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Electrical Apparatus, Jun 1999

You are invited to send questions to this department. Your identity will be kept confidential, but we ask that you include your name, affiliation, postal (not e-mail) address, and telephone number If you'd like a pre-publication reply, enclose a stamped envelope and we'll try to respond promptly. We cannot provide personal consultation or supply winding data.

Surge protection for two motor voltages

A pipeline operator has a 1750 hp 3600 rpm motor that was built for 2400/4160 volt operation, for use at different pump stations supplied by one voltage or the other. The winding has failed a couple of times. The operators are blaming lightning, so they want to mount surge arresters on the motor. How should this be done to suit both voltage ratings?

This is basically a 4160 volt machine, insulated between turns and to ground for that voltage; it just has extra leads brought out for reconnection to 2400 volt operation if desired. So the same 4.5 kV class arresters intended to protect the 4160 volt insulation will be suitable for either connection.

Testing 50 Hz motors on 60 Hz power

For an export job, we are rewinding several standard three-phase motors (IS to 30 hp) for 50 Hz. We'd like to check them with a load test on our small dynamometer but aren't sure how to relate a 60 Hz shop test to the 50 Hz calculated performance. Do you have any recommendations?

Run whatever load tests you want, at 60 Hz, but apply 6/5 of the new nameplate voltage, and consider full load as 6/5 of the nameplate horsepower. The results should be close enough to check out your redesign.

Purpose of equalizers in a-c motor winding

We had a 300 hp, 590 rpm motor in for rewind, with a three parallel star connection. Each leg of the winding had an extra jumper tying together the center points of all the parallels. We replaced those jumpers as they were, and the motor performed properly, but we wonder why they were there. Can you explain?

Sometimes called balancing connections, these "equalizers" (not to be confused with d-c armature connections of the same name) have the electromagnetic effect of increasing the number of actual parallels in the winding, by equalizing the potential at the midpoints of each one. The result is a reduction in any unbalanced magnetic pull in the machine that results from unavoidable tolerance in the small air gap normally used at such a low speed.

Why take a single-phase locked-rotor test?

Your recent article on no-load testing (EA, April 1999) mentioned a NEMA standard provision for locked-rotor current to be measured as either three-phase or single-phase. What would be the purpose of a single-phase test on a three-phase motor? How is single-phase locked-rotor current related to the three-phase value?

The single-phase test reduces the current and power required from the supply. Single-phase locked-rotor current is 87% of the three-phase value, and power is only half.

How is line current related to horsepower?

We have been using a-c ammeter readings to check the loading on induction motors in various pieces of machinery. Our assumption has been that current is proportional to the horsepower. But the readings sometimes don't appear consistent with that, and we're no longer sure how to interpret the measurements. Should we be taking something else into account?

Current is easily measured. However, at any given load, it will be (very nearly) inversely proportional to voltage-at the motor terminals, not somewhere upstream on the circuit. For three-phase machines, phase balance also influences current; you need to check amperes in all three phases.

Finally, at loads below about half the rating, the current may vary much less rapidly than the power output, especially for motors 900 rpm and below. A linear relationship between current and load can be assumed (at balanced nameplate voltage) only for horsepower output within about 25% of the nameplate value.

How often can solid-state starters be operated?

For lower maintenance and easier starting, we've put new solid-state soft starters on several motors with good results. One of these starters, however, on a 75 hp granulator, has given us a lot of trouble. The motor checks out okay, but the starter had an overheat indicator that kept tripping, and the SCRs eventually failed. The supplier couldn't find anything wrong with the control circuits. A replacement unit is starting to act the same way. What could we look for in the application as a source of trouble?

Keep track of how often you're starting that motor In published solid-state starter ratings, the allowable frequency of starts is seldom stated. A "standard" starter may be good for 300% current for 30 seconds, whereas a "heavy duty" or "crusher duty" unit can take 450% or 500% current for that period-but nothing is said about how often that can be repeated. In this size range, the possibility is that starter operation cannot safely be repeated as often as the motor can be started You'll have to check that capability with the starter manufacturer.