View Full Version : Power Sag

05-04-2010, 07:15 PM
We are using a 40hp phase converter and are experiencing a voltage sag of about 20 volts when running our cnc lathe. Only when ramping the spindle up to speed for less than one second. Anyone know of a device that will smooth out the sag?

05-04-2010, 07:18 PM
Is this on the artificial phase or the 1 phase input also?
If so, it could be length of run and conductor size from the distribution point.

05-04-2010, 07:49 PM
Both the generated leg and the 1 phase leg.

05-04-2010, 08:46 PM
Then most likely length or wire gauge,or combination of both from service entry point.

05-05-2010, 06:24 AM
Hi Lyn

The power sag you are experiencing can be countered in several ways.

1. Assuming the twenty volts droop is not experienced when measuring at the power companies distribution board / meter. Then increasing the cross sectional area of the cable between the machine and the meter board would help. However the power taken for a 40 hp motor accelerating hard will be in excess of 30Kw or 260 amps. To get a lower voltage drop implies very large cables and this may not be practical. Assume you have tried running with reduced acceleration and found this unacceptable.

2. Wire all other circuits back to the power supply companies board independently from the wiring supplying the 40 hp motor supply. This will ensure lights and computers etc are recieving the least brown out or power droop due to the motor starting up.

3. Use a constant voltage transformer to feed any equipment - computers - lighting that is being caused problems by the power droop caused by the motor start up.

Constant voltage transformers are readily available for loads upto 3 or 4 Kw - do a google search for local suppliers.

Consider using a UPS to support computers and possibly lighting as these are becomming widely used to replace constant voltage transformers. However the UPS contains batteries and hence has a maintenance liability due to battery replacement every few years.

Whilst you are sorting out the wiring of your shop it might be the right time to look at checking or providing those essential wall mounted emergency stop buttons and contactor/s to interupt the power to the macinery. In particular is the contactor capable of interupting the supply to that motor. (Do not under any circumstances interupt the wires between the electronics of the VFD and the motor or you will wreck the electronics ONLY switch the input to the VFD from the main fuse board.)

Here are some links to get you thinking about constant voltage transformers.



For UPS ideas look at http://www.apc.com/

As an after thought as the motor start up surge is so high it might be causing problems to other users in neighbouring property. I which case the power supply company may increase the size of the neighbourhood cable to each property. Different companies have different approaches to how the supply cable upgrade is carried out. You may find a clause in your supply contract that covers excessive power fluctuations and this could result in you having to switch to a three phase supply. I ran into this problem with my last house but was lucky as the out buildings had been used by a farmer for a grain drier and there was a capped off three phase supply available that only required activating by the supply company.

Hope this helps - regards Pat