Number of starts/stops per hour when using VFDs

[cokeguy]

There was an interesting remark within the "Starting a used 22kW motor" thread about having no restriction with the number of starts/stops when using VFDs. Could somebody comment on that? For example, when we bring in somebody to dynamically balance our fans which have 200 HP 4-pole motors, we are always cautious about not attempting more than 4 starts per hour, even though VFDs with slow acceleration curves are used in those fans. Could we safely make more than 4 starts per hour in that case? How many? Thanks...

 

[CJCPE]

With a VFD, torque is fairly closely proportional to current. The motor can deliver close to rated torque at any speed without exceeding rated current. If the current is limited to rated current with a current limit function or by limiting the acceleration rate, the motor can be started and accelerated to full speed as often as the ramp-up / ramp-down sequence time permits. If the current is allowed to exceed rated current, determining the number of starts per hour would be a matter of calculating the effective value of the current averaged over time.

 

[itsmoked]

CJCPE is spot on!

Only gotcha is a lot of slow speed time would mean normal cooling is lacking a bit.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Bingo to both of you, but I'll add to Keith's comment that external motor cooling fans can overcome that issue and besides, on a Variable Torque load such as a fan, low speed operation usually means very little current anyway, which means very little heat to begin with. Still, all of these issues should be considered.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ozmosis]

It also depends on how you are starting the motor with the VFD. If you are using the control inputs (the digital inputs) to start/stop the motor (i.e. the VFD is already powered up) then there are no restrictions other than the time it takes to accelerate/decel. If, however, you are applying power to the VFD and either relying on the auto-start function of the VFD or simply then closing the RUN command, then you will have limitations on most VFD's as to how many times you can power it up per hour. This is due to restrictions on controlling the inrush to the pre-charge circuits within the VFD. The latter is not too common and not generally recommended but people do it all the same.

 

[CJCPE]

sed2developer has a good point. In the situation described by cokeguy it is likely that the VFD input power will be shut down for safety when people are actually working on the fan. In this case, it is best to check with the VFD supplier to determine limitations on power-up cycles. The other alternative would be to put a lock-out disconnect switch between the VFD and the motor and leave the VFD energized.

 

[cokeguy]

You are right CJCPE, we do shut down the VFD for safety reasons when balancing, so I´ll check with Saftronics (we use 200HP GP10 drives on those fans) about that. Thanks everyone.

 

[bacon4life]

Marke also has a good explaination in

http://www.eng-tips.com/viewthread.cfm?qid=96292.