Motors & Drives Information

Questions concerning various AC drives often arise. Discussions with and answers from various users indicate that the operational behavior of ac drives are still a mystery. This article will clarify some of the concerns raised by typical user’s questions.

Typical Questions

1. When there is a short-circuit in the cable between motor and the AC drive, I always see that the DC bus voltage increases. Why?
2. What is vector control and is it possible to work without motor tuning (measurement of motor equivalent circuit)?
3. The manufacturer of an inverter suggests a maximum cable length of shielded cable between inverter and motor. Why? I believed that the maximum length is valid for unshielded cables only.
4. Every drive has a measurement of the AC output voltage to the motor. Is this the RMS value of the voltage?
5. I’m using AC drives in residential applications, with either 115- or 230-volt single phase input driving a 3-phase motor for pump use. Since water is involved, I’m fairly certain that GFI are required per NFPA 70. But AC drives trip standard GFIs, so it seems a catch-22.

Discussion Group Answers

Answers to question 1:

• The AC drive shuts down when it detects a short. So the DC link is not loaded any more. The voltage you see is mains voltage times sqrt(2).
• The DC bus voltage, when the drive is off, is the peak voltage of the mains: normally the RMS value times sqrt(3). However most input supplies have some distortion, and the resulting DC bus voltage is generally higher than expected. As long as the value is below the overvoltage trip point of the drive, no serious problems should occur. One concern does exist. That is when any stored energy finds its way back to the DC bus from the load. If the DC bus is too close to the design trip point, energy from the load can result in an overvoltage trip.
• When the drive output leads are shorted, virtually all drives will fault out in milliseconds, so I find it unlikely that the DC bus voltage could be read under those conditions.

Answers to question 2:

You can not run a vector control successfully without motor tuning. Magnetizing current and dynamic properties are the first problems—and there will be more. But, you can usually switch to scalar control (U/f) and get up and running without entering motor parameters (you should enter rated power, current, voltage and frequency to get the right operating point for the motor).

Vector control requires the characteristics of the connected load. It is important to remember that the cable is part of the connected load. Entering only motor parameters will allow operation but will not yield optimum performance.

Answers to question 3:

The problem here is that cable capacitance loads the inverter. A shielded cable has a higher capacitance and the length has to be limited or you have to derate the inverter. Reducing the switching (carrier) frequency also helps.

Unshielded cable results in undefined return paths from switching frequency contained within the current waveform. Distance and location of unshielded cable results in unwanted electrical noise in the facilities electrical system. Shielded cable reduces these conditions if correctly installed but will place predictable demands on the AC drive.

Answers to question 4:

The positive work being done is a result of the RMS value of the fundamental frequency. Any other measurement would be totally useless. If you measure the motor voltage with a True RMS meter, you will measure fundamental and high frequency components from the switching action (PWM). That is why you always get a higher reading when you read the motor voltage with an external DMM. Trust the read-out on the inverter.

Answers to question 5:

Personnel GFCI’s are not required (by NEC) for permanently wired circuits.

If you really want to have such protection, you need to use the equipment protective GFCI, which essentially are less sensitive, tripping at 30mA or more, compared to 4-6mA sensitivity for personnel GFCIs required in bathrooms or kitchen.

Application of an isolation transformer should not be ruled out. It is used for electric lawn mowers and similar electrical appliances outdoor. It should also be adequate for electrical pumps. However, the ground fault monitoring is needed.