC- and use the scope math function to do the A-B thing to get line voltage info. Electric isolation between scope GND and inverter GND is VERY important otherwise scope and/or inverter may be damaged. Also, the scope should be capable of handling the inverter voltage. If these are nor possible, the first option given above is the way to go.