Designing in the right cable features

Dec 5, 2007 9:52 AM

Proper lengths, terminations and shielding, and sleeve materials make up the tripod of strengths on which good cable performance stands.

Connectors

Choosing connectors affects both the selection of cable style and overall reliability. With motors, either the cable must be terminated inside the connector, or an inter-cable connector must be supplied. In a gantry system (typical for cutting applications and electronic assembly equipment), the two axes of motion, X and Y, require an interconnecting cable assembly that traverses the moving tracks.

Cable insulation comparison
	INSULATION AND JACKET MATERIAL
PROPERTY	PVC	ETHYLENE PROPYLENE	NEOPRENE	POLYURETHANE	TEFLON
Abrasion resistance	XXX	XXXX	XXXX	XXXXX	XXXXX
Tear and cut resistance	XXXX	XXXX	XXXXX	XXXXX	XXXXX
Low temperature flexibility	XXX	XXXX	XXXX	XXXXX	X
UV resistance	XXXX	XXXXX	XXXXX	XXXXX	XXXXX
Ozone resistance	XXXXX	XXXXX	XXXX	XXXXX	XXXXX
Water resistance	XXXXX	XXXX	XXXXX	XXXXX	XXXXX
Transformer oil resistance	XXXX	XX-XXX	XX-XXX	XXXXX	XXXXX
Gasoline resistance	X	XX	XXX	XXXXX	XXXXX
Kerosene resistance	X	XXX	XXX	XXXX	XXXXX
Bleach	XX	XXXXX	XXXX	XX	XXXXX
Ethylene glycol	XXX	XXXXX	XXXX	XX	XXXXX
E = XXXXX VG = XXXX G = XXX F = XX P = X
Polyurethane and Teflon have the best properties for machine applications, but material selection is application-specific and depends on outside factors such as cost, life, and serviceability. PVC is most often used for motion control applications.

Recommended conductor current
Conductor size AWG or MCM	Conductor cross-sectional area, mm2	Ampacity per 75°C, NEC table 310-16, A
20	0.5	5
18	0.8	7
16	1.3	10
14	2.1	15
12	3.3	20
8	8.4	50
6	13.3	65
4	21.2	85
2	33.6	115
1	42.4	130
1/0	53.5	150
2/0	67.4	175
3/0	85.0	200
4/0	107.2	230
250MCM	126.6	255
300MCM	152.0	285
350MCM	177.4	310
400MCM	202.7	335
The conductor's cross-sectional area increases proportionally with current, and the cable has an ampacity of at least 125% full-load current. As the wire's thickness increases, so does the amount of current flowing.

Bend but don't break

In many motion control applications, the motor, feedback device, or both move relative to the controller and require special, high-flex cables. High and continuous-flex cables are a combination of conductors, insulation material, shields, and a jacket that can withstand mechanical impact. A typical continuous flex cable has many fine, bare copper strands covered with extra flexible PVC insulation and a PU jacket.

Bend radius requirements determine the service life of a continuous flex cable. The smaller the radius, the shorter the life. The minimum allowable cable bend radius is specified as a factor N multiplied by the cable outside diameter — for example "12 x cable diameter" — where N = 12. Properly selected and installed continuous flex cables have a life expectancy of several million cycles. Special flat cables have also been developed to decrease the limit on the dynamic bend radius.

The end

Any cable can fail prematurely when not properly terminated. One scenario is using hand tools to make electrical and mechanical connections between the conductor and contact when machine crimping is not available. Further, many stepper and servomotor drives come with terminal blocks for power cable termination. Alternately, a large number of stepper systems come with IDC connections. These must be limited to static applications, as motion will compromise the connector.

Proper shield grounding is required to reduce emissions, increase immunity, and prevent personal injury from ground currents. A safe practice is to bond shielded motor cables to the drive's back panel with metal cable clamps.

On the opposite end of the spectrum, a bad practice, is installing cable that is "just long enough" because it puts unnecessary stress on cable termination points and can form extremely sharp bends that reduce cable reliability. On the other hand, excessive cable length increases overall system costs and can degrade performance. Long cables degrade feedback signals, generate more heat in power connections, and encourage crosstalk, due to the cable's resistance, inductance, and capacitance. Excessively long, coiled power cables reduce drive voltages at the motor terminals and act as antennas, which radiate electrical noise interference.

For more information, call Danaher Motion at (866) 993-2624 or visit www.danahermotion.com