Voltage drop on VFD output at low speeds

[philly]

I wanted to figure out how to calculate the voltage drop to a motor that is on the output of a VFD, for the motor operating at low speeds.

Lets say that we have a motor/vfd combination that at full speed and full curerent has aprox a 3-5% voltage drop which is usually what is deemed as acceptable. We know that this voltage drop will be a function of bus voltage, cable size, motor current, and cable distance.

What happens however when the VFD is operating at low speeds and thus the voltage output from the drive is reduced to maintain a constant V/Hz? Lets also assume that we are dealing with a constant torque load, so it is possible that full current is needed at low speeds in order to produce full torque. Now since we have the same size cable, the same distance, and the same current, we will still have the same amount of voltage dropped across the cable, however this drop will now be a larger portion of the output voltage to the motor.

So this same voltage drop which may have been 3-5% of the output voltage at full speed and full drive output voltage (bus voltage) will now be a larger % of the reduced drive output voltage at low speeds.

Is this increased % voltage drop something to be concerned about at low speeds? Can this reduce the V/Hz at the motor and cause issues? Is this why drives have voltage boosts at low speeds?

 

[StephenSDH]

What happens however when the VFD is operating at low speeds and thus the voltage output from the drive is reduced

The drive maintains a fairly constant bus voltage. The drive then uses PWM (pulse width modulation) to drive the motor. It doesn't recreate a 5hz sine wave at 40V Ac. It reduces the % duty cycle at low speeds, but it is still using the full bus voltage. I doubt someone can give you a clear answer.

You want minimize the distance between the drive and the motor if possible. When the motor is a far enough distance to worry about voltage drop you may likely have other issues. At longer distances you may run into reflected wave issues, potentially doubling the voltage at the motor terminals and causing insulation failure.

AB article on Reflected Waves
http://www.ab.com/drives/techpapers/ieee/pcic.pdf

 

[Strahan]

I wanted to figure out how to calculate the voltage drop to a motor that is on the output of a VFD, for the motor operating at low speeds.

Lets say that we have a motor/vfd combination that at full speed and full curerent has aprox a 3-5% voltage drop which is usually what is deemed as acceptable. We know that this voltage drop will be a function of bus voltage, cable size, motor current, and cable distance.

What happens however when the VFD is operating at low speeds and thus the voltage output from the drive is reduced to maintain a constant V/Hz? Lets also assume that we are dealing with a constant torque load, so it is possible that full current is needed at low speeds in order to produce full torque. Now since we have the same size cable, the same distance, and the same current, we will still have the same amount of voltage dropped across the cable, however this drop will now be a larger portion of the output voltage to the motor.

So this same voltage drop which may have been 3-5% of the output voltage at full speed and full drive output voltage (bus voltage) will now be a larger % of the reduced drive output voltage at low speeds.

Is this increased % voltage drop something to be concerned about at low speeds? Can this reduce the V/Hz at the motor and cause issues? Is this why drives have voltage boosts at low speeds?

Most V/HZ drives can be modified to boost the voltage applied to the motor while running or starting. This will force more current through the stator and in effect increase the starting and running torque at low speeds. In AB powerflex drives this boost function is only available while operating in the custom V/HZ control mode.
If your application requires you to run at low speeds and still maintain close torque regulation you may want to try operating in sensorless vector control mode. In this mode you will not be required to provide any feedback from the motor. Current and voltage reflected from the motor to the drive provides more control of motor speed and torque at low speeds.
Calculating your voltage drop at low speeds first you will have to know your V/HZ ratio; which is calculated as VH=V/F where VH=Ratio, V=nameplate voltage rating of motor, and F=operating frequency.
An example 460V rated motor operating at 60HZ would give you a ratio of 7.67; so for an increase or decrease in 1HZ voltage will change 7.67VAC.
The reflected wave phenomenon can be reduced by adding line reactors or filters to the conductors motor or drive. This phenomenon will produce a voltge of approximately twice the output voltage of the drive and will be reflected at the motor. This can be due to long runs of conductors usually specs say any runs over 30feet. I would look close at your distance and also look at what type of motor you are using standard or inverter duty. Inverter duty will provide motor windings that are rated to 1600volts higher than the standard ratings of 1000volts. Running a standard motor at low speeds will also cause inadequate cooling of the motor. Some things to consider.

 

[weressl]

I wanted to figure out how to calculate the voltage drop to a motor that is on the output of a VFD, for the motor operating at low speeds.

Lets say that we have a motor/vfd combination that at full speed and full curerent has aprox a 3-5% voltage drop which is usually what is deemed as acceptable. We know that this voltage drop will be a function of bus voltage, cable size, motor current, and cable distance.

What happens however when the VFD is operating at low speeds and thus the voltage output from the drive is reduced to maintain a constant V/Hz? Lets also assume that we are dealing with a constant torque load, so it is possible that full current is needed at low speeds in order to produce full torque. Now since we have the same size cable, the same distance, and the same current, we will still have the same amount of voltage dropped across the cable, however this drop will now be a larger portion of the output voltage to the motor.

So this same voltage drop which may have been 3-5% of the output voltage at full speed and full drive output voltage (bus voltage) will now be a larger % of the reduced drive output voltage at low speeds.

Is this increased % voltage drop something to be concerned about at low speeds? Can this reduce the V/Hz at the motor and cause issues? Is this why drives have voltage boosts at low speeds?

The concern has been taken away by the ASD software. They - the drive designers - are interested in delivering the required voltage AT the motor terminals for the desired operation and are aware the 'obstacles'. The volatge drop calculation would be also be made more complex by the carrier frequency, since resistance of the materials are give to DC or AC with a sinusodial waveforem and 56/60Hz. When you start talking about 8000 or 16000 Hz pulsed DC carrier frequency your normal resistance values are not longer valid, not to mention the capacitive leakage, the magnetic or nonmagnetic conduit question, etc.

Nothing that slick math won't take care of....

That is just one reason why drive manufacturers are jealously guarding their operating software.

 

[philly]

Is it necessary to use load reactors on a drive that has a inverter duty rated motor that is aprox 150ft away from drive. Motor and drive are 40hp.

 

[weressl]

Is it necessary to use load reactors on a drive that has a inverter duty rated motor that is aprox 150ft away from drive. Motor and drive are 40hp.

Vey unlikely, but the drive manufacturer will list in their manual the limitations for the various cable installation options and you should follow that.