Chuck Yung 
EASA Technical Support Specialist 

A good friend recently reminded me of a tip that can save you a lot of trouble when repairing motors with an aluminum frame: Never hot-dip a stator with an aluminum frame. 

In the bake oven, the aluminum frame expands faster than the encased steel stator core. The core is loose inside the aluminum frame and, if dipped hot, varnish seeps into the gap and cures.

That destroys the concen­tricity between the stator bore and the bracket fits. The critical airgap between the stator bore and rotor becomes eccentric, and the rotor might even drag on the stator bore. Even if it does not rub, the eccentric airgap is likely to cause electrical noise when the motor runs. 

That reminded me of other valuable tips worth sharing. So that is what this article is about: Tips to help you do your job faster with less effort and to avoid mistakes; and a few that you could classify as opportunities to improve a customer’s motors. 

Unequal grouping/consequent-pole 
Four members called last month about a recent design issue: the combination of unequal grouping and consequent-pole connections. This is never a good idea, because the pole-phase spacing varies more than is allowable. The worst case is when such a winding is the low speed of a multiple-speed, 2-winding motor. If you encounter a consequent-pole connection with uneven grouping, the safest course of action is to redesign it to a salient-pole winding. 

Coils with high turn counts 
Sometimes you have to wind a solenoid coil, brake coil, or DC field coil with thousands of turns of small wire. To cut the time in half, wind half the turn count using two wires in hand, then connect the two strands in series (finish to start) and the coil will have the correct turns. If you said to yourself, “Gee, I can use four in hand and save even more time,” you’re right. Just balance the time saved against additional connection effort. 

Use the same tip when converting from metric to AWG wire, instead of winding half the turns and splicing to another wire size. For example, a coil wound with 1000 turns of 1.25 mm wire could be rewound with half the turns using one #16 and the other half using one #16.5 AWG wire. Rather than wind 250 turns with the #16, and splicing the #16.5, simply loop 500 turns with both wires, then connect them in series. 

Changing speed and hp(kW) rating 
If a customer asks us to redesign a motor for a different speed, but wants to know “How much horsepower can I get from this redesign?” here is the short answer: For a constant torque change, the original hp times the original poles = the new hp times the new poles. 
Poles₁ x hp_l/Poles₂ = hp₂ 

The phrase “constant torque” is important: Torque is proportional to airgap density squared. With the same airgap density, the hp changes in inverse proportion to the pole change. 
Next time someone asks how much horsepower you can get from his 250 hp, 4-pole motor if redesigned to 6 poles, the answer is simple: 
(250 x 4)/6 = 166; we can prob­ably get 175 hp with no problems. 

The rest of the story 
When considering a pole change, other items that must be considered are: 

• The stator slot-rotor bar combina­tion. 
• Backiron: The fewer the poles, the more backiron is required. 
• Rotorconstruction and maximum safe speed. 
• Current density (circular mils/amp).

While a change from 4 to 8 poles may look viable, a change in the other direction is likely to be restricted by the backiron dimension. Changes to or from 2 poles will almost always require a large reduction in torque. When changing poles, always check the stator slot-rotor bar combination against the the EASA Technical Manual. Die-cast rotors are usually safe up to 2-pole operating speeds, but fabricated rotors may not be able to withstand a redesign that includes a speed increase. 

Turns and circuits can be changed in proportion without affecting the flux (or motor perfor­mance.) In other words, a winding connected 2-delta with 6 turns per coil and five #15 AWG wires in hand is equivalent to a winding with a 1-delta and 3 turns/coil using ten #15 wires. The flux, and therefore the torque, would duplicate the original design. You might prefer fewer circuits to simplify the connection, or more circuits to reduce the wire in hand. 

When increasing the number of circuits, watch for these pitfalls: 

• Unequalgrouping. 
• Poles must be divisible by circuits. 
• Voltage stresses in random-wound coils change in proportion to the number of circuits. 

Unequal (“odd”) grouping 
When a motor has an unequal coil grouping (not all groups have the same number of coils) each parallel path must have the same number of coils. Otherwise, circulating currents will cause heating and increased magnetizing current. 

When considering a pole change, if the number of slots divided by (poles x 3 phases) results in a whole number (i.e., no fraction) then we can use as many circuits as poles. 

Poles must be divisible by circuits 
We know that you cannot use a 4-circuit connection on a 6-pole motor, but sometimes a winder gets in a hurry, makes a set of coils with twice the turns and suddenly we’re trying to connect a 2 & 4-delta on a 6-pole winding. It won’t work. 

Delta and wye 
Dual-voltage NEMA motors are usually designed for a 2:1 ratio, such as 230/460v. Most of the rest of the world uses a v3 ratio such as 230v/ 400v. Where those in the U.S. might use a 9-lead (e.g., 1 & 2-circuit) connection, everyone else is using a delta/wye connection. Here is where the trouble starts: An international manufacturer checks the volts/Hz ratio for the 400v 50 Hz versus 480v 60 Hz and determines that the 400v 50 Hz motor will operate fine on a 480v 60 Hz supply. (400v 50 Hz is proportional to 480v 60 Hz.) So far, no problem – but then your customer connects it for low voltage. Now, we have a problem. 

The IEC motor has 6 leads, and performs as expected when con­nected wye for 480v 60 Hz. The problem goes back to the v3 voltage ratio for the delta/wye connection: 480v divided by v3 is 277v. Our customer runs the motor on 230v, and the torque drops—by the square of the voltage ratio—to only 75% of rated. 

Even though the speed increased by 20% (60/50 Hz = 1.2), the motor is still underpowered for the applica­tion at 83% of the original rating. 
(230/277)2 = 0.69; 1.2 x 0.69 = .83 

So when a customer sends in a 6­lead, dual-voltage motor for repair, find out the operating voltage. If the motor is operating on 230v, either redesign it for the correct voltage or reconnect it for 9 leads. For ex­ample, if the winding is connected 1-delta/1-wye (230v/400v) for 50 Hz, reconnect it as a 1 & 2-wye (230v/460v) 60 Hz. If the nameplate of a wye/delta connected motor is labeled 230v/460v, then the motor nameplate is misleading. 

The more common application of this is the 2300v/4000v winding, which is connected delta for 2300v volts or wye for 4000v. 

To change voltage from wye to delta, the turn multiplier is v3, or 1.732. To change from a delta to a wye connection, we use the inverse of that, or .58, as the multiplier (1/ (v3) = .58). We can sometimes take advantage of that ratio when a customer requires a non-standard voltage rating. For example, we may have a 12-lead motor that is connected 2-wye for use at 480v. We can externally connect it 1-delta for 550v operation: 
480 x 0.58 = 278.4; 278.4 x 2 = 557v. 

While within the +/- 10% pre­scribed by NEMA MG1, should a customer require a motor to operate at both 460v and 575v, we can redesign a slight compromise so that it performs satisfactorily at both ratings. 

Random wound medium-voltage 
Not all random-wound 2300v motors last as long as our customers would like. They were pleased to find such an economical first-cost motor, but the winding rarely lasts as long as a comparable motor with form coils. To improve the reliability for your customer, convert the core to accept form coils. Have replacement laminations made by one of the companies that specializes in lamina­tions. EASA Technical Support can work with you to accomplish this improvement. 

The winding will last longer, the customer saves money compared to a replacement and you get to perform the repair. 

Wedges and form coil windings 
When rewinding form coil motors, we use a bottom stick to protect the bottom coil from damage, and a separator between the coil sides. Some winders also use a topstick filler, for the wedge to slide against as it is driven into position. 

Other winders chamfer the bottom edge of one end of each wedge instead. Both methods protect the coil from damage, but the chamfer makes the wedge easier to start and allows you to cut one less piece of material per slot. 

For on-site rewinds, another tip is to put a thin strip of saturated felt on top of the coil, with a thin topstick on top of that for the wedge to slide against. If the wedges are really tight, brush on some resin to ‘grease’ the wedge. By reducing friction, the wedges are easier to drive. Plus the resin, once cured, will glue the wedges in place. 

For large bore, low speed ma­chines, especially those with hard coils, cut a wood ramp to support the span coils. 

Categories: Repair procedures & tips

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