Vfd As Soft Starter?

[Amal]

Hello everyone.

 

I am having an equipment with two 22kW motors controlled by a single 80 kW VFD. Since my process speed has stabilized to a very small range, I am thinking of by-passing the VFD and also about replacing the motors by Eff1 motors.

However, I wish to use the VFD as a soft starter. Can this be done? What sort of precautions I must take to prevent damage to the drive. I will have to avoid backfeeding, right? Any ideas on design of the by pass circuit?

 

Also, am I likely to get an improvement in the electrical efficiency by doing all these? Can the gain be around 10%?

 

Thanks.

 

Amal

[jraef]

Hello everyone.

 

I am having an equipment with two 22kW motors controlled by a single 80 kW VFD. Since my process speed has stabilized to a very small range, I am thinking of by-passing the VFD and also about replacing the motors by Eff1 motors.

However, I wish to use the VFD as a soft starter. Can this be done? What sort of precautions I must take to prevent damage to the drive. I will have to avoid backfeeding, right? Any ideas on design of the by pass circuit?

 

Also, am I likely to get an improvement in the electrical efficiency by doing all these? Can the gain be around 10%?

 

Thanks.

 

Amal

Hi Amal.

 

It's possible, but trickier than most people realize. If you leave the VFD in the circuit, you are losing energy compared to running across the line because of the losses in the VFD. Those losses are typically in the order of 3-5%, but it also depends on if you have to do harmonic mitigation, which can add a few more percent. There are also some additional losses in the motor and if the motor and VFD are in an air conditioned space, those losses all add up to energy consumption to remove the heat. It's difficult to put an overall value on the losses, but 10% would be unlikely, probably closer to 7% overall.

 

So if you plan to bypass the VFD, here is the tricky part. You can NEVER apply line voltage to the output terminals. You must have an isolation contactor on the VFD output that is never closed at the same time as the line starter. Since an AC motor will become a generator if the windings are excited and the rotor speed is above the theoretical line voltage source, you must allow a second or two for the motor's magnetic field to collapse before re-energizing. That means there is going to be a moment, however brief, where the motor is disconnected from any power source while you change over. So when you open that VFD isolation contactor and wait a second to close the line (bypass) contactor, the motor will slow down. When re-connected across the line then, the flux in the motor needs to be reestablished and there is an inrush of current again. Not for long, but potentially just as high as if the motor was at a standstill. This can cause a torque spike as well. Depending on the load, this may or may not be a problem. If it is a high friction load, the slowdown is greater and the effects are worse. High inertia loads are often not a problem at all. Pumps are hit and miss, it all depends on a lot of factors. 2 motors on one drive is a bigger potential problem though, because differences in the motors may make one of them keep exciting the other, meaning you will be better off with a longer transition delay and possible repercussions.

 

Can you just replace the VFD with a soft starter? All those issues go away when you do that.

 

[Amal]

Off course, it is better to replace the VFD with a soft starter. But, that would be an additional cost. I thought that I might just use the VFD to cut some cost.

Hi Amal.

 

It's possible, but trickier than most people realize. If you leave the VFD in the circuit, you are losing energy compared to running across the line because of the losses in the VFD. Those losses are typically in the order of 3-5%, but it also depends on if you have to do harmonic mitigation, which can add a few more percent. There are also some additional losses in the motor and if the motor and VFD are in an air conditioned space, those losses all add up to energy consumption to remove the heat. It's difficult to put an overall value on the losses, but 10% would be unlikely, probably closer to 7% overall.

 

So if you plan to bypass the VFD, here is the tricky part. You can NEVER apply line voltage to the output terminals. You must have an isolation contactor on the VFD output that is never closed at the same time as the line starter. Since an AC motor will become a generator if the windings are excited and the rotor speed is above the theoretical line voltage source, you must allow a second or two for the motor's magnetic field to collapse before re-energizing. That means there is going to be a moment, however brief, where the motor is disconnected from any power source while you change over. So when you open that VFD isolation contactor and wait a second to close the line (bypass) contactor, the motor will slow down. When re-connected across the line then, the flux in the motor needs to be reestablished and there is an inrush of current again. Not for long, but potentially just as high as if the motor was at a standstill. This can cause a torque spike as well. Depending on the load, this may or may not be a problem. If it is a high friction load, the slowdown is greater and the effects are worse. High inertia loads are often not a problem at all. Pumps are hit and miss, it all depends on a lot of factors. 2 motors on one drive is a bigger potential problem though, because differences in the motors may make one of them keep exciting the other, meaning you will be better off with a longer transition delay and possible repercussions.

 

Can you just replace the VFD with a soft starter? All those issues go away when you do that.

 

[jraef]

Short term initial expenditure, yes. But over the long run, that 3% energy loss will add up to a lot of money every year... forever.