Motor control circuit

[vanskat]

Curiously, how is the size of the control transformer in a motor control circuit calculated. Would it involve the starter coil and any circuit loads (eg relays) only?

Is there any documented procedure on how this is done?

 

[purplepete]

I'd look at inrush currents: need to know the contactor coil will be able to pull in OK. Other loads eg relays, may not need much but a large contactor coil must be able to pull in quickly.

 

[dpc]

Manufacturers typically have standard CPT sizes based on the starter size. You can specify additional CPT VA if you have a lot of auxiliary relays, lights and other junk powered from it.

 

[vanskat]

I have seen and used these manufacturer specs but I was just curious about the actual calculations from first principle.

 

[busbar]

Micron Industries [and surely others... Hammond or Acme?] have an application note for machine-tool control-power transformer sizing that incorporates inrush and low-pf loads in the unit’s sizing/regulation. Don’t forget hi-side fusing.

 

[jbartos]

Suggestion: Normally, any transformer load is calculated by obtaining the load Resistance R, Inductance L, Capacitance C, Power Factor, HP, Lock Rotor Currents of motors, and load timings or diversity since not all loads may be on at the same time. Then, the real power is added and reactive power is added to obtain VA=(Watt**2+Vars**2)**0.5 in VAs. The PF=Watt/VA (if not considering harmonics). The control transformers have relatively small motor load (fan, pump motor, etc.). Once, you obtain VA, some design VA margin is usually added unless the controls load is not anticipated to grow. Design margin can be as high as 50% especially, if some additional heating is needed to be added.

 

[busbar]

Without some review, sidestep NEMA 5-15R receptacles served from 120V-secondary starter CPTs, unless the transformer is sized for existing load plus the full receptacle rating. Some starter manufacturers routinly offer machine-tool transformers in their combination starters with an additional 100VA capacity for a "work light."

A rural research site had a fair quantity of indoor and outdoor electrical gear. Along the way combination size-2 starters were installed for some auxiliary pump motors away from process buildings, and it was later proposed that a heat tape was needed for a short run of above-ground piping. There was adequate capacity in the starter CPTs for the additional ~50-watt 120V load. The heat tapes came with molded 5-15P caps, and so electricians installed out-of-the-way receptacles, which allowed disabling heat tapes for warmer seasons.

At one point later the client facility was turning in trouble tickets for non-operational pumps. The ~2-amp CPT-secondary fuses were claimed to be opening whether the pumps has been started or not, but repeated spot load checks and a couple of 24-hr logger runs showed nothing of interest. The client group was a little miffed at the poor performance, fearing that pipe freezing and dead pumps could cause not only lost production—but worse—a new threat of winning an administratively taboo citation for material release. It was only after another fuse swap and again searching for the cause of open fuses, that when leaving the area, the electrician noticed a carpenter passing him on the small road to one of the pumps. He was indeed building timber access platforms for the client facility, and doing it in ‘spare time,’ so his work on the platforms was somewhat random.

The next nearest receptacle was ~100 feet distant, so understandably the carpenter didn’t want to lay out extension cords for his Skilsaw. The carpenter had found the receptacles at several pumps, and—of course—had recurrently and innocently popped CPT fuses. Other than disliking the additional tasks involved, he thought nothing significant of his electric saw dying after a fraction of a second.