×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Number of starts/stops per hour when using VFDs

Number of starts/stops per hour when using VFDs

Number of starts/stops per hour when using VFDs

(OP)
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...

RE: Number of starts/stops per hour when using VFDs

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.

RE: Number of starts/stops per hour when using VFDs

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

RE: Number of starts/stops per hour when using VFDs

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 pirate

RE: Number of starts/stops per hour when using VFDs

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.

RE: Number of starts/stops per hour when using VFDs

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.

RE: Number of starts/stops per hour when using VFDs

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources